Direct time-domain observations are reported of a low-power, self-induced modulation of the transmitted optical power through a high-Q silicon microdisk resonator. Above a threshold input power of 60 µW the transmission versus wavelength deviates from a simple optical bistability behavior, and the transmission intensity becomes highly oscillatory in nature. The transmission oscillations are seen to consist of a train of sharp transmission dips of width approximately 100 ns and period close to 1 µs. A model of the system is developed incorporating thermal and free-carrier dynamics, and is compared to the observed behavior. Good agreement is found, and the self-induced optical modulation is attributed to a nonlinear interaction between competing free-carrier and phonon populations within the microdisk. IEEE J. Quantum Electron. 22, 873-879 (1986 , 1985). 25. G. Cocorullo and I.Rendina, "Thermo-optical modulation at 1.5 µm in silicon etalon," Electron. Lett. 28, 83-85 (1992). 26. K. J. Vahala, "Optical Microcavities," Nature (London) 424, 839-846 (2003). 27. Here we assume that the coupling to each of the standing-wave modes is identical. In general, the coupling can be different, although experimentally we have noticed only small differences in coupling. 28. R. Claps, V. Raghunathan, D. Dimitropoulos, and B. Jalali, "Influence of nonlinear absorption on Raman amplification in silicon waveguides," Opt. Express 12, 2774-2780 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-12-2774. 29. Note that the confinement factor and effective mode volume for the two standing-wave modes are identical, hence we drop the c/s subscript. 30. For TPA with the standing wave modes one has an additional term dependent upon the product U c U s , with cross-confinement factor Γ c/s,TPA and cross-mode volume 3V c/s,TPA pre-factors. For FCA, described below, one cannot write the total absorption just in terms of products of powers of the cavity energies, but rather the mode amplitudes themselves must be explicitly used. 31. R. A. Soref and B. R. Bennett, "Electrooptical Effects in Silicon," IEEE J. Quantum Electron. 23, 123-129 (1987 (Academic Press, Boston, MA, 1985). 35. M. Dinu, F. Quochi, and H. Garcia, "Third-order nonlinearities in silicon at telecom wavelengths," Appl. Phys.Lett. 82, 2954-2956 (2003). 36. D. K. Schroder, R. N. Thomas, and J. C. Swartz, "Free Carrier Absorption in Silicon," IEEE Trans. Electron.Dev. 25, 254-261 (1978