Comment on “Nonreciprocal Light Propagation in a Silicon

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. In (1), Feng et al. consider a two-modewaveguide with a spatially varying but time-independent dielectric constant (Fig. 1), and numerically and experimentally demonstrate a one-waymodal conversion effect: The odd mode is strongly excitedwhen an evenmode is incident along the backward direction from the right end of the waveguide (Fig. 1b). In contrast, the odd mode is not excited when the same even mode is incident along the forward direction from the left end (Fig. 1A). Related to (1), we note that a sim-ilar one-way modal conversion effect, with the same pattern of spatially varying dielectric con