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Annual Meeting Optical Society of America

Experimental distinctions between population inversion gain and hyper-Raman gain in xenon

M. Rankin, Jon P. Davis, Christopher Giranda, M. Squicciarini, Lloyd C. Bobb

Published Jan 1, 1989• 0 citations
Publisher

Abstract

Three-level systems in which the middle level has opposite parity from the remaining levels can develop optical gain on the excited-state transition when a two-photon resonant field couples the ground state with the uppermost state. Both population gain, which leads to amplified spontaneous emission, and hyper-Raman gain, which leads to stimulated hyper-Raman scattering, occur.1 Evidence for both types of process appears in two-photon-excited bidirectional emission in atomic xenon.2 This paper presents and compares threshold scaling characteristics, temporal behavior, directional energy distribution patterns, and tuning behavior as evidence for the gain mechanism in xenon. Near threshold, clear evidence for population inversion gain exists. Above threshold, axial emission energy exceeds off-axial (spontaneous emission) energy by a factor which increases with excitation energy. This excess of energy indicates the presence of hyper-Raman and/or four-wave mixing gain.

DOI
10.1364/oam.1989.mkk1

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Annual Meeting Optical Society of America•Jan 1, 1991•0 citations
Three-level systems in which the middle level has opposite parity from the remaining levels can develop optical gain on the excited-state transition when a two-photon resonant field couples the ground state with the uppermost state. Both population gain, which leads to amplified spontaneous emission, and hyper-Raman gain, which leads to stimulated hyper-Raman scattering, occur.1 Evidence for both types of process appears in two-photon-excited bidirectional emission in atomic xenon.2 This paper presents and compares threshold scaling characteristics, temporal behavior, directional energy distribution patterns, and tuning behavior as evidence for the gain mechanism in xenon. Near threshold, clear evidence for population inversion gain exists. Above threshold, axial emission energy exceeds off-axial (spontaneous emission) energy by a factor which increases with excitation energy. This excess of energy indicates the presence of hyper-Raman and/or four-wave mixing gain.
Excitation of xenon near the two-photon resonance between the ground state and the 6p(1/2,0) state produces forward conical emission at a wavelength close to the 828-nm 6p(1/2,0)—6s(3/ 2,1) excited-state transition. We describe experimental features of this process as excited by 600-ps pulses of 250-nm radiation from a frequency-doubled dye laser with intensity up to 1.3 × 108 W/ cm2 in xenon at pressures of 1-20 Torr. The conical emission occurs most prominently when the driving field is a few gigahertz above the frequency at which spontaneous emission from the excited 6p state peaks. The cone angle increases with xenon pressure in the 1-10-Torr range. The dependence appears consistent with the predictions based on noncollinearly phase-matched four-wave mixing. In this interpretation, the fourth wave would be near resonance with the 146.9-nm first resonance transition of xenon and detuned to the low-frequency side. The conical emission co-exists (and may compete with) an axial stimulated process which may be either two-photon driven amplified spontaneous emission or stimulated hyper-Raman scattering. The different tuning characteristics for the two processes make the extent of their competition unclear.
Both on-axis and conical emissions have been observed following two-photon pumping of the 5p states of krypton and the 6p', 7p, 8p, and 4f states of xenon. In the former case, coherent emissions from the 5p states to the 5s are observed, and in the latter case, many p..-->..s, d..-->..p, and f..-->..d cascade emissions are observed. By analogy to the well-studied alkali and alkaline earth examples, the emissions are discussed in terms of amplified spontaneous emission (ASE), stimulated hyper-Raman scattering, and parametric four-wave mixing. The physical processes responsible for the conical emission and for intensity anomalies in the xenon p..-->..s emissions are not understood at present. Interference effects due to coherent cancellation between competing excitation pathways may be occurring. 4 refs., 3 figs.
It is well known that driven two-level atoms can exhibit optical amplification.1 This fact has led to the recent demonstration of single-photon lasing in cell and atomic-beam type ensembles of two-level atoms.2 We show that the same simple two-level system can also be utilized as a two- or more-photon amplifying medium. The underlying physics can be understood in terms of the atom field dressed-state picture. When driven by a field detuned from the atomic resonance, significant population inversion between the dressed-state doublets can be created. In the dressed picture, single-photon lasing occurs between inverted sublevels of adjacent dressed-state doublets. Transitions between nonadjacent doublets are still inverted, but require two or more photons. In the degenerate case, the multiphoton emission frequencies differ from the single-photon emission frequency and can hence be selectively enhanced using an optical cavity. Due to the peculiar structure of dressed levels, the multiphoton emissions are all connected through near resonant intermediate states. The multiphoton emissions can also be viewed as a stimulated hyper-Raman scattering of successively higher order.
Both on‐axis and conical emissions have been observed following two‐photon pumping of the 5p states of krypton and the 6p’, 7p, 8p, and 4f states of xenon. In the former case, coherent emissions from the 5p states to the 5s are observed and in the latter case, many p → s, d → p, and f → d cascade emissions are observed. By analogy to the well‐studied alkali and alkaline earth examples, the emissions are discussed in terms of amplified spontaneous emission (ASE), stimulated hyper‐Raman scattering, and parametric four‐wave mixing. The physical processes responsible for the conical emission and for intensity anomalies in the xenon p → s emissions are not understood at present. Interference effects due to coherent cancellation between competing excitation pathways may be occurring.
Measurements of stimulated Raman gain in rubidium vapor are made and compared with a detailed model. Amplification of a weak probe laser is observed in a \ensuremath{\Lambda}-type three-level system that is driven by a strong cw coupling laser. The intensity of the coupling laser is sufficiently strong that it both optically pumps the atomic levels, causing a population inversion in the ground-state hyperfine levels, and drives the stimulated Raman process producing gain. A theory that models all aspects of the gain mechanism is presented. The model includes all interactions between the external fields and the atomic transitions, including those processes that involve multiple fields interacting with the same transition. Numerical calculations are compared with gain measurements made at several coupling laser intensities and detunings from the D1D1D1 line in 85Rb.{}^{85}\mathrm{Rb}.85Rb.
We consider a four-level atomic system with a degenerate or quasidegenerate ground-state doublet and two excited states. One of the upper levels is coupled to the ground doublet by a coherent field of variable strength, which we refer to as the Raman field, whose role is to create atomic coherence between the driven atomic states. The other higher lying state is coupled to the ground states when laser action develops. This model includes an incoherent pump mechanism to create some population in the top lasing state. In appropriate conditions, for the incoherent pump rate and the Raman field strength, the absorption profile of the laser transition develops gain features. We discuss the physical origin of this gain which we find to be the consequence of a population inversion between dressed atomic states. It is interesting to compare the results of this analysis with a similar study of a three-level lambda system which was previously recognized to yield lasing without inversion.1 In this case the appearance of gain can be attributed to the induced atomic coherence.