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IEEE Journal of Quantum Electronics

Intense spontaneous violet emission due to coherent two-photon excitation in potassium

N. Tan-no, Ken-ichi Kan’no, Kenichi Yokoto, Humio Inaba

Published Mar 1, 1973• 7 citations
Publisher

Abstract

This paper describes the measured result of intense violet emission at 4048 Å that is due to the strong coherent two-photon excitation in potassium vapor. Anomalous delay and narrowing of the emitted pulse was observed in contrast to the incident pulse. The intensity dependence on the atomic density also is reported.
DOI
10.1109/jqe.1973.1077488

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Two simultaneous laser pulses incident on a cell of potassium vapor at 350°C generate an intense (1-kW) beam of violet light at about 4000 Å. The incident pulses are a 2-MW ruby beam and a 300-kW ruby-generated Stokes beam nearly resonant with the potassium 4P3/2- 4S1/2transition. A simplified theory is presented which treats the problem as a four-wave parametric interaction, and accounts for such experimental details as the doublet stucture of the violet light and the fact that the components of the doublet have different intensities.
Conical emission, redshifted from the atomic resonance, is observed when a ∼2-ps laser beam, blueshifted from resonance, propagates through potassium vapor. In contrast, no cone is visible when a 150-fs pulse is used. The 2-ps-pulse-induced cone has a unique, small, angular spread of Δθ/θ≈0.1, compared with Δθ/θ≈0.3 from other pulsed experiments, which used 25-ps–5-ns-excitation pulses. In addition, the fraction of the 2-ps-pulse power emitted in the cone reached 7.5%, far exceeding this fraction from the longer-pulse experiments. An additional, unexpected result is that the spectral width of the laser pulse is narrower, without significant intensity loss, after propagation through the vapor. The vapor densities and laser detunings that yield cones in the present experiment are similar to those from the longer-pulse experiments. Although the 2-ps-pulse spectrum partially overlaps the resonance, the cone spectrum and cone angles versus density and laser detuning are similar to those previously reported for nanosecond pulses. The fourth (blue-detuned) wave required for satisfying four-wave mixing is absent, again equivalent to what is observed in the nanosecond experiments. The physical origin of cone emission by pulsed excitation of atomic vapors is still largely unknown, and this high efficiency and narrow angular spread of a spectrally broad cone further contradicts existing models, perhaps ultimately providing a clue to a correct explanation.
It is shown that by proper use of the excitation spectra, the emission spectra of the transition and the far-field patterns it is possible to distinguish between competing processes involved in the two-photon excitation to the 6S state of potassium, namely: (a) resonant fluorescence emission associated with the cascading spontaneous emission (ASE) and (b) the four-wave mixing parametric process. The effect of the buffer gas on the intensities of the spectra of the radiations emitted is discussed. Conical emission and spatial instabilities of the excitation radiation and the emission were also observed for high excitation and high vapour pressure of potassium.
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.
The interaction of a laser with potassium atoms is investigated by monitoring the radiation of the 5P-4S transition under the two-photon 4S(1/2)-6S(1/2) excitation. For low number density and laser intensity the observed radiation is parametric axial or conical, depending on the laser detuning from the two-photon resonance, the number density and the laser intensity. With increasing laser intensities and number densities, an additional delayed component is observed corresponding to a two-step four-wave mixing with the participation of internally generated photons. A two-photon quantum interference effect is observed for the total (axial and conical) parametric four-wave mixing emission. A deviation from the proposed model under certain experimental parameters is attributed to a two-step four-wave mixing and the phase mismatch of the parametric processes.
The two‐photon excitation of 4S1/2–7S1/2 transition of potassium atoms is studied. Several coherent emissions and processes are possible, such as parametric four‐wave (PFWM), parametric six‐wave (PSWM) mixing and competition with the stimulated hyper Raman (SHRS) and the amplified spontaneous emission (ASE). The radiations at the transitions 6P3/2,1/2−4S1/2, 6S1/2−4P3/2,1/2 and 5P3/2,1/2−4S1/2 are emitted only in the forward direction (indicating a parametric process), while the radiation at the transition 4P3/2,1/2−4S1/2 is emitted in the forward and in the backward direction, indicating an ASE process.