Nonresonant Four-wave Mixing Research Articles

Spectral line shape of nonresonant four-wave mixing in Markovian stochastic fields

The spectral line shape of the four-wave mixing (FWM) signal, induced by broad bandwidth laser fields, is derived in the case of a nonresonant interaction. The laser fields are assumed to have a Lorentzian spectrum arising from Markovian stochastic fluctuations described by the phase-diffusion, chaotic field, and Gaussian-amplitude models. The usual ``phase conjugating'' geometry for FWM is considered where the nonresonant interaction is determined only by the statistics of, and the correlations between, the pump and probe fields. For each one of the four possible correlation states between the three input fields and for each of the three fluctuating field models, the line shape of the FWM signal is calculated and shown to be particularly sensitive to the field statistics.

Time resolved four- and six-wave mixing in liquids. II. Experiments

Femtosecond four- and six-wave mixing is employed to study intermolecular motion in liquids, using CS2 as a working example. Nonresonant four-wave mixing yields the total spectral response associated with the low-frequency motions in the liquid. The results of optical Kerr effect and transient grating scattering experiments can be modeled equally well by homogeneously and inhomogeneously broadened intermolecular vibrations. Femtosecond nonresonant six-wave mixing, where two independent propagation times can be varied, contains a temporally two-dimensional contribution that provides information on the time scale(s) of these intermolecular dynamics. The six-wave mixing signal of CS2 shows distinctly different behavior along the two time variables. When the first propagation time is varied, both librational motion at short times and a picosecond diffusive tail are observed. Along the second propagation time, there is no sign of diffusive response and the signal is solely determined by the librational motions. Its shape depends on the first propagation time, when it is varied between 0 and 500 fs, but it is unaffected by further increase of that delay. This is a strong indication for a finite correlation time of the fluctuations in the intermolecular potentials. The interplay between the initial coherent motions and the diffusive behavior on longer time scales is far from clear. A widely used model in which these are treated as independent harmonic processes fails to describe the results.

Strong asymmetry of non-resonant optical four-wave mixing in PbSe and PbSnSe

Non-resonant four-wave mixing experiments were performed for a determination of the free carrier intraband energy relaxation time in semiconductors. The signal intensity was measured as a function of the pump beam detuning Delta omega . For PbSe and PbSnSe this line shape showed an unexpected strong asymmetry; that is, the Stokes signal dependence on Delta omega was different from that of the anti-Stokes signal. The authors explain this asymmetry by an interference with resonant scattering due to a shallow impurity with binding energy of the order of a few cm-1.

Rotational distributions of CO from the 193 nm photodissociation of BH 3CO using nonresonant four-wave mixing for VUV generation

BH 3CO is photolyzed using an excimer laser operating at 193 nm and the nascent rotational energy distributions in CO ν″ = 0 and ν″ = 1 are measured by using laser-induced fluorescence of the fourth positive system of CO near 140 nm. The rotational energy distributions in the two manifolds can be characterized by temperatures of 362 ± 22 and 175 ± 8 K, respectively. The tunable VUV is generated using a nonresonant four-wave mixing process in Hg vapor. This technique has enabled us to extend the range of laser-induced fluorescence measurements in the vacuum ultraviolet using a single dye laser.

Phase-conjugating mirror with continuous-wave gain

We demonstrate a phase-conjugating mirror that has a continuous-wave power reflectivity much greater than unity (gain ~100). This mirror uses nonresonant degenerate four-wave mixing in a single crystal of barium titanate (BaTiO3). With our mirror we have (1) observed cw self-oscillation in an optical resonator formed by this mirror and a normal mirror, (2) demonstrated a cw oscillator that, in spite of phase-distorting material placed inside the resonator, will always emit a TEM00 mode, and (3) demonstrated an optical image amplifier. This mirror will work at any visible wavelength and with weak (milliwatt or weaker) pump beams.