Mid-infrared (MIR) detection technology has a wide range of applications in the fields of environmental monitoring, astronomical observation, biomedicine and security warning. Compared with the visible light band, the mid-infrared photon energy is small, and the corresponding photodetector usually adopts narrow bandgap semiconductor materials, such as mercury cadmium telluride, indium antimonide, etc. At room temperature, the noise equivalent of mid-infrared detectors is limited by severe dark current and thermal noise. At room temperature, infrared detectors are limited by severe dark current and thermal noise, and their noise equivalent power is generally on the order of pW/Hz1/2, and their sensitivity is far from being able to cope with the application scenario of sparse photons. For a long time, researchers have been committed to the development of infrared detection technology with higher sensitivity and lower noise to meet the urgent needs of infrared measurement and control under extremely weak microlight illumination.
In recent years, infrared upconversion detection technology has attracted a lot of attention, which converts the infrared light field to the visible or near-infrared wavelengths through a nonlinear process, and then utilizes high-performance silicon-based detectors to obtain infrared information, with high sensitivity, fast response speed, room temperature operation and other advantages. Usually, upconversion detectors can be divided into two categories according to the pumping method. One is based on pulsed light pumping, which utilizes the characteristics of high pulse peak power and narrow time window to significantly improve efficiency and suppress noise. However, the pulsed pumping scheme is only suitable for active detection under cooperative targets, and the application scenarios are greatly restricted. The other uses continuous optical pumping to apply to a wider range of passive infrared detection scenarios. Compared with the pulse pumping scheme, continuous pumping can effectively improve the overall detection efficiency of time-domain random infrared signals. However, the program in the average power acquisition and parametric noise suppression puts forward more stringent requirements, the development of high-efficiency, highly sensitive passive upconversion detection is still quite challenging.
To this end, the team of Prof. Heping Zeng and Kun Huang designed and developed a high-efficiency, low-noise, large dynamic range mid-infrared detection system by combining the external cavity pump enhancement technology and nonlinear frequency conversion technology. By optimizing the mode matching and impedance matching of the optical cavity, the researchers obtained a 36-fold pump power enhancement; using fully automated digital cavity locking technology based on FPGA, the power jitter is controlled within 1%; in the cavity under the average power of 55 W, the quantum conversion efficiency of infrared photons reached 22%; combined with a high suppression ratio of the filtering system and a highly sensitive silicon-based single-photon counters, to achieve a low to 0.3 fW/Hz1/2 noise-equivalent power, an improvement of at least one order of magnitude over previous records. To further improve the detection dynamic range at single-photon sensitivity, the researchers employed a multi-pixel photon counter to effectively broaden the linear response interval of the incident signal. In addition, the modular separation design of the pump source and optical cavity circumvents the complex solid-state laser design and allows full utilization of compact and robust fiber lasers.
It is worth mentioning that the adopted pump light source has an ultra-narrow linewidth with a single longitudinal mode, which can realize high-fidelity mid-infrared spectral mapping, providing the possibility of high-precision molecular spectral analysis. In the future, the system is expected to be expanded to the long-wave infrared and even terahertz bands, providing strong support for highly sensitive detection, spectroscopy and imaging.
This work was jointly supported by the Ministry of Science and Technology, the Foundation, Shanghai Municipality, Chongqing Municipality and East China Normal University, and was recently published in Advanced Photonics Nexus, 3, 046002, (2024).
Figure (a) Device diagram of the mid-infrared frequency upconversion detection system with external cavity pumping enhancement; (b) Photograph of the enhancement cavity and a schematic of the materials used in the various parts; (c) Background counts and equivalent noise power with increasing pump power in the cavity; (d) Schematic diagram of the digital locking process of the optical cavity.
Link to paper: Highly sensitive mid-infrared upconversion detection based on external-cavity pump enhancement
