Peng Bai

The development of a photon-type very long-wavelength infrared (VLWIR) detector that simultaneously achieves broadband spectral response and elevated operating temperatures remains a formidable challenge. GaAs/AlGaAs quantum ratchet structures have emerged as a promising architecture to circumvent conventional semiconductor bandgap limitations, enabling multispectral detection while effectively suppressing dark current. In this work, metal-organic chemical vapor deposition (MOCVD)-grown GaAs/AlGaAs heterojunction quantum ratchet photodetectors (QRPDs) are demonstrated. Theoretical and experimental studies of the system were conducted to investigate its spectral response and transport properties. The multiperiod quantum ratchet structure exhibits broadband detection covering the near-infrared to very long-wavelength infrared bands. The asymmetric low-dimensional quantum structure also leads to a photovoltaic-like effect with zero bias response and bias-tuned spectral characteristics. A high peak specific detectivity of 2.4 × 1011 cm·Hz1/2 W–1 was achieved at zero bias. Thanks to the suppression of dark current by the ratchet barrier, the BLIP temperature of the QRPD reached 40 K. This work not only demonstrates the significant potential of quantum ratchet devices for broadband high-temperature operation but also verifies the feasibility of MOCVD in the mass production of quantum ratchet devices, offering a promising solution for cost-effective product iteration and broader application scenarios in photon-type infrared detection.

Enduring infinitesimal development from a research standpoint, the very long wavelength infrared (VLWIR) band has aroused intense interest with the best overall performance in adverse weather and atmospheric turbulence. Meanwhile, a quantum ratchet infrared photodetector (QRIP), ensuring an ultrabroadband photocurrent spectrum at wavelengths ranging from near-infrared to THz (1–75 μm), was recently proposed to acquire a potential broadband photon-type detection including the VLWIR regime. However, the limited VLWIR responsivity obstructs further applications of GaAs-based ratchet structures. Exploiting metal-dielectric-metal resonant cavities, we demonstrate that the broadband coupling efficiency theoretically achieves a 20-fold enhancement across the 16–20 μm VLWIR range compared to the mesa reference device. This improvement leads to a significant elevation in responsivity (achieving a peak value of 1.18 AW⁻¹), concurrently reducing the noise equivalent power to a minimum of 0.6 pWHz^−1/2 and increasing the specific detectivity to a peak of 8.26 × 10¹⁰ Jones. Through theoretical simulations, we reveal that the broadband enhancement can be attributed to the geometric structure of the diagonal-placing patchwork antenna, which not only restricts the light field within the double-metal region but also leads to an effective modulation of patch sizes to broaden the enhancement coverage. Our discovery evidences efficient arrayed spacing-perturbed antennas for the high-performance QRIP which can be modified to capture not only VLWIR but also IR/THz electromagnetic signals.

Self-mixing interference in a terahertz quantum-cascade laser has been demonstrated to be suitable for the detection of weak signals scattered or reflected by the target. This technology has achieved the high-sensitivity detection of complex refractive indices, surface/interface morphologies and molecular feature spectra. Here, a set of terahertz quantum-cascade lasers with different lasing frequencies is used to inspect a tiny amount of powder concealed inside a polytetrafluoroethylene tablet by using self-mixing interferometry combined with the penetration properties of terahertz waves. Multicolor spectral images were acquired, which were synthesized by absorption contrast images obtained at different lasing frequencies. They enable the detection of the spatial distribution of hidden objects which are totally opaque in visual light and allow for them to be identified with spectral absorption characteristics. Self-mixing interference technology can also obtain phase information when a terahertz wave interacts with a tablet, showing the difference between the hidden object and surroundings from another dimension. Our research may provide a strategy for the development of terahertz multispectral imaging technology for the inspection of hidden trace residues.

Broadband upconversion has various applications in solar photovoltaic, infrared and terahertz detection imaging, and biomedicine. The low efficiency of the light-emitting diodes (LEDs) limits the broadband upconversion performance. In this paper, we propose to use surface microstructures to enhance the electroluminescence efficiency (ELE) of LEDs. Systematical investigations on the cryogenic-temperature performances of microstructure-coupled LEDs, including electroluminescence efficiency, luminescence spectrum, and recombination rate, have been carried out by elaborating their enhancement mechanism and light emitting characteristics both experimentally and theoretically. We have revealed that the reason for the nearly 35% ELE enhancement of the optimized structure under cryogenic temperature and weak injection current is the efficient carrier injection efficiency and the high recombination rate in the active region. We also compare studies of the surface luminescence uniformity of the optimized LED with that of the unoptimized device. This work gives a precise description, and explanation of the performance of the optimized microstructure coupled LED at low temperatures, providing important guidance and inspiration for the optimization of broadband upconverter in the cryogenic temperature region.

High performance infrared imaging devices have extensive applications in environmental monitoring, surveillance, and national defense. Pixelless upconversion imaging devices offer promising potential for large-format infrared imaging. In this work, we propose a pixelless upconversion imaging device enhanced by a microsphere (MS) structure. Fabricated through a convenient, low-cost, and efficient process, our device successfully achieved pixelless upconversion imaging of blackbody hot spots. Compared to prior methods, this design significantly improves the light extraction efficiency (LEE) while achieving an upconversion efficiency comparable to traditional grating structures. Theoretical analyses reveal that the external quantum efficiency (EQE) of the light-emitting diodes primarily drives the upconversion efficiency rather than the coupling efficiency of the quantum well detector. Finally, we present an optimized MS-coupled device with enhanced upconversion efficiency, providing insights for future development of large-format, high-efficiency, high-speed pixelless upconversion infrared imaging.

High-performance infrared thermal imaging devices are widely used in military, biomedical and other fields. Upconversion pixel-less imaging is promising for infrared imaging. In this paper, we propose a hybrid metasurface to achieve high upconversion efficiency of the integrated quantum well infrared photodetector and light-emitting diodes (QWIP-LED). Systematical investigations on the performance of the QWIP-LED, including optical coupling efficiency, light extraction efficiency, and upconversion efficiency, have been carried out via theoretical simulation. We also present the integration time for different devices with different optical coupling structures. Numerical results show that 45° edge-coupled QWIP-LED is not suitable for imaging applications for the low upconversion efficiency. Traditional grating-coupled QWIP-LED can be optimized for real-time thermal imaging. The hybrid-metasurface-based QWIP-LED can achieve a high frame rate above 300 Hz due to the enhanced upconversion efficiency. This work gives a precise description of QWIP-LED performance with different device structures and opens the way for large format upconversion pixel-less imaging.

Achieving a photodetector with high bandwidth, high sensitivity, high speed, and high operating temperature is a common goal for researchers, especially for infrared and terahertz regions due to its important application in various fields like military defense and biochemical analysis. Among photon-type terahertz (THz) detectors, heterojunction interfacial work function internal photoemission (HEIWIP) detectors demonstrate high bandwidth, high sensitivity, and high speed, but are limited by the low operating temperature. In this work, we developed an effective strategy for raising its operating temperature by introducing ratchet structures. By comparing with traditional HEIWIP, the ratchet HEIWIP exhibits a low dark current and high detectivity while retaining the ultra-broad response bandwidth. The operating temperature has been raised from 7 to 20 K in the terahertz region, which is meaningful progress in THz photon-type detection. The universality of this scheme makes it possible to be further migrated to general photon-type semiconductor detectors.