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International Laser Science Conference

Use of incoherent light to generate coherent excitation

S.R. Hartman

Published Jan 1, 1986• 0 citations
Publisher

Abstract

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.

DOI
10.1364/ils.1986.tuc3

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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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Near-resonant four-wave light mixing in Na vapor with inert buffer gases reveals collision-induced coherences when the difference frequency between two incident light beams equals the hyperfine splitting or the Zeeman splitting between two states of the 32S3^{2}S32S ground-state manifold. The Doppler broadening of the corresponding resonances decreases with increasing buffer-gas pressure. The width of the resonances becomes significantly narrower than the spontaneous linewidth of the near-resonant intermediate state and is determined by spin-exchange collisions between ground-state atoms. The dependence of the intensity of these collision-induced resonances on buffer-gas pressure and detuning from one-photon resonances is measured. Selection rules for various polarizations of the incident light fields and directions of an externally applied magnetic field are established.
Coherent signals induced by fundamentally incoherent phenomena through a four-wave mixing process are among the most intriguing and subtle effects in nonlinear spectroscopy. These effects were initially observed by Bloembergen and his co-workers in the form of pressure-induced extra resonances in four-wave mixing. Using standard perturbance techniques based on the optical Bloch equations, they suggested that these collision induced resonances were the result of a destruction of the destructive interference existent between two alternate time-ordered pathways. Subsequently, Prior et al realised that fluctuations in the incident radiation fields could play essentially the same role as the collisionally induced dephasing. In practice, of course, both collisional dephasing and field fluctuations are present in a typical atomic vapor pulsed four-wave mixing experiment. If the timescales governing these two effects approach one another, one can expect to gain valuable new insight into this fascinating process of incoherently inducing a coherent signal
Time-delayed four-wave mixing using incoherent light has been proposed as a new method to measure the ultrafast population relaxation time T1. Assuming three input beams of incoherent light with much longer duration than T1, the theoretical calculation has shown that the decay curve by T1 appears in the signal profile and that the time resolution is determined by the correlation time of the light. The decay time to be observed is twice that of the transient four-wave mixing with short pulses. We have experimentally confirmed the prediction by observing picosecond population relaxations of organic dye solutions, using nanosecond pulses from a normal broadband dye laser. An advantage of the present method over the pump-probe method using incoherent light is also discussed.