Waveguide-integrated van der Waals heterostructure photodetector on a lithium niobate on insulator platform.

Lithium niobate (LN) photonics has gained significant interest for their distinct material properties. However, achieving monolithically integrated photodetectors on lithium niobate on an insulator (LNOI) platform for communication wavelengths remains a challenge due to the large bandgap and extremely low electrical conductivity of LN material. A two-dimensional (2D) material photodetector is an ideal solution for LNOI photonics with a strong light-matter interaction and simple integration technique. In this work, a van der Waals heterostructure photodiode composed of a p-type black phosphorus layer and an n-type MoS2 layer is successfully demonstrated for photodetection at communication wavelengths on a LNOI platform. The LNOI waveguide-integrated BP-MoS2 photodetector exhibits a dark current as low as 0.21 nA and an on/off ratio exceeding 200 under zero voltage bias with an incident power of 13.93 µW. A responsivity as high as 1.46 A/W is achieved at -1 V bias with a reasonable dark current around 2.33 µA. With the advantages of high responsivity, low dark current, and simple fabrication process, it is promising for the monolithically integrated photodetector application for LNOI photonic platforms at communication wavelengths.

Analytical Transient Responses and Gain-Bandwidth Products of Low-Dimensional High-Gain Photodetectors.

Low-dimensional photodetectors, in particular those in photoconductive mode, often have extraordinarily high photogain. However, high gain always comes along with a slow frequency response. The gain-bandwidth product (GBP) is a figure of merit to evaluate the performance of a photodetector. Whether the high-gain photoconductors can outperform standard PIN photodiodes in terms of GBP remains an open question. In this article, we derived the analytical transient photoresponses of nanowire photoconductors which were validated with the simulations and experiments. Surprisingly, the fall transients do not follow a simple time-dependent exponential function except for some special cases. Given the analytical photogains that were established previously, we derived the theoretical GBP of high-gain nanowire photoconductors. Analysis of the analytical GBP indicates that nanoscale photoconductors, although having extremely high gain, will never outperform typical PIN photodiodes in terms of GBP.