Broadband Mid-infrared Single-photon Upconversion Spectroscopy Based On Quantum Correlation

Driven by the demand of multiple fields,mid-infrared spectroscopy has become a major international research hotspot in recent years.As the core technology in biological application,biochemical reaction,environmental monitoring,material analysis,clinical medicine and other aspects,the importance of mid-infrared spectroscopy has been gradually highlighted.It is one of the frontiers of infrared technology to realize the highly sensitive and ultra-broadband spectrum measurement at the single-photon level in mid-infrared band.In this thesis,the measurement sensitivity of broadband mid-infrared spectroscopy is advanced to the single-photon level and a new technique of mid-infrared quantum correlation spectroscopy based on broadband frequency conversion is developed.The primary findings of the experiment are as follows:（1）Practice the efficient broadband mid-infrared frequency upconversion technology.Based on chirp-poling lithium niobate（CPLN）crystals,the configuration of linearly increased poling periods was designed.The quasi-phase-matching process at different frequencies was simulated,and the time walk-off caused by group velocity dispersion was calculated theoretically.With synchronous pulsed pumping of 30 MHz stable repetition rate,nonlinear frequency upconversion process converts mid-infrared photons carrying spectral information from 2.45μm to 3.85μm into near-infrared band.The mid-infrared spectrum is of 1.4μm bandwidth.The spectral resolution after deconvolution is 4 cm^-1.The scheme expands the application advantage of maturely developed single-photon detectors in near-infrared band,and successfully avoids the shortcomings of existing mid-infrared detectors.The noise of the spectral detection system is less than 10 counts per pixel element per second.（2）Build the experimental platform of mid-infrared quantum correlation spectroscopy.To further improve the sensitivity of mid-infrared upconversion spectroscopy,a broadband correlated photon pair source based on the CPLN crystal is constructed via spontaneous parametric down conversion.The correlated photon pair source with a high pair generation rate of 6.76×10⁶counts?s^-1?m W^-1 not only covers a wide band in mid-infrared,but also provides non-classical correlation that improves the robustness of the spectral detection.The mid-infrared signal photons cover the mid-infrared band of 3.14～3.80μm,providing a spectral detection window greater than660 nm.The ultra-sensitive mid-infrared frequency upconversion based on quantum correlation guarantees the optical components such as the spectrometer working in the band far away from the mid-infrared,and enhances the sensitivity of the mid-infrared spectroscopy to 0.09 photons per pulse.（3）Develop the scheme of nonlocal time-stretch mid-infrared upconversion spectroscopy.In this scheme,single-pixel detectors are used to obtain the mid-infrared single-photon spectrum,eliminating the mechanical tuning of the grating.Based on the group velocity dispersion in single-mode fiber of 10 km and the time-frequency quantum correlation of photon pairs,the mid-infrared upconversion spectrum is non-locally mapped to the time domain,which results in ultra-sensitive,broadband,and high spectral resolution mid-infrared spectroscopy in the wavelength range of2.76～3.94μm.Under the condition of 0.21 mid-infrared photon flux per pulse,the entire exposure time of coincidence measurement is reduced to 30 minutes with an average signal-to-noise ratio of 28.3 d B.The transmission spectrum of samples accords well with the measurement results of Fourier transform infrared spectroscopy with spectral resolution of 0.5 cm^-1.To sum up,the thesis demonstrates ultra-sensitive mid-infrared single-photon spectroscopy with broad bandwidth,low power consumption and strong robustness.The experiment proves that the mid-infrared frequency upconversion spectroscopy is an effective alternative to the direct mid-infrared spectral detection.The detection system of the sensitive mid-infrared spectroscopy is expected to be used in leading fields such as materials and medical treatment.