Multimode resonances in square-shaped optical microcavities

Square-shaped two-dimensional optical microcavities ($\mu$-cavities) were investigated for possible applications as filters for dense wavelength-division multiplexing. Multimode cavity resonances were observed in the elastic scattering of $\approx 200‐\mu \mathrm{m}$ square-shaped $\mu$-cavities in fused silica. Based on a two-dimensional $k$-space representation, we accounted for the multimode spectrum by different normal modes with rays confined by total internal reflection. The cavity-mode trajectories need not be closed after each round trip. Single-mode spectra are expected from smaller square-shaped $\mu$-cavities.