Promoting the sensitivity of mid-infrared (MIR) spectroscopy to the single-photon level is a critical need for investigating photosensitive biological samples and chemical reactions. MIR spectroscopy based on frequency upconversion is a compelling pioneer allowing high-efficiency MIR spectral measurement with well-developed single-photon detectors, which overcomes the main limitations of high thermal noise of current MIR detectors. However, noise from other nonlinear processes caused by strong pump fields hinders the development of the upconversion-based MIR spectroscopy to reach the single-photon level. Here, a broadband MIR single-photon frequency upconversion spectroscopy is demonstrated based on the temporal-spectral quantum correlation of non-degenerate photon pairs, which is well preserved in the frequency upconversion process and is fully used in extracting the signals from tremendous noise caused by the strong pump. A correlation spectrum broader than 660 nm is achieved and applied for the demonstration of sample identification under a low incident photon flux of 0.09 average photons per pulse. The system is featured with non-destructive and robust operation, which makes single-photon-level MIR spectroscopy an appealing option in biochemical applications.
Mid-infrared (MIR) frequency upconversion presents a powerful tool for MIR photon detection at room temperature. However, the introduction of strong pump light will exacerbate the limitations of upconverted thermal radiation and upconverted parametric fluorescence on detectability in broadband MIR frequency upconversion. Here, a broadband synchronized MIR upconversion spectrometer system based on a step-chirped poled LiNbO3 crystal is demonstrated. This system is integrated with spontaneous parametric downconversion (SPDC) and sum-frequency generation (SFG) to form a broadband MIR light generation and synchronous pulsed upconversion, which are simultaneously driven by the same pulsed pump laser. The spectral coverage is close to 1.4 μm from 2450 to 3850 nm and the resolution of the system is about 4 cm−1 after deconvolution, as confirmed by a Fourier transform infrared spectrometer. In conjunction with synchronous pumping-based temporal gating, the system noise is suppressed to less than 10 counts per second per nm. This broadband synchronized SPDC–SFG configuration can be extended to different infrared wavelengths for low-noise broadband spectroscopic measurement.
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