Recent Advances Toward Optical Devices in Semiconductor-Based

INVITED PAPER

Recent Advances Toward Optical Devices in Semiconductor-Based Photonic Crystals Photonic crystals are being developed to perform fast optical switching, filtering and routing, and optical frequency conversion, and to form spectrometers-on-a-chip and several types of lasers. By Henri Benisty, Jean-Michel Lourtioz, Senior Member IEEE, Alexei Chelnokov, Sylvain Combrie´ , and Xavier Checoury

ABSTRACT

|

Photonic crystals, artificial, wavelength-scale

multidimensional periodic structures, have given birth to a number of realizations in semiconductors. Photonic integrated circuits, especially around new integrated lasers, are challenging directions of research for miniaturization and new functions in optical telecommunications. We review the basic physics behind such applications and underline the current status of this very active research field worldwide. KEYWORDS

| Demultiplexers; lasers; optoelectronic; photonic

crystals (PhCs); photonic integrated circuits (PICs)

I. INTRODUCTION A. Brief History and Basic Concepts of Photonic Crystals (PhCs) This introductory part will give an overview of PhC concepts and the challenges of miniature photonic circuits before detailing the outline of the rest of this paper. We

Manuscript received May 26, 2005; revised September 15, 2005. This work was supported in part by the FUNFOX FP6-IST 04582 European project and in part by the CRISTEL French project. H. Benisty is with the Laboratoire Charles Fabry de l’Institut d’Optique, Orsay Cedex 91403, France. J.-M. Lourtioz and X. Checoury are with the Institut d’Electronique Fondamentale, UMR 8622 du CNRS, Orsay Cedex 91405, France (e-mail: [email protected]). ´ is with Thales Research and Technology TRT France, Domaine de S. Combrie Corbeville, Orsay Cedex 91404, France. A. Chelnokov is with CEA CEA-LETI, Grenoble 38054, France. Digital Object Identifier: 10.1109/JPROC.2006.873441

0018-9219/$20.00 Ó 2006 IEEE

assume that the reader is familiar with basic Bragg reflection physics, as occurs in fiber Bragg gratings (FBGs), distributed Bragg reflectors (DBRs) and distributed feedback (DFB) devices. Light–matter interaction is the heart of optoelectronic devices. A core aspect of this is refraction, the fact that materials react with their dielectric constant. When properly used, e.g., in DFB lasers under the form of a one-dimensional (1-D) grating, it profoundly shapes the device emission. PhCs are essentially a multidimensional generalization of periodic structures. Two notable features distinguish this field from the previous studies of optical gratings. First, most of the physics based on PhCs reveals its interest for large index contrast. Second, PhCs represent an unprecedented link between solid-state physics and optics. From the former, they borrow most notably the bandgap that makes them optical analogues to semiconductors. From the latter, they borrow the Boptical physics[ background from interference to beam shaping, thereby bridging the gap between lasers and ultimate sources exploiting quantum electrodynamic effects. They are indeed one of the preferred options toward the ultimate control of light, down to spontaneous emission itself. Yablonovitch [1] and John [2] in 1987 marked the emergence of the field. John’s idea, simplified, was that light scattering in a disordered assembly of very strong scatterers could turn to localization of light, trapped in subparts of the medium. Yablonovitch’s idea was that the Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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Benisty et al.: Recent Advances Toward Optical Devices in Semiconductor-Based Photonic Crystals

three-dimensional (3-D) propagative states of light could be separated by bandgaps forbidding propagation, just as for electron in semiconductors (Si, Ge, GaAs, InP, GaN, etc.). Spontaneous emission would then be inhibited for an emitter having its natural spectrum within this photonic bandgap (PBG). Furthering this idea, the ideal electrooptic converter was some kind of thresholdless laser whereby light emission was forbidden in all but the one desired mode. However, the route from the concept to the device was long and inhibition was not the whole story. First of all, in order to achieve such ultimate devices, a very small mode volume is necessary. The use of dielectric and semiconductors is to be preferred to metals because of their much lower losses at optical wavelengths. Despite the recent emergence of Bplasmonic[ devices at subwavelength scales [2], semiconductors are indeed a sound choice, thanks to their mastered emission properties. Today, spontaneous emission control is best achieved in semiconductor-based microcavities that can be mostly viewed as engineered defects in a periodic structure (with the clear exception of microdisks). A resonant mode is thus created at a specific frequency in the structure bandgap. The opening of an omnidirectional bandgap is still a touchstone of PhC studies. Two-dimensional (2-D) and 3-D structures are the salt of the story. Multidimensional periodic media were familiar in optics, as, for instance, holograms, but the index contrast was not strong enough to provide a full bandgap. Only a directional gap arose. Actually, the bandgap condition is akin to the standard Bragg condition of X-rays; the path difference in the scattering by two planes/rows in succession should be an integer multiple of the wavelength. Fig. 1(a) represents the quasi-normal incidence situation of a wave impinging on a stack of alternate materials. This stack is nothing but a high-reflection coating. If it has a small index contrast, the band opening and the reflection stopband are narrow [Fig. 1(c) and (d)]. Upon tilting the angle of incidence, the peak frequency evolves with a classical cosðÞ factor in simple cases. Hence, the initial quest was to find whether dielectrics could be structured enough to create very wide bandgaps. If wide enough, a shift with direction will still leave a sizable overlap between all the directional bandgaps. As illustrated in Fig. 1(e) and (f), the goal is to find periodic structures, e.g., arrays of cylinders that possess an omnidirectional gap in 2-D or 3-D. In 1987, everything was to be proven in 3-D. The use of the vector Maxwell equations in 3-D soon revealed that the simpler close-packed structures could not have a true PBG. A desired band splitting was forbidden by symmetry at the W point of the fcc lattice’s first Brillouin zone. This prompted a thorough understanding of the nature of the bands and their associated electromagnetic field distribution. Two variants of the diamond lattice emerged from this. On the theoretical side, a diamond-like array of high998

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index spheres was proved to be the simplest system. A clever version imagined by Yablonovitch consisted in drilling three sets of holes that mimic the (111) oriented Bgalleries[ of silicon as displayed in crystallography textbooks. This indeed resulted in the first demonstration of a bandgap at microwave frequencies. Another clever structure imagined by the team at Iowa State University is the Bwoodpile[ or Blayer-by-layer[ stack [4]. PhCs became increasingly popular after these successes. Large investigations of these novel structures toward optical frequencies started around 1995. Simultaneously, with the prospect of mass production, self-assembly was sought to obtain large arrays of spheres. However, despite impressive fabrication results, only some degrees of functionality toward optical devices have been demonstrated yet, and the field of 3-D PhCs in 2005 appears as essentially academic to the telecommunication and optoelectronic community. Conversely, 2-D PhCs have been easier to explore using existing planar technology. In order to achieve fine structures with 200–800-nm periods and mark-space ratios often different from unity, several tools were available, such as e-beam lithography and, to a lesser extent, the deep-UV lithography stepper projectors of mainstream microelectronics. The first convincing demonstration of 2-D near-infrared PhCs was made in 1996–1997 [5], [6]. The subsequent years rapidly brought to light more complex structures of interest for telecommunications such as microcavities, waveguides, and their various combinations. This paper is focused on these structures, as they lead to consider PhCs as promising candidates either for future miniature photonic integrated circuits (PICs) or for ultimate sources based on the extreme confinement of light. Actually, there is hardly a competitor to PhCs in the nanophotonic arena, displaying such a fascinating physics. The basic physics of 2-D bandgap is summarized in Fig. 1(e), (g), and (h). Fig. 1(e) represents the main directions along which a wave can impinge on a triangular array of holes drilled in a high-index matrix ðn 3:36Þ. The hexagonal Brillouin zone has six equivalent K and M points. The band structure in Fig. 1(g) represents the frequencies of allowed photon modes in such a lattice. Only the TE modes with the magnetic field H along the hole axis are shown here for simplicity. The allowed modes are also called Bloch modes because the Bloch theorem used for solid-state crystals also applies to PhCs. The absence of resemblance with more familiar guided wave diagram is a consequence of Bfolding.[ Each mode is indeed composed of interrelated Fourier components of wavevectors k; k þ G1 ; k þ G2 . . . , where fG1 ; G2 ; G3 ; . . .g is the discrete set of 2-D vectors of the reciprocal lattice. Although the gap along K direction lies at higher frequencies than the one in the M direction (K is further away from ), they have a sizable overlap: the full photonic


Fig. 1. Basics of PhCs. (a) The 1-D case, in quasi-normal incidence, with Bragg-type reflection. (b) Wavevector representation. (c) Dispersion relation of waves in the periodic stack. (d) Reflection stopband. (e) Left: omnidirectional reflection on a piece of 2-D PhC with a hexagonal lattice. Right: first Brillouin zone of the lattice. (f) Inhibition of emission in a 3-D PhC. (g) TE band structure of a 2-D triangular lattice of holes in a matrix of dielectric constant 11.3, with a 30% air filling factor. (h) Gap map for the two polarizations.

