Objective Mid-wavelength infrared band covers a key atmospheric window and contains characteristic absorption lines of numerous gas molecules, driving strong demand fortrace gas analysis, target detection, and free-space optical communication. Developing novel mid-infrared detector materials and heterostructures has become a major focus in optoelectronics. The quantum cascade detector (QCD), a photovoltaic device based on intersubband transitions, employs a multi-quantum-well structure. Electrons in the absorbing well are excited from the ground state to an excited state by infrared photons. Subsequently, through a staircase of subband levels formed by coupled quantum wells with energy spacings matching the longitudinal optical phonon energy, photogenerated carriers are directionally transported via LO phonon scattering. Under zero bias, vertical collection of photogenerated carriers is achieved, offering advantages such as zero-bias operation, low dark current noise, and flexible wavelength tunability. However, the low responsivity of QCD devices is mainly limited by two factors. First, the absorption region adopts a quantum-well structure with a small absorption coefficient, making it difficult to fully utilize the incident photons. Second, the multiperiod cascade structure makes the responsivity inversely proportional to the number of periods, requiring the consumption of multiple photons for each electron collected by the external circuit. In addition, thermal backfilling and scattering losses during photogenerated electron transport further reduce the carrier collection efficiency. This work focuses on the issue of low absorption coefficient and reports a mid-infrared QCD structure and device employing a double-well-coupled structure.

Methods A double-well coupled structure was designed to enable multichannel transitions from a strongly coupled ground state to several near-degenerate excited states, and the corresponding device is fabricated. Theoretical calculations validate the feasibility of this structure. The QCD structure is grown on a InP substrate via metal-organic chemical vapor deposition, and device fabrication is completed using lithography and etching. The photocurrent spectrum is measured using a Fourier-transform infrared spectroscopy system. Responsivity is calibrated with a blackbody radiation source and a lock-in amplifier. Dark current and R₀A are characterized by an IV source meter, and the detectivity is subsequently derived.

Results and Discussions Theoretical calculations indicate that the device achieves an extraction efficiency of 86.5%, a significantly increased transition matrix element, and an absorption intensity of 1.8%, corresponding to a theoretical peak responsivity of 44.6 mA/W. Experimental results show that the fabricated double-well-coupled mid-infrared quantum cascade detector exhibits a peak response wavelength of 4.9 μm and a peak responsivity of 16 mA/W at 77 K (Fig.3). The R₀A value is 6×10⁵ Ω·cm², and the peak Johnson-noise-limited detectivity reaches 2×10¹¹ Jones (Fig.5), which is consistent with the theoretical design values. The discrepancy between the experimental and theoretical results is likely attributable to inferior epitaxial growth quality compared with theoretical predictions.

Conclusions A double-well-coupled quantum cascade detector with an operating wavelength of 4.9 μm at 77 K was designed. By optimizing the energy level structure, the theoretical responsivity was successfully increased, and the performance improvement was experimentally demonstrated. The detector exhibits high detectivity and responsivity at 77 K, while still maintaining a response at room temperature, indicating that the double-well-coupled structure offers advantages in terms of flexible energy level design and enhanced device performance. In the future, further improvements in epitaxial material quality will be pursued to achieve even higher performance.