Is one-way modal conversion sufficient for optical isolation

We show that the novel one-way photonic modal conversion effect, as demonstrated by Feng et al (Reports, 11 August, 2011, p. 729), is insufficient in itself to enable optical isolation, since the underlying dielectric structure possesses a symmetric scattering matrix. Moreover, one cannot construct an optical isolator, by enclosing this structure with a system containing any combination of components or signal processing elements, as long as the overall system is linear, and is described by a static scalar dielectric function. Instead, to achieve complete optical isolation, one would need to augment the present design so that the final system is described by a non-symmetric scattering matrix.