band gap. Fig. 1(h) shows the evolution of the full PBG versus the air-filling factor f of the crystal for both TE and TM polarizations, the latter being far more delicate to obtain (especially due to the tiny veins between the holes

at high f values). Conversely, for the TE gap, hitting a predetermined frequency range is well achieved by stateof-the-art nanofabrication, whose first-order accuracy on the f value is about 5%. Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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B. The PIC Concept In the area of Bconventional[ integrated optics, confinement is obtained by using a core material of higher index than the surrounding medium. The index step is a crucial parameter, determining losses at circuit bends, propagation losses in waveguides and coupling losses to fibers. These quantities are expressed in dB/90 , dB/cm and pure dB, respectively. The photonic lightwave circuit (PLC) platform refers to doped silica on silica, with minute index steps of n ¼ 0:01. The minimum bend radii are about R ¼ 10–40 mm to keep the losses below 1 dB/90 . Meanwhile, the mode profile is very close to that of fibers, as the index step n and the core size ( ¼ 5–10 m) are comparable. Scattering losses at the interfaces are also very weak. Their basic scaling law can be derived from perturbation theory. The polarization induced by the unperturbed field E in a microbump of volume V is P ¼ ðEÞ ¼ ð"EÞ. As the power radiated into a homogeneous environment goes like the square of the induced dipole p ¼ ðVPÞ, incoherent scattering losses then scale like ð"Þ2 ðnnÞ2 if " 2nn is used. Increasing the index step thanks to, e.g, silicon nitride, or semiconductors, bend radii can be drastically reduced, but tighter mode profiles are obtained, hence a more delicate coupling to fibers and a higher sensitivity to surface roughness. The almost extreme case is that of silicon-on-insulator (SoI) with large n (2–2.5), the value of which depends on the cladding used on both sides of the silicon guides. The use of such high-index steps allows not only the fabrication of sharp bends, but also that of compact splitters (Y junctions) and compact frequency selective devices (demux). For example, the area needed to implement a phasar (or AWG) on SoI can be as small as 100 100 m2 instead of several square centimeters on silica [7]. Compact single-frequency devices such as cavities closed by two distributed Bragg reflectors can be designed as well. Continued efforts are devoted to optimize the mirror performances in confined wave geometries. Regarding active devices that accomplish functions such as wavelength conversion, reconfigurable add–drop filtering, signal regeneration and clock recovery in PICs, it might be believed that a minimum physical interaction length is needed to operate them in proper conditions. For instance, a length of 300 m is typical of edge-emitting lasers. This being, if optimally designed in-plane mirrors are used, one can produce edge-emitting, integrated lasers as short as a vertical cavity surface-emitting laser (VCSEL) with the same material gain. More generally, the mastering of the in-plane confinement of light opens large opportunities to revisit active and passive devices as well as to explore new phenomena as mentioned in Section I-A. Thus, it has been progressively recognized that innovative solutions could stem from the ultimate control of light by wavelength-scale structures. In this review, we shall give a flavor of several novel effects that go well 1000


beyond the simple index-step scaling, tackling, for instance, the management of group velocity and dispersion, or the generation of single photons on demand. Fig. 2 presents some basic aspects of PhC building blocks, along with a futuristic PIC. Fig. 2(a) and (b) show a localized defect consisting of: (a) one missing hole and (b) three missing holes. Holes in the neighborhood of the defect are of reduced size in (a) as compared to the rest of the crystal, while in (b) two side holes are shifted (see later). Such defects support tightly localized modes, quickly decaying in the cladding (see Section II-C). Fig. 2(c) and (d) show two uses of a line-defect in a PhC. In (c), the defect is used as a waveguide along the line of missing holes. In (d), it is used as an in-plane Fabry– Perot (FP) cavity for a wave that propagates transversally. The essential difference with the classical guiding mechanism based on total internal reflection is pictured in (e)–(f). PhC waveguiding actually occurs because all the elementary waves scattered by Bphotonic atoms[ interfere constructively in the defect region, and destructively in the outer PBG regions. The difference between the two types of guiding is obvious in the Bdielectric rod[ configuration, whereby PhC guiding takes place in the low index medium. Conversely, for the Bair hole[ situation in a high-index matrix, the PhC represents a medium of average index well below the core index. Hence, index guiding and photonic bandgap effects can be combined. (See Section II-A). Having substantiated some simple PhC building blocks, it is possible to envision the advent of ultracompact PhCbased circuits [Fig. 2(g)]. Most elements are those of a general PIC except for in/out functions (ideally wavelength-scale tapers) needed to couple to external fibers. In the present example, photodiodes are placed near the Bbar[ channel on top in order to monitor specific wavelengths, namely, those of wavelength division multiplexing (WDM) or coarse WDM (CWDM) channels (see Section III for more detail). Bends and couplers are used to derive a fraction of the input signal toward a switching system. Selected wavelengths are mixed with those from integrated lasers. Next, they are amplified in a slow wave amplifier specially designed to achieve high gain in a miniature footprint. Note that in order to combine active and passive functions on a chip, the electronic bandgap requires adjustment, or very ingenious arrangement, in order to overcome the intrinsic absorption of unpumped active material.

C. PhC Versus Previous Breakthroughs The development of semiconductor heterostructures, one of the pioneering advances in the 1960s, was prompted by the need of a first level of confinement for photons and electrons in optoelectronic devices. A second decisive progress in the 80–90s has been the development of quantum well and quantum dot structures with a much radical improvement of the carrier confinement also leading to a much wider exploration of optoelectronic


Fig. 2. (a) Defect forming a microcavity (note the smaller holes in the immediate neighborhood of the defect). (b) An elongated microcavity design (here an SEM view is used) proposed by Noda et al., note the deliberate shift of the arrowed hole. (c) Line defect as a waveguide. (d) As an FP cavity. (e) Classical index guiding. (f) PBG guiding, by construction of all interferences. (g) A futuristic PIC with many PhC elements: in/out coupling, couplers, bends, add–drop filters, dichroic splitters, slow-wave active devices, tunable lasers.

materials. However, contrary to electronic integration, optical integration has not developed at the fast pace of Moore’s law: no more than two or three functions can currently be cascaded on sixteen channels, for example. In this context, the concept of PhC arises considerable expectations. They indeed lead to envision the possibility of realizing optical circuits to channel, analyze and combine an increasing number of optical signals in a more compact form than previous solutions of integrated optics. Even if catching up Moore’s law remains beyond reach, the breakthrough of full wave control would lead to

a decisive acceleration of integration. The practical impact of PhCs in the domain of optical telecommunications is already recognized with the development of PhC fibers, which have definitely reached the market place. In the field of semiconductors, going from concept to actual applications has taken more time than had been expected. Fortunately enough, most of the technological difficulties are in the process of being progressively solved thanks to the advances, which are taking place in the fields of microtechnology and nanotechnology. This is the main purpose of this paper to survey the progress that has Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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recently been accomplished in the development of semiconductor-based PhC components and circuitries.

D. Description of the Paper Contents The rest of this paper is divided into four sections and a conclusion. Section II deals with building blocks of semiconductor-based photonic circuits such as waveguides, bends, and microcavities. State-of-the-art performances are reported for these elements with a special attention to waveguide propagation losses and microcavity Q factors. Sections III and IV deal with systems closer to components. Passive components are first considered in Section III. It is shown how the use of bandgap and dispersion properties of PhCs can help us to take up some of the great challenges of optical communications, namely, the control of group velocity dispersion at small length scales and the design of miniature filters and spectrometers for wavelength selection. Active components including lasers and nonlinear devices are considered in Section IV. It is shown that 2-D PhCs are potentially applicable to a large variety of laser systems from ultimate photon sources to VCSELs and efficient single-mode miniature lasers. New opportunities are also open for nonlinear devices and low-threshold all-optical switches thanks to the tight confinement of light in PhCs. All these device perspectives are illustrated through recent works. Section V briefly presents the recent advances in the field of 3-D PhCs. Section VI concludes the paper.

II. SEMICONDUCTOR-BASED PHC CI RCUIT RY A. Vertical Confinement of Light in 2-D PhCs Whereas 2-D PhCs are ideally adapted to planar semiconductor technology for an in-plane confinement of light, an obvious question arises: what is the confinement of light in the third direction? In the simple case of an unstructured slab cladded by media with index nclad , the guided modes possess propagation constants, which are necessarily larger than nclad !=c. The dispersion curves of these modes lie outside the light cone k ¼ nclad !=c. In the case of a periodically structured slab, band folding occurs, and dispersion branches that previously were outside the light cone may now partly or completely lie inside it. Correspondingly, the guided modes may become leaky modes when the periodic modulation (lattice vector G) can generate a Fourier component at a k value smaller than nclad !=c [Fig. 3(a)]. This leaky component is, in turn, eliminated using a smaller value of nclad [Fig. 3(b)]. As could be expected, a high-index contrast between the slab waveguide and the claddings is more desirable to provide lossless guided modes over a wide range of frequencies. In practice, there are two main approaches of 2-D PhCs in planar optics. The most standard approach for III–V optoelectronics is the substrate approach where periodic 1002


holes are etched through multilayer waveguides with a low or moderate index contrast between the core and cladding layers [Fig. 3(c)]. A deep etch process is required to reduce the diffraction losses at the PhC holes. Because of the high index of the cladding layer, the light line typically intersects the Brillouin zone edges below the gap [Fig. 3(e)]. In other words, defect modes created in gap will be leaky modes. The second approach relies on a strong vertical confinement as provided by a III–V semiconductor membrane in air [Fig. 3(d)] or by the SoI technology as well. The etch process is much less demanding on account of the limited depth. Truly lossless defect modes can exist in the gap [Fig. 3(e)]. In contrast, the pure membrane approach is penalized by an increased influence of heating effects and a difficult implementation of electrical excitation. If we now consider the linear waveguide of Fig. 2(d) inserted in a slab [Fig. 3(g)], the issue of the light line arises again, but only one dimension ðkinplane Þ is concerned. In a generic case and focusing on a single dispersion branch of the waveguide, the situation will be that of Fig. 3(f). A mode in the waveguide manifests itself as a dispersion curve in the diagram, while the bands of the ideally infinite PhC surrounding it, once projected, take the form of a continuum. Let us consider, for instance, the membrane case with an air cladding. There are many different regions along the frequency axis. Region 1 still has no mode at all in the gap (below the light line), region 2 is only part of the waveguide mode dispersion curve below the light line. In region 3, the guided mode is intrinsically leaky, and radiates in the air cladding. Finally, in region 4, the mode has the same characteristic as region 3, but modes of the PhC continuum are present at the same frequency, so that the Blight insulating[ properties are lost. Light injected at the guide entrance may then spread into the surrounding PhC rather than to couple only to the desired mode. It is thus clear that the vertical confinement has a strong interplay with guiding properties. Nevertheless, when losses are acceptably low, the behavior of the system can be largely thought as 2-D.

