  We demonstrate an efficient nonlinear photodetector (NLPD) with quadratic
response based on a few-layer InSe p-n homojunction, which is beneficial from
the strong second harmonic generation (SHG) process in InSe and effective
harvest of photocarriers actuated by the high-quality homojunction. The NLPD
can sense light with photon energy smaller than InSe electronic bandgap because
the SHG process in InSe doubles the frequency of incident light, extending InSe
photodetection wavelength range to 1750 nm. The InSe p-n homojunction, which is
electrostatically doped by two split back gates, presents a rectification ratio
exceeding 106 with a dark current down to 2 pA and a high normalized
responsivity of 0.534 A/W2 for the telecom-band pulsed light at 1550 nm. The
photocurrents of the SHG-assisted photodetection have a quadratic dependence on
the optical powers, making the NLPD highly sensitive to light intensity
variation with improved spatial resolution. As examples, the NLPD is employed
to precisely determine the localization point of a focused laser beam waist and
implement spatial imaging with an improved resolution compared with the linear
photodetector. These features highlight the potential of the proposed NLPD in
developing advanced optical sensing and imaging systems.
