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Physical Review Letters

Laser Oscillation without Population Inversion in a Sodium Atomic Beam

Gunasiri G. Padmabandu, George R. Welch, Ivan N. Shubin, Edward S. Fry, Dmitri E. Nikonov, Mikhail D. Lukin, Marlan O. Scully

Published Mar 18, 1996• 269 citations
Publisher

Abstract

Continuous wave (cw) amplification and laser oscillation without population inversion have been observed for the first time in a $\ensuremath{\Lambda}$ scheme within the sodium ${D}_{1}$ line. This is also the first demonstration in which the lasing medium was an atomic beam; this is an approach which, in addition to elucidating the physics, lays a foundation for extensions into the ultraviolet. Calculations using realistic atomic structure were critical to the choice of experimental approach. Observations agree with full density-matrix calculations and clearly show there was no population inversion.

DOI
10.1103/physrevlett.76.2053

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Continuous wave (cw) amplification and laser oscillation without population inversion have been observed for the first time in a \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ scheme within the sodium D1{D}_{1}D1​ line. This is also the first demonstration in which the lasing medium was an atomic beam; this is an approach which, in addition to elucidating the physics, lays a foundation for extensions into the ultraviolet. Calculations using realistic atomic structure were critical to the choice of experimental approach. Observations agree with full density-matrix calculations and clearly show there was no population inversion.
The importance of lasing without inversion (LWI) in comparison with the use of nonlinear optical techniques and techniques based on high-intensity femtosecond light pulses to generate high-frequency coherent radiation is outlined. The original development and utilization of an experimental facility based on semiconductor diode lasers to study the fundamental aspects of LWI on the D1 line of the Cs atom, which is rarely attempted, is described. This experimental facility is used to observe inversionless amplification on the D1 line of the Cs atom by use of a Λ atomic scheme. A combination of EIT and broadband pumping of atoms from the lower to the upper level of the probe transition allowed the observation of the inversionless gain. Justification for the absence of inversion in the probe transition is provided. We report a single-pass inversionless gain of ≈11% with relatively weak incoherent pumping. The results show improvement over the previously reported inversionless gain observed in Na, Rb, and Cs, illustrating the potential advantage of using the D1 line of Cs.
The most commonly used lasers operate with population inversion. This requires an incoherent pumping power (from lamps, electrical discharges, etc.) that rapidly scales with the frequency of the generated laser radiation. In practice, this is the main obstacle for the operation in the continuous-wave regime of short-wavelength lasers in the UV, X-rays, etc. But lasers can also operate without population inversion using atomic coherences excited in the laser medium by coherent pumping with another laser. We present in this paper a short review of the field of lasing without inversion (LWI). In particular, we summarize the basic physics of LWI in the framework of both semiclassical theory and quantumjump formalism to describe light-matter interaction. We also review the experimental state-of-the-art in LWI. Most proof-of-principle demonstrations of LWI have used atomic vapours, so far. But recently gain without inversion (a pre-requisite for lasing) has been observed also in a technologically relevant medium: semiconductor nanostructures that can be tailored to have “artificial atom” electron states.
Laser oscillation without population inversion is demonstrated experimentally in a V-type atomic configuration within the D1andD2{D}_{1}\mathrm{and}{D}_{2}D1​andD2​ lines of Rb vapor. It is shown that the effect is due to the atomic interference. The experimental results, as first predicted by careful theoretical analysis, are in a good agreement with detailed calculations.
Summary form only given. Intensive theoretical work, carried out in the past decade, has indicated that optical properties of matter can be dramatically affected by atomic coherence and interference effects. In particular, atomic coherence and interference was shown to lead to such intriguing phenomena as electromagnetically induced transparency, lasing without population inversion (LWI), and resonant enhancement of refractive index accompanied by vanishing absorption. In the present paper we report the first experimental observation of LWI oscillation and the first demonstration of index enhancement.
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.