B. Waveguides 1) Waveguide Dispersion Playground: Fig. 3(f) shows that PhCs present various kinds of dispersion characteristics. There is actually a vast playground for waveguide dispersion in these systems, much more than in nonperiodic photonic wires. A first-order design, that of canonical structures, is obtained by not drilling one or several rows of holes. These waveguides are commonly termed BWn[ for n missing rows ðn ¼ 1; 2; . . .Þ along the K axis, i.e., the dense rows of period a. The dispersion relation of a W1 waveguide in a 2-D PhC is shown in Fig. 4(a), with the same general features as Fig. 3(f) (projected bands, etc.). Even in this narrow


Fig. 3. (a) A guided mode and the effect of periodicity G in k space, inducing leakage. (b) Not inducing leakage for small index cladding. (c) The substrate approach. (d) The membrane approach. (e) Light line superimposed on the 2-D PhC dispersion relation. (f) Dispersion relation of a line-defect, and the various frequency region defined by the light line and the gap. (g) A line defect defined into a semiconductor slab.

waveguide, two guided modes exist in gap. One presents an even field pattern at various points of the dispersion curve, whereas the other possesses an odd symmetry of the magnetic field distribution. Let us concentrate on singlemode domains. The single-mode domain of low frequency/ high k is actually the only one extending outside the light cone, thus in the Blossless[ region. A large dispersion arises in this domain, as the slope of the dispersion curve, the group velocity of light, goes rapidly to zero. For 1.5-m applications, there are sizable frequency ranges, say tens of gigahertz, within which the group velocity can be of the order of c/100 to c/1000. It is believed that this performance, low group velocity on sizable spectral span, is inseparable from the large index contrast approach. In the

single-mode domain, the group velocity dispersion (GVD) reaches values larger than 100 ps/nm/mm, i.e., values 107 times larger than those of optical fibers. This is simply because the same dispersive retardation, say 1 ps/nm, is attained within one tenth of a mm (100 m, or even less) in the PhC instead of 100 m in the optical fiber. The price to pay for this rather exceptional guiding behavior is a strong modification of the mode profile with a large extension into the surrounding PhC [Fig. 4(c)], making it more sensitive to structural imperfections. Birefringence is another aspect of PhC waveguides. Due to the very nature of the bandgap, the two polarizations have drastically different behaviors. The TM gap is much smaller than the TE one [Fig. 1(h)]. On the other Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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Fig. 4. (a) Typical dispersion relation of a W1 waveguide (matrix index 3, air filling factor 30%), with projected bands shaded in gray. (b) Same for a W3 waveguide. (c) Magnetic field maps of three selected modes along the slow branch fraction of the W1: note the spreading in the surrounding crystal as the continuum band is approached. (d) Micrograph of a taper between the two kinds of waveguides (see [8]).

hand, guiding in both polarizations can be achieved in the absence of gap thanks to index guiding of the TM mode while the TE mode is guided by a bandgap effect. Birefringence can then be huge, with beat length of a few micrometers. For more complex structures, with slanted hole walls, polarizations can mix, and the picture is more complex. The exploitation of such effects is sought for polarization conversion and polarization diversity. Periodicity may induce specific gaps on the dispersion relation. This is apparent in Fig. 4(b), for the case of a W3 waveguide. The several branches in the gap are associated to modes that are the various intermediates between those of Fig. 2(c) and (d). These modes possess different propagation constants along the guide. Different signs of slopes arise from the folding at the Brillouin zone edge. As a result, branches tend to cross. However, as shown by an arrow in Fig. 4(b), anticrossing may occur instead, leading to a small Bminigap[ or Bmini-stopband[ in the dispersion relation of the dispersive mode, e.g., the fundamental mode of W3 with the largest negative slope on this diagram. At such specific frequency and wavevector values, energy is exchanged between the fundamental mode and a higher order mode, as these modes share the same frequency and the same momentum modulo a reciprocal wavevector shift of k ¼ 2 =a. The energy launched in the fundamental waveguide mode can then be 1004


Btransferred[ into the laterally oscillating mode. This is a novel game, not feasible in classical devices based on Bragg reflection. We will report later on its exploitation in microspectrometer chips (see Section III-D). The periodic modulation of the waveguide width can be an asset if one wishes to play with dispersion. Transitions and tapers are thus welcome. One such example is given in Fig. 4(d) for a practical realization based on the introduction of a few holes of variable diameter [8]. Once more, it is the beauty of strong index contrast systems that such a short taper (a few micrometers) may work almost perfectly in a sizable frequency range of tens of nanometers around 1500 nm. Practitioners in the domain are thus facing a delicate task in theoretically exploring the various PhC structures. Let us give here a brief list of the available tools. The planewave expansion method is certainly the most familiar tool for calculating the dispersion relations. A free version exists from the Massachusetts Institute of Technology (MIT), Cambridge1 [9]. This method has been used, for instance, to calculate the dispersion relations of Figs. 1(g) and 3(e). It amounts to write the field’s scalar component, say the vertical component of the magnetic field Hz , as a 1 [Online]. Available: http://www.opticsexpress.org/abstract. cfm?URI= OPEX-8-3-173


sum of plane waves on the basis {G} with adequate coefficients. A 2-D supercell has to be used to periodize the nonperiodic dimension of a waveguide, provided the guided modes do not couple each other through the PhC Bwalls[ of the supercell. The use of a 3-D supercell is more cumbersome for 2-D waveguide applications, since the fictitious periodicity along the Bvertical[ axis normal to the waveguide induces many artifacts and parasitic mode couplings. In any case, the plane wave method is limited to finding mode frequencies and patterns of an infinite system (in the periodic directions), but is not adapted to describe the fate of a particular external wave impinging onto a finite structure. A number of methods exist to treat finite problems while remaining in the frequency domain. Many of them can be collectively assigned as scattering matrix methods [10], resting on various basis functions such as Bessel functions for spherical scatterers and Fourier components in most of the other systems. These treatments are also tightly related to advanced diffractive optic treatments and grating theory [11]. Alternatively, readers familiar with solid-state physics may find interesting to look at an application of the Fermi golden rule to efficiently calculate scattering losses from a guided mode to a continuum [12]. The eigenmode expansion freely available at Ghent2 is also commendable. Finally, the finite-element method and associated tools such as the popular BHFSS[ software of microwave practitioners are also of current use. In the time domain, the finite-difference time-domain (FDTD) method is extremely popular. Freewares are available.3 FDTD calculations can indeed produce snapshots of the strange field behavior in these novel materials. Precautions with boundary conditions must be taken to avoid spurious reflections, by using, for instance, the socalled perfectly matched layers. The FDTD method is, however, a modest design tool for the early step of conception, as it completely ignores the modal picture. 2) Waveguide Propagation Losses a) Predictions: Losses in PhC waveguides are of course a crucial question. The light line story only tells the basis, i.e., whether radiation losses are possible or not for a guide of infinite length. In practice, the guide is of finite length with possible transitions to other photonic devices. It has also imperfections such as irregularities in hole diameter and position, nanometric roughness of the walls, etc. Hence, even outside the light cone, losses may be induced by these imperfections. Three-dimensional modeling incurs a large computation penalty. Accounting for the third dimension in a fictitious manner in 2-D simulations is thus of great interest. This has been done through an imaginary part of

the dielectric constant in the air holes [13], [14] and applied to several structures. Within this approach, scattering is described as a mere dissipation and, therefore, all coherent effects, including the light line, are ignored. This approach is thus most appropriate for PhC structures carved in substrates [Fig. 3(c)]. Practical loss figures are known for state-of-the-art deep-etching as explored in the PCIC project.4 The imaginary part of the dielectric constant in the air holes may be as low as "00 ¼ 0:02. For a homogeneous loss dielectric medium, the field amplitude would decay as expðn00 ð!=cÞrÞ, and the power attenuation would be given by A "00 4:34 ð!=cÞ=n0 (decibels per unit length) where n0 is the modal index and "00 ¼ 2n0 n00 . However, for waveguides, the modal overlap of the fundamental mode with the PhC holes may be tiny. This alleviates the penalty, since we should now take A "00 4:34 ð!=cÞ=n0 . T h u s , t h e v a l u e o f "00 ¼ 0:02 translates into losses of 10–20 dB/cm for W3 waveguides ð 0:01Þ, and about 10–20 dB/mm for W1 waveguides (larger ). Hence, propagation through 100 periods (40 m) of a W1 waveguide still leaves, in principle, a quite reasonable signal (0.5-dB loss). As for membranes, theory predicts that losses for the mode of Fig. 4(a) and (c) are huge above the light line (say 1 dB for ten periods!), and zero below. Imperfections of the structure may give rise to scattering. There is still a debate whether the roughness, for instance, will lead to losses comparable toVor smaller thanVthose of photonic wires with a similar degree of lateral confinement: the average medium being highly structured around the PhC waveguide, hand-waving arguments are delicate. Optimistically, one may assume that spontaneous emission inhibition implies that an irregularity only gives rise to a lossless evanescent wave. Systems with intermediate index contrast such as SoI and reported membranes pose different problems. Not only is there a slightly smaller room below the light line than in the pure membrane case, but also there is no definite mode symmetry and polarization, since the top and bottom claddings now differ. This couples previously uncoupled TE and TM branches at their numerous crossings. b) Measurements: Measurements of PhCs and related waveguides recourse to various methods. A generic approach is the Bpseudocutback[ method, whereby the transmissions of guides of different lengths L are compared under the same excitation and assuming the same conditions of fabrication. The results of measurements are fitted to an exponential law expð LÞ from which the modal loss is deduced. Let us consider the excitation by an external laser focused onto the cleaved facet of the studied sample [endfire technique of Fig. 5(a)]. In practice, ridge access

2

[Online]. Available: http://camfr.sourceforge.net/ E.g., at KTH, Sweden. [Online]. Available: http://www.imit.kth.se/ info/FOFU/PC/F2P/ 3

