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Quantum Electronics

Collision-induced amplification of radiation in inversionless two-level systems

А. И. Пархоменко, Anatolii M. Shalagin

Published Dec 31, 2009• 5 citations
Publisher

Abstract

Amplification of probe radiation by an inversionless two-level system in the 'red' wing of its spectral line is studied theoretically during the resonance absorption of intense cw laser radiation. The effect is caused by the inequality of the spectral densities of the Einstein coefficients for absorption and stimulated emission outside the absorption line under conditions when homogeneous broadening, due to the interaction of particles with the buffer gas, significantly exceeds the natural broadening (at high pressures of the buffer gas). It is found that the larger the inversionless gain, the higher the buffer gas pressure and the pump radiation intensity. It is established that at high enough pump intensity, the probe field is amplified along the entire region of the 'red' wing of the line (at any negative detunings of the probe field frequency).

DOI
10.1070/qe2009v039n12abeh014134

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Amplification of probe radiation by an inversionless two-level system in the 'red' wing of its spectral line is studied theoretically during the resonance absorption of intense cw laser radiation. The effect is caused by the inequality of the spectral densities of the Einstein coefficients for absorption and stimulated emission outside the absorption line under conditions when homogeneous broadening, due to the interaction of particles with the buffer gas, significantly exceeds the natural broadening (at high pressures of the buffer gas). It is found that the larger the inversionless gain, the higher the buffer gas pressure and the pump radiation intensity. It is established that at high enough pump intensity, the probe field is amplified along the entire region of the 'red' wing of the line (at any negative detunings of the probe field frequency).
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.
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.
The spectral densities of Einstein coefficients for absorption and stimulated emission in a two-level quantum system are not equal to each other beyond the absorption (emission) line if the homogeneous broadening caused by interaction with a thermostat is much larger than the natural width. In this study, a relationship between these coefficients is derived on the basis of general thermodynamic requirements. This relationship is associated with new effects such as population inversion upon the nonresonant absorption of continuous radiation and radiation amplification without population inversion.
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 ).
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 .
When the gas particles, which can be considered as a two level quantum system, are affected by frequent collisions with buffer gas particles (thermal reservoir), the spectral densities of the Einstein coefficients for absorption and induced emission are unequal to each other far from the absorption (emission) line. The relationship between the spectral densities at the specified condition reveals new physical phenomena. The mostly convincing example is the fact that population inversion is established in a two level system under nonresonance optical excitation. This effect is experimentally observed as a lasing on sodium resonance transitions.