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.

1. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics.” Nature Photonics, vol. 4, pp. 535-544, July 2010.
2. J. Bravo-Abad, E.P. Ippen, and M. Soljacic. “Ultrafast photodetection in an all-silicon chip enabled by two-photon absorption.” Appl. Phys. Lett., vol. 94, no. 241103, pp. 1-3, June 2009.
3. T. Tanabe, H. Sumikura, H. Taniyama, A. Shinya, and M. Notomi. “All-silicon sub-Gb/s telecom detector with low dark current and high quantum efficiency on chip,” Appl. Phys. Lett., vol. 96, no. 101103, pp. 1-3, March 2010.
4. M. Dinu, F. Quochi, and H. Garcia. “Third-order nonlinearities in silicon at telecom wavelengths,” Appl. Phys. Lett., vol. 82, pp. 2954-2956, May 2003.
5. K. Ikeda,Y. Shen and Y. Fainman. “Enhanced optical nonlinearity in amorphous silicon and its application to waveguide devices,” Opt. Express, vol. 15, 17761-17771, December 2003.