4

[Online]. Available: http://www.ist-optimist.org/proj.asp

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Fig. 5. (a) Measurement of a PhC device by the end-fire method, using ridge access waveguides. (b) Measurement of locally scattered light intensity , and logarithmic plot. (c) Internal light source technique, based on embedded quantum wells or quantum dots in the heterostructure. (d) Corresponding results for 15 rows of PhC along the K direction. Data from various periods are stitched together on the normalized frequency axis.

waveguides have to be implemented to accommodate the very different lengths involved in the sample: a few tens of micrometers for the PhC waveguide, and about 1 mm for the cleaved chip itself. A careful fabrication is then needed, since proximity effects in electron-beam lithography jeopardize fabrication uniformity. A reproducible coupling to external fibers is also needed to perform the pseudocutback method with a good accuracy. Another method relies on Btop view[ measurements [Fig. 5(b)]. The uniformity of the light scattering mechanism is built-in for leaky mode (unlike roughnessinduced scattering). The light scattered by the PhC is measured along the guide to be studied. SoI structures have been investigated by this method, and Bloch modes were evidenced through their standing wave patterns in a top view arrangement [15]. Alternatively, the reflections on the uncoated cleaved facets can be advantageously exploited. FP cavities are indeed formed between the cleaved facets and the ridge/PhC transitions. The analysis of the FP fringes contrast gives access to the cavity losses and the corresponding exponential factor expð LÞ as well. The use of an internal light source instead of external injection may offer an additional flexibility for character1006


izing either a large number of structures or structures of high complexity. Quantum well or quantum dot layers embedded in the waveguide core can be photoexcited for this purpose. This technique holds mostly for III–V structures (GaAs, InP), and Ge islands on Si. Fig. 5(c) shows the schematic arrangement exploiting the collection of light through the cleaved edge. It also shows how microcavities can be probed by measuring the front photoluminescence diffracted at the cavity edges. The main drawbacks of this method are the limited sensitivity (weak photon fluxes, 1 nW-1 pW) and the restricted spectral range (5%–20% in relative units). The latter impediment can be circumvented by probing structures of different lattice constants a, and then stitching together the spectra plotted versus the dimensionless frequency a= Fig. 5(d) shows the experimental results for 15 rows of crystal probed along K. Waveguides losses are a benchmark of choice. The lowest values measured in the substrate approach for a W3 guide are 15 dB/cm [16]. They fall to 2 dB/cm in the broader W7 structure. In turn, the lowest values in W1 are around 100–200 dB/cm. Deep-etching of highaspect-ratio PhC holes, crucial to the obtainment of low losses, is achievable using the inductively coupled plasma


Fig. 6. (a) Modified bend in a W3 geometry: calculated transmission (bold line) and measured data (gray line). (b) Same for a more modified bend. (c) Optimized bend in a W1 geometry: snapshot of the magnetic field. (d) SEM of a corresponding fabricated structure. (e) Use of a W3 to W1 taper to implement a bend in a monomode section of the waveguide. (f) SEM of a corresponding realization.

(ICP) technique, the electron cyclotron resonance (ECR) reactive ion etching (RIE), as well as chemically assisted ion beam etching (CAIBE). Waveguide losses in the membrane approach have been extensively measured for the canonical W1 system. They are around 1000 dB/cm for points above the light line. In contrast, recent efforts from teams in the United States and Japan [17], [18] have resulted in losses as low as 1–8 dB/cm below the light line. This is indeed a tremendous achievement, reaching the regime where surface roughness associated to the etching process becomes the only limiting factor, with negligible structural errors. 3) Waveguide Bends: Strategy and Results: Sharp bends at the wavelength scale are one of the major expectations from photonic-crystal-based PICs. Bends break the translational symmetry of waveguides. Even in the absence of in-plane losses due to bandgap, bends can lead to in-plane reflection as well as to scattering in the third direction A near-unity transmission was predicted in 1996 by an MIT group for a sharp bend in a quasi-single-mode waveguide [19], but it was in the case of a lattice of dielectric rods in air. The effective wavelength in the guide core was large enough to ignore wall corrugations and out-of-plane

diffraction. The situation is less favorable for the practical case of a lattice of air holes in a dielectric. Moreover, in multimode waveguides, many in-plane modes are excited resulting in a transmitted beam of poor quality in general. One solution is to Bsmooth[ the waveguide bend over a few micrometers [Fig. 6(a) and (b)]. The excitation of multiple modes is still the general case but there is a Bquiet[ region quite immune to the phenomenon where the transmission is maximized. Another elegant solution consists in using a taper to a single-mode W1 ðWi ! W1Þ at the bend entrance while using the inverse mode converter ðW1 ! WiÞ at the bend output [Fig. 6(e) and (f)]. The optimization of bends in W1 waveguides has been investigated in detail. Such a bend can be seen as a short PhC structure whose modes have to be matched to the modes of the connected waveguide arms. More simply, this can also be seen as an impedance matching problem. Finally, different strategies rely on a general shape optimization, devoid of physical assumption, using some figure of merit such as reflection and bandwidth. The results of Fig. 6(c) and (d) are not intuitive. A high transmission of the bend is predicted on a sizable bandwidth, which is a delicate requirement [20]. Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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4) PhC Waveguides and Their Competitors: All in all, issues of PhC waveguiding are similar to those met for photonic wires in the SoI technology, and the compacity/bandwidth compromise may be not much better. Deeply etched ridge waveguides or photonic wires can indeed provide an almost perfect guiding, although there is no gap to stop the in-plane irregularities. Typical losses of 500-nm-wide silicon wires [21], [22] are close to those of W1 membrane PhC waveguides. Bent photonic wires with small curvature radius of a few micrometers have also been realized with loss inferior to 0.1 dB over a bandwidth of 250 nm. This performance is hard to achieve in PhC systems whose TE gap width is typically of 250 nm [Fig. 1(h)]. In turn, the PhC technology is certainly the best adapted to the fabrication of ultranarrow waveguides (e.g., W x with x G 1) with an extreme confinement of light. PhC waveguides also present a smoother surface topology, which is more adapted to large-scale integration: only one level of metallization is required a priori for the integration of active components.

C. Microcavities 1) Microcavity Modology: Any defect surrounded by a PhC with a band gap defines a cavity. The modes of this cavity, however, have no simple properties as compared to those of, e.g., rectangular boxes, and they do not have either, in general, a simple relation with the modes of the surrounding crystals. One exception is that of shallow modes, which arise for slight modifications of the PhC (chirping the lattice [23], [24], etc.). As for dopants in semiconductors, these modes are of Bdonor[ or Bacceptor[ type, with a field pattern closely similar to that of the surrounding crystal. The number of modes per unit frequency, or within the bandgap interval, can be roughly predicted from the density of states (DOS) of the defect Bmaterial,[ which is simply bulk dielectric in many instances. In triangular lattices, the simplest mode symmetry is of the Bmonopole[ type, invariant by sixfold rotations. Fig. 7(a) depicts a mode pattern in the case of a H2 cavity, with a high but distinct symmetry, close to a microdisk-type mode. Among the various cavities, it should be noted that the B1-D[ cavities of Fig. 2(b) are still a good tool to quantify reflectivities, a delicate task otherwise. In the substrate approach, this latter is now around R ¼ 0:97, which qualifies it for a number of in-plane confinement applications in lasers notably. In membranes, microcavities are often different in nature, but modal reflectivities at the closed end of W1 waveguides can apparently reach 99.99% in extremely optimized cases, where the Q factor is of several tens of thousands. The Kyoto team of Noda has realized several breakthroughs recently using specific Bclosed W1[ designs [L3 cavity in Fig. 7(a)] [25] or even more subtle Bheterostructured W1[ designs [26] for 1008


ultrahigh Q. We discuss below the physics underlying the obtainment of very high quality factors Q. 2) Microcavity Q Factor: As can be seen from Fig. 7(a), there has been a sustained progress in PhC microcavity Q factors over the last years. The main strategy of improvement has been to expel the in-plane Fourier transform of the resonant mode outside the light cone. However, even if this task is performed, the strength of the radiation is not only connected to the mode Fourier components, the problem being rather akin to that of an arrayed antenna with interacting elements, a still controversial topic. Breaking the PhC symmetries is another aspect, achieved either by modifying a few holes in the crystal or by deliberately using an elongated form of cavity, a most successful approach to date. The latter has allowed Noda’s team to Bsteadily[ gain three decades since 2000 [25], [26], while other pioneering groups, notably at Cal’tech, have also proposed successful alternative designs [23], [27]. These results show that Q factors of PhC microcavities can be increased to become comparable to- (and even higher than-) those obtained for micropillars [Fig. 7(b), typical Qs around 103 –104 ] and microdisks [Fig. 7(c), typical Qs around 104 –105 ]. The insertion of emitters is thus on the verge of being fully exploited yet. 3) PhC Microcavities Versus Other Microcavities a) Q=V Versus Q or Q 2 =V Challenges for Fundamental and Practical Investigations: The enhancement of the peak DOS at the resonant mode gives rise to the Purcell effect [28], thus accelerating spontaneous emission rate. The relevant factor of merit is Q=V, where V is the mode volume. In a planar FP, this quantity is basically the finesse of the cavity. For 3-D microcavities of more complex geometry, recent results provide a clearer vision of the compromise to be made between Q and V for optimizing the factor Q=V. It is likely that Purcell effects (essentially a measure of peak DOS enhancement) of over 100 could be reached soon, opening a way to make future LEDs as Bfast[ as lasers. The strong-coupling regime is more concerned with thepstrength of the modal field itself, which is related to ffiffiffi 1= V rather than to the volume [29]. The factor to be maximized is then Q 2 =V, making the high Q still more a challenge than the small volume. However, it is not yet clear how strong coupling, e.g., with quantum dot emitters, could be put to use in devices. b) Fabrication Accuracy and Roughness Limits: Even with a perfect design, fabrication has various limits. The very position of interfaces, at the level 0.1–10 nm, may be a trouble for ultrahigh Q structures. The role of the lateral scale of roughness has yet to be established. Some Bsmoothing[ techniques exist (oxidation of Si, electrochemical wet etching, etc.) to reduce small-scale roughness, but they have to respect the overall interface positions.


