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Physics of Wave Phenomena

Manifestations of the Dicke Narrowing at Fast Recording of High-Resolution Absorption Spectra Using Frequency-Tunable Lasers

В. В. Лагунов, Vladimir N Ochkin, A. I. Volkova

Published Oct 1, 2023• 1 citation
Publisher

Abstract

Abstract The possibility of performing quantitative absorption measurements of particle concentrations using frequency-tunable lasers is investigated. At fast frequency scanning, when the recording time of spectrum is shorter or comparable with its formation time, well-known time-dependent interference interactions between the radiation incident on an absorbing medium and the radiation induced in it are observed. Under these conditions steady-state absorption spectra are distorted, and the classical relations lying in the basis of absorption measurements are violated. The character of the distortions depends on the type and density of particles, their absorption state, the mechanisms of spectra formation, and the laser beam power and geometry. In this paper, we report the results of studying the manifestations of Doppler profile narrowing caused by the Dicke effect in time-dependent spectra and their influence on the results of measuring the concentrations of absorbing particles. It is shown that the static spectrum can be reconstructed and quantitative measurements by integrated absorption spectroscopy can be performed under these conditions.

DOI
10.3103/s1541308x23050060

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Abstract The possibility of performing quantitative absorption measurements of particle concentrations using frequency-tunable lasers is investigated. At fast frequency scanning, when the recording time of spectrum is shorter or comparable with its formation time, well-known time-dependent interference interactions between the radiation incident on an absorbing medium and the radiation induced in it are observed. Under these conditions steady-state absorption spectra are distorted, and the classical relations lying in the basis of absorption measurements are violated. The character of the distortions depends on the type and density of particles, their absorption state, the mechanisms of spectra formation, and the laser beam power and geometry. In this paper, we report the results of studying the manifestations of Doppler profile narrowing caused by the Dicke effect in time-dependent spectra and their influence on the results of measuring the concentrations of absorbing particles. It is shown that the static spectrum can be reconstructed and quantitative measurements by integrated absorption spectroscopy can be performed under these conditions.
Abstract— The applicability of the light absorption method for measuring particle concentrations during fast recording of absorption spectra in different ranges of frequency tuning rates and in different measurement schemes has been considered. If the molecular resonance time is shorter than the decay times of induced particle polarization in the open space of the medium, scanning of the transmission radiation frequency is accompanied by well-known transient coherent effects, the stationary spectrum is distorted, and oscillations arise. In this case the conventional total absorption methods can hardly be applied directly. A method based on reconstructing the stationary spectrum from the wing of the experimentally observed absorption line is proposed. When carrying out highly sensitive measurements of absorption in the medium inside of a high-Q external optical cavity, absorption line distortions arise as well, but at much lower frequency tuning rates. The observed spectra exhibit asymmetry, oscillations are not observed, and the violation mechanism is related in this case to the limitation of the probe-light coherence length (in comparison with the effective optical path length) and finite photon lifetime in the cavity. It is shown that direct application of the classical relations linking the total absorption with the particle concentration yields adequate results in this situation. The role of instrumental distortions in integral measurements is discussed.
A single photon, emitted in a transition between two states, has a frequency distribution of intensity that is given by a Lorentzian if the transition is only naturally broadened and the period of observation TTT is long compared to the lifetime T1{T}_{1}T1​ of the excited state. However, when the observation time TTT is short or comparable to T1{T}_{1}T1​, the frequency spectrum is appreciably broadened. If only the delayed part of the emitted radiation field is detected, then the radiation spectrum does not change. However, if the radiation field is transmitted through a resonant absorber and then detected, the transmission spectrum of the delayed radiation field is narrowed. We show that this narrowing is due to the interference of the spectral components of the incident and coherently scattered fields. Experimental spectra of absorption of M\"ossbauer radiation, obtained by the coincidence technique, confirm this conclusion.
The absorption (amplification) spectrum of a weak probe field by two-level atoms located in a strong resonant laser field and colliding with buffer-gas atoms is analyzed theoretically. The analysis is performed for low collision frequencies compared to the Doppler absorption linewidth (low gas pressure) and with allowance made for an arbitrary change in the phase of the radiation-induced dipole moment at elastic collisions between gas particles. The phase memory effects have been found to lead to a strong qualitative and quantitative transformation of the probe-field spectrum even at rare collisions, when the well-known Dicke manifestation mechanism of the phase memory effects (the removal of Doppler broadening due to the restriction of the spatial particle motion by collisions) is inoperative. The strong influence of the phase memory effects on the spectral resonances at low gas pressures stems from the fact that the phase-conserving collisions change the velocity dependence of the partial refractive index n ( v ) (the refractive index for particles moving with velocity v ).
We analytically obtained the frequency dependences of the absorption coefficient profile, taking into account the combined influence of the macroscopic motion of matter, Doppler and Voigt mechanisms of spectral line broadening. The effect of rotation of a spherical plasma on the formation of resonant fluorescence by barium ions upon absorption of laser radiation at a wavelength of 455.4 nm was studied. The deformation of the frequency shape of the laser radiation passing through the plasma and the emission spectrum is due to a shift in the absorption coefficient profile and its broadening caused by the rotational motion of the plasma. An increase in the resonant photons escape from the medium stipulated by the broadening of the emission spectrum of the resonance line is predicted.
Effects caused by fast laser frequency sweeping across absorption lines are investigated. Oscillations in the time dependence of intensity of radiation generated by the medium, which are caused by beats between oscillations at variable excitation frequency and constant eigenfrequencies, are discovered. The time intervals between local maxima of the oscillations are inversely proportional to the difference between the eigenfrequency of the atomic oscillator and the instant frequency of the external radiation. The method of using fast frequency sweeping for determining indices of absorption at high optical densities (k 0 z ~ 100) is proposed.
The spectrum of weak probe field absorption (amplification) by two-level atoms experiencing collisions with buffer gas atoms in a strong resonance laser field is studied theoretically. Analysis is carried out for systems with a weak Doppler broadening under relatively mild constraints on the strong field intensity for the general case of an arbitrary change in the phase of the radiation-induced dipole moment in elastic collisions of gas particles. It is shown that, in spite of uniform broadening of the absorption line, the probe field spectrum exhibits a clearly manifested anisotropy to mutual orientation of the wavevectors of strong and probe radiation. It is found that the width of resonances in the probe field spectrum under definite conditions (that can easily be created in experiments) is proportional to the diffusion coefficient for atoms interacting with radiation. This fact can form the basis of the spectroscopic method for measuring the transport frequencies of collisions between particles absorbing radiation and buffer particles. It is shown that phase memory effects in collisions strongly modify the probe field spectrum both qualitatively and quantitatively. Simple operative formulas proposed for the probe field spectrum are convenient for experimental data processing.