Enhanced Pockels Effect in AlN Microring Resonator Modulators Based on AlGaN/AlN Multiple Quantum Wells

AlN-on-sapphire is a promising integrated photonic platform for operation over a wide wavelength range from ultraviolet to infrared. However, this platform suffers from a weak second-order electro-optic effect, also known as the Pockels effect. Here, we demonstrate an enhanced Pockels effect by utilizing AlGaN/AlN multiple quantum wells (MQWs) regrown on the AlN layer. This enhancement is attributed to the large built-in polarization field in the MQWs, which results in a higher second-order susceptibility in the MQW layer due to the electric-field-induced second-order effect overlapping with the optical mode of the waveguiding device. To investigate this enhancement, we design and fabricate separate AlN microring resonator modulators (MRMs) with MQWs operating at two different wavelengths, 1550 and 780 nm. The resonance shift due to the Pockels effect is characterized by applied voltages and compared with the AlN MRM of similar dimensions but without MQWs. We observe enhanced resonance shift factors of 2.16 (at 1550 nm) and 1.56 (at 780 nm) for resonators with MQWs compared to those without MQWs. Through a modal overlap analysis between the MQW layers and the optical mode of the resonator, we extract the second-order susceptibility in the MQW regions, which is shown to be 20 (at 1550 nm) and 10 times (at 780 nm) higher compared to that of AlN. Our study paves a promising path for realizing III-nitride-integrated photonic modulators with a stronger Pockels effect by employing MQWs with an optimal overlap with the optical mode of the modulator.