Nonlinear absorption in silicon and the prospects of mid-infrared silicon Raman lasers

1 Introduction By circumventing the problem of indirect bandgap, Raman scattering offers a path to optical amplification and lasing in silicon [1]. To create the recently demonstrated silicon Raman lasers [2, 3] nonlinear losses due to absorption by free carriers that are generated by two-photon absorption (TPA) had to be eliminated using pulsed pumping or reverse-biased carrier sweep-out. Even when such measures are taken, operation in the near IR band is limited to low duty cycle pulse pumping, or in the case of CW operation, to low output powers and poor efficiency. The latter limitation originates from the fact that at high pump intensities, the use of PN junction is only partially effective because free carriers screen the junc-tion's electric field [4]. In this paper, we report the first measurements of nonlinear absorption in silicon in the mid-IR wavelengths and show that two-photon and free-carrier absorption processes can be reduced to negligible levels by using pump lasers with photon energy less than half the band gap. This implies that when pumped with laser wavelength longer than ~2.2 µm, the two-photon absorption process is elimi