Bi-periodic nanostructured waveguides for wavelength-selectivity of hybrid photonic devices A. TALNEAU,1,* X. POMMAREDE,1 A. ITAWI,1 K. PANTZAS,1 A. LUPU,2 AND H. BENISTY3 1
Laboratoire de Photonique et de Nanostructures, route de Nozay, F-91460 Marcoussis, France IEF, CNRS, UMR 8622, Univ. Paris-Sud, 91405 Orsay Cedex, France 3 Laboratoire Charles Fabry, Institut d’Optique Graduate School, Univ PSud, 2 Avenue Augustin Fresnel, F-91127 Palaiseau, France *Corresponding author:
[email protected] 2
Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX
A bi-periodic nanostructuration consisting of a superperiodicity added to a nanohole lattice of sub-wavelength pitch is demonstrated to provide both modal confinement and wavelength selectivity within a hybrid III-V on Silicon waveguide. The wavelength selective behavior stems from finely-tuned larger holes. Such biperiodic hybrid waveguides have been fabricated by oxide-free bonding III-V material on Silicon and display well defined stopbands. Such nanostructured waveguides offer the versatility for designing advanced optical functions within hybrid devices. Moreover, keeping the silicon waveguide surface planar, such nanostructured waveguides are compatible with electrical operation across the oxide-free hybrid interface. © 2015 Optical Society of America OCIS codes: 230.3120 Integrated optics devices; 130.7408 Wavelength filtering devices; 130.5296 Photonic crystal waveguides; 220.4241 Nanostructure fabrication. http://dx.doi.org/10.1364/OL.99.099999
Integrated photonics will boost the performances of optical networks links. Within photonics integrated circuits –PIC-, emission/amplification and isolation are two major optical functions to be included. Si-based approaches can provide monomode sources including rare-earth doped waveguides operating under optical pumping [1], whereas tandem-modulator-based isolator performs the optical isolation [2]. The most widely used approach leading to the best present performances relies upon hybrid silicon photonics, wherein III-V semiconductors provide light amplification and emission, while materials such as garnets provide optical isolation. When both materials are associated with a very thin intermediate layer to silicon, the optical guided mode is mainly selected by the chosen silicon structure, usually a rib waveguide [3-4]. If any advanced optical function is needed, an extra processing step is required to carve the adequate added structure: for instance a grating on top of the guide [5] or a lateral grating [6] for wavelength selective lasing operation. We propose here to carve the silicon guiding layer prior to bonding, specifically implementing guidance with a two-dimensional Photonic Crystal (2D-PC) operating essentially below its photonic gap. When properly tailored, thanks to the huge range of effective indices that can
be obtained in silicon/air composites, such a nanostructure can favor virtually any spatial or spectral characteristics for the propagated optical mode. A sub-wavelength, below band-gap, periodicity, in particular, is compatible with a broad range from the lasing longwavelength to the other short-wavelength limits (multimodal onset, silicon or III-V absorption, etc.). This sub-λ silicon waveguide patterning allows one to implement advanced optical functions in a single technological step, and also retains a plane Si bonding surface, which is oxide-free bonding compatible. This allows electrical injection through the hybrid interface, of great interest for the future of hybrid devices operation [7]. In the present contribution, a sub-λ 2D photonic bandgap structuration is designed for modal confinement, and a bi-periodic structuration in the propagation direction is added to provide wavelength selectivity. We calculate and demonstrate experimentally that this bi-periodic patterned waveguide operates as a wavelength selection mechanism. For modal simulation, being in the sub-photonic band gap regime, Effective Medium Theory (EMT) has been implemented to represent the nanopatterned material. EMT is of interest since simulation of the exact geometry requires a tiny meshing which leads to highly demanding computational resources. EMT allows fast and accurate investigation for large ranges of the several geometrical parameters under consideration. Then a 2D Modal analysis is performed to calculate the effective index of the fundamental mode. For the spectral behavior, a 3D-FDTD is performed to determine the geometry of the added super-periodicity. Both SOI waveguides, as well as hybrid InP/SOI oxide-free bonded waveguides are fabricated and measured. Such a design has the potential of being included for single-mode longitudinal operation in hybrid III-V/SOI DFB lasers, providing on a single technological step both the modal confinement and the wavelength selectivity. More generally, according to the unit-cell shape, a large number of optical functions can be addressed [8]. This approach offers the great potential of being compatible with any kind of hybrid environment. The optical waveguide has a generic shallow ridge shape, its core is not structured and both lateral claddings are composed of the same two-dimensional Photonic Crystal (2D-PC) operating below its photonic gap. Here the 2D-PC is a square array of air holes drilled in the silicon guiding layer. To investigate a large number of geometrical parameters with reasonable computing resources, the Effective Medium Theory is implemented to represent the 2D-PC material, Fig. 1(a), propagation occurs along the z direction. EMT has been
demonstrated to fully represent a periodic nanostructured medium when its pitch Λ is much smaller than the wavelength λ, α = Λ/λ
