Construction And Improvement Of Femtosecond Time-resolved Transient Fluorescence Spectrometer Based On Optical Parametric Amplification Mechanism

Under the normal condition,from a microscopic perspective,the physical and chemical changes within the substances induced by the radiation field are usually manifested as the movement of atoms or molecules,the transfer of electrons,and the change of energy level.If the phenomenon of the interaction between radiation field and matter in a certain wave range was very significant,these processes can be reflected by absorption spectrum or emission spectrum.When studying the dynamical processes on the ultrafast time scale,the absorption spectra are easily measured by pump probe technique,but they are often accompanied by excited state absorption,stimulated emission,ground state bleaching,and many other processes which are difficult for the experimentalist to control.Spectral components are messy,which brings some trouble to the analysis of the real physical process;While the transient fluorescence emission spectrum reflects the dynamic process of the excited state relaxation,whose spectral composition is simple and direct,physical image is clear and intelligible.Studying the ultrafast change within the substances by transient fluorescence spectroscopy is an effective way and a common technique in ultrafast spectroscopy research.Combined with the main research direction of our research group,the original energy transfer process of photosynthesis,considering that the fluorescence of biological samples is particularly weak and the process is often quite short,we analyzed the advantages and disadvantages of several mainstream transient fluorescence measurement techniques and found that the femtosecond time-resolved transient fluorescence spectral measurement technique based on the mechanism of optical parametric amplification is the most promising to achieve the ultrafast time resolution and the extremely weak fluorescence detection.Our research group has been cultivating in this direction for many years,hoping to further develop this technique and uncover the mystery of many unresolved scientific issues on the ultrafast time scale.A set of transient fluorescence spectral measurement device with femtosecond time resolution based on the mechanism of optical parametric amplification and a noncollinear structure was built by adopting the lateral collection&dot-like amplification scheme.After the performance improvement,a set of optimal optical path configuration parameters was obtained,and its two technical indicators broke the records of the research group,including the gain exceeding 10⁷ and the limit concentration of the detectable DCM ethanol solution（standard sample）down to5×10^-6 mol/L;But the noise to signal ratio of this device is still in the range of 1/300～1/100,which indicates that it is still disturbed by quantum noise,which is the fluctuation of parametric superfluorescence.By studying the random characteristics of parametric superfluorescence,combined with the idea of statistical average,the cone-shaped collection&ring-shaped amplification scheme is proposed and its effectiveness is experimentally verified,which is a new scheme that uses spatial superposition to suppress the quantum noise.With this scheme,the three technical indicators all broke through the records of the research group.Among them,the gain of this device exceeds 10⁷;The limit concentration of the detectable DCM ethanol solution is reduced to the order of 10^-6 mol/L with the femtosecond time-resolved transient fluorescence spectroscopy and fluorescence decay dynamics successfully obtained;The noise to signal ratio is reduced to about 1/700.These technical indicators indicate that the extremely weak fluorescence detection has been initially realized,which is well prepared for the further technical upgrade and the scientific research in the future.