Phase memory effects in probe-field spectroscopy of two-level systems at low collision frequencies

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).     

Light-induced drift of aerosol particles in gas mixtures

We analyze the motion of an aerosol particle in a gas mixture in which the molecules of one of the components have been selectively excited as to velocity by resonant optical radiation. We derive expressions for the force with which the gas acts on the particle and for the velocity of particle motion in the Knudsen regime. We also examine the dependence of the force and velocity of photophoresis on the offset of the radiation frequency from the center of the absorption line and on the concentration and mass ratios of the molecules of the absorbing and buffer gases. Zh. éksp. Teor. Fiz. 116, 1313–1328 (October 1999)     

激发光线宽对原子光致漂移速率的影响

在光致漂移效应的研究中,激发光线宽会改变原子激发的速度选择性,进而影响漂移速率的大小.本文以原子光致漂移速率方程理论为基础,利用强碰撞模型描述原子与缓冲气体的碰撞作用,运用数值方法对速率方程进行求解计算,研究了激发光线宽对原子漂移速率的影响.研究结果表明,其他条件相同时,随着线宽的增大,漂移速率的值呈现先增大后减小的趋势.存在一个最佳的激发光线宽,使得原子的漂移速率达到最大值.最佳线宽与激发光功率密度、温度和缓冲气体压强有关.为了获得最佳的光致漂移效果,激发光应工作在最佳线宽条件下.当激发光线宽在最佳线宽附近波动时,设置激发光线宽略大于最佳线宽可减少线宽波动对漂移速率的影响,对获得较大漂移速率更为有利.     

Study of the rates of collisional decay of population, orientation, and alignment by stimulated photon echo in a molecular gas

A stimulated photon echo technique with specially selected linear polarizations of the coherent resonant driver pulses is used to study depolarizing collisions in the molecular gas SF₆ and in mixtures of it with buffer He and Xe. The collisional decay rates of the population, orientation, and alignment in an ensemble of gas particles are determined for the first time in a single experiment. These relaxation rates are measured as a function of the longitudinal translational velocities of the resonant particles. To within the experimental accuracy, no significant dependence of the collisional decay rates on the translational velocities of the particles was observed. This result confirms the conventional theoretical approach to depolarizing collisions. In pure SF₆ the decay rates for the orientation and alignment were lower than the relaxation constant for collisions involving a change in the longitudinal velocity (elastic collisions) that is known from experimental observations of the ordinary photon echo. This means that only some of the elastic collisions participate in destroying the multipole moments of the levels. Evidence is found that the relaxation of the multipole moments created by polarized radiation in a resonant medium of molecular SF₆ gas depends on j, the total angular momentum of the level. Zh. éksp. Teor. Fiz. 113, 826–833 (March 1998)     

Light-induced drift of ions in a magnetic field

The effect of a magnetic field on the ion drift in a weakly ionized gas under the combined action of the light-induced drift and light pressure is studied theoretically. It is shown that, under the action of light, a component of ion drift velocity transverse to the direction of propagation of radiation may appear in a weakly ionized gas upon the application of an external magnetic field. It is shown that the Lorentz force acting on ions in the magnetic field radically changes the dependence of the ion drift velocity on the radiation frequency detuning. It is predicted that the ion drift velocity component along the direction of radiation must reverse its sign upon an increase in the magnetic field and an anomalous light-induced drift may be observed.     

Anomalous light-induced drift of lithium atoms in a mixture of noble gases

This paper is a theoretical study of the spectral features of the velocity of light-induced drift (LID) of lithium atoms (⁷Li and ⁶Li) in a binary mixture of noble gases: Ne + Ar, Ne + Kr, and Ne + Xe. The spectral shape of the LID signal is predicted to depend strongly on the fraction ξ of neon in the buffer mixture in the range ξ≈0.8–0.9 (ξ=N _Ne/N _b, where N _Ne is the neon concentration, and N _b is the total concentration of the buffer particles). When the velocity of anomalous LID is treated as a function of the radiation frequency, it is found to have one, three, five, or seven zeros and to differ substantially from the dispersion-curve-like behavior with one zero predicted by the standard LID theory with velocity-independent transport collision rates. The reason for these additional zeros of the drift velocity is the alternating-sign dependence on the lithium-atom velocity of the relative difference of transport rates of collisions between buffer particles and excited and unexcited atoms. What is also established is that the anomalous LID of lithium atoms can be observed at almost all temperatures, depending on the value of ξ. At a fixed temperature, anomalous LID can be observed only in a narrow range of values of the fraction of neon in the buffer mixture, Δξ≈0.02. The results make possible highly precise testing in the LID experiments of the interatomic potentials used in calculations of the velocity spectrum of anomalous LID. Zh. éksp. Teor. Fiz. 116, 1587–1600 (November 1999)     

