Reconfigurable optofluidic silicon-based photonic crystal components Christian Karnutsch1*, Uwe Bog1, Cameron LC Smith1, Snjezana Tomljenovic-Hanic1, Christian Grillet1, Christelle Monat1, Liam O’Faolain2, Tom White2, Thomas F Krauss2, Ross McPhedran1, Benjamin J Eggleton1 1 Institute of Photonics and Optical Science (IPOS), Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), School of Physics, University of Sydney, NSW 2006, Australia 2 School of Physics and Astronomy, University of St Andrews, St Andrews, Fife, Scotland ABSTRACT We report reconfigurable optofluidic photonic crystal components in silicon-based membranes by controllably infiltrating and removing fluid from holes of the photonic crystal lattice. Systematic characterizations of our fluidicallydefined microcavities are presented, corresponding with the capability to increase or decrease the span of the fluid-filled regions and thus alter their optical properties. We show initial images of single-pore fluid infiltration for holes of diameter 265 nm. Furthermore, the infiltration process may employ a large range of optical fluids, adding more flexibility to engineer device functionality. We discuss the great potential offered by this optofluidic scheme for integrated optofluidic circuits, sensing, fluorescence and plasmonic applications. Keywords: Microfluidics, microphotonics, optofluidics, tuneability, photonic integration, microcavity, sensors
1. INTRODUCTION Photonic crystals (PhCs) represent a class of materials that display a periodic arrangement of dielectric constants [1]. In contrast to a homogeneous medium, the dielectric modulation causes the dispersion of a PhC to be highly frequency dependent. The abrupt spectral variations in the associated photonic band structure imply that moderate shifts in the refractive index – accomplished through an external perturbation – can substantially modify the optical properties of the PhC at a particular frequency. This offers the potential for creating flexible and dynamic optical functionalities, which could be favorably used in applications such as optical information processing (switching, routing, buffering), quantum electrodynamics experiments and highly sensitive and localized optical sensing [2]. Many of the relevant properties of PhCs require a large refractive index contrast, which is provided e.g. by air silicon interfaces. A variety of materials have been introduced in PhCs, such as liquids [3-6], organic liquids [7], liquid crystals [8-10], polymers [11, 12], nanoparticle-based composites [13], colloidal quantum dots [14, 15] and fluorescent organic dyes [16-20]. Because there exists a range of liquid materials featuring a wide array of optical properties, PhC infiltration opens up many different opportunities associated with the particular characteristics of the infused material [21, 22]. The idea of PhC infiltration has recently been expanded through the concept of selective fluid filling. Introducing liquid crystals into individual air pores of a planar PhC can potentially create various tunable photonic elements (Y-junctions, bends, waveguide intersections and beam splitters) integrated in a PhC circuit [23]. Planar PhCs can confine light in three dimensions by combining a 2D PhC lattice and a step index waveguide (e.g. a thin silicon slab) [24, 25]. Hence their fabrication is compatible with the mature microelectronic manufacturing techniques, while they provide a suitable platform for creating a variety of optofluidic devices that can be readily integrated onto a single chip [26]. Planar PhC components (e.g. waveguides and cavities) are realized by introducing a local ‘defect’ in the periodic lattice. While these defects generally consist of removed or displaced air holes, they can alternatively be created through the selective infiltration of air holes with a liquid. Light can be effectively routed along these fluidically defined paths [27]. Although the resulting light confinement is shallower due to the reduction of the PhC index contrast, effective light guiding is possible even around tight bends [27]. Fig. 1 shows a schematic vision of such an integrated optofluidic photonic circuit, where
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[email protected]; phone: +61-2-9351 3958; fax: +61-2-9036 7158 Silicon Photonics IV, edited by Joel A. Kubby, Graham T. Reed, Proc. of SPIE Vol. 7220, 72200K · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.811083
Proc. of SPIE Vol. 7220 72200K-1
infiltrated holes act as waveguides, microcavities, light sources, filters or switches. The different components of this integrated photonic circuit could not only be modulated based on the tunable properties of the fluid, but also completely erased and subsequently rewritten. This flexible platform will therefore provide a dynamic control over the guiding of light as well as full reconfigurability of the resulting photonic integrated circuit. Double-Heterostructure Cavities
Optical Filters or Switches Integrated Light Source
Waveguide
Figure 1 Schematic of a microfluidic reconfigurable photonic circuit made of a planar silicon photonic crystal. The device integrates various components – all achieved through selective infiltration of air holes – onto the same platform.
Among the attractive properties of PhC structures lies the ability to tightly confine light within highly compact microcavities [28, 29]. Planar PhC microcavities represent a versatile platform for realizing various small-scale optical components, such as low threshold lasers [30, 31], optical switches [32, 33], narrow-band filters [34], and slow-light structures [35]. For this wide range of applications, design rules generally aim at generating high quality factors, Q and small modal volumes (V