Fig. 7. (a) Trend of Q factor increase over last seven years, with pictures of cavities as inset ; Q=V follows about the same trend, although a factor of 5 exists between the various PhC cavity volumes displayed here. (b) A micropillar type microcavity, where a mode resonates like in a VCSEL. (c) A microdisk type microcavity, with the mode Btrajectory[ at the disk periphery sketched.

The learning curves of both design and technology, given the current trends, are equally concerned, making further progress avenues both challenging and exciting.

II I. PASSIVE COMPONENTS I N SEMI CONDUCTOR-BASED 2 -D PHCS A. Full PhCs as Dispersive Elements 1) Superprisms: When used in transmission bands instead of optical bandgaps, a piece of 2-D PhC may readily serve as a dispersive device, much like a glass prism or a grating. However, the 2-D nature of the arrangement and the strong index contrast between air and semiconductor result in very nontrivial dispersion relations of Bloch modes, opening in turn the way to more sophisticated devices. Fig. 8(a) shows the isofrequency contours of a heuristic 2-D PhC with square geometry:

! ¼ !ðkx ; ky Þ. A wave of frequency !0 impinging on the crystal will be transmitted in such a way as the k component along the interface (here ky for the superprism input) will be conserved modulo G ¼ 2 =a. The transmitted Bloch wave group velocity in the PhC is then normal to the isofrequency curve !ðkx ; ky Þ ¼ !0 at the conserved ordinate k ¼ ky . A giant dispersion of the group velocity direction is obtained when the incident light is coupled to those bands singular around the Brillouin zone edge kx ¼ 2 =a [Fig. 8(b)]. Such a Bsuperprism[ effect [Fig. 8(c)] has been reported for the first time by Kosaka et al. in 1999 [30]. Dispersions as high as 5 =nm were demonstrated. Recent experiments have been conducted in integrated optics [Fig. 8(d)] [31], [32] leading, for instance, to dispersions of 1:3 =nm in the SoI system. However, two main difficulties subsist. First, a very accurate crystal fabrication is needed to operate near a singular point of the isofrequency curve. Second, the collimated input beam Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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Fig. 8. (a) Isofrequency curves for a heuristic dispersion relation of a square lattice crystals, with singularities and flat regions, for three close frequencies. The conservation of the tangential k component is represented in dashed lines. (b) Zoom on the singular zone edge. (c) Real space picture: beams at different frequencies have a large walk-off. (d) Supercollimator whose input waves are indicated by gray arrows in (a). All k-components have nearly the same group velocity inside the crystal: an output beam from a narrow waveguide will experience almost no diffraction upon traversing the supercollimator. (e) Generic implementation, showing the conserved k component (results from T. F. Krauss, St Andrews University). (f) Negative refraction at a bulk/PhC interface and its use in a Bflat lens[ for imaging a point source inside and outside the PhC.

must be wide enough to limit the ky dispersion at the entrance. This may lead in turn to bulky PhC devices with poor transmission figures and poor crosstalk performances. The future of this approach is thus unclear. 2) Supercollimators and Negative Refraction: The converse operation, Bsupercollimation,[ is obtained when employing a particularly flat portion of the dispersion relation [33] [Fig. 8(d)]. Then, a bunch of rays diverging at the PhC entrance may display group velocity Bfocusing[ along the specific normal to the flat dispersion contour. Notomi [34] has proposed illuminating discussions of this effect as well as of the superprism effect. Negative refraction is yet another interesting case; it arises when the isofrequency curves are nearly circles, and the group velocity, normal to the curve, is oriented toward its interior [Fig. 8(f)]. The crystal then refracts as a homogeneous medium with a negative effective index would do: a bunch of rays diverging at the PhC entrance is focused within the crystal and also beyond it [Fig. 8(f)]. One application of negative refraction is to focus light outside a chip without using any integrated lens, but rather just by inserting a piece of PhC between the guide output and the chip facet [35]. Let us stress that the concept of 1010


negative refraction may carry a misleading analogy as the same phenomenon occurs in the so-called left-handed materials. These latter are a special class of metamaterials, where " and are both negative, thanks to specific resonances in LC-type resonators [36].

B. Optical Delay Lines The interest of slow modes with low group velocity in regions near gaps has already been mentioned for waveguides (Section II-B1). To date, convincing results of slowing down light in full PhCs are scarce. It can be reasonably admitted that the losses incurred in slow regions are intrinsically large, being naively proportional to the wave Bdwell time.[ Waveguides can, in turn, slow down waves without interacting too much with them. This is illustrated, for instance, in the case of elongated microcavities, which are but closed W1 waveguides (Section II-C2). Very low losses are obtained for these cavities whose mode corresponds to a slow mode of the open W1 waveguide. Further exploration is needed to fully exploit this fascinating effect into real-world devices. One application of slow PhC waveguide modes is to embody an optical memory. Optically storing bits of information is clearly of great interest to manage


Fig. 9. (a) Mode spectrum measured at the output of a W3 waveguide when photoexciting an H7 cavity on it side. The cavity is separated from the guide by either two or three PhC rows. (b) Results for an elongated cavity on the side of a W1 waveguide in the membrane approach [25]: dropped signal intensities for different shifts of the holes at the cavity ends. (c) Q variation versus shifted hole coordinate. (d) Principle of coupling between guides based on the transfer to a higher order mode. (e) Predicted results for a 11-m-long device showing the large loss tolerance of the device. (f) Device based on contradirectional coupling between a W0.8 and a W1 waveguide [46]. (g) Predicted and measured results for the dropped spectrum of the first prototype with ¼ 8 nm resolution (see [45]).

transparent networks. The typical time constant needed is 25 ps per bit for a 40-GHz operation. A time delay of 100 ps in a 30-m-long device represents a velocity c/1000, a value that seems attainable at a single frequency. An interesting way to go to slow modes is the coupling of a string of cavities. This new waveguide type proposed by Stefanou in 1998 [37] was further termed Bcoupled resonator optical waveguide[ (CROW) by Yariv in 1999 [38]. PhCs represent indeed a wonderful opportunity to implement this concept, as was demonstrated

soon after in [39]. Other configurations of interest are those based on the use of chirped PhCs [40]. These latter could also be used as laser mirrors for pulse compression in mode-locked semiconductor lasers (Section IV-A2c). The ability to manage dispersion on a large wavelength span ð 10–100 nm) makes chirped PhCs attractive components compared either to Bragg gratings in fibers or to multilayer mirror stacks that are tedious to grow accurately. Chirped PhCs can also be applied to phasematching in nonlinear optics (Section IV-B1). Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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Fig. 10. Spectrometer-on-chip (wavelength monitor) based on a wedged PhC waveguide. (a) Principle: in each section, the fundamental mode at a specific wavelength is converted into a higher order mode that tunnels through the thinned waveguide barrier. (b) Micrograph of a realization at HHI (Berlin, courtesy of K. Janiak), with superimposed light paths toward photodiodes.

C. Add–Drop Filters Add–drop filtering is a much desired function in PICs, above all if reconfigurable. One needs a bus and a selective element. Moreover, the selective element should support two orthogonal quasi-degenerate modes to ensure directionality in the add/drop line. In microdisks, this is naturally the case of the two counterpropagating modes. For basic single-mode microcavities, one may use a pair of them. The MIT team proposed theoretical implementations of this concept [41], but none that would translate into feasible semiconductor systems at 1.5 m. In the current status of the topic, the roads offered by PhCs can be divided into two kinds. First configurations are based on guide-cavity coupling, whereas the second ones use a direct coupling between guided modes. 1) PhC Guide-Cavity Coupling: The cavity should lie on the side of the input/(output) guide(s) to let all the unaffected frequencies propagate along the device; Fig. 9(a) and (b) exemplifies pioneering realizations. The optical coupling of the cavity to free space (vertical radiation loss) and the optical couplings between the cavity and the waveguide(s) can be cast into the effective Q factors Qv , Qin (and Qout ), respectively. Optimization of the Q values actually depends on the filter design and its application. In 2000, Noda’s team achieved impressive results by using straight waveguide coupled to a point defect H1 microcavity [42]. The dropped wavelength was extracted vertically (no output guide), and equal values were chosen for Qv and Qin in such a way as half the photons flowing in the guide were emitted vertically. The overall Q reached about 500. In 2004, the same team used a different configuration where the elongated cavity shown in Figs. 2(b) and 7 was coupled to W1 input/output guides [Fig. 9(b) and (c)] [25], [43]. The overall Q reached 30 000 (and even the ultrahigh value of 600 000 in a new design [26]). The achievement of a high Qv is crucial 1012


for the use of such a device in dense WDM. Recent progress in integration and performances have been reported by the NTT team [44]. In the substrate approach, early results on nonultimate cavities [Fig. 9(a)] [45] showed a clear potential, furthered by alternative clever approaches. One of these approaches is depicted in Fig. 9(f) and (g), while it actually pertains to the case of coupling between guides (here W0.8 and W1) [46] as discussed in the next section. 2) PhC Guided Mode Coupling: In a family of proposals, the coupling between two guides in PhC was used to implement selective functions [47], [48]. Due to the periodicity, PhC waveguide couplers are natural gratingassisted waveguide couplers, whereas the bandgap effect suppresses in-plane radiation loss. A short beat length between two codirectionally coupled waveguides is a typical feature attainable with proper design. Another recent scheme exploits multimode sections to convert the fundamental guided mode into a specific highorder mode [see Fig. 4(b)] that can easily tunnel through the PhC barrier between guides [49] [Fig. 9(d) and (e)]. This configuration indeed constitutes a fault-tolerant and loss-tolerant filter. The illustrations of the two waveguidecoupling based devices [Fig. 9(d)–(g)] suggests that this is a privileged road to be further exploited.

