Polarization Properties of Metal-Dielectric Composite Subwavelength Gratings on GaN Substrate

A novel type of GaN substrate for light emitting devices with a highly polarized output using metal-dielectric composite subwavelength grating structure is proposed. Characteristics of both optical transmission and polarization extinction ratio of the polarized GaN-based LED with a double-layer grating structures are investigated. It is found that both TM transmission (T_TM) and the extinction ratio(ER) of the LED output can be effectively enhanced by incorporating a dielectric transition grating between the metal grating and GaN substrate with a lower refractive index than that of the GaN substrate. Flat sensitivity of the T_TM on the period, duty cycle of the grating, and the wide range of operating wavelength have been achieved. Up to 0.7 high duty cycle of the grating can be employed to achieve ER > 56dB while T_TM maintains higher than ~93%. The results provide guidance in designing, optimizing and fabricating the novel integrated GaN-based and polarized photonic devices.