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Journal of the Optical Society of America B

Polarization four-wave mixing technique for measuring ultrashort pulses

Victor R. Blok, Gennady M. Krochik

Published Nov 1, 1990• 2 citations
Publisher

Abstract

We treat the problem of four-wave resonant mixing of ultrashort laser pulses in gaseous media. We show that for some transitions, for example, quadrupole transitions, the polarization effects can be strongly manifested. For four-wave two-photon resonant mixing we find that the signal-wave plane rotation depends on the parameters of the resonant medium and the pulses mixed. We show that for long pump pulses and short signal pulses in the ultrashort-pulse time domain the angle of rotation of the plane of polarization of the signal plane can reach several dozen degrees. We show that the effect of altering the state of polarization of the short signal pulse can be used for measuring its shape and duration. Since the polarization measurements are precise and the proposed regimes of four-wave mixing do not depend on phase and amplitude fluctuations of the measuring pulse, the polarization four-wave mixing technique can be more accurate than the technique widely used for such purposes, measurement of the second-order intensity correlation function. We also discuss new nonlinear spectroscopy for measuring fast resonant susceptibilities (in particular, second-order polarizabilities) of the medium, based on the rotation of the polarization plane or the polarization ellipse of the signal pulse.

DOI
10.1364/josab.7.002192

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We treat the problem of four-wave resonant mixing of ultrashort laser pulses in gaseous media. We show that for some transitions, for example, quadrupole transitions, the polarization effects can be strongly manifested. For four-wave two-photon resonant mixing we find that the signal-wave plane rotation depends on the parameters of the resonant medium and the pulses mixed. We show that for long pump pulses and short signal pulses in the ultrashort-pulse time domain the angle of rotation of the plane of polarization of the signal plane can reach several dozen degrees. We show that the effect of altering the state of polarization of the short signal pulse can be used for measuring its shape and duration. Since the polarization measurements are precise and the proposed regimes of four-wave mixing do not depend on phase and amplitude fluctuations of the measuring pulse, the polarization four-wave mixing technique can be more accurate than the technique widely used for such purposes, measurement of the second-order intensity correlation function. We also discuss new nonlinear spectroscopy for measuring fast resonant susceptibilities (in particular, second-order polarizabilities) of the medium, based on the rotation of the polarization plane or the polarization ellipse of the signal pulse.
The main regularities of induced change of light polarization in the field of powerful ultrashort pulses in the resonant gaseous medium are investigated. Frequency- tunable ultrashort pulses allows to observe the polarization plane rotation at large values of detuning, in the different schemes of single-photon and two-photon interaction. On the other hand, the high intensity of these pulses allows to investigate the nonresonant interaction in the crystalline and solid transparent media. Our experiments carried out permit to propose the polarization technique for forming the given parameters pulses or train of pulses. In the observing medium placed between crossed polarizers, at counter propagation, powerful pump and linearly polarized probe pulses interact, because of which the probe pulse polarization changes. The polarization rotation angle as a time function is determined by the intensity of ultrashort pulse, the parameters of medium and frequencies of interacting waves. At the definite choice of the experimental conditions, requiring a probe wave after a crossed polarizer we can obtain the pulse which duration is determined by the length of medium, where interaction occurs. Using a nonresonant transparent media we have the possibility of forming the pulses of given duration tunable in a wide spectral region.
Two-photon resonant and near-resonant four-wave difference-frequency mixing in gases in the interaction regime when laser pulse durations are comparable to or shorter than the medium polarization relaxation time ${T}_{2}^{\ensuremath{'}}$ is investigated. The results of experimental studies of the process in Ar and Kr using pump pulses from the ArF-excimer laser are presented demonstrating a generation of tunable short pulse radiation in the range 102--124 nm. The results are discussed in terms of a theoretical model based on a self-consistent solution of the Bloch equations for the atomic transitions and the Maxwell equations for the fields. This enables one to interpret specific nonstationary resonant and quasiresonant phenomena involved in the frequency conversion process. It is shown that the femtosecond-pulse four-wave frequency-mixing technique with probe pulses significantly shorter than the pump pulses makes it possible to study the coherent dynamics of an atomic transition exposed to an intense field. Using atomic Kr as the nonlinear medium, coherent Rabi oscillations and the subsequent phase relaxation of excitation were observed under the condition of two-photon interaction of Kr with femtosecond 193-nm laser pulses. The obtained information is important for controlling and optimizing processes of two-photon resonant frequency conversion and for time-resolved studies of Rydberg states in atoms and molecules.
We show that four-wave mixing in optically dense media is very different than in an optically thin medium. In time-resolved degenerate four-wave mixing we experimentally observe ``negative'' time delay response, fast decay rates at short delay times, and broad shoulders at long delays. The observations are explained very well in terms of pulse propagation effects, and a theoretical framework is presented for the analysis of the nonlinear interaction. We treat both the incident and generated fields self-consistently, and solve for the interaction of short pulses with a resonantly absorbing medium. The implications for the extraction of relaxation rates from four-wave-mixing experiments involving ultrashort pulses in optically dense matter are discussed.
Four-wave mixing processes with double and triple resonances are discussed in detail. Explicit expressions for the resonant nonlinear susceptibilities of various cases are derived. It is shown that in some cases, four-wave mixing can yield a spectrum with reduced inhomogeneous broadening. Possible applications of multiresonant four-wave mixing as a spectroscopic technique are considered. They include high-resolution spectroscopy, deduction of transition matrix elements, study of transitions between excited states, measurement of longitudinal relaxation times, distinction between resonant Raman scattering and resonant fluorescence, etc. Selective polarization arrangement allows us to suppress the nonresonant background, separately measure the real and imaginary parts of the resonant nonlinear susceptibility, minimize the effects of absorption and laser intensity fluctuations, and greatly enhance the signal-to-noise ratio.
The optical density of a resonant medium significantly affects the output signal in time resolved nonlinear optical interactions. The resonant interaction with a medium reshapes any propagating short pulse, and the mutual interaction of several such pulses, as in four-wave mixing, must be treated self-consistently. We present a theoretical framework for the proper handling of the propagation of all pulses, input as well as generated, in the small area limit. For optically thick resonant absorbers, negative time delay signals are observed, and the apparent decay rate of the induced polarization is faster than the rate observed for thin samples. We experimentally measure degenerate four-wave mixing in an atomic medium as an example, and demonstrate the quality of the theoretical model by the excellent fit to measured signals over several orders of magnitude. The improved understanding enables us to provide a simple, but surprisingly accurate, estimate for the apparent decay rate in homogeneously broadened optically thick media: If the absorption is given by \ensuremath{\alpha}, the propagation length is L, and the transverse relaxation time is T2{\mathit{T}}_{2}T2​, the apparent decay rate 2/Ta{\mathit{T}}_{\mathit{a}}Ta​ of a time resolved four-wave mixing signal is given by 2/Ta{\mathit{T}}_{\mathit{a}}Ta​=(2/T2{\mathit{T}}_{2}T2​)(1+\ensuremath{\alpha}L/2).
Considering an electronic two-level system coupled to vibrational degrees of freedom and driven by short and intense non-overlapping laser pulses, we introduce the concept of nonperturbative response functions. These response functions are expressed in terms of effective electronic transition dipole moments which depend on the strength of the field-matter coupling and on the pulse durations. It is shown that the nonlinear polarization representing four-wave-mixing signals can elegantly be expressed in terms of these nonperturbative response functions to all orders in the field-matter coupling. The nonperturbative response functions generalize the interpretative power of the familiar third-order response functions to four-wave-mixing experiments with intense laser pulses.