D. Spectrometer-on-Chip Mode coupling in a PhC multimode waveguide can also be used to build up an integrated spectrometer-on-a-chip. Fig. 10(a) illustrates the principle while Fig. 10(b) shows a first prototype fabricated in a collaboration of one of the authors with HHI and Alcatel [50]. A wedged waveguide provides the mode-coupling situation at different places along the guide for the different wavelengths. Light injected at a given wavelength into the fundamental mode of the guide is redirected into a specific lateral channel after conversion to a higher order


Fig. 11. Surface-emitting PhC lasers. (a) Cross-sectional SEM image of the electrically driven single-cell PhC laser demonstrated in [24]. Inset shows the chirped pattern size in top view. Regions from I to V correspond to PhC holes of increasing diameter. (b) Schematic view of the bandedge PhC laser reported in [65] : an InP-based perforated membrane with four active InAsP quantum wells is bond onto silica on silicon. The inset shows a magnified part of the PhC structure.

mode, whose strong leakage is favored by its deep penetration into the thin PhC walls. Integrated photodiodes can be placed to monitor the various signals. PhC waveguides offer such a unique situation where light confinement and grating-type action can coexist to provide a mini-stopband at specific values of k and !. Other Bselective[ structures are selective either in k (a simple corrugated waveguide acts on all frequencies) or in ! (a point cavity acts for all directions of impinging waves). Q factors of 300 have been obtained for the spectrometer prototype. Q factors of 1000 are expected from optimized versions. Such performances are of interest for coarse WDM networks, where the wavelength spacing is ¼ 20 nm.

IV. ACTIVE COMPONENTS IN SEMI CONDUCTOR-BASED 2 -D PHCS A. Lasers 1) Surface-Emitting Lasers a) PhC Microcavity Lasers: The perspective of achieving a better control of spontaneous emission in optoelectronic devices has been a major impetus to research on PhCs in its early development [1]. Ideally, the full control of spontaneous emission could lead to thresholdless lasers where all injected electrons would be converted to photons emitted in a single cavity mode [51]. More realistically, the increase of the spontaneous emission factor into a given mode and the simultaneous reduction of the active layer volume can provide ultralow-

threshold lasers with original properties including low noise and fast dynamics [10], [52], [53]. A reasonable approach consists in creating a small defect in a 2-D photonic-bandgap slab. Because the k-spectrum of the confined in-plane field possesses sizable components down to k ¼ 0 within the light cone, there is a certain leakage of light along the third (vertical) direction, which actually constitutes the useful laser output. The tradeoff is then to achieve a high-Q cavity and a low laser threshold while keeping a sufficient level of vertical emission and an acceptable beam shape. The studies of surface-emitting defect mode lasers have closely accompanied those of PhC defect microcavities (Section II-C). The first PhC laser emission near ¼ 1:5 m was reported in 1999 by the Caltech group [54]. A modified H1 microcavity was used where two holes among the six ones surrounding the defect were designed with a larger diameter to lift the intrinsic mode degeneracy. The laser was operated under pulsed optical pumping with an estimated mode volume of 0.09 m3 . A recent and decisive progress has been achieved by the Kaist group in Korea, demonstrating an electrically driven single-cell PhC laser at room temperature [24]. As seen from Fig. 11(a), the introduction of a central post under the PhC slab allowed the current injection into the active region while it did not notably degrade the quality factor Q of the H1 defect microcavity. The latter was optimized by using a chirped pattern size around the defect (Fig. 11(a), inset), thereby leading to Q 2500 and a mode volume of 0:68ð=nÞ3 ¼ 0:058 m3 . A record value of 0.25 was estimated from the light–current characteristics. In contrast, the threshold current ð 260 AÞ was rather Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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larger than the previously reported 36-A record in a vertical-cavity surface emitting laser (VCSEL) [55]. Identification of several current leakage paths in the fabricated structure actually shows that there is a wide room for further improvements. Clearly, the above results represent meaningful steps toward ultimate photon sources. The use of a single quantum dot emitter instead of quantum wells in PhC microcavities offers additional opportunities, especially toward on-demand single photon sources for quantum cryptography applications. Another challenge would be to funnel most of the emitted photons into a neighboring lowloss PhC waveguide instead of extracting them vertically. A first result by Noda’s team [43] on a passive membrane structure will be certainly soon translated to active structures in III–V materials. b) 2-D PhCs for VCSELs: The configuration of defect mode lasers as seen in the former section is essentially based on an optimized PhC slab whose thickness optimizes in-plane guiding. A somewhat more complex structure consists in adding a vertical confinement of light with Bragg multilayers for optimizing the vertical extraction as well as the laser output mode profile. This structure is only a VCSEL with a lateral PhC. Previous studies have shown that the efficiency of surface-emitting LEDs can be increased beyond the limit values reached in planar cavity by using 2-D PhCs at the periphery of the emitting area [56]. The recycling of photons guided in the active layers and/or their extraction by vertical diffraction mainly explained the improvement in that case. A similar use of 2-D PhCs was also proposed in resonant cavity LEDs (RCLEDs) where the resonant vertical cavity is formed by a pair of multilayer Bragg mirrors [57]. The situation for VCSELs equipped with highly reflective Bragg mirrors is different due to the dynamics of the lasing mode(s) and of the laser populations. If the lateral 2-D PhC is used within its gap, the VCSEL mode(s) is (are) indeed confined both in plane and vertically . The mode properties will obviously depend on the respective strengths of the confinements in the different directions. In the case of broad emitting (unstructured) areas, lasing would rather start in the horizontally guided modes thus losing most of the advantages of VCSELs. In contrast, for narrow emitting areas, 3-D calculations are needed for analyzing the modal properties of the cavity, but bigger potential can be foreseen for applications. This latter configuration represents indeed an intermediate step toward lasers with a 3-D confinement of light. The practical fabrication of such structures is still restrained by the requirement of highaspect-ratio dry etch of submicrometer patterns through a multilayer stack of typically 5-m thickness. A somewhat thinner structure has recently been proposed where the top mirror consisted of a 1-D PhC membrane [58], but this configuration has not been experimentally demonstrated yet. 1014


From a simpler and more practical viewpoint, 2-D PhCs can rather be used to laterally modulate the refractive index of VCSELs than to create a 2-D photonic bandgap. In this case, the PhC VCSEL Bmimics[ a slice of microstructured fiber with all the benefits associated to the transverse mode control. For a sufficiently large emitting area, the essential modal properties of the PhC VCSEL can be understood by the decoupling of the vertical and lateral confinements. Guided photons are vertically confined by the distributed Bragg reflection, but the transverse mode profile is determined by the lateral modulation of the refractive index. In the same way as microstructured fibers can be single-mode even with a core of much larger diameter than the wavelength, large-emitting-area PhC VCSEL can be designed to support the propagation of only the fundamental mode. Typical periodicity of the structure can be of the order of several micrometers as in microstructured fibers, while the etched patterns should not necessarily traverse the whole thickness of the VCSEL mirrors [59], [60]. Such PhC VCSELs are thus of great promise for the achievement of single-mode operation even at high injection currents. The control of spontaneous emission and light polarization can also be strongly enhanced as compared to ordinary VCSELs. Single-mode powers in the 1–10-mW range have recently been obtained from PhC-VCSELs [61] and VCSELs with holey structure in the GaAs/AlGaAs system [62]. These very encouraging results should be further extended to other material systems. c) Band-Edge Lasers: The simplest surface-emitting PhC laser structure is that of Bloch mode lasers, also called band-edge lasers, where the PhC is used as a whole instead of being located at the periphery of the active region. Actually, such a structure makes an intentional use of the radiative losses of modes, which propagate in the 2-D PhC. Lasing preferably occurs on slow modes, notably at the , K, and M points of the band diagram where light strongly interacts with the active medium [63]. To some extent, band-edge lasers can then be viewed as a 2-D extension of popular DFB lasers. However, it has to be noticed that the 1-D gratings of DFB lasers are always buried and furthermore seldom etched to the very active layer. To date, most of the band-edge lasers have been fabricated within the membrane approach. The K and M points are then below the light line, and the vertical extraction of light only results from the finite PhC size and/or the presence of fabrication errors in that case. By contrast, the vertical emission is more naturally favored at the point. Low lasing thresholds have been reported by several authors for InP-based structures of small emitting area [64]–[66]. For instance, the effective threshold pump power was estimated to be below 50 W for a graphitelike PhC structure of 7 m2 [66]. One important improvement in these experiments stems from the transfer of the thin multiquantum-well InP-based heterostructure onto a silicon host wafer [Fig. 11(b)], thus


Fig. 12. SEM images of in-plane emitting PhC waveguides. (a) W1 defect waveguide laser demonstrated in [67]. (b) Coupled resonator optical waveguide laser demonstrated in [73].

providing a more efficient cooling of the active region than in the free standing membrane case. 2) Edge- (or In-Plane) Emitting Lasers: Edge-emitting lasers are workhorses of optical telecommunications. They easily deliver single-mode output powers in the 10– 100-mW range. They can also be integrated with other optoelectronic components within planar technology appraoches. Regarding this point, the development of PhC waveguide lasers with an in-plane emission opens unprecedented perspectives for the large-scale integration of lasers with miniature guided-optical devices [Fig. 2(f)]. The simplest geometry of a PhC waveguide laser is that of a canonical PhC waveguide formed by one or several rows of missing holes in a triangular lattice PhC [Fig. 12(a)]. Another interesting geometry is that of a CROW [Fig. 12(b)] first proposed by Stefanou [37]. In any case, one advantage of the PhC laser structures compared to those of more traditional ridge waveguide lasers stems from the fact that the laser fabrication does not require any regrowth step, which is a critical fabrication step. Another advantage of PhC-based integrated optics around laser sources results from the fact they operate in a single polarization (TE) mode. This relaxes the constraints on the behavior of the optical chain in the TM polarization, in a sharp contrast with the requirements imposed on a receiving system. a) Canonical Waveguide Lasers: Despite the apparent simplicity of canonical waveguide lasers, different situations may occur depending on the waveguide width (one, three, or more missing rows; see Section II-B), on the crystal direction ðK; M; . . .Þ and on the vertical structure (membrane or substrate). In any case, lasing preferably occurs at low-group-velocity points (band edges, point) where light-matter interactions are enhanced [63]. The in-gap situation is also preferable,

