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Conference on Lasers and Electro-Optics

Stimulated effects in interferometric four-wave mixing experiments

Lde.S. Menezes, Valery P. Kozich, Cid Bartolomeu de Araujo

Published Jan 1, 2000• 0 citations
Publisher

Abstract

Summary form only given. Nonlinear techniques using broadband light have been developed and applied for studying dephasing processes, population and orientational dynamics of molecules. These techniques are based on the fact that in wave-mixing processes one may have the time resolution determined by the laser coherence time instead of the laser pulse duration. One limiting factor of the measurement sensitivity is a constant signal (pedestal), which is present even when the delay time between the exciting broadband beams is substantially longer than their coherence time and relaxation time under study. A technique has been developed, which uses not "white" detection, but spectral filtering in field-correlated four-wave mixing, the so called interferometric four-wave mixing (IFWM), which gives a signal that reveals oscillatory modulation, enabling us to discriminate it more easily from the pedestal. The technique allows to determine rather precisely the frequency of a molecular vibration and its dephasing time, as well as to study damped molecular rotations in liquids. We report the first measurements of population relaxation rates affected by stimulated emission in the presence of the pump field applying the IFWM technique.

DOI
10.1109/cleo.2000.907173

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Summary form only given. Nonlinear techniques using broadband light have been developed and applied for studying dephasing processes, population and orientational dynamics of molecules. These techniques are based on the fact that in wave-mixing processes one may have the time resolution determined by the laser coherence time instead of the laser pulse duration. One limiting factor of the measurement sensitivity is a constant signal (pedestal), which is present even when the delay time between the exciting broadband beams is substantially longer than their coherence time and relaxation time under study. A technique has been developed, which uses not "white" detection, but spectral filtering in field-correlated four-wave mixing, the so called interferometric four-wave mixing (IFWM), which gives a signal that reveals oscillatory modulation, enabling us to discriminate it more easily from the pedestal. The technique allows to determine rather precisely the frequency of a molecular vibration and its dephasing time, as well as to study damped molecular rotations in liquids. We report the first measurements of population relaxation rates affected by stimulated emission in the presence of the pump field applying the IFWM technique.
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.
The theory of an interferometric six-wave mixing quasiecho signal from a modeled molecular liquid using broadband (``noisy'') light is reported. The line-shaping function is treated by a standard mixture of a dynamic (homogeneous) dephasing process and a static (inhomogeneous) Bohr frequency distribution. Simulations are presented in either limit of the overall process to demonstrate the rephasing aspects of this nonlinear spectroscopy. The general aspects of the two-dimensional signal are inferred from factorized time correlator (FTC) diagrams. These diagrams pictorially represent the correlation functions of the random noise (associated with the broadband light) that arise upon stochastic averaging at the signal level. The entire set of FTC diagrams is seen to be isomorphic with those that describe a nonecho interferometric noisy light spectroscopy at fifth order, and the third-order electronically resonant four-wave mixing. Similarities between these three very different processes are discussed in terms of this one-to-one correspondence.
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.
We study theoretically ultrafast modulation spectroscopy in Doppler-broadened cascading three-level and four-level systems. The pump beams consist of two frequency components that drive two-pathway excitation. The four-wave mixing (FWM) signal exhibits damping oscillation as the relative time delay τ between the two pump beams is varied, from which the resonant frequencies between energy levels can be deduced. Furthermore, Doppler-free precision in the measurement of the resonant frequencies can be achieved when the pump beams have either narrow-band or broadband linewidths. We study two-photon resonant FWM induced by time-delayed correlated fluctuating fields in a Doppler-broadened cascading three-level system. It is related intrinsically to the sum-frequency trilevel echo when the pump beams have broadband linewidths. However, unlike the corresponding incoherent-light stimulated photon echo, the maximum of the FWM signal is shifted from τ≃0. If we consider the fluctuations of the pump beams, the FWM signal exhibits damping oscillation when the laser frequency is off resonant from the two-photon transition.
New radiation-matter phenomena in four-wave mixing spectroscopies, given a theoretical basis in the preceding paper [Dugan and Albrecht, Phys. Rev. A 43, 3877 (1991)], are examined experimentally here. A new class of Rabi detuning oscillations is recorded in interferometric coherent Stokes Raman scattering (CSRS). Two identical nanosecond incoherent light fields are mutually delayed, while simultaneously mixing in a nonlinear medium with a narrow-band field of a different color. Spectral filtering of the CSRS signal is essential. From such signals the frequencies and the dephasing rate constants can be a measure of the Raman-active vibrations in both the electronic ground state and the electronic excited state of the scattering chromophore. In principle, analogous probing of vibronic transitions is possible. Detuning oscillations for multimode systems are presented. It is shown how they are transformed into quantum beats, exactly those seen with femtosecond CSRS, when the narrow-band field is made broad and/or the detection is ``white.'' Finally, an example of the sub-Raman linewidth spectral feature, predicted in the preceding paper, is presented. It determines a Raman frequency, but does not carry its linewidth.