Mode engineering for circular-side square microcavity lasers

Whispering-gallery mode (WGM) microcavities with the merits of small mode volumes and high quality (Q) factors have attracted great research interests as potential low-power-consumption light sources for photonic integration. We propose and demonstrate deformed square microcavity lasers with the flat sidewalls replaced by circular arcs as converge mirrors to control the WGMs inside the laser cavity. The ray dynamic analysis results indicate that the circular-sides can confine the light rays with stable islands, although full chaotic dynamics are observed under certain deformation. With the numerical simulation of the circular-side square microcavities, ultrahigh-Q modes are obtained owing to the elimination of the scattering losses from the vertices, and a reduction of mode Q factors due to the chaotic ray dynamics is also observed. Different transverse modes have distinct light trajectories, which results in a difference of the effective roundtrip length and a controllable transverse mode interval. Low threshold lasing is achieved experimentally due to the high Q factors of the WGMs. The lasing spectra can be engineered by designing the cavity geometry for the waveguide-coupled circular-side square microcavity lasers. The robust structure and ultrahigh-Q of the waveguide-coupled microlasers provide a potential solution for the compact light sources in photonic integrated circuits.