Two-level Gas Research Articles

Increasing the Generation Efficiency of a Two-Level Gas Laser by Switching from Single-Sided to Double-Sided Transversal Diode Pumping

Increasing the Generation Efficiency of a Two-Level Gas Laser by Switching from Single-Sided to Double-Sided Transversal Diode Pumping

Two-level gas laser with transverse diode pumping

We theoretically study a new method for generating laser radiation by a two-level system without population inversion in the ‘red’ wing of its spectral line under resonant absorption of broadband radiation from pump laser diodes. A two-level system simulates the atoms of an active gas in an atmosphere of a high-pressure buffer gas. The effect results from the fact that in the ‘red’ wing of the spectral line, the probability of stimulated emission exceeds the probability of absorption if the homogeneous broadening due to the interaction of particles with the buffer gas significantly exceeds the natural one (at high pressures of the buffer gas). Analytical formulae are obtained that describe the operation of a two-level gas laser with transverse diode pumping. It is found that the longer the active medium, the higher the buffer gas pressure and pump radiation intensity, and the smaller the width of the pump radiation spectrum, the greater the efficiency of conversion of pump radiation into laser radiation. In a sufficiently long active medium (the length of the medium is 50 times its width), the conversion efficiency can reach 44 % at a buffer gas pressure of 5 atm, a pump diode radiation intensity of 3 kW cm−2, and a half-width of its spectrum of 1 cm−1. A two-level gas laser with transverse diode pumping is capable of generating continuous optical radiation with a very high (up to 100 kW) power. The frequency of laser radiation can be tuned by several tens of cm−1.

Refractive index of two-level gas in quantized field of counterpropagating waves

An expression for the refractive index of a gas of two-level atoms in the quantized single-mode field of counterpropagating waves is calculated. It is shown that in the limit of large numbers of photons the result reproduces the quasiclassical result in only the case of standing wave where intensities of both waves are equal. Explanation of this “strange” situation where the abilities of quantum field theory turn to be more restricted than those of classical theory is given.

The Hanle effect in forward scattering in excited media

A new method for calculating the density matrix of a quantized electromagnetic field which interacts with the environment in the presence of kinetic processes in the medium is suggested. This method accurately takes into account photon-photon quantum correlators and possesses certain symmetry properties, which substantially facilitate the summation of the Feynman diagrams that arise in the calculations. Forward scattering of resonance radiation by two-level gas atoms in a magnetic field is considered as an application. Insufficiency of a semiclassical description of this coherent process in excited media with the use of the unique tensor index of refraction is shown to follow from quantum electrodynamics. The introduction of one more function depending on the frequency of irradiation and the concentration of excited atoms is necessary. Experiments that simultaneously determine both this additional function and the standard refractive index of the medium are specified.

Effect of phase memory under collisions on field work and light-induced drift of gas particles

The influence of phase memory under collisions on the field work and the light induced drift (LID) effect of two-level gas particles is studied. It is shown that the phase memory effects may lead to the case where about half of the particles interacting with radiation amplify the incident radiation. The integral absorption coefficient for this remains positive due to growing radiation absorption by another half of the particles. It has been found that the phase memory strongly affects the LID under v ≫ kv̄ (v is the collision frequency, kv̄ is the Doppler line width): calculation of LID without regard for the phase memory can lead to underestimation of the LID effect value by a factor of (v/kv̄)2 ≫ 1. Interaction of the particles in the hard sphere model is used to illustrate that the phase memory begins to affect the LID when the relative difference of the transport cross-sections of the particles is ground and excited states is small: Δσ/σ ≲ λ̄/R, where λ̄ is the de Broglie wavelength at thermal velocities, R is the sphere radius.

Spectral and kinetic manifestations of a phase memory in collisions during interaction of radiation with a Lorentz gas

A study is reported of the influence of a phase memory in the presence of collisions on the work done by a field and on a light-induced drift of two-level gas particles in the Lorentz model of a gas (heavy buffer particles). It is shown that a phase memory may result in amplification of the incident radiation by particles at certain velocities in the absence of a population inversion (when practically all the particles are in the ground state). A phase memory is shown to have a strong influence on the light-induced drift if the collision frequency is much higher than the Doppler width of an absorption line: the drift calculated ignoring the influence of a phase memory may be strongly underestimated.

Approach of gas and radiation to equilibrium

Einstein described equilibrium between two-level gas molecules and radiation in terms of a Brownian motion model involving frictional and agitational effects arising from photon absorptions and emissions. This paper extends his treatment to the case of disequilibrium when molecules and radiation are in cavities with either black or perfectly reflecting walls. The precise role of the black walls in promoting equilibrium is explicated in terms of Einstein’s formula for the frictional effect and the nature of changes in radiation patterns caused by Doppler shifts of photon frequencies. It is then shown that equilibrium between the two-level gas molecules and radiation cannot be achieved for a container with perfectly reflecting walls. The blockage of equilibrium occurs because a change in the slope of the Planck spectrum at the resonant frequency causes a change in the frictional force.

New nonlinear resonances in a gas in the presence of a strong optical field

The present paper offers the solution to the probe of the nonlinear interaction of a weak standing wave with a two-level gas in the field of a strong travelling wave. It turns out that a number of new nonlinear resonances with a homogeneous width arise on the background of linear absorption.

Angular dependence of phase conjugation in SF_6

Optical phase conjugation by degenerate four-wave mixing in SF(6) was studied with a cw CO(2) laser. The intensity and angular dependence of reflectivity were measured for the 10.6-microm P(18) line. The results verify the predictions of the inhomogeneously broadened resonant two-level gas model.

Manifestation of the resonance radiation pressure in a gas

The influence of the radiation pressure effect on the behavior of two-level gas in a field of a traveling quasi-resonance electromagnetic wave has been studied. The peak formation in the distribution function in a collisionless gas and the gas cooling in a collisional gas are shown.