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Figure 1: (a) Through transmission of 40 µm radius ring with 200 nm coupling gap for four different in-waveguide input powers. (b) Measured through transmissions (circles) on resonance normalized to off-resonance transmission, and fits (lines) for 20 µm rings, and (c) for 40 µm rings with various coupling gaps g; along with the gap, the fitted β is labeled for each sample.
Nonlinear optics comprises a rich body of physics that could enable a number of interesting functionalities in integrated silicon photonics [1] . We are currently interested in the phenomenon of two-photon absorption (TPA) as a means to achieving sub-bandgap photodetection in silicon. Though TPA is a relatively weak absorption mechanism in crystalline silicon, the fact that absorbed power scales quadratically with intensity implies that its strength can be increased for device applications by concentrating a large amount of light energy in a small mode-volume resonator, for example in photonic crystal microcavities [2] [3] .
Further enhancement of TPA may be expected when polycrystalline Si is used instead of crystalline Si as the detector material, due to a larger TPA coefficient (β) resulting from mid-gap electronic states at grain boundaries. Though β has been fairly well characterized for crystalline Si [4] , an understanding of two-photon absorption in the polycrystalline phase is lacking. We have therefore measured β in polycrystalline Si waveguide devices by monitoring the peak through transmission dip in ring resonator devices as a function of input power and fitting the results to a model with β as a fitting parameter; data from such measurements are shown in Figure 1. These measurements indicate β = 310 ± 70 cm/GW, a value over two orders of magnitude larger than that in crystalline Si at 1550 nm and on the same order of magnitude as the value observed in amorphous silicon [5] . As polysilicon is widely available in advanced CMOS processes, these results suggest that high-density, integrated devices utilizing nonlinear absorption at relatively low powers should be achievable.
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