Resonant Four-wave Mixing Research Articles

Four-wave mixing in the far infrared from free carriers in n-type indium antimonide

Far-infrared techniques have been developed that make possible the first observation to our knowledge of resonant four-wave mixing of submillimeter-wave radiation in n-type InSb maintained at 2 K. The dependences of the emission strength on dc magnetic field, circular polarization, and carrier concentration demonstrate that the resonant nonlinear process involves the collective excitations of the electron gas.

Two-dimensional mapping of temperature in a flame by degenerate four-wave mixing in OH

Phase conjugate images are produced by resonant degenerate four-wave mixing from rotationally excited OH molecules in a premixed, laminar methane/air flame. By comparing signal intensities in the images produced using transitions from different rotational levels, the temperature map of a planar section of the flame is derived.

Dynamic characteristics of coherent and population grating processes in resonant degenerate four-wave mixing

Dynamic characteristics of coherent and population grating processes in resonant degenerate four-wave mixing

Studies of resonant and preresonant femtosecond degenerate four-wave mixing in unoriented conducting polymers

Degenerate four-wave mixing with femtosecond time resolution is used to measure the magnitude and transient response of the third-order nonlinear optical susceptibility χ(3) (ω;ω,−ω,ω) at 620 nm in nonoriented conducting polymers including polydiacetylene, <m1;38p>polyacetylene, polyaniline, polydiethynylsilane, polythiophene, and polythiophene derivatives. <m1;40p>Resonant and nonresonant excitations influence the magnitude and transient response of χ(3). <m1;39p>The electronic response is instantaneous for preresonant excitation, but for resonant excitation it <m1;38p>has ultrafast and slow components which illustrate photoexcitation dynamics. The <m1;38p>magnitude of χ(3) for all of the polymers is in the range from 10−10 to 5×10−8 esu depending on the energy difference between the laser excitation and the polymer absorption maximum.

Generation of tunable coherent XUV radiation in a hydrogen gas jet by two-photon resonant four-wave mixing

Tunable coherent XUV radiation down to 75 nm was generated by two-photon resonant four-wave mixing of a tunable ArF excimer laser with a dye laser in a molecular gas jet of hydrogen. The obtained XUV spectral profiles reflected the double resonance effect at Rydberg states in H 2 molecules. This scheme has a possibility to extend the shortest wavelength limit of the tunable radiation down to 66 nm by using a frequency-doubled dye laser.

Time-dependent phase conjugation in plasmas: Numerical results and interpretation

The salient features of time-dependent phase conjugation and four-wave mixing in a plasma are summarized and developed numerically in an extension of the work of Goldman and Williams [Phys. Fluids B 3, 751 (1991)]. It is shown that plasma nonuniformity can reduce the decay time of a conjugate wave responding to a delta-function signal without significant reduction in the maximum amplitude of the phase-conjugate wave. A phase-conjugating nonuniform plasma may therefore offer advantages over other media in the tracking or self-targeting of moving objects. The temporal approach to steady-state of an amplified phase-conjugate wave is found numerically for the case of resonant four-wave mixing driven by pumps of unequal intensity; the temporal response above threshold for absolute instability is also studied. Related absolute instabilities pumped by Langmuir waves are briefly described. Effects of nonuniformity are considered in connection with amplified phase conjugation.

Enhanced selectivity for spectrochemical measurement by mode selection in fully resonant nonlinear mixing

In this paper, we demonstrate the feasibility of a new family of mode-selective spectroscopies based on multiresonant nonlinear mixing. Three tunable lasers are used for fully resonant four-wave mixing so the output signal depends on the simultaneous contribution from three resonances. Mode selection is accomplished by tuning the lasers to a particular vibrational resonance

Raman-enhanced nondegenerate four-wave mixing

It is well known that resonant four-wave mixing can be used for high-resolution laser spectroscopy. Here we report an extensive study of Raman- enhanced nondegenerate four-wave mixing (RENFWM) in benzene. Instead of using a broadband laser source and single-shot measurement, as in the first experiment by Saha and Hellwarth,1 we measured the spectrum by scanning the wavelength of a narrowband dye laser used as the probe beam. Our results demonstrate the feasibility of using RENFWM as a practical and powerful tool for coherent Raman spectroscopy.

Stable isotope ratio analysis at trace concentrations using degenerate four-wave mixing with a circularly polarized pulsed probe beam

Stable isotope analysis based on vectorial optical-phase conjugation by resonant degenerate four-wave mixing (D4WM) is reported by using a D4WM method with vertically polarized pump beams and a circularly polarized probe beam. Since the polarization of the signal beam is different from that of the pump beams, the background radiation is suppressed more effectively. Excellent sensitivity, high spectral resolution, and efficient optical detection make this an effective and unusually convenient nonlinear spectrometric method for the analysis of trace amounts of stable isotopes. Using an excimer-pumped pulsed dye laser, the fine structures of lithium are examined. A detection limit of 2.5 ng/mL lithium is observed while a Doppler-free resolution is maintained by using transient "coherent-grating" based D4WM spectroscopy.

Doppler-Free Spectrum of the Barium 1S0−1P1 Transition by Degenerate Four-Wave Mixing Using an Air/Acetylene Flame

Doppler-free flame spectrometry based on resonant degenerate four-wave mixing (D4WM) is shown to be an effective analytical tool for the determination of barium at high spectral resolution. Optical phase conjugation is produced in an air/acetylene flame, resulting in an optical signal that is a coherent time-reversed replica of the probe beam. The degenerate four-wave mixing spectrum of barium for the 6 s21 S0-6 s6 p1 P1 transition at 18,060.19 cm−1 is collected with a linewidth of 0.08 cm−1 (FWHM) with the use of a convenient continuously flowing atmospheric-pressure flame atomizer. A preliminary detection limit of 5 μg/mL barium, while a Doppler-free linewidth is maintained at 0.08 cm−1, is reported.

Saturation splitting of line shapes in noncollinear four-wave mixing in SF_6

Phase-conjugate emission by resonant degenerate four-wave mixing with two counterpropagating weak pump beams and a noncollinear strong object beam was calculated for a Doppler-broadened two-level system. Singlepeak Doppler-free lines with small saturation broadening at a collinear configuration became split for a wide angle. The effect is demonstrated with the use of a CO(2) laser and SF(6). Conversely, calculated line shapes with Stark splitting in a collinear arrangement became single peaked for a wide angle.

Distortion of phase-conjugation in degenerate four-wave mixing with phase fluctuation of pump and signal

The temporal evolution and behaviour of the backscattered wave in resonant degenerate four-wave mixing is strongly influenced by phase fluctuation in pump and signal and by the mutual coherence of the incident light beams. In particular, the conjugation process is distorted by such fluctuations where the distortions can be diminished by applying fast-response media as conjugators.

Time Dependence of Degenerate Four-wave Mixing with Broad Bandwidth Pulsed Lasers

Abstract A time dependent theory of resonant degenerate four-wave mixing (DFWM) with broad bandwidth lasers is outlined and used to calculate the temporal evolution of DFWM signals generated by pulsed lasers. Experimental measurements of saturation-induced pulse-shortening and reshaping effects are reported for DFWM in atomic Na vapour and found to be in quantitative agreement with theoretical predictions.

Analysis of vibrational correlations and couplings in the lowest two singlet states of pentacene by high resolution, fully resonant, coherent four-wave mixing spectroscopy

Coherent, fully resonant four-wave mixing (FRFWM) spectroscopy is used to probe the vibrational and vibronic state of the pentacene S0 and S1 electronic states. High resolution is obtained using mixed molecular crystals at cryogenic temperatures. The FRFWM spectra combined with conventional absorption and fluorescence spectra allow correlation of the S1 fundamentals to the corresponding ground state normal coordinates. Couplings between modes are indicated and may be related to Fermi resonance of the pentacene vibrations by means of host librational phonons.

Resonant photodiffractive four-wave mixing in semi-insulating GaAs/AlGaAs quantum wells

We have performed photodiffractive four-wave mixing in semi-insulating multiple GaAs/AlGaAs quantum wells at a wavelength of 0.83 microm. The quantum wells were made semi-insulating by proton implantation, which introduces defects that are available to trap and store charge during holographic recording. The experiments demonstrate how photodiffractive behavior using the large resonant nonlinearities of quantum-confined excitons yields highly sensitive material for optical image processing. When pump powers of 1 mW/cm(2) are used, the measured sensitivity is 2 orders of magnitude greater than that of bulk, nonresonant photorefractive semiconductors.

Analysis of vibronic mode coupling in pentacene by fully resonant coherent four-wave mixing

This paper develops the theoretical framework for the analysis of relative peak intensities in fully resonant, three laser, coherent four-wave mixing spectra of molecular vibrational and vibronic levels. For a Franck–Condon system, the relative vibronic peak intensities are shown to scale as the square of the absorption spectrum for all resonances not associated with the normal mode selected by the fixed vibrational resonance. A large enhancement of vibronic resonances involving the selected mode is predicted for typical potential well offsets. Higher order mode coupling causes deviations from these predictions by allowing other coupled resonances to be enhanced as well. Vibrational peak intensities are similarly related to the emission spectrum. Pentacene in benzoic acid mixed crystals at 2 K are used to experimentally study the mode mixing effects. It is shown that important coupling occurs between the 747.7 and 790.8 cm−1 vibronic modes of the pentacene S1 electronic state. Two possible mechanisms for this coupling are suggested. This work shows that the relative intensities of vibrational and vibronic resonances in fully resonant four-wave mixing spectra can serve as a useful probe of mode coupling.

TUNABLE VACUUM-ULTRA VIOLET RADIATION GENERATED BY TWO-PHTON RESONANTFOUR-WAVE DIFFERENCE MIXING IN Hg VAPOR

Vacuum-ultra violet radiation is generated by 61S0-61D2 or 61S0-63D2 two photon resonant four-wave difference mixing (ω4=2ω1-ω3) in Hg vapor. VUV radiation is tunable when one of the input wave (ω3) is tuned, the tuning range is 184.9-187.5 nm. The mechanism of this frenquency conversion, for example, the two-photon and single-photon resonant enhancement and phase matching have been investigeted. The competing processes and saturation effect on conversion efficiency have been analyzed. Using the generated tunable VUV radiation, we have recorded the absorption spetrum of O2 in air.

Quantized-field approach to pressure-induced resonances.

Using a fully quantized dressed-atom approach, we present a theory and interpretation of the pressure-induced resonances in four-wave mixing (PIER4) that may arise when three incident fields interact with an ensemble of two-level atoms. The PIER4 resonances are seen to arise from a collisionally created modulation of a dressed-state population when an operator approach is used to solve the problem and from a level crossing between collisionally populated dressed states when an occupation-number formalism is used.

Revised theory of resonant degenerate four-wave mixing with broad-bandwidth lasers.

We present a revision of the theory of finite laser bandwidth effects on degenerate four-wave mixing (DFWM) of Alber, Cooper, and Ewart (ACE) [Phys. Rev. A 31, 2344 (1985)]. The same model is used for intense, broad-bandwidth pump lasers interacting with a weak monochromatic probe in a medium composed of two-level atoms. The density-matrix equations describing the time evolution of the atomic polarization coupled to fluctuating fields are solved using an appropriate decorrelation approximation. A steady-state analytical solution is found for resonant and near-resonant DFWM, for intensities that do not saturate the medium. For more intense fields it is found to be necessary to form a more complicated set of differential equations for all products of the field and atomic density-matrix elements. The results show that as in the ACE theory, increasing the bandwidth b leads to an increased effective saturation intensity. However, we find the DFWM reflectivity scales as 1/${b}^{2}$ when the probe is monochromatic and not 1/b as in the ACE theory. Furthermore, the saturation behavior of nonresonant DFWM is found to differ from the predictions of ACE. Atomic motion effects are shown to yield a Doppler-broadened line shape and also to affect the saturation behavior of the signal.

Efficient phase conjugation of a cw low-power laser diode in a short Cs vapor cell at 852 nm

Up to 50% cw phase-conjugate reflectivity has been obtained with a 30-mW diode laser at 0.85 microm by means of resonant degenerate four-wave mixing on an optically thick, 1-mm-long Cs vapor cell. Self-oscillation of a Fabry-Perot