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Quantum Electronics and Laser Science Conference

Phase-matching in gases simultaneously pumped by Bessel and Gaussian beams

Virgilijus Vaičaitis

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. The picosecond Nd:YAG laser with passive mode-locking and passive negative feedback has been used for the experiments.For the excitation of sodium we have used the second harmonic beam of this laser focused with an axicone (Bessel pump) together with the signal wave of the KH/sub 2/PO/sub 4/ (KDP) optical parametric generator (OPG). When the wavelength of the OPG output beam was about 962 nm, the two-photon resonance 3S-3D of sodium was excited and resulted in four-photon parametric superfluorescence.

DOI
10.1109/qels.2001.961954

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Summary form only given. The picosecond Nd:YAG laser with passive mode-locking and passive negative feedback has been used for the experiments.For the excitation of sodium we have used the second harmonic beam of this laser focused with an axicone (Bessel pump) together with the signal wave of the KH/sub 2/PO/sub 4/ (KDP) optical parametric generator (OPG). When the wavelength of the OPG output beam was about 962 nm, the two-photon resonance 3S-3D of sodium was excited and resulted in four-photon parametric superfluorescence.
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.
Three-photon resonance enhancement of third-harmonic generation and multiphoton ionization have been studied in xenon under excitation by the Bessel and the focused annular laser beams. It has been shown that the transformation of a Gaussian beam into a Bessel or an annular one leads to significant changes of third-harmonic excitation profiles and ionization spectra. These changes result from a principal difference of the beams, where either collinear (Gaussian beams) or noncollinear (Bessel and focused annular beams) excitation geometries are realized. It has been shown that the noncollinear ring-type phase matching shifts the maximum of the generated third harmonic to the longer wavelength toward the atomic resonance. Numerical calculations of third-harmonic excitation spectra have been carried out and compared with experimental results. The efficiency of frequency tripling in collinear and noncollinear excitation geometries have been compared. Cancellation of the three-photon 6s6s6s resonance and the influence of harmonic photons on the excitation of the four-photon 4f4f4f resonance in the Bessel beams are discussed.
Laser detection technologies offer obvious benefits for the standoff detection of hazardous or energetic materials where safe detection at a distance is the goal. Of the many optical standoff detection methods available, multiphoton fluorescence techniques have been studied less extensively. Multiphoton fluorescence allows high selectivity relative to the background while preserving the larger signal of laser induced fluorescence (LIF). Using sodium vapor as a test system, we demonstrate that stimulated Raman adiabatic passage (STIRAP) is capable of providing more than a factor of ten improvement in population transfer efficiency to the final state when compared to stimulated emission pumping (SEP). The two sodium transitions used are the 3p (2P1/2) ← 3s (2S1/2) and 5s (2S1/2) ← 3p (2P1/2). The light used to couple the states was produced with two synchronously pumped OPG/OPAs pumped by the 355 nm light from a picosecond tripled Nd:YAG.
We have observed optical second harmonic generation (SHG) in atomic sodium when the fundamental is tuned in the vicinity of the two-photon allowed transition 3s-4d. This result is obtained with the fundamental in a single loosely focused laser beam in the absence of any external fields. Although doubling in atomic media using noncollinear inputs1 or external fields2 has been reported by several groups, there remain only two previously reported doubling processes involving collinear beams and no fields.3 Both of these previous experiments reported both weak conversions and narrowly tunable outputs.