On the collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field

The collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field is investigated theoretically. It is shown numerically that the transfer effect is weak. This makes it possible to use simpler approximate one-dimensional quantum kinetic equations instead of three-dimensional equations to solve spectroscopy and light-induced gas kinetics problems, where it is important to take account of the velocity dependence of the collision frequency. It is shown for anomalous light-induced drift, calculations of which are most sensitive to neglecting the transfer effect, that in a wide range of spectroscopy and light-induced gas kinetics problems the transfer of nonequilibrium can be neglected without risking the loss of important fine details of the phenomena being described.     

Modeling of flowing gas diode pumped alkali lasers: dependence of the operation on the gas velocity and on the nature of the buffer gas

A simple, semi-analytical model of flowing gas diode pumped alkali lasers (DPALs) is presented. The model takes into account the rise of temperature in the lasing medium with increasing pump power, resulting in decreasing pump absorption and slope efficiency. The model predicts the dependence of power on the flow velocity in flowing gas DPALs and checks the effect of using a buffer gas with high molar heat capacity and large relaxation rate constant between the P_3/22 and P_1/22 fine-structure levels of the alkali atom. It is found that the power strongly increases with flow velocity and that by replacing, e.g., ethane by propane as a buffer gas the power may be further increased by up to 30%. Eight kilowatt is achievable for 20?kW pump at flow velocity of 20 m/s.     

Diffusion-broadened line shape near a turning point

The absorption spectrum of a gas with a large Doppler width and soft collisions between particles is studied. The particles are assumed to have a nonlinear dependence of the resonance frequency on velocity. The shape of the narrow peak in the spectrum resulting from an extremum of this dependence is calculated analytically for the first time. In the absence of collisions it has a characteristic asymmetric shape. Collisions are shown to broaden the line and change its shape. The profile of the probe-field spectrum is obtained for a three-level system with the strong field at an adjacent transition. The components of the Autler-Townes doublet are found to spread and repel each other because of collisions. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 27–32 (10 July 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Drift of gases induced by nonmonochromatic light

The phenomenon of light-induced drift of gases influenced by a nonmonochromatic light with different spectral properties is under consideration. The dependence of the drift velocity on the spectrum shape, bandwidth, detunings and light intensity is studied. The largest drift velocity is found to be obtained at the bandwidth of the spectrum being of the order of the Doppler atomic transition width, and when the wings of the spectrum drop rapidly. It is shown that nonmonochromatic light of the same intensity as a monochromatic one allows to reach a much larger drift velocity. Principal possibilities to obtain supersonic light-induced drift velocities are shown.     

Superluminal light propagation assisted by Zeeman coherence

We have observed a dispersionlike absorption (or gain) spectrum at the D1 transition in a Rb vapor cell filled with a buffer gas, due to Zeeman coherence of the ground states in a double Lambda configuration. Meanwhile, we have also observed superluminal pulse propagation. It is experimentally demonstrated that the front speed of a light pulse still equals the light speed c in vacuum, although the group velocity of the light pulse is(-2.2+/-0.6) x 10(4) m/s.     

Ultrafine particle formation by resonant laser excitation of potassium molecules

In a heat pipe oven containing potassium and a noble gas we have observed a large production of metallic potassium particles upon resonant laser excitation of the K₂ molecules. This is a new method for particle production, based on the strong local heating of the vapor due to the high absorption rate of the laser light. As a consequence of the disequilibrium caused at the gas/vapour boundary by this heating, a supersaturated region is created where the vapor condenses and produces ultrafine particles.     

Light-induced drift of neon under optical excitation from a metastable state

A running light wave, resonant with the neon transition 1s₅-2p₂, has been found out to affect the spatial distribution of neon atoms in a positive discharge column. The effect has shown an antisymmetric dependence (typical of the light-induced drift) on the tuning out of the radiation frequency relatively the absorption line centre, as well as on the direction of the light wave vector.     

Origin of the resonant optical Kerr nonlinearity in Cd(S,Se)-doped glasses and related topics

After having reported preliminary results related to saturation, we first theoretically consider the various mechanisms contributing to the resonant optical Kerr effect in Cd(S, Se)-doped glasses. We obtain the expression for the expected effective susceptibility in different possible cases. This nonlinearity is studied experimentally using optical-phase conjugation in the low-intensity regime. We show that, by time resolving the nonlinear response of such glasses having experienced various degrees of photodarkening, we can clearly assess the origin of the resonant optical Kerr effect in these materials. Usually, a combination of a fast free-carrier contribution due to particles without traps and of a slow trapped-carrier one due to particles with traps is observed. For the free-carrier contribution, induced absorption is observed to be almost as important as absorption saturation. We also report frequency-dependent measurements and discuss the change in absorption spectrum and the increase of the nonradiative decay rate that accompany darkening.     

