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Le Journal de Physique Colloques

GENERATION OF COHERENT IR, VISIBLE AND UV RADIATIONS BY TWO-PHOTON PUMPING IN ATOMIC BARIUM

M. TALEB, G. Fabre, R. Stringat, Jean D'Incan, Christiane Effantin

Published Dec 1, 1987• 0 citations
Publisher

Abstract

Barium atomic vapours were obtained in a cross-shaped heat pipe in the presence of argon. In the temperature range explored (900°-1050°C), the population was ~ 1016 to 1017 cm-3. The transverse and axial (relative to the laser beam) emissions were recorded with the aid of a photomultiplier coupled to an SRS Model 280 boxcar averager. Filters and dichroic mirrors were used to suppress the laser line during axial observations. The dye laser was pumped by a doubled or tripled Nd-YAG laser operating at 10 Hz with 13 ns pulses. The domain explored (725nm > λ > 490nm) correspond to the dyes LDS 698, Rh 590, F548 and C 500. The pulse energies (100-300 µJ) and focussing conditions correspond to an intensity of 100-300 MW/cm2. The dye laser beam was linearly polarised.

DOI
10.1051/jphyscol:19877137

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Barium atomic vapours were obtained in a cross-shaped heat pipe in the presence of argon. In the temperature range explored (900°-1050°C), the population was ~ 1016 to 1017 cm-3. The transverse and axial (relative to the laser beam) emissions were recorded with the aid of a photomultiplier coupled to an SRS Model 280 boxcar averager. Filters and dichroic mirrors were used to suppress the laser line during axial observations. The dye laser was pumped by a doubled or tripled Nd-YAG laser operating at 10 Hz with 13 ns pulses. The domain explored (725nm > λ > 490nm) correspond to the dyes LDS 698, Rh 590, F548 and C 500. The pulse energies (100-300 µJ) and focussing conditions correspond to an intensity of 100-300 MW/cm2. The dye laser beam was linearly polarised.
A pulsed dye-laser beam (ωL) pumped by a frequency-doubled Nd:YAG laser is focused into a heat-pipe oven containing potassium vapor (nk ≈ 1016 cm−3). Two-photon resonant four-wave and six-wave mixing processes using the potassium 8s 2S1/2 state are investigated. Thirty-two coherent UV radiation lines are generated within the range of 300–400 nm through the processes ωUV = 2ωL ± ωIR and ωUV = 2ωL − ω′IR − ω″IR ± ω‴IR, where ωIR are frequencies of laser-induced IR waves generated in the vapor near the transitions 8S–7P, 7P–7S, 7P–5D, 8S–6P, 7S–6P, 5D–6P, etc. The influence of resonant enhancement on the UV line intensities and the phase-matching condition are analyzed.
Multiphoton ionization (MPI) of barium atom using an excimer-pumped pulsed dye laser has been studied in the wavelength range 570-608 nm at a laser intensity approximately 10.0 GW cm -2 . Resonances in MPI were recorded by monitoring either the ions produced in the multiphoton process using a thermionic detector or fluorescence from the ions formed in their various excited states of Ba + . The observation of photoion and fluorescence spectra were made on Ba vapour in the heat-pipe oven in the moderate-density regime ( approximately 10 13 atoms/cm 3 ) so that the collisional effects are negligible. Fine-structure resolution of the final ionic state (6p 2 P 1/2,3/2 ), produced after four-photon autoionizing decay was obtained. The decay of the Ba atom after four-photon absorption preferentially populates the 6p 2 P 3/2 state relative to the 6p 2 P 1/2 state.
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 .
Time-resolved measurements of fluorescence and level populations have been made on barium vapour illuminated by an intense pulse of laser radiation (1 MW cm -2 , 50 ns FWHM) tuned to the resonance transition of Ba I, 6s 2 1 S 0 -6s6p 1 P 1 , at 5535 AA. Barium number densities between 10 13 -10 15 cm -3 were investigated, using a buffer gas of helium at 100 Torr. Fluorescence was measured with two different geometrical arrangements, one allowing partial spatial resolution. In neither case did the observed fluorescence saturate with increasing laser power, although the maximum flux was more than sufficient to burn through the vapour column, as confirmed by transmission measurements. The power loss from the laser beam was found to be compatible with the scattered radiation from the pumped column in the absence of radiation trapping. In contrast to other similar experiments, no significant ionisation was detected (<5%). Time-resolved population measurements by anomalous dispersion on the 6s6p 3 P 2 and 6s5d 1 D 2 metastable levels yielded quenching cross sections for both Ba-Ba and Ba-He collisions, but gave anomalous result for the peak populations. A qualitative explanation of the fluorescence anomaly is proposed on the basis of the AC Stark effect, and it is shown that the same effect can inhibit the onset of stimulated emission to the metastable D levels.
Dense barium vapour (10 19 -10 21 m -3 ) produced in a heat-pipe oven is irradiated by pulsed laser light tuned to the Ba I, 6s 2 1 S 0 -6s6p 1 P 1 0 , resonance transition at 553.55 nm. The transient population densities of various Ba I and Ba II states are measured and time-resolved by means of the hook method. Simultaneously, the IR forward scattering (1.5, 1.13, 2.55 mu m) in the barium vapour is measured. Emphasis is laid on the investigation and the interpretation of the IR emission, the 'anomalous' 6p 3 P 2 0 level overpopulation, and of processes which are responsible for the initiation of ionisation. In addition, the radial diffusion of excitation and ionisation energy in a barium vapour pumped with a focused laser pulse is investigated, as well as the energy transfer in a Ba-Sr vapour.
Two dye laser beams copropagate in barium vapour (N approximately 10 17 cm -3 ). One is tuned to the frequency ( omega L ) of the two-photon transition 6s 2 1 S 0 -6d 1 D 2 (661.2 nm). The frequency of the second ( omega L ) is tuned around that of the transition 5d6p 1 P 1 0 -5d 1 D 2 (528.6 nm). A new coherent wave at lambda UV approximately 350 nm, tunable on about 1000 cm -1 , with frequency omega UV = omega L + Omega (5d 1 D 2 -6s 2 1 S 0 ) is generated in the forward direction by a six-wave mixing parametric process.