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AIP conference proceedings

Competing non-linear and cooperative light emissions

K. J. Boller, H.-J. Lau, J. Sparbier, P. E. Toschek

Published Jan 1, 1988• 0 citations
Publisher

Abstract

Two‐photon excitation (λ1,λ2) of lithium vapor generates light emission by four processes involving the 3d,3s levels: 1. Two‐photon emission, 3d−2s, 2. superfluorescence, 3d−2p, 3. parametric mixing, and 4. two−photon scattering. Upon stepwise tuning λ2, emission spectra are recorded close to 610 and 812 nm, and thus the sum frequency ω1+ω2 is scanned across the 4f resonance. Light observed at 610 nm is superfluorescence. The light at 609.2 nm varies in wavelength upon a shift of λ1, but not of λ2: It is two‐photon emission. Two more emission peaks shift in step with λ2: Parametric emission by five‐wave mixing, and hyper‐Raman‐Stokes scattering of two photons. Interference of these emissions has been observed. The pulse fluctuations of parametric emission are caused by the pump light, while quantum fluctuations dominate the other processes.

DOI
10.1063/1.37396

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Laser excitation and ionization processes in dense (1–10 Torr) sodium vapor have been studied for laser wavelengths near the two-photon allowed nd 2D(n=3, 4, 5) and ns 2S(n=4, 5, 6) states. In particular, the 1 3Σ+g→1 3Σ+u excimer emission in Na2, predicted in 1980 by Konowalow and Julienne and observed recently by Dinev et al, was studied here in greater detail. Strong excimer emission (∼830 nm) was observed for two-photon pumping to both sides of the unresolved 4D states, and weak excimer emission was seen when pumping near the 5D levels. The excimer emission exhibits a complicated pump laser profile with a pronounced ‘‘dip’’ at the 4d 2D two-photon resonance. Similarly, [2+1] photon ionization via the 3d 2D and 4d 2D states shows a dramatic decrease as the sodium density increases. These results can be attributed either to depleted 3d 2D or 4d 2D population due to stimulated electronic Raman scattering (SERS) or to the interference effects recently reported by Malcuit et al. and Krasnikov et al. and treated theoretically by Manykin and Afanas’ev and by Agarwal. It is argued that both mechanisms are operative. Strong ionization and SERS signals were observed at the hybrid resonances corresponding to 3p 2P3/2, 1/2 →4d 2D transitions; however, no excimer lasing at 830 nm was detected. No excimer emission was detected upon two-photon pumping near or at the 3d 2D or ns 2S(n=4, 5, 6) states. Based on these and other observations, the 1 3Σ+g→1 3Σ+u excimer emission is attributed to a molecular Raman process involving stimulated emission or six-wave mixing via a pathway of the type 1 3Σ+u→2hν 3Δu→ j 3Πg→(k 3Πu, l 3Σ +u)→1 3Σ+g→1 3Σ +u .

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Two-photon resonant, stimulated processes in krypton and xenon

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•Nov 1, 1988•0 citations
Two‐photon excitation (λ1,λ2) of lithium vapor generates light emission by four processes involving the 3d,3s levels: 1. Two‐photon emission, 3d−2s, 2. superfluorescence, 3d−2p, 3. parametric mixing, and 4. two−photon scattering. Upon stepwise tuning λ2, emission spectra are recorded close to 610 and 812 nm, and thus the sum frequency ω1+ω2 is scanned across the 4f resonance. Light observed at 610 nm is superfluorescence. The light at 609.2 nm varies in wavelength upon a shift of λ1, but not of λ2: It is two‐photon emission. Two more emission peaks shift in step with λ2: Parametric emission by five‐wave mixing, and hyper‐Raman‐Stokes scattering of two photons. Interference of these emissions has been observed. The pulse fluctuations of parametric emission are caused by the pump light, while quantum fluctuations dominate the other processes.
We report observations of 4‐wave and 6‐wave mixing processes based upon two‐photon pumping of the potassium 4S→6S and 4S→4D transitions in a sodium‐potassium mixture. Additionally we observe coherent emissions at the potassium 3D→4P transition frequencies for pump wavelengths between 725 and 760 nm. The excitation spectra for these emissions show several broad peaks, each of roughly 10 nm width. A series of sharp dips is superimposed on the most intense peak due to depletion of the 3D population by laser‐induced absorption to Rydberg F levels. Although the 3D→4P emissions can be observed in both the forward and backward directions, they are ∼50 times more intense in the forward direction. We believe the forward emission is primarily due to 6‐wave mixing while the backward emission can most likely be attributed to amplified spontaneous emission following two‐photon molecular photodissociation. However, at present, the exact mechanisms responsible for these 3D→4P emissions are not fully understood.
Two-photon resonant or nearly resonant optical processes have been investigated in atomic barium vapour ( approximately 10 17 cm -3 ) with a pulse dye laser (frequency omega L ) covering the range 735-415 nm. These interactions result in strong coherent emissions emitted either forward and backward, or forward only. The optical processes responsible for these coherent emissions have been identified. Seventy five lines (frequency omega ES ) emitted from 1000 to 360 nm are due to optically pumped stimulated emissions or stimulated electronic hyper-Raman scattering. Twenty eight lines (frequency omega sigma ) observed from 795 to 250 nm are generated through four-wave-mixing parametric processes with omega sigma =2 omega L +or- omega ES , and nine- (795-260 nm) through six-wave-mixing parametric processes with omega sigma =2 omega L - omega ES1 - omega ES2 +or- omega ES3 .
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.
Laser excitation and ionization processes in dense (1–10 Torr) sodium vapor have been studied for laser wavelengths near the two-photon allowed nd 2D(n=3, 4, 5) and ns 2S(n=4, 5, 6) states. In particular, the 1 3Σ+g→1 3Σ+u excimer emission in Na2, predicted in 1980 by Konowalow and Julienne and observed recently by Dinev et al, was studied here in greater detail. Strong excimer emission (∼830 nm) was observed for two-photon pumping to both sides of the unresolved 4D states, and weak excimer emission was seen when pumping near the 5D levels. The excimer emission exhibits a complicated pump laser profile with a pronounced ‘‘dip’’ at the 4d 2D two-photon resonance. Similarly, [2+1] photon ionization via the 3d 2D and 4d 2D states shows a dramatic decrease as the sodium density increases. These results can be attributed either to depleted 3d 2D or 4d 2D population due to stimulated electronic Raman scattering (SERS) or to the interference effects recently reported by Malcuit et al. and Krasnikov et al. and treated theoretically by Manykin and Afanas’ev and by Agarwal. It is argued that both mechanisms are operative. Strong ionization and SERS signals were observed at the hybrid resonances corresponding to 3p 2P3/2, 1/2 →4d 2D transitions; however, no excimer lasing at 830 nm was detected. No excimer emission was detected upon two-photon pumping near or at the 3d 2D or ns 2S(n=4, 5, 6) states. Based on these and other observations, the 1 3Σ+g→1 3Σ+u excimer emission is attributed to a molecular Raman process involving stimulated emission or six-wave mixing via a pathway of the type 1 3Σ+u→2hν 3Δu→ j 3Πg→(k 3Πu, l 3Σ +u)→1 3Σ+g→1 3Σ +u .
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.