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

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_laser²). 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 W², 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.