Quadratically Nonlinear Photodetector Enables In-Sensor Mixing and Optoelectronics with Telecom-Band Light

Developing on-chip photodetectors capable of nonlinear photoresponse represents a significant step towards more sophisticated photonic integrated circuits, enabling functions like in-sensor processing and optoelectronic mixing, areas currently lacking in advanced devices. Yu Zhang, Xiaoqing Chen and Mingwen Zhang, from Northwestern Polytechnical University, alongside colleagues, now demonstrate a novel on-chip quadratically nonlinear photodetector, achieving this capability through a unique InSe p-i-n homojunction integrated onto a silicon waveguide. This innovative design efficiently converts telecom-band light into visible light via second-harmonic generation within the InSe material, subsequently generating photocurrent, and importantly, establishes a quadratic relationship between photocurrent and light power. The resulting device exhibits a high normalized responsivity and remarkably low dark current, substantially exceeding the performance of previously reported nonlinear photodetectors, and allows for the direct electrical monitoring of light-light interactions, exemplified by a 16-pixel array capable of functioning as a fully integrated, single-shot autocorrelator for picosecond pulse measurement.

InSe Homojunction Enhances Nonlinear Light Detection

This research details the development of a novel nonlinear photodetector based on an InSe p-n homojunction. The researchers created a photodetector leveraging the strong second harmonic generation (SHG) effect in InSe, combined with a p-n homojunction to achieve a quadratic photoresponse, where the detector’s output signal is proportional to the square of the input light intensity. This offers advantages for applications requiring high-resolution imaging and on-chip optical signal processing. The fabricated device demonstrates a clear quadratic dependence of photocurrent on input optical power, confirming its nonlinear detection capability and enhancing spatial imaging resolution compared to traditional linear detectors.

Integration with a silicon waveguide enables on-chip optical signal processing and potential for compact photonic integrated circuits, and the device functions successfully as an on-chip optical autocorrelator, comparable to established commercial devices. The research also highlights the potential for using the detector for upconversion photodetection, expanding its spectral range. The detector utilizes monolayer or few-layer InSe, chosen for its strong SHG properties, with a p-n homojunction formed within the InSe flake to enhance charge separation and improve performance. The device is fabricated using standard microfabrication techniques and integrated onto a silicon-on-insulator wafer, and its performance is characterized through current-voltage measurements and spatial imaging experiments. This work represents a significant step towards developing compact, high-performance nonlinear optical devices with potential applications in high-resolution imaging, optical signal processing, optical communication, spectroscopy, and photonic integrated circuits.

InSe Homojunction Enables On-Chip Nonlinear Detection

Scientists engineered an on-chip quadratically nonlinear photodetector (QNPD) by fabricating an InSe p-i-n homojunction directly onto a silicon waveguide, creating a novel platform for sophisticated photonic integrated circuits. The device operates by coupling telecom-band light into the InSe layer, where it undergoes frequency up-conversion via second-harmonic generation (SHG) into visible light, which is then absorbed to generate photocurrent under a built-in electric field, establishing a quadratic relationship between photocurrent and input optical power. The fabrication process utilizes multilayer InSe, approximately 46nm thick, to maximize SHG efficiency, and a 51nm-thick h-BN dielectric layer to ensure a clean interface and prevent unwanted doping. Researchers established the p-i-n homojunction by electrostatically doping the InSe layer using dual silicon back-gates, creating p-type and n-type regions flanking an intrinsic channel, approximately 6μm in length, for enhanced light-matter interaction.

Electrical characterization demonstrated clear ambipolar behavior, with the drain current varying significantly, indicating effective control of carrier type and density. The team achieved a high normalized responsivity of 37.1A/W2, a significant improvement over previously reported nonlinear photodetectors, and demonstrated an array of 16-pixel QNPDs functioning as a fully single-shot on-chip autocorrelator, precisely measuring picosecond pulses with a sensitivity of 6.1*10-10 W2 without external cameras.

Indium Selenide Enables Efficient Nonlinear Detection

Scientists have developed a new on-chip quadratically nonlinear photodetector (QNPD) integrating indium selenide (InSe) with a silicon waveguide, achieving a normalized responsivity of 37.1A/W2 and a remarkably low dark current of 1 pA. This device efficiently up-converts telecom-band light into visible light through InSe’s second-harmonic generation (SHG), generating photocurrent within a built-in electric field established by an InSe p-i-n homojunction, and experiments demonstrate a quadratic relationship between photocurrent and optical power. The team fabricated a device with a 6μm InSe channel and a 36μm light-InSe interaction length, utilizing a 46nm-thick multilayer InSe structure to maximize SHG efficiency.

A 51nm hexagonal boron nitride layer ensures a clean interface and prevents unwanted doping, contributing to the device’s high performance. Electrical characterization reveals the InSe channel exhibits clear ambipolar behavior, with drain current varying significantly, demonstrating effective control of carrier density. Measurements of the p-i-n homojunction demonstrate high rectification and low current consumption, validating the quality of the fabricated diodes. Furthermore, the researchers designed a 16-pixel QNPD array to implement a fully single-shot on-chip autocorrelator, capable of precisely measuring picosecond pulses with a sensitivity of 6.1*10-10 W2, surpassing commercial devices by more than three orders of magnitude and enabling pulse width characterization at low peak powers.

Indium Selenide Boosts On-Chip Photodetection

This research presents a new on-chip quadratically nonlinear photodetector, created by integrating an indium selenide p-i-n homojunction with a silicon waveguide, and represents a significant advance in photonic integrated circuits. The device efficiently converts telecom-band light into visible light through second-harmonic generation within the indium selenide, then generates photocurrent via the homojunction’s built-in electric field, resulting in a quadratic relationship between the photocurrent and the incident optical power. The team achieved a high normalized responsivity and remarkably low dark current, exceeding the performance of previously reported nonlinear photodetectors based on two-dimensional materials. Demonstrating the device’s capabilities, researchers fabricated a 16-pixel array functioning as a fully integrated autocorrelator, successfully measuring picosecond pulses with high sensitivity without requiring external cameras or bulky optics.

This compact, low-power solution holds considerable promise for large-scale integration within photonic integrated circuits and applications such as in-sensor computing and optical sampling. The authors note that while some existing on-chip photodetectors exhibit nonlinear photoresponsivity, their relationship between photocurrent and optical power varies, whereas their device maintains a consistent quadratic function, ensuring reliability and signal fidelity. Future work may explore broader applications of this direct photoelectric conversion of all-optical mixing signals within integrated systems.