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Acta Physica Sinica

INJECTED PARAMETRIC OSCILLATION IN LITHIUM VAPOR

ZHU BAO-QIANG, Zongju Xia, ZOU YING-HUA, (1)北京大学物理系,北京,100871; (2)北京大学物理系,北京,100871;北京大学物理系光学教研室

Published Jan 1, 1991• 0 citations
Publisher

Abstract

The injected 2s-3d two-photon resonant parametric oscillation in lithium vapor has been studied theoretically and experimentally. It's output, compared with general pamametric oscillation, has better monochromaticity and appears as a "sharper" conical radiation. Furthermore, the injected parametric oscillation is tunnable as we change the frequency of the seed beam on the red side of the 2s-2p transition of lithium atom.

DOI
10.7498/aps.40.926

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•Aug 1, 1987•0 citations
The injected 2s-3d two-photon resonant parametric oscillation in lithium vapor has been studied theoretically and experimentally. It's output, compared with general pamametric oscillation, has better monochromaticity and appears as a "sharper" conical radiation. Furthermore, the injected parametric oscillation is tunnable as we change the frequency of the seed beam on the red side of the 2s-2p transition of lithium atom.
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.
There is an interest in consideration of the propagation of the Gaussian monochromatic light beams through a cell filled with the medium of two-level atoms (in our case, the atomic potassium vapor), which has (at small detuning from resonant transition to low or high frequencies) the refractive index other than unity (n > 1 for v < v01 and n < 1 for v > v01, where v is the light frequency and v01 is the resonant-transition frequency). In this case, the properties of the radiation passed through the cell are changed. In the very close proximity of the resonance, the nonlinear processes (the three-photon electronic stimulated Raman scattering (SRS) and the six-photon parametric scattering (SPS)) play a crucial role. It is precisely these processes in the two-level medium that determine the irreversibly transformed spectral and angular structure of the light beams at the output of the cell.
An investigation was made of the spectral properties of four-photon parametric oscillation when the frequency of the generated field is tunable near an atomic resonance line. An ω4 = ω1–ω2+ω3 generation process involving coherent anti-Stokes scattering in thallium vapor was achieved experimentally. Biharmonic pumping was provided by resonant excitation of a 6P1/2–6P3/2 two-photon transition at the frequency ω1–ω2. Measurements were made of the oscillation power when the frequency ω3 was tunable near the frequency of a 6P3/2–7D3/2 transition at various thallium vapor pressures. It was found that the spectral properties of resonant parametric oscillation depend strongly on the concentration of active-mixture atoms and also on various saturation effects.
We present theoretically and experimentally a novel type of parametric fluorescence and oscillation in which four optical waves in off-axis, diagonal directions simultaneously build up under resonant counterpropagating pump-laser excitation in atomic sodium vapor. Two of the four are anti-Stokes shifted and the other two are Stokes shifted, and they are strongly coupled owing to electromagnetically induced diffraction. Theoretically, the Liouville-Maxwell equations are solved to derive four-wave propagation equations. Experimentally, we have observed four-wave parametric oscillation when a weak off-axis seed beam is introduced into otherwise axially symmetric parametric fluorescence patterns. Temporal waveforms are also analyzed, and strong correlations between any pairs of the four emissions are identified.
An analysis was made of four-photon interaction processes under conditions of two-photon resonance excitation of the 3 D sodium level by two-frequency radiation from a single-pulse picosecond YAG:Nd laser with frequency doubling and a KDP optical parametric oscillator. The spatial and frequency spectra of the four-photon parametric scattering (FPPS) were recorded and studied experimentally at various sodium vapor densities (10 15 –10 17 cm −3 ) and also using collinear and noncollinear excitation. It was found that the observed conical structure of the FPPS radiation may be interpreted by an analysis of whether the frequency and spatial phase-matching conditions were satisfied. Dependences of the intensity of the FPPS radiation on the intensity of the exciting radiation, the sodium vapor density, and the detuning of the exciting radiation frequency from the two-photon resonance were obtained.
This work examines the use of higher order multiphoton resonances in higher harmonic generation together with judicious exploitation of coherent interaction properties to achieve efficient harmonic generation. A detailed experimental study on third harmonic generation in two photon resonant coherent interaction and a theoretical study on four photon resonant coherent interaction have been conducted. Two photon resonant coheren propagation in lithium vapor (2S-4S and 2S-3D interaction) has been studied in detail as a function of phase and delay of the interacting pulse sequence. Under coherent lossless propagation of 90 phase shifted pulse pair, third harmonic generation is enhanced. A maximum energy conversion efficiency of 1% was measured experimentally. This experiment shows that phase correlated pulse sequence can be used to control multiphoton coherent resonant effects. A larger two photon resonant enhancement does not result in more efficient harmonic generation, in agreement with the theoretical prediction. An accurate (to at least 0.5 A°) measurement of intensity dependent Stark shift has been done with the newly developed "interferometric wavemeter." Stark shifts as big as several pulse bandwidths (of picosecond pulses) result in a poor tuning of multiphoton resonance and become a limiting factor of resonant harmonic generation. A complete theory has been developed for harmonic generation in a four photon resonant coherent interaction. A numerical application of the theory to the Hg atom successfully interprets the experimental observations in terms of the phase dependent stimulated Raman scattering. With the intensity required for four photon resonant transition, the calculation predicts a dramatic Stark shift effect which completely destroys the resonance condition. This model provides a basis for the development of future schemes for efficient higher order coherent upconversion.