Circularly-Polarized Laser Equivalent to Two Phase-Shifted Linearly-Polarized Sources

Compact circularly-polarized (CP) light sources are important for chirality-based applications at the chip level. The CP light is conventionally generated by passing the linearly-polarized (LP) one through quarter wave plates which are too thick for packed integrations. In this work, the in-plane submicron phase shifter substitutes the quarter-wave plate in the creation of CP light. After the lasing field of a holed microdisk is coupled into the nearby waveguide and phase shifter, it is guided into two grating couplers that output orthogonally LP waves vertically. The two LP waves are phase-shifted by pi/2$\pi /2$ and combine into the coherent CP radiation in the far-field zone. Experimentally, the fabricated devices exhibit a degree of circular polarization as high as 74%. This device scheme is simple and effective in that it relies on no extrinsic materials, large-area auxiliary structures, or specialized operation conditions critical for CP radiation. The compatibility issues for integrated photonic systems are of less concern in such schemes. Circularly-polarized light is essential for chiral-photonic applications. However, it is difficult to scale down the conventional setup for generating such lights. In this work, compact circularly-polarized lasers are demonstrated by combining the vertical radiations from two phase-shifted light sources. Free from elements incompatible with integrated photonic circuits, the device naturally fits into photonic chips.image