Radially Polarized Orbital Angular Momentum Beam Emitter Based on Shallow-Ridge Silicon Microring Cavity

Radially polarized orbital angular momentum (OAM) beams could be applied in optical manipulation and optical microscopy. An integrated OAM beam emitter with radially polarized radiation is proposed on shallow-ridge silicon microring with azimuthally distributed gratings etched on top of the ring waveguide. Two key structural parameters, the number of grating elements, and the dimension of each grating element are optimized by considering both the OAM order purity (C_p) and the total radiation efficiency (η_all). With elaborate design, C_p = ~98%, η_all = ~49% are achieved by simulations, and the calculated energy ratio between the far-field radially and azimuthally polarized component is more than 11 dB, which indicates that the radially polarized component is predominant in the radiated electromagnetic field.