as in-plane scattering is inhibited. However, an in-gap situation does not necessarily ensure low 3-D radiation losses, especially in the substrate approach where the guided modes essentially lie above the light line. Moreover, carrier recombination at the sidewalls can also impact a lot on the mode selection, since it penalizes modes extending in the crystal regions. This effect obviously favors the emission on the fundamental mode, which is the best confined in the guide as in the case of classical waveguide lasers. i) Ultracompact waveguide lasers (W1) An ultranarrow waveguide formed by one row of missing holes is but a very elongated microcavity. To date, lasing of a triangular lattice PhC W1 waveguide has only been achieved by photopumping and using the membrane approach [67]. The emitting laser area was 0.5 13.6 m2 [Fig. 12], and the pump threshold was estimated to be 690 W. The laser emission was considered to occur on the mode of lower energy in the gap near the K point [see the band diagram of Fig. 4(a)]. The dispersion curve of the fundamental mode actually intersects the Brillouin zone edge at a lower energy below the gap. Lasing on the fundamental mode of a narrow W1 waveguide has been obtained in the case of a square lattice [68]. The PhC waveguide was fabricated on InP using the substrate approach [69]. Experiments were conducted under optical pumping. Despite the absence of a complete gap, a low-loss situation occurs at the second folding of the fundamental mode ( point), where the laser emission is intrinsically single-mode [Fig. 13(a)]. The selection mechanism based on band-edge dependent losses is actually similar to that reported earlier for a second-order DFB laser by Henry and Kazarinov [70]. Here, only one of the two bandedge DFB modes is well confined in the guide core. This illustrated in Fig. 13(b), which shows the calculated field pattern of the confined mode. Unlike the lasing mode of the W1 triangular lattice waveguide [Fig. 13(c)], the field does not spread in the crystal region. ii) Medium size waveguide lasers (from W3 to W5) For a K orientation of the guide in the triangular lattice and a standard air-filling factor ( 30%), the fundamental mode does not fold in the gap whatever the width of the guide is. In other words, the fundamental mode is not a slow mode in the gap region. Other slow modes exist in this region [Fig. 4(a) and (b)], but their fields penetrate more deeply in the PhC claddings [e.g., Fig. 4(c)]. If the substrate approach is used with a modest vertical confinement of light, all these modes also suffer out-of-plane losses, and the in-gap situation is not so advantageous. The laser action rather takes place out-of-gap at the folding points of the fundamental Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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Fig. 13. (a) Calculated band diagram of a square lattice W1 waveguide with an air filling factor of 26% and a refractive index of 3.21. Inset zooms of the second folding of the fundamental mode. (b) Calculated H-field pattern of the fundamental mode at the second folding in the Brillouin zone. (c) Calculated H-field pattern of the lasing mode of the triangular lattice W1 waveguide laser [67].

mode. This is indeed observed for the W3 laser [Fig. 14(a) and (b)], whose emission preferably occurs above gap at the second folding of the fundamental mode ( point). The laser was experimentally found to be single mode with a side-mode-suppression-ratio larger than 40 dB [71]. This single-mode behavior is analogous to the one previously described for the square lattice W1 laser (Fig. 13). iii) The same behavior should be obtained, in principle, for wider waveguides such as W5 and W7. However, a more direct solution for selecting the fundamental mode is to create a periodic modulation of the guide in such a way as to fold the dispersion curve of this mode into the gap [Fig. 14(e)]. Such a laser has been demonstrated where the basic W5 guide was constricted to a W3 geometry every six periods of the PhC matrix [72]. A continuous-wave (cw) single-mode emission has been obtained under electrical pumping with a wavelength selectivity better than 25 dB [Fig. 14(f)] and an external efficiency over 0.15 W/A [Fig. 14(g)]. As a major result, this work has demonstrated the absence of fundamental impairments toward a large efficiency from PhC waveguide lasers in cw. iv) The periodic modulation of the waveguide width is somehow built-in for guides oriented in the M direction. For instance, the width of the W2–3 waveguide is alternately determined by two and three missing holes. For a 30% air filling factor of the lattice, the third folding of the fundamental mode now occurs in gap [Fig. 14(c)], thereby allowing a genuine DFB laser emission at the Mpoint [71]. Typical output spectra measured for the 1016


W2–3 laser either with or without anti-reflecting coating are shown in Fig. 14(d). The large separation between the two DFB components of the laser emission reveals an equivalent coefficient as high as 400 cm1 . b) CROW Lasers: W2–3 structures can be seen as CROWs, albeit with low-Q resonators. PhCs actually allow exploring a lot of CROW systems with various types of resonators and various types of coupling between resonators. A familiar system is that formed by coupled hexagonal cavities [38]. Such a device has been fabricated at the University of Wu¨rzburg [Fig. 12(b)] [73]. Stable singlemode lasing has been obtained at 1:53 m with a sidemode suppression ratio greater than 40 dB. The laser emitted up to 2.6 mW under cw operation at room temperature. One formal interest of the CROW concept is that it leads to dispersion relations, which are both simple and remarkable for the guided modes. The coupling of the individual resonator modes creates minibands within the photonic gap, each of them being almost centered on a mode of the isolated resonator. Adjusting the coupling strength (e.g., the number of rows between resonators) allows varying the group velocity of guided modes. In standard PhC waveguides, there does not exist such a simple guideline for the resolution of certain inverse problems, especially that of finding shapes and frequencies that slow down the group velocity of propagating waves. One possible drawback of CROW lasers stems, in turn, from the fact that inescapable vertical losses may arise. c) PhC Laser Systems and Applications: Another interest of PhC mirrors/guides lies in the possibility of revisiting earlier single-mode laser configurations, like the so-called C3 laser, or cleaved coupled cavity laser [74]. First investigated by Happ et al. [75], the PhC versions


Fig. 14. Calculated band diagrams of medium-size PhC waveguides and measured output spectra of the corresponding waveguide lasers. (a)–(b) W3 laser. (c)–(d) W2–3 laser. (e)–(g) W5 laser with a periodic constriction of the waveguide width [72]. In each case, the lasing frequency is indicated by an arrow. The zig-zag plot in (e) schematizes the fundamental mode folding due to the larger periodicity introduced in the W5 laser. The light–current characteristic of this laser is shown in (g), with a 0.15-A/W efficiency.

have been recently used in an elaborate system of a twochannel tunable PhC laser diode developed at the University of Wu¨rzburg [Fig. 15(a), (b), and (c)] [76]. Each laser source is based on the contradirectional coupling of two independently contacted PhC waveguide segments of slightly different lengths. The waveguide sections are oriented in the M direction of the PhC, and are separated by an intermediate PhC mirror of one lattice period (Fig. 15(b), left). The frequency of each laser can be tuned by separately adjusting the injection currents in the two

laser sections, respectively [Fig. 15(c)]. The outputs of the two tunable lasers are coupled into a single waveguide using a PhC Y-coupler structure oriented in the K direction (Fig. 15(b), right). Quasi-continuous tuning has thus been achieved in a 30-nm window with 36 WDM channels spaced 0.8 nm apart (ITU grid). The simplicity of fabrication as well as the promising output characteristics should indeed make this tunable laser design an interesting source for monolithic integration into highly integrated photonic circuits. Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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Fig. 15. (a) Schematics of the two-channel tunable PhC laser diode developed in [76]. (b) Aggregated SEM images of the device. Left: PhC coupledcavity laser source with waveguides oriented in the M direction. Right: PhC Y coupler in K orientation. (c) Output spectra under simultaneous operation. Laser 1 is tuned while the wavelength of laser 2 is fixed. (d) Schematics of the MOPA system developed in [77] with an SEM view of PhC subcomponents.

Two-dimensional PhCs can also be used for the fabrication of laser subcomponents in the more traditional semiconductor laser technology. This is illustrated in Fig. 15(d), which shows the two-section ridge waveguide master oscillator power amplifier (MOPA) system developed in [77]. The use of a 2-D PhC side reflector allows operating the MOPA on a narrowband while simultaneously isolating the two cleaved facets. The use of a PhC reflector at one cavity end provides a high reflectivity in the desired band while it is compatible with the integration of a power monitoring photodiode. Two-dimensional structures are preferable to 1-D grating because they are less dependent on the roughness and details of etched profiles and they offer more flexibility for integration. The PhC-MOPA system fabricated in [77] was capable of delivering 0.6 W in a narrow spectral band of 2 nm. The wavelength shift was found to be less than 10 nm from threshold to 3-A injection current. 1018


The potential of 2-D PhCs for confining light with a simultaneous control of the group velocity dispersion finally opens new perspectives for high-speed waveguide lasers. The achievement of very high power densities in small laser structures can lead, for instance, to fast laser dynamics and wideband modulation [78]. The control of the lasing mode group velocity dispersion can be exploited for short pulse compression and short pulse generation as well [40], [79].

B. Toward Nonlinear PhC Devices 1) Frequency Conversion: The two conditions for an efficient frequency conversion process in a nonlinear optical material are the existence of a high value of the nonlinear susceptibility and the possibility of phase matching between the interacting waves. The first condition is quite fulfilled in bulk III–V materials, but phase matching is not allowed due to their cubic


Fig. 16. Schematic representation of a SHG process in a PhC (unfolded dispersion diagram). Phase matching creates a coupling between TE and TM slow modes at ! and 2!, respectively.

symmetry. Phase matching is, in turn, obtainable when structuring the refractive index of III–V materials. Actually, microstructured materials such as PhCs not only allow a genuine engineering of the refractive index, but also provide the simultaneous control of the group velocity dispersion at the different wavelengths. This is schematized for second harmonic generation (SHG) via the unfolded dispersion diagram of Fig. 16. Practically, the phase matching condition is obtained by appropriately designing the band structure of the 2-D PhC and using its birefringence properties: the energy carried by a TE wave at ! and k! can be transferred to the harmonic TM wave at 2! and k2! ¼ 2k! in the example of Fig. 16. The existence of slow PhC modes at both frequencies reinforces the mutual interactions of the two waves with the nonlinear material. Thus, the SH conversion efficiency achieved at 1500 nm in a 1-D PhC composed of alternate AlGaAs/Alox layers has been found to increase almost as N 6 , where N was the number of layers [80]. This result is of great promise for very compact frequency converters in semiconductor-based PhCs. The extension from a 1-D PhC to W1 waveguides in 2-D PhCs has been discussed in [81], and application to full 2-D PhCs has been reported in [82]. More generally, all semiconductor sources that use a nonlinearly stimulated process can benefit from the intrinsic properties of 2-D PhC waveguides and resonators. For a given optical power injected in a resonant device of volume V and quality factor Q, the circulating intensity is proportional to Q=V. Thanks to the recent performances of PhC resonators, intensities larger than 100 MW/cm2 are achievable at pump powers less than 1 mW. This can be exploited in third-order nonlinear processes including, for instance, stimulated Raman emission and four-wave mixing. Self-phase modulation in narrow PhC waveguides could also be used for short-pulse broadband emission, following previous demonstrations in PhC fibers [83]. 2) Fast Optical Switching in Reconfigurable PhCs: Among third-order nonlinear processes, refractive index nonlinea-

rities (Kerr effects) are known as a powerful means of controlling light by light in semiconductors. Very low thresholds can be expected from PhCs structures that provide a strong confinement of light. This opens the way toward miniature versions of fast all-optical switches and routers in integrated optics. The progress in this field is illustrated by the recent work of Raineri et al. [84], where a narrow (0.4 nm) reflection band of a 2-D PhC was blueshifted by more than 8 nm under optical pumping in the 0.1–1-mW range. Fig. 17 shows a schematic view of the sample used in these experiments (left) along with the results of spectral measurements (right). The InP-based PhC slab included four InGaAsP quantum wells whose refractive index was varied with pump intensity. Pump and probe beams were incident perpendicularly to the slab surface. The 8-nm blueshift of the slab reflectance was obtained at a rather constant level of reflectivity, whereas a strong amplification of the vertical probe was observed for pump intensities above 4 kW/cm2 [85]. All-optical PhC switches as the one of Fig. 17 can also be seen as reconfigurable or tunable PhC wavelength filters. Of particular interest for the domain of optical telecommunications is the possibility of achieving in-plane versions of these devices. All-optical bistable switches using 2-D-PhC nanocavities fabricated on SoI have recently been demonstrated in the thermal regime [86]. Extension of this work to III–V devices employing fast optical nonlinearity can be expected soon.

V. 3-D PHOTONIC CRYSTALS The original dream of being able to manipulate the flow of light in all three dimensions, and ultimately to control the spontaneous emission from a single emitter with a complete 3-D photonic bandgap, is still very much alive despite the difficulties in fabricating 3-D structures. Recent results obtained on the association of light emitters with 3-D crystals are encouraging in that sense [87], [88]. Three-dimensional PhC microcavities represent indeed an ideal configuration for thresholdless lasers and singlephoton sources (Section IV-A1). Several approaches exist for the fabrication of 3-D PhCs in semiconductors. One approach consists in creating a 3-D template in a low-index dielectric, and then in-filling the template with semiconductor. Either artificial opals made of small silica spheres [89] or 3-D structures fabricated by X-ray lithography in a polymer [90] can be used as templates. Infilling of artificial opals has been successfully achieved with various semiconductors, although not of epitaxial quality [91]–[93]. However, the template approach does not allow an easy incorporation of guides and microcavities into the 3-D periodic structures. Another approach based on thin-film technologies consists in constructing 3-D PhCs layer by layer much in the same way as one stacks a woodpile. This other approach has been successfully demonstrated on silicon [94] and III–Vs [95]. Vol. 94, No. 5, May 2006 | Proceedings of the IEEE

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Fig. 17. Left: schematic view of the PhC slab used in the pump-probe configuration of [84]. Right: reflection spectra of the PhC slab measured at various pump intensities around the PhC resonance.

The insertion of microcavities and waveguides is straightforward in its principle, but still delicate in its realization. Thanks to the continuous progress in planar microtechnologies, there is no doubt that sophisticated 3-D structures will be realized soon within this technique. For small-scale fabrication in laboratory, other technologies were also demonstrated such as chemically assisted ion beam etching [96], focused ion beam etching [97], and autocloning [98].

VI . CONCLUSION We have shown that the understanding of the physics of semiconductor-based PhCs and the required technologies are now all making rapid progress. At the beginning of 1998, nobody could imagine an ultrahigh Q cavity (Q ¼ 600 000 in [26]) or single-cell electrically injected lasers on a membrane [24]. Although these are futuristic devices for real-world applications, there are many other entrance points for functional devices such as PhC-VCSELS and various lasers, or spectrometers on a chip, for which the market place becomes a reality more than a dream. With proper design, semiconductor based 2-D PhCs can indeed bring new functionalities to optoelectronic devices, as, for instance, the dispersion control for which a breakthrough is not unlikely in the forthcoming decade. Perhaps still more important than their application to new isolated devices is the fact that semiconductor-based PhCs and, more generally, semiconductor-based highindex-contrast structures are a real opportunity to bring REFERENCES [1] E. Yablonovitch, BInhibited spontaneous emission in solid state physics and electronics,[ Phys. Rev. Lett., vol. 58, pp. 2059–2062, 1987. [2] S. John, BStrong localization of photons in certain disordered dielectric superlattices,[

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the large scale level of integration into the world of photonics. PhC components take advantage of the nanostructuring technologies developed for microelectronics. From this point of view, the successful technological approach of III–V nanostructures for active devices, on one hand, and that of SoI for low losses and high confinement, on the other hand, should be pushed further for future developments. An additional step of optimization would be to combine these approaches more systematically as it has already been done, for instance, in the report of III–V membranes on SoI. Ideally, an ultimate solution for 2-D integrated optics would be to have access to a III–V semiconductor-on-insulator technology equivalent to the SoI one. Telecommunications remain the most relevant field of application of PhC structures. As such, telecommunications also prompt the studies of PhC themselves, their limits and the underlying physics. This wonderful synergy is, fortunately, likely to continue for a long time. h

Acknowledgment The authors would like to acknowledge the discussions with the members of the FUNFOX European project and those of the former PCIC and PICCO FP-IST European projects. The authors would like to thank the members of the French RNRT project CRISTEL for their helpful collaboration, with a special mention to the technological support of Thales-Alcatel.

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ABOUT THE AUTHORS Henri Benisty was born in Casablanca, Morocco, in 1963. He received the Ph.D. degree in accumulation layers at Si interfaces from Ecole Normale ´rieure, Paris, France, in 1989. Supe He was with the University of Versailles until 2002 and is now with Laboratoire Charles Fabry de l’Institut d’Optique, Orsay, France. Since receiving his Ph.D. degree, his research topics have been first nanostructure growth and physics (Thales, then Thomson, Orsay) and lamellar III–VI compounds (Paris 6 University). Since 1994, he has been involved in research on planar cavities (mainly for LEDs) and in photonic crystals in two dimensions on III–V, with both experimental and theoretical `re Condense é approaches at the Laboratoire de la Physique de la Matie in Ecole Polytechnique, Palaiseau. He currently investigates applications of photonic crystals to LEDs, biophotonics (he cocreated the start-up Genewave in 2001), and miniature photonic integrated circuits.

Jean-Michel Lourtioz (Senior Member, IEEE) was born in Lens, France, in 1948. He graduated from Ecole Centrale, Paris, France, in 1971 and received the equivalent of the M.S. degree in physics and the Ph.D. degree from the University of Paris in 1975 and 1981, respectively. Since 1976, he has been with CNRS and has worked at the Institut d’Electronique Fondamentale (IEF), University of Paris-Sud, France. He is currently Directeur de Recherche at CNRS and is the head of IEF, which includes 125 permanent researchers, technicians, and administratives and 80 Ph.D. students and postdoctorals. From 1996 to 2001, he coordinated in France the research studies on photonic crystals and microcavities. Since 2002, he has coordinated the French network on nanophotonics. His current research interests include optical and fast electronic devices, semiconductor nanostructures, photonic crystals, and microcavities.

´ received the Diploma degree in Sylvain Combrie ´le éngineering from the Ecole Nationale des Te communications in Paris and the M.S degree from the University Pierre et Marie Curie, Paris, France, in 2002. He is currently working toward the Ph.D. degree in nanooptics devices at Thales Research and Technology in Orsay. His research interests include design, fabrication (e-beam lithography, ICP plasma process), and characterization of high-Q cavities in photonic crystals.

Xavier Checoury was born in Boulogne-Billancourt, France, in 1974. He received the engineering ´rieure des degree from the Ecole Nationale Supe ´ le ´ communications in 1998. He is currently Te working toward the Ph.D. degree at the Institut d’Electronique Fondamentale (IEF), University of Paris-Sud, France. From 1999 to 2002, he worked as an R&D engineer at EADS Telecom (formerly Matra Nortel Communications). His research interests include photonic crystals, semiconductor lasers, and numerical modeling.

Alexei Chelnokov was born in St. Petersburg, Russia, in 1965. He received the B.S. degree from St. Petersburg Polytechnical Institute in 1988 and the Ph.D. degree for work on dynamics of highpower semiconductor lasers and on planar erbium doped amplifiers from the Institut d’Electronique Fondamentale (IEF), University of Paris-Sud, France, in 1995. He worked for three years as a member of research staff at A.F. Ioffe Physico-Technical Institute, St. Petersburg, on short pulse generation from semiconductor lasers. From 1996 to 2001, he spent five years as a CNRS researcher at IEF, working on microwave and optical photonic crystals. In 2001, he joined Fontainebleau Research Center, Corning SA, France to develop semiconductor optical amplifiers, rapid photodetectors, and photonic crystals. Since 2003 he has been at the CEA Leti laboratories, Grenoble, France. He is author and coauthor of over 40 scientific papers and over 40 conference presentations. His research interests include integrated optics, III/V optoelectronic devices, and nanooptics.


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