NANOFLUIDICALLY DEFINED DEFECTS IN PHOTONIC CRYSTALS

Optofluidics” represent a new class of photonic devices in which microand nano-scale fluidics are exploited to deliver liquids directly into optical structures either for dynamic reconfiguration or for exploitation in integrated microscale total analysis systems as biosensors. Optical feature sizes in these devices are typically on the order of 100nm with spacing of several 100nm and thus targeted fluidic delivery, into a single defect resonant cavity for example, poses a significant challenge. In this work we demonstrate a technique for dynamically tuning and removing defects in periodic photonic structures using softlithography patterned nanofluidics. 1.0 INTRODUCTION Optofluidic devices seek to take advantage of recent advancements in microand nanoscale fluidics to deliver liquids directly into the optical structures. This provides an order of magnitude increase in the achievable ∆n/n, over traditional techniques, enabling flexible fine-tuning and even dynamic reconfiguration of optical circuits in addition to traditional microfluidic capabilities such as distribution of chemicals to be analyzed, and temperature stabilization. Examples of such devices include adaptable photonic crystal lasers [1], fluidfluid waveguides [2] amongst others. At present such devices rely on manipulating optical properties at scales significantly greater than the wavelength of light. Development of a technique which can fluidically modulate optical properties at smaller scales could enable a broad range of new functionalities such as dynamic reconfiguration of photonic circuits, or targeted delivery directly into resonant cavities for bio-detection. In this paper we describe the use of multilayer soft-lithography [3] nanofluidics to dynamically modulate the index of refraction of targeted defects within a photonic crystal [4] at sub-wavelength length scales. 2.0 NANOSCALE OPTOFLUIDIC INTEGRATION Figure 1 shows a general overview of the fabrication procedure. The bottom layer of the device consists of an SOI substrate on which we define the photonic crystals through standard e-beam lithography and dry etching. The photonic crystals design used here had a triangular lattice of holes constant a = 434nm, hole radius r = 140nm and height h = 207nm. In the experiments presented here we increase the radius of the holes within the central row of the photonic crystal to 203nm in order to introduce a reduced index guided mode into the band gap created by the otherwise regular crystal lattice. This geometry was selected on the basis of a series of numerical experiments as having transmission properties that were most sensitive to local changes in index of refraction. To create the fluidic layer we first define the nanofluidic array (here consisting of an array of 350nm wide channels spaced with 5μm period) in positive relief on a separate SOI substrate. A microfluidics engine structure (with a channel height of 10μm and width of 50μm) is superimposed on the same nanofluidic substrate (in photoresist using standard photolithographic processing). The engine provides the microfluidic functionality which can more conveniently be performed at the micron length scale. The multiscale approach also simplifies fluidic coupling into the nanochannel array. The master is then cast in RTV elastomer and bonded to a valve control layer which provides the active control. The fluidics were then interfaced and bonded with the photonic structure by placing them in conformal contact after a 7s air plasma oxidation. To align the nanochannels with the targeted row of holes we create an array of 30 photonic crystals and vary the spacing between the structures by one half the diameter of the central row of holes. When interfaced with the regularly spaced array of nanochannels, this ensures that at least one nanochannel will be aligned with the central row of at least one photonic crystal. In addition to manufacturing simplicity, building the fluidic layer separately from the optical layer in a soft polymer such as RTV has several inherent advantages including air permeability and the ability to make a conformal seal over the photonic structures. Figure 2 shows the results of an alignment and seal integrity check where a concentrated solution of 5% CTAB surfactant was infused into the nanochannel array and allowed to dry overnight forming a cast of the channel. As can be seen very precise, subwavelgnth scale alignment and sealing is achieved. Figure 1: Nanoscale Optofluidic Integration fabrication and assembly procedure (a) Nanophotonic structure (b) Nanofluidics with Microfluidic Coupling (c) Valve control layer (d) Device assembly (e) Photograph of actual Chip. Figure 2: SEM image of photonic crystal after precipitation of CTAB from solution and removal of PDMS fluidics 1μm Aligned nanochannel 3.0 NANOSCALE FLUIDIC MODULATION OF PHOTONIC CRYSTALS Figure 3a shows the normalized quasi-TE mode transmission through the photonic crystal with a fluidically modulated waveguide (i.e. central row aligned with a nanofluidic channel) for DI water and 5M CaCl2. The data presented has been smoothed to remove higher frequency Fabry-Perot resonances. The results show a shift in the peak transmission of the guided mode from a/λ = 0.291 to a/λ = 0.289 (corresponding to a ∆λ=15nm). While higher index of refraction solutions are available commercially (e.g. Methylene Iodide formulations) for this work we selected solutions of aqueous CaCl2 as being the most appropriate due to their relatively low viscosity and miscibility within the low index solution, thereby minimizing the difficulties resulting from the dominance of capillary pressures on the 100nm scale. Figure 3b demonstrates dynamic modulation of the transmitted power by fluidically switching between the water and CaCl2 solutions. The timescale for switching demonstrated here is on the 10s of seconds, limited here by the use of pressure driven flow and the nanoscale channel size. 4.0 SUMMARY In this work we have demonstrated the integration of nanofluidics and nanophotonics at the subwavelength scale. A general fabrication procedure and architecture for nanoscale optofluidic integration was presented along applied to the fluidic modulation of a single row of holes within a photonic crystal. 5.0 REFERNECES [1] Maune, B. Lonkar M. , Witzens J., Hochberg M. , Baehr-Jones, T. , Psaltis, D. , Scherer, A. , Qiu Y. , "Liquid-crystal electric tuning of a photonic crystal laser" App. Phys. Lett. , 85, 360-362 (2004). [2] Wolfe, D. , Conroy, R. , Garstecki, P., Mayers B. , Fischbach, M. , Paul, K. , Prentiss, M. , Whitesides, G. , "Dynamic control of liquid-core liquid-cladding optical waveguides" PNAS-USA, 101, 12434-12438 (2004) [3] Unger, M. , Chou, H. , Thorsen, T. , Scherer, A. , Quake, S. , “Monolithic microfabricated valves and pumps by multilayer soft lithography” Science 288 113-116 (2000). [4] Joannopoulos, J., Meade, R. , Winn, J., “Photonic Crystals: Molding the Flow of Light. ” Princeton University Press, Princeton, New Jersey (1995). Figure 3: Nanofluidic addressing of single row of holes within a photonic crystal (a) Measured transmission spectrum for DI water and 5M CaCl2 (b) dynamic modulation of transmitted power at a/λ = 0.291. 0 20 40 60 80 100 120 0 5 10 15 20 Time [s] N or m al iz ed T ra ns m itt ed P ow er [d b] 0.285 0.29 0.295 0 5 10 15 20 25 30 a/λ N or m al iz ed T ra ns m itt ed P ow er [d b]