Mid-infrared spectroscopy has become an important means of analysing the characteristic patterns of vibrational transitions of numerous molecules, and the continuous improvement of its detection sensitivity is of positive significance to meet the low illumination application scenarios, such as trace detection, infrared remote sensing and environmental monitoring. However, existing mid-infrared detectors are noisy at room temperature, which severely limits the detection sensitivity of conventional spectrometers. In addition, scan-free mid-infrared spectrometers usually require focal plane array detectors, which are expensive while the spectral resolution is limited by the pixel scale. Therefore, how to achieve highly sensitive and high-resolution mid-infrared spectroscopic detection in a wide spectral range is of considerable interest.
In recent years, the time-stretching spectroscopy technique is in the ascendant, which maps the spectral information into the time dimension, eliminates the spatial dispersion element, and achieves high-throughput and high-resolution spectroscopic measurements with a single-pixel detector without mechanical scanning. However, highly sensitive time-stretched spectroscopy is currently limited to the visible or near-infrared wavelength bands, and the lack of low-loss, high-dispersion media and high-bandwidth optical detectors has seriously hindered its development into the mid-infrared spectral band. Therefore, it is still a challenge to realise wide-band, high-resolution mid-infrared single-photon time-stretching spectroscopy.
To this end, the team of Prof Heping Zeng and Kun Huang designed and implemented a broadband mid-infrared single-photon time-stretching spectrometer based on high-fidelity frequency upconversion and time-dependent photon counting techniques. Using broadband nonlinear and frequency processes, the researchers converted the supercontinuum spectrum covering 2.4-4.2 μm into the near-infrared region to make full use of the low-loss single-mode fibre and high-performance silicon-based detectors available in this band, providing a solution for stretch transformation and sensitive detection. In the experiment, the arrival time of the upconverted photons was precisely recorded by a low-timing jitter photon counter, enabling a high spectral resolution of about 0.5 cm-1 to be obtained at single-photon illuminations as low as 0.14 photons/nm/pulse.
The research team has realised mid-infrared single-photon spectroscopy in the time domain for the first time, combining the advantages of up-conversion detection and time-stretched spectroscopy, overcoming the stringent requirements for multi-pixel array detectors in precision spectroscopic measurements, and realising high-throughput and high-resolution single-photon spectroscopic acquisition with single-pixel photon detectors.
In the future, the spectral resolution can be further improved by using a medium with larger group velocity dispersion and a detector with lower time jitter. In addition, the proposed method can be extended to the long-wave infrared or terahertz bands to meet the urgent needs for high-sensitivity and high-resolution spectroscopic measurements in this spectral band, which will provide a new means of infrared spectroscopic analysis for the fields of materials, chemistry, biology, medicine and so on.
The work was jointly supported by the Ministry of Science and Technology, the Foundation, Shanghai Municipality, Chongqing Municipality and HUST, and was published in Laser & Photonics Reviews (DOI: 10.1002/lpor.202301272), with the first author being Sun Ben, and the corresponding authors being Huang Kun and Professor Zeng Heping.
Fig. (a) Conceptual diagram of mid-infrared single-photon time-stretching spectroscopy; (b) Device diagram of mid-infrared single-photon time-stretching spectroscopy system; (c) Wide-band mid-infrared upconversion spectroscopy detection with spectral resolution of 0.5 cm-1; (d) Detection sensitivity up to 0.14 photons/nm/pulse.
Source：Research News丨Mid-infrared Single-Photon Time-Stretch Spectroscopy
Link to the paper: Single-Photon Time-Stretch Infrared Spectroscopy
