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Optics Letters

Temperature diagnostics for cold sodium atoms by transient four-wave mixing

Masaharu Mitsunaga, Makoto Yamashita, Masato Koashi, Nobuyuki Imoto

Published Jun 1, 1998• 27 citations
Publisher

Abstract

Transient four-wave mixing signals were observed in cold sodium atoms. This phenomenon is interpreted as Bragg diffraction of a probe pulse by a population grating of the ground-state hyperfine level induced by pump pulses. The decay time of this grating is directly related to the temperature of the cold atoms and can serve as a novel technique for temperature diagnostics.

DOI
10.1364/ol.23.000840

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In this study, we extended the observation of the effect of the coherence of a laser field to four-wave mixing interactions in atomic media. These measurements will help us understand quantitatively recent techniques employing broad-band nanosecond laser pulses for measurements of sub-picosecond dipole relaxation lifetimes in a variety of media. The coherence of the laser field in this work is controlled using random phase/frequency modulation techniques, creating a field in the laboratory known as the phase diffusion field. The four-wave mixing signal is generated by an ensemble of two-level atoms (Na) interacting with degenerate broad-band, correlated, pump and probe fields. This two-level system is produced by optically pumping the sodium atoms into a single hyperfine component of the ground state. Data sets were taken without and with noise applied to the optical field. For the latter, we measured the dependence of the power of the four-wave mixing signal on the delay times between the pump and probe beams. These data sets were compared with the computer simulation results.

•Jan 1, 1997•0 citations
Transient four-wave mixing signals were observed in cold sodium atoms. This phenomenon is interpreted as Bragg diffraction of a probe pulse by a population grating of the ground-state hyperfine level induced by pump pulses. The decay time of this grating is directly related to the temperature of the cold atoms and can serve as a novel technique for temperature diagnostics.
Optical-pumping-induced population-grating transfer between hyperfine levels of the cesium D2{D}_{2}D2​ line is observed through four-wave mixing in a sample of cold atoms. Diffraction efficiencies of order of 1% have been measured for a large range of angular apertures. We have studied the angular dependence of the diffracted signal in the limit of small atomic velocities and discussed its application for a nondestructive diagnostic of the trap dynamic. Image processing with nearly degenerate frequency conversion was also demonstrated using this specific mechanism.
We have observed a new component in the signals produced by degenerate four-wave mixing of 5-nsec optical pulses by the DDD-line resonances of sodium vapor. This component is comparable in intensity to that previously observed from a grating pattern of excited sodium atoms, but arises rather from a grating of coherently aligned (but not optically pumped) ground-state atoms. This component exists when there are no spatial intensity beats between input waves.
In femtosecond degenerate four-wave mixing experiments in sodium vapor, quantum beats are observed at the wavelength of the third harmonic when the fundamental beams are three-photon resonant. The period of beating, which corresponds to fine structure energy-level splitting, does not change with the atomic number density, buffer gas pressure, or laser intensity. Surprisingly, the third-harmonic signal is observed only at negative time delays for the transient grating pulse sequence. We relate this to the well-known interference effect between excitation pathways involving the fundamental and the third-harmonic fields, which leaves atoms virtually unexcited and prevents formation of population grating.
In this study, we extended the observation of the effect of the coherence of a laser field to four-wave mixing interactions in atomic media. These measurements will help us understand quantitatively recent techniques employing broad-band nanosecond laser pulses for measurements of sub-picosecond dipole relaxation lifetimes in a variety of media. The coherence of the laser field in this work is controlled using random phase/frequency modulation techniques, creating a field in the laboratory known as the phase diffusion field. The four-wave mixing signal is generated by an ensemble of two-level atoms (Na) interacting with degenerate broad-band, correlated, pump and probe fields. This two-level system is produced by optically pumping the sodium atoms into a single hyperfine component of the ground state. Data sets were taken without and with noise applied to the optical field. For the latter, we measured the dependence of the power of the four-wave mixing signal on the delay times between the pump and probe beams. These data sets were compared with the computer simulation results.