Polycrystalline Germanium Photoconductive Detector for Integrated Lithium Niobate Platform

The thin-film lithium niobate (TFLN) is widely used in optical communications, microwave photonics, terahertz communications, and quantum photonics because of its strict mode confinement, high optical nonlinearity, and broad transparency window. However, lithium niobate itself is not suitable for use as a photodetector. In this work, a polycrystalline germanium (poly-Ge) photoconductive detector was fabricated on the Lithium Niobate on Insulator (LNOI) platform, which simultaneously demonstrates low manufacturing complexity, low cost, and the ability to detect 1550 nm wavelength. Furthermore, the newly proposed and developed lithium niobate waveguide structure enhances the detector’s linearity and responsivity, providing new insights into the layout design of thin-film detectors on the LNOI platform. The responsivity and bandwidth of the fabricated detector increase as the bias voltage rises. The highest responsivity of 10.5 mA/W (bias voltage = 50 V) is measured at 1550 nm, and the -3 dB bandwidth ranges from 100 to 182 MHz between 30 and 50 V. This work offers a practical solution for integrating high-linearity, low-cost photodetectors on the LNOI platform for 1550 nm wavelength.