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We show that the structure demonstrated by Feng et al. (Reports, 5 August 2011, p. 729) cannot enable optical isolation because it possesses a symmetric scattering matrix. Moreover, one cannot construct an optical isolator by incorporating this structure into any system as long as the system is linear and time-independent and is described by materials with a scalar dielectric function.

Numerical analysis predicts that continuous-wave Raman lasing is possible in silicon-on-insulator (SOI) waveguides, in spite of the detrimental presence of two-photon absorption and free-carrier absorption. We discuss in particular the dependence of the lasing characteristics of SOI Raman lasers on the effective lifetime of the free carriers generated by two-photon absorption. It is shown that the pump-power-dependent cavity losses lead to a rollover of the output-power characteristics at a certain pump-power level and that there exists an upper shutdown threshold at which the laser operation breaks down.

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