Population Inversion Induced by Collisions in a Two Level System under Nonresonance Optical Excitation

Abstract

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.

Thermalization of coupled atom-light states in the presence of optical collisions

“…(16) is compatible with existing theories of spin-boson interaction with thermostat particles (due to collisions in our case) discussed by Leggett et al in [28]. In particular, according to our approach in the ideal case, neglecting spontaneous emission terms in (16), the dressedstate population imbalance S z should reach its thermodynamically equilibrium value S (eq) z with the rate 2w due to collisions with buffer gas atoms [see (21)], which is in agreement with experimentally tested approaches to OCs based on the solution of Boltzmann-like (rate) equations [11,26]. In addition, spontaneous emission described by the rates i→j (i,j = 1,2) causes a transfer between levels of different manifolds.…”

Spectroscopy of atomic rubidium at500−barbuffer gas pressure: Approaching the thermal equilibrium of dressed atom-light states

“…From Eqs. 1 we obtain f e = 1 − f g = 1/(1 + c e→g /c g→e ) so that the following relation of the rate constants must apply: c e→g /c g→e = e −δ/kB T , as also shown in [14]. Note that the sign of the laser detuning in this formula depends on the definition of δ. Physically, the increased rate towards a population of the energetically lower dressed state is due to the larger phase space that becomes available to the reservoir with the associated energy gain of |δ|.…”

“…The effect of collisional redistribution has shown the influence of state changing collisions [13]. In an interesting experiment, deviations from the Einstein coefficients for absorption and stimulated emission have been observed for far off-resonant excitation in a collisionally broadened system [14]. Dressed state approaches have proven to be an elegant way to treat collisional processes in the presence of laser radiation [15].…”

Two-level lasers operating on the transitions to the ground state

“…Both theoretical and experimental works on developing nonequilibrium two-level systems generating on the transitions to the ground state known today reduce mainly to optical pumping of atomic systems. For example, population inversion in such systems is formed via collisions of alkali-metal (K, Na, Rb) atoms with buffer gas atoms under non-resonant optical pumping [5][6][7]. In our previous publications [8,9] we showed that it is possible to develop two-level systems with thermal pumping due to the peculiarities of the thermal evaporation process of rare earth metals (REMs).…”