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Physical Review Letters

Attosecond beats in sodium vapor

D. DeBeer, Eric W. Usadi, Sven R. Hartmann

Published Mar 28, 1988• 31 citations
Publisher

Abstract

Transient, time-delayed, four-wave mixing (TDFWM) experiments have been performed on the Na D doublet with use of a novel angled-beam geometry. The effects of petahertz superposition-state modulations have been observed in the integrated TDFWM signal as a function of the time delay. As the time delay is varied, the lowest-order mixing signal modulates with a period of 980 as---corresponding to the sum frequency of the two Na D lines. Higher-diffraction-order mixing signals contain modulation components at integral multiples of the doublet sum frequency.

DOI
10.1103/physrevlett.60.1262

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Attosecond beats in sodium vapor

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Time-delayed four-wave mixing signals in Na vapor using intense 7-ns pulses of 12-Å wide incoherent light centered at the Na D doublet display a 1.9-ps beat corresponding to the 6-Å splitting of the Na D lines. Collisional relaxation from an argon buffer gas is observed for relaxation times as short as the inverse bandwidth of the excitation noise. The effect of noise field induced relaxation is also examined.

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Transient, time-delayed, four-wave mixing (TDFWM) experiments have been performed on the Na D doublet with use of a novel angled-beam geometry. The effects of petahertz superposition-state modulations have been observed in the integrated TDFWM signal as a function of the time delay. As the time delay is varied, the lowest-order mixing signal modulates with a period of 980 as---corresponding to the sum frequency of the two Na D lines. Higher-diffraction-order mixing signals contain modulation components at integral multiples of the doublet sum frequency.
Transient time‐delayed four‐wave mixing (TDFWM) experiments have been performed on the Na D doublet and modulations of 980 attoseconds have been observed. This period corresponds to the sum frequency of the two Na D lines. The experiments were performed using 7 ns pulses of light with frequency components at each of the D‐line transitions. The effects of superposition‐state modulations are observed in the integrated TDFWM signal as a function of the time delay. As the time delay is varied, the lowest diffraction order mixing signal is modulated. Higher diffraction order mixing signals contain modulation components at integral multiples of the beat frequency.
Two transient time-delayed four-wave mixing experiments have been performed on the Na D doublet and modulations of 1.9 ps and 980 as have been observed. These periods correspond to the difference and sum frequencies of the two Na D lines. Both experiments were performed using 7-ns pulses of light with frequency components at each of the D-line transitions. The effects of superimposition-state modulations are observed in the integrated TDFWM signal as a function of the time delay. As the time delay is varied, the lowest diffraction order mixing signal is modulated. Higher diffraction order mixing signals contain modulation components at integral multiples of the beat frequency.
Photon-echo modulation spectroscopy is extended into a new regime to study coherences between states that are widely separated in frequency compared with the bandwidth of the exciting laser pulses. Time-delayed four-wave mixing (TDFWM) is observed on the Na DDD doublet by use of 7-nsec pulses of light spectrally peaked at each of the DDD-line transitions. As the two double-frequency, 7-nsec light pulses are delayed with respect to each other a 1.9-psec beating of the integrated TDFWM signal is observed, even for delays greater than the excitation pulse width.
Time-delayed four-wave mixing (TDFWM) on the 3S-3P transition in Na vapor has been studied using intense 7-nsec pulses from a broadband laser. Large-scale 1.9-psec modulation associated with the fine structure of the 3P state is observed throughout. At low buffer-gas pressures and low Na-vapor temperatures, pulse-induced relaxation effects are observed while at higher Na-vapor temperatures an anomalous response develops. In the normal region the TDFWM signal is interpreted in terms of either accumulated free decays or photon echoes. We estimate the relaxation induced by the excitation pulses.
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.