Dark state lasers.

We propose a new type of laser resonator based on imaginary energy-level splitting (imaginary coupling or quality factor Q-splitting) in a pair of coupled microcavities. A particularly advantageous arrangement involves two microring cavities with different free-spectral ranges in a configuration wherein they are coupled by far-field interference in a shared radiation channel. A novel Vernier-like effect for laser resonators is designed in which only one longitudinal resonant mode has a lower loss than the small-signal gain and can achieve lasing while all other modes are suppressed. This configuration enables ultrawidely tunable single-frequency lasers based on either homogeneously or inhomogeneously broadened gain media. The concept is an alternative to the common external cavity configurations for achieving tunable single-mode operation in a laser. The proposed laser concept builds on a high-Q "dark state," which is established by radiative interference coupling and bears a direct analogy to parity-time symmetric Hamiltonians in optical systems. Variants of this concept should be extendable to parametric-gain-based oscillators, enabling widely tunable single-frequency light sources.