Quadratically Nonlinear Photodetector Based on InSe/Si p–n Heterojunction

Abstract A highly efficient quadratically nonlinear photodetector (QNPD) is demonstrated based on an InSe/Si p–n heterojunction that uniquely exploits the strong second‐order nonlinearity of few‐layer InSe integrated onto silicon substrates. Through second‐harmonic generation (SHG)‐assisted frequency upconversion, the QNPD can detect photons with energies below the InSe/Si electronic bandgap, extending the photodetection range to 1750 nm. Under 1550 nm pulsed excitation, it achieves a high normalized responsivity of 2.3 × 10 −3 A W −2 with a clear quadratic photocurrent dependence ( I ph ∝ P laser 2 ). Leveraging this quadratic response, the device functions as a compact optical autocorrelator, enabling accurate characterization of ultrashort pulses of 2.5 and 8.5 ps with a sensitivity of 5 × 10 −5 W 2 , surpassing commercial systems by an order of magnitude. Furthermore, the QNPD's nonlinear response is integrated as an activation function within convolutional neural networks, achieving a digital image classification accuracy of over 98.7% on the MNIST dataset. With the well‐developed large‐scale growth and transfer of 2D materials with Si, the demonstrated QNPD promises broad potential in integrated nonlinear photonics, ultrafast optical diagnostics, and neuromorphic computing.