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Soviet Journal of Quantum Electronics

Establishment of population inversion by photoionization of inner electrons in atoms

Vladislav B Rozanov

Published Mar 31, 1971• 0 citations
Publisher

Abstract

It is shown that the photoionization of the inner-shell electrons can yield metastable ions. Therefore, the accumulation of excited particles becomes possible at practically realizable optical pumping powers. This method of population inversion may be effective because a large part of the emission spectrum of an optically transparent plasma in a radiation source is in the form of "hard" photons, which are emitted when electrons are captured and atoms or ions go over to their ground state.

DOI
10.1070/qe1971v001n03abeh003071

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Journal of Experimental and Theoretical Physics•Aug 1, 2008•0 citations
It is shown that the photoionization of the inner-shell electrons can yield metastable ions. Therefore, the accumulation of excited particles becomes possible at practically realizable optical pumping powers. This method of population inversion may be effective because a large part of the emission spectrum of an optically transparent plasma in a radiation source is in the form of "hard" photons, which are emitted when electrons are captured and atoms or ions go over to their ground state.
We present a detailed analysis of the creation of population inversion and coherent UV radiation with a new scheme, i.e., nonlinear excitation of an inner atomic electron in a strong laser field, accompanied by filling the resulting inner-shell vacancy with an external-shell electron. Our analysis is given for the experimental results obtained by Boyer et al. in 1984 [ J. Opt. Soc. Am. B1, 3 ( 1984)]. The principal new features of the scheme under consideration are the following: (1) the two-electron character of atomic excitation with subsequent emission of a high-energy photon, (2) the initial absence of multiphoton resonance between the ground state and the level to be excited, and (3) strong-field-induced level crossing and excitation of the upper working level by means of Landau–Zener-type transitions.
POPULATION RATE EQUATIONS From the discussion in the preceding chapter, it is clear that population inversion is the basic condition for an optical gain. For any system in its normal state in thermal equilibrium, a low-energy level is always more populated than a high-energy level, hence there is no population inversion. Population inversion in a system can only be accomplished through a process called pumping by actively exciting the atoms in a low-energy level to a high-energy level. If left alone, the atoms in a system relax to thermal equilibrium. Therefore, population inversion is a nonequilibrium state that cannot be sustained without active pumping. To keep a constant optical gain, continuous pumping is required to maintain population inversion. This condition is clearly consistent with the law of conservation of energy: amplification of an optical wave leads to an increase in optical energy, which is possible only if the required energy is supplied by a source. Pumping is the process that supplies energy to the gain medium for the amplification of an optical wave. There are many different pumping techniques, including optical excitation, electric current injection, electric discharge, chemical reaction, and excitation with particle beams. The use of a specific pumping technique depends on the properties of the gain medium being pumped. The lasers and optical amplifiers of particular interest in photonic systems are made of either dielectric solid-state media doped with active ions, such as Nd:YAG and Er:glass fiber, or direct-gap semiconductors, such as GaAs and InP. For a dielectric gain medium, the most commonly used pumping technique is optical pumping using either an incoherent light source, such as a flashlamp or a light-emitting diode, or a coherent light source from another laser. A semiconductor gain medium can also be optically pumped, but it is usually pumped by electric current injection. In this section, we consider the general conditions for pumping to achieve population inversion. Detailed pumping mechanisms and physical setups are not addressed here because they depend on the specific gain medium used in a particular application. The net rate of increase of the population density in a given energy level is described by a rate equation. As we shall see below, pumping for population inversion in any practical gain medium always requires the participation of more than two energy levels.
It was found that by observing the fluorescent intensity of ruby along its longitudinal axis during pumping, an appreciable amount of information may be obtained. It was observed that the time at which population inversion occurs during a pump cycle is directly observable from the time-resolved fluorescent intensity. A method for establishing the actual population in the metastable state from the fluorescent intensity is demonstrated and cross correlated with two other methods for measuring population inversion. The rate equation describing population inversion has been solved including the nonlinear stimulated emission loss term, and complete agreement with experiment is demonstrated. For the particular pumping geometry used, maximum energy available, and ruby quality and size, maximum metastable population of 1.21×1019 ions/cm3 was achieved, representing 75% inversion.
Abstract We extend the 4-step analytical model of resonant enhancement of high harmonic generation to the systems possessing resonant transitions of inner-shell electrons. Resonant enhancement is explained by lasing without inversion in a three-level system of ground, excited and shifted resonant states, which are coupled to the fundamental field and its high harmonics. The role of inelastic scattering is studied by simulation of an excited state’s population dynamics. It is shown that maximal gain is achieved when the energy shift between the excited state and resonant state is close to the energy of the fundamental photon. To prove the concept we demonstrate the enhancement of harmonics in the In plasma using different pumps.
A method is proposed to produce population inversion on transitions involving the ground state of atoms. The method is realized experimentally with sodium atoms. Lasing at the frequency corresponding to the sodium D2{D}_{2}D2​ line is achieved in the presence of pump radiation resonant to the D1{D}_{1}D1​ line with helium as a buffer gas.
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.