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Journal of Experimental and Theoretical Physics

Collision-induced amplification of short-wave radiation on transitions to the ground state of atoms

А. И. Пархоменко, Andrey N. Usoltsev, Anatolii M. Shalagin

Published Aug 1, 2008• 0 citations
Publisher

Abstract

A new version of obtaining short-wave lasing on transition between highly excited states and the ground state of active atoms in the buffer gas atmosphere is studied theoretically. The mechanism of obtaining population inversion on such a transition is associated with the establishment of the local Boltzmann distribution of populations in a group of highly excited levels due to frequent collisions. If the excitation of the upper-lying level is performed by two laser radiation sources with frequencies ω 1 and ω 2 , short-wave lasing can be obtained at a frequency close to the total frequency ω 1 + ω 2 . The conditions for the emergence of population inversion are analyzed and simple analytic formulas are derived. It is shown that collision-induced enhancement of short-wave radiation can occur for pumping intensities on the order of 100 W/cm 2 . For pumping intensities on the order of 1000 W/cm 2 , the amplification factor for short-wave radiation may attain values of 3 cm −1 (for an active atom concentration of N ∼ 10 15 cm −3 ), which is sufficient for the development of lasing per path through an active medium (superradiance condition) for a length of the active medium on the order of 10 cm.

DOI
10.1134/s1063776108080013

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Acta Physica Polonica A•Jan 1, 2002•6 citations
A new version of obtaining short-wave lasing on transition between highly excited states and the ground state of active atoms in the buffer gas atmosphere is studied theoretically. The mechanism of obtaining population inversion on such a transition is associated with the establishment of the local Boltzmann distribution of populations in a group of highly excited levels due to frequent collisions. If the excitation of the upper-lying level is performed by two laser radiation sources with frequencies ω 1 and ω 2 , short-wave lasing can be obtained at a frequency close to the total frequency ω 1 + ω 2 . The conditions for the emergence of population inversion are analyzed and simple analytic formulas are derived. It is shown that collision-induced enhancement of short-wave radiation can occur for pumping intensities on the order of 100 W/cm 2 . For pumping intensities on the order of 1000 W/cm 2 , the amplification factor for short-wave radiation may attain values of 3 cm −1 (for an active atom concentration of N ∼ 10 15 cm −3 ), which is sufficient for the development of lasing per path through an active medium (superradiance condition) for a length of the active medium on the order of 10 cm.
Lasing at the resonance transitions ( D 1− and D 2− lines) of sodium was observed in the superradiance mode upon nonresonance optical excitation in the presence of a buffer gas. The dependences of the lasing intensity on the exciting radiation intensity and on the detuning of its frequency from the frequencies of resonance transitions were studied. It is found that, under specific conditions of the experiment (high pressure of a buffer gas and a rather high radiation intensity), in the case of a large positive detuning of the exciting radiation frequency from the resonance (“working”) transition frequency, the population inversion is produced at the “ working” transition, which results in lasing.
We report a theoretical study on the possibility of amplification and generation of radiation by an inversionless two-level system in the ‘red’ wing of its spectral line under resonant diode pumping. The two-level system is used to simulate atoms of an active gas, while it is in the atmosphere of a high-pressure buffer gas. The effect results from the fact that the probability of stimulated emission in the ‘red’ wing of the spectral line 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). It is found that the higher the buffer gas pressure and the higher the pump radiation intensity, the greater the inversionless amplification. It is shown that the gain in a two-level system in the ‘red’ wing of its spectral line can reach 0.011 cm −1 when the radiation intensity and the width of the radiation spectrum of pump diodes are 5 kW cm −2 and 4 cm −1 , respectively. The use of transverse diode pumping of an active medium placed in an optical cavity will make it possible to obtain lasing with frequency tuning and thereby solve the problem of converting incoherent broadband radiation into a coherent laser light in a gas of two-level active particles.
Lasing on the sodium resonance transitions (D1 and D2 lines) at the superluminosity regime was observed upon the nonresonance excitation in the presence of a buffer gas. Dependences of the lasing intensity on the pump radiation intensity and its frequency detuning from the frequencies of resonance transitions were examined. It was found that under the specific experimental conditions (high buffer gas pressure, sufficiently high intensity of pump radiation) upon the large positive frequency detuning of pump radiation with respect to the frequency of resonance (working) transition, contrary to ingrained conceptions, population inversion for the working transition is raised. That results in observed phenomena.
Laser Generation without Population Inversion in the Red Wing of the D1{D}_{1}D1​-Line of Alkali Metal Atoms under Resonant Diode Pumping
A new method of laser radiation generation without population inversion in the red wing of the D1D_{1}D1​ -line of alkali metal atoms under resonant absorption of broadband radiation of pump laser diodes at the D1D_{1}D1​ transition is studied theoretically. The appearance of laser generation is caused by the fact that the probability of stimulated emission in the red wing of the spectral line exceeds the probability of absorption if homogeneous broadening due to interaction between active particles and the buffer gas significantly exceeds the natural broadening (under high pressures of the buffer gas). Analytical formulas representing the operation of a transversely diode-pumped alkali metal vapor laser are obtained. It is found that cesium atoms are the most promising object for observation of laser generation without population inversion in the red wing of the D1D_{1}D1​ -line of alkali metal atoms. In a sufficiently long active medium (the length of the medium is 50 times its width), the efficiency of pumping radiation conversion into laser radiation can reach 57% at a buffer gas pressure of 5 atm, a pump diode radiation intensity of 5 kW/cm 2{}^{2}2 , and a half-width of its spectrum of 1 cm −1{}^{-1}−1 . The laser radiation frequency can be tuned by several tens of cm −1{}^{-1}−1 .
The theory of amplification and lasing without population inversion in a three-level medium with inhomogeneous broadening via the formation of an open V configuration is elaborated. The conditions for energy transfer from the infrared into the visible spectral range, i.e., the conditions of up-conversion n b >n c >n a , and the external field required for saturation of the b⇄a transition are established. Two-photon resonant Raman transitions in ensemble of mobile atoms of a gas-discharge plasma are analyzed. The frequency shift of the probe field spectrum as a whole is shown to be governed by the frequency shift of the pump field multiplied by the ratio of the wave numbers of the probe amplification field and the pump field. The interaction of atoms through Ne transitions with the pump field (λ=1.15 εm, 2p 4-2s 2 transition) and the lasing field (λ=0.6328 εm, 3s 2-2p 2 transition) with an increase in the lasing frequency by a factor of 1.82 with respect to the absorbed radiation is calculated.
an ov and A .M. Sh ala g in In st i t ut e of Auto m ati on an d El ect ro metr y Si beri a n Bra nch of R ussian Aca dem y of Science Pr.A k. Ko ptyu ga 1, No vosibi rsk, 630090, R ussia L asing on t he sodi um resonan ce transition s ( D 1 and D 2 lines) at the superlumi nosit y regime was obser ved up on the non-resonance excitation in the presence of a bu ˜er gas.Dep endences of the lasing intensity on the pump radiation intensity and its fre quency detuning from the frequencies of resonance transition s were examined.It was found that under the speciÙc exp erimental condition s (high bu˜er gas pressure , su£ciently high intensity of pump radiatio n) up on the large positiv e frequency detuning of pump radiation with resp ect to the frequency of resonance ( \w or king ") transition, contrary to ingrained conceptions , populati on inversion for the \w orking" transition is raised.T hat results in the observed phenomena.