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Journal of the Physical Society of Japan

Laser Induced Population Grating in Sodium Vapor

Yukio Fukuda, Koichi Yamada, Tsuneo Hashi

Published Apr 1, 1980• 7 citations
Publisher

Abstract

By using two intersecting resonant pulses from a laser, we produced Laser Induced Population Gratings in atomic sodium vapor, and with an aid of a delayed probe pulse, observed “Zeeman beats,” anomalous effects of Na–Ar collision and a new type of echo. These new phenomena can be interpreted in terms of the dynamics of LIPG's. The description of the Zeeman beat experiment is given in some details.

DOI
10.1143/jpsj.48.1403

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Time Development of the Population Grating in Zeeman Sublevels in Sodium Vapor–Detection of Zeeman Quantum Beats

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Physical Review A•May 1, 1994•7 citations
By using two intersecting resonant pulses from a laser, we produced Laser Induced Population Gratings in atomic sodium vapor, and with an aid of a delayed probe pulse, observed “Zeeman beats,” anomalous effects of Na–Ar collision and a new type of echo. These new phenomena can be interpreted in terms of the dynamics of LIPG's. The description of the Zeeman beat experiment is given in some details.
The population gratings were created by two intersecting resonant light pulses in the ground state 3 2 S 1/2 and the excited state 3 2 P 1/2 in sodium vapor, and their developements in a static magnetic field were monitored by the diffraction of the resonant probe pulse. Intensity modulations of the diffraction signals at the Zeeman frequencies (Zeeman beats) were observed, which are due to the oscillation of the LIPG's. The detailed analysis of the creation, detection and the time development of the LIPG was made in a simple model system with total angular momentum J =1/2 in both the ground and the excited states. The observed beat patterns were compared with the theoretical calculations including the effects of hyperfine interaction in both states. Polarization characteristics were also examined. The agreement between theoretical predictions and experimental results was quite satisfactory.
Optical four-wave mixing resonances in sodium vapor induced by collisions with inert buffer gases (He, Ar, Xe, N2{\mathrm{N}}_{2}N2​) are characterized at high resolution ( 5 MHz) using lasers tuned near the DDD lines of Na. These resonances are interpreted in terms of coherent scattering from a population grating established by collisional absorption of the incident radiation. The roles of homogeneous and inhomogeneous contributions to the resonance line shapes are clarified. The line shapes exhibit structure sharper than the predicted linewidth, that characteristic of the 3P3P3P-to-3S3S3S transition rate. We show that this is caused by collision-induced optical pumping which introduces the ground-state lifetime into the grating relaxation physics. These long-lived gratings are exemplified dramatically by experiments in the presence of N2{\mathrm{N}}_{2}N2​ which collisionally quenches the Na 3P3P3P states. Similar results are obtained in Xe where it is argued that excimer formation with the 3P3P3P states plays a role in the population dynamics. Model calculations of the effects of sodium level degeneracy on the grating dynamics are presented to support these conclusions. Finally we present data illustrating interference between the collision-induced population grating resonances detailed here and collision-induced coherent Raman resonances between Zeeman sublevels of the ground state.
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.
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.
levels of atomic sodium using ultrashort laser pulses in a stepwise excitation scheme. Two broadband femtosecond laser pulses are used to excite the 6S level from the 3S ground level in sodium vapor. The first one is simultaneously resonant with the D1 and D2 lines of the 3S→3P transition, and the second one with the 3P→6S transition. The delay between the two pulses is scanned in order to probe the dephasing between the two excited intermediate levels. Quantum beating is observed by monitoring the fluorescence signals at 330 nm and 515 nm, namely transition 4P→3S and 6S→3P, respectively. Signals are compared to a semi-classical model that reproduces the oscillations with a very good agreement.
Degenerate four-wave mixing in dense sodium vapor under conditions of strong pressure-induced Zeeman pumping is investigated. Thermal diffusion and its interplay with the Zeeman light shift are found to be responsible for the generation of a grating mechanism that is not sensitive to thermal washout. The effect can be observed experimentally after eliminating the influence of radiation trapping by the use of nitrogen as a buffer gas. Under these conditions large nonlinearities are produced which also influence the propagation of the signal wave via collision-aided circular birefringence.