Grating Coupled Waveguide Biosensor Based on Porous Silicon
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Grating Coupled Waveguide Biosensor Based on Porous Silicon Xing Wei and Sharon M. Weiss Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USA ABSTRACT Porous silicon waveguides with integrated porous silicon grating couplers are demonstrated as small molecule biosensors. Two fabrication methods are presented for the grating couplers: standard electron beam lithography with reactive ion etching and a new technique based on direct imprinting of porous substrates. Although the gratings fabricated using standard lithographic techniques have steeper sidewalls and enable a larger available sensing surface area inside the waveguide, the imprinted gratings have the advantage of rapid and lowcost fabrication. Both the lithographically and imprinted sensors are shown to have waveguide losses on the order of 10 dB/cm, and both are demonstrated for detection of 16mer nucleic acids. INTRODUCTION Porous silicon (PSi) is a crystalline form of silicon with nanoscale void spaces introduced into its microstructure. It was first discovered by A. Uhlir in 1956 [1], but did not initially receive much attention. Starting in the late 1990’s, PSi began to be recognized for its potential to serve as a host material in biochemical sensor applications [2, 3] due to its enormous internal surface area (up to 800m2/g, [4]) that can accommodate the immobilization of a large number of biomolecules. Although many PSi configurations have been utilized for a variety of sensing demonstrations [5], PSi waveguide structures have the key advantages of strong electric field confinement in the porous waveguide film layer where biomolecules are captured and in-plane light propagation, which result in more sensitive detection of small molecules and excellent potential for monolithic integration with photonic integrated circuits. Moreover, porous waveguides can outperform evanescent wave based biosensors [6, 7] by taking advantage of the large mode overlap of a well-confined waveguide mode and molecules inside the waveguide instead of relying on the interaction of the exponentially decaying tail of a guided mode with surface-captured biomolecules. Here we demonstrate a grating coupled waveguide structure made entirely of PSi as a compact platform for biosensing. Two different methods are employed for the grating fabrication. In one case, electron beam lithography is used to define photoresist gratings on top of the waveguide followed by reactive ion etching to transfer the grating patterns into the PSi underneath. In the second case, the PSi gratings are formed by a new direct imprinting method where a silicon grating stamp is used to locally deform the PSi film [8]. In both instances, the gratings on top of the waveguide act as couplers to enable light to enter and subsequently propagate inside the waveguide where the light can interact with captured molecules. The condition for coupling is satisfied only for a particular wavelength and incident angle of light that satisfies the grating equation, neff = n
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