Tunable photonic dark modes in coupled cavity chains

We study the steady-state photon distributions in two coupled systems consisting of three or four single-mode cavities. We find that it is viable to realize a single or double dark mode with vanishing photons in one cavity while arbitrary photons in other cavities under specific conditions. Such dark modes arise from perfect destructive interference between different coupling pathways and are in analogy to atomic dark states in the Λ or tripod systems. It is of more interest that a sharp dark-mode dip may be attained in the three-cavity system and two dark modes may interplay to yield a sharp bright-mode peak in the four-cavity system. The flexible tunability of such dips and peaks is beneficial to design sensitive photonic devices of switching or measuring, in a way different from coupled-resonator-induced transparency and absorption.