Optical Frequency Comb-Based Cavity-Enhanced Fourier-Transform Spectroscopy:Application to Collisional Line-Shape Study

Direct-comb spectroscopy techniques uses optical frequency combs(OFCs)as spectroscopic light source.They deliver high sensitivity,high frequency resolution and precision in a broad spectral range.Due to these features,the field has burgeoned in recent years.In this work we constructed an OFC-based cavity-enhanced Fourier-transform spectrometer in the nearinfrared region and used it for a line-shape study of rovibrational transitions of CO perturbed by Ar.The highly sensitive measurements spanned the wavenumber range from 6270 cm^-1 to 6410 cm^-1,which covered both P and R branch of the second overtone band of CO.The spectrometer delivers high-resolution surpassing the Fourier-transform resolution limit determined by interferogram length,successfully removing ringing and broadening effects caused by instrumental line shape function.The instrumental-line-shape-free method and high signal-to-noise ratio in the measurement allowed us to observe collisional effects beyond those described by the Voigt profile.We retrieved collisional line-shape parameters by fitting the speed-dependent Voigt profile and found good agreement with the values given by precise cavity ring-down spectroscopy measurements that used a continuous-wave laser referenced to a stabilized OFC.The results demonstrate that OFC-based cavity-enhanced Fouriertransform spectroscopy is a strong tool for accurate line-shape studies that will be crucial for future spectral databases.

Spectrum evolutions of spontaneous and pump-depleted stimulated Brillouin scatterings in liquid media

A theoretical model for calculating spontaneous and stimulated Brillouin scattering（SBS） spectra is described. An empirical formula for the Stokes output spectral linewidth, a function of spontaneous Brillouin linewidth and the exponential gain coefficient, is obtained by the calculated data fitting. The formula holds true for two cases involving pump undepletion and depletion. The lineshape change from spontaneous to highly pump-depleted SBS spectra is also investigated. The result shows that for the pump power below the SBS threshold, the Stokes output spectral lincshape evolves from Lorentzian to approximately Gaussian as the pump power increases. For the pump power near or beyond the threshold, the SBS spectrum is in the form of a steady Gaussian profile, and the spectral linewidth comes to a certain value about 7 times narrower than the spontaneous one. The theoretical results are experimentally demonstrated by using several common liquid media.

Analysis of asymmetry of the D_(α)emission spectra under the Zeeman effect in boundary region for D-D experiment on EAST tokamak

In 2015 campaign,deuterium atomic emission spectra(D_(α))under the Zeeman effect in boundary region had been measured by a high resolution optical spectroscopic multichannel analysis(OSMA)system based on passive spectroscopy during the deuterium plasma discharge on EAST tokamak,and part of the works about the Zeeman effect on D_(α)spectra had already been done.However,the asymmetric phenomena of D_(α)emission spectra under the Zeeman effect were observed in process of analyzing the spectral data.To understand the asymmetric phenomena and acquire the useful local plasma information,an algorithm was proposed and used to analyze the asymmetry of the emission spectra under the Zeeman effect with all polarization components(πand±σ).In the algorithm,the neutral atoms were considered to follow the Maxwell distribution on EAST,and I+σ=I-σwas considered and set.Because of the line-averaged spectra along the viewing chord,the emission spectra were considered from two different regions:low-field side(LFS)and high-field side(HFS).Each spectral line was classified into three energy categories(the cold,warm,and hot)based on different atomic production processes in boundary recycling.The viewing angleθ(between the magnetic field B and the viewing chord),magnetic field B at two spectral emission positions(HFS and LFS)and the Doppler shift of all three energy categories of each spectral line were all considered in the algorithm.The effect of instrument function was also included here.The information of the boundary plasma were acquired,the reason for the asymmetric phenomena was discussed,and the boundary recycling during the discharge were studied in the paper.Based on fitting a statistical data of acquired fitting results,an important conclusion was acquired that the ratio of the spectral line intensity in HFS and LFS was proportional to the square of that of the corresponding magnetic field.

Quantum mechanical solution to spectral lineshape in strongly-coupled atom-nanocavity system

The strongly coupled system composed of atoms,molecules,molecule aggregates,and semiconductor quantum dots embedded within an optical microcavity/nanocavity with high quality factor and/or low modal volume has become an excellent platform to study cavity quantum electrodynamics(CQED),where a prominent quantum effect called Rabi splitting can occur due to strong interaction of cavity-mode single-photon with the two-level atomic states.In this paper,we build a new quantum model that can describe the optical response of the strongly-coupled system under the action of an external probing light and the spectral lineshape.We take the Hamiltonian for the strongly-coupled photon-atom system as the unperturbed Hamiltonian H_(0)and the interaction Hamiltonian of the probe light upon the coupled-system quantum states as the perturbed Hamiltonian V.The theory yields a double Lorentzian lineshape for the permittivity function,which agrees well with experimental observation of Rabi splitting in terms of spectral splitting.This quantum theory will pave the way to construct a complete understanding for the microscopic strongly-coupled system that will become an important element for quantum information processing,nano-optical integrated circuits,and polariton chemistry.