Ultrasensitive infrared detection plays an important role in both basic scientific research and cutting-edge innovative applications, such as quantum optics, laser ranging, optical communications, environmental monitoring and biomedicine. In recent years, nonlinear frequency upconversion technology has been rapidly developed and has become one of the effective means of infrared detection, which is able to realise the detection of infrared band signals by using high-efficiency, high-sensitivity, and high-speed silicon-based detectors to significantly enhance the performance of infrared detection and imaging.
Usually, frequency upconversion detection technology can be roughly divided into two categories. One class is based on the nonlinear frequency transformation of the infrared light field, such as through the nonlinear sum-frequency or four-wave mixing effect, the infrared photons will be converted to the visible light band. The other category is based on the nonlinear effect of the detector's own material, through the two-photon or multi-photon absorption process, to achieve the effective excitation of carriers, thus generating a detectable photocurrent. Comparatively speaking, the latter does not require phase matching, which makes the optical path easy to adjust and enables broadband spectral response. In particular, the non-simplified two-photon absorption infrared detection technology adopts a strong pumping light field as an aid, so that the infrared signal can still obtain significant two-photon absorption under extremely weak conditions, thus significantly improving the detection efficiency. In addition, studies have shown that the interaction between the pumping and signalling optical fields with non-simple wavelengths can enhance the two-photon absorption coefficient by orders of magnitude.
Professor Zeng Heping's group has been engaged in infrared photon measurement and control for a long time. Recently, the group has deeply developed the two-photon absorption infrared detection technology with long-wavelength pumping, which has improved the detection sensitivity by 1-2 orders of magnitude compared with the previous reports. Specifically, the team's innovative use of a 3 μm mid-wave infrared laser as the pumping light field can overcome the severe two-photon background noise of the pumped light in a silicon detector under the conventional scheme, thus dramatically improving the detection signal-to-noise ratio and dynamic range. It supports important applications such as remote ranging, bio-imaging and sensitive spectroscopy.
Fig. (a) Energy level diagram of an indirect bandgap silicon detector, which gives direct absorption (LA) at 1030 nm, two-photon absorption (D2PA) at 1550 nm, simple merged three-photon absorption (D3PA) at 3070 nm, and non-simple merged two-photon absorption (ND2PA) at 1550 and 3070 nm. (b) Variation of photon count rate with incident pulse energy. It can be seen that the background noise induced by the three-photon process is roughly 5-6 orders of magnitude smaller than that of the two-photon process at the same pump power.
Currently, in order to further suppress the background noise caused by three-photon absorption, the team proposes to use a longer wavelength pump field, aiming to advance the detection sensitivity to the single-photon level. In addition, the technology will also be directly applied to the subsequent ultra-sensitive infrared imaging research, through the use of silicon electron-multiplying CCD camera, is expected to be both room temperature operation, broad spectral response, high speed frame rate, high spatial and temporal resolution and other superior performance. This work was published in Physical Review Applied 14, 064035 (2020), with the full collaboration of Wu Jing.