室温条件下掺铒光纤中光脉冲群速可控特性的研究

利用相干布居振荡技术在介质吸收光谱上产生烧孔，孔宽大约为基态粒子数恢复时间的倒数. 由增益理论分析得到不同抽运光功率对介质吸收状态的影响. 在介质的吸收区域，振荡导致光脉冲经历饱和吸收，脉冲传输延迟；在介质的增益区域，振荡又导致光脉冲经历增益饱和，脉冲传输超前. 应用此技术在掺铒光纤中实现了光速人为可控. 在掺铒光纤晶体中观测到了最慢为2.857×10³m/s的光速减慢传输，相应感生群折射率为10.5×10⁴. 根据布居振荡效应及增益理论，由速率方程出发，得到了 关键词： 光谱烧孔 相干布居振荡 饱和吸收 慢光     

Effect of argon buffer gas and the dimer component on the optical properties of sodium vapor

We have theoretically studied the nature of strong optical losses in the red region of the spectrum that occur in sodium vapor. We take into account the effect of the Ar buffer gas and Na₂ dimers on the resulting absorption of light by the system. Based on the proposed approach, we determine the interatomic distance in the dimers and achieve good agreement with quantum calculations and direct measurements. We show that intrinsic transitions in atomic argon may be the reason for the appearance of a broad absorption peak in the range 700–1000 nm.     

Stimulated processes in sodium vapour in the presence of molecular buffer gas systems

The influence of hydrocarbon compounds such as methane, ethane and ethylene on collision-induced amplified emission in sodium vapour has been investigated. The intensity dependence of the stimulated resonant emission and stimulated Raman effects on the buffer gas and sodium vapour pressures and the intensity of the excitation radiation has been obtained. The dependence of the stimulated resonant emission bandwidth on the buffer gas pressure has also been measured. It is shown that saturated hydrocarbon molecules can be used as an efficient buffer gas for the generation of stimulated emission in other alkali atoms such as rubidium and caesium.     

Theory of light-induced drift of a binary gas mixture in a capillary

The drift of a binary gas mixture in capillaries induced by resonant light is studied theoretically. The surface and bulk mechanisms of flow of the mixture components are analyzed for arbitrary values of the Knudsen number and the rate parameter (the ratio of the radiative decay rate of an excited molecular level to the intermolecular collision rate). Finally, the theoretical results are compared with the experimental data. Zh. éksp. Teor. Fiz. 114, 1709–1722 (November 1998)     

Kinetic theory of inhomogeneous diffusion

This paper theoretically investigates particle diffusion in a medium where the diffusivity depends on position. We exclusively consider continuous-time, continuous-space transport and our working tool is the linear kinetic theory pertinent to guest particles in a passive host medium (Lorentz’s picture of transport). The host medium may or may not thermalize the guest particles. It may be inhomogeneous in two ways: either particle scattering features depend on position in an explicit way (geometric inhomogeneity), or they depend on position through the medium’s local temperature (thermal inhomogeneity). When the inhomogeneity is geometric, it is found that Fick’s law is valid and the particle-current equation exhibits drift without current. When the inhomogeneity is thermal, current without drift is possible, but there is no generally valid pattern for the current equation. The consistency of our results with non-equilibrium thermodynamics is brought out. The results shed light on thermodiffusion (the Ludwig-Soret effect), which often combines inhomogeneities of both kinds. Finally, a limitation of the Lorentz picture of transport in accounting for thermodiffusion is outlined.     

Design, simulation and structural optimization of a longitudinal acoustic resonator for trace gas detection using laser photoacoustic spectroscopy (LPAS)

The design of the acoustic resonator is critical for the optimization of the sensitivity of laser photoacoustic spectroscopy (LPAS) in trace gas detection applications. In this paper, an LC circuit model is used for the simulation of a 1D acoustic resonator. This acoustic resonator is designed for CO photoacoustic spectroscopy. The effects of the structural parameters, quality factor and resonant frequency on the performance of the device are theoretically analyzed. The role of the buffer volume as an acoustic filter is investigated and optimized dimensions of the buffer volume, to achieve minimum noise transmission coefficient, are calculated. The effects of the ambient temperature, variety of pressure and gas flow velocity on the resonant frequency of photoacoustic resonator and PA signal are simulated. The temperature dependence of the microphone sensitivity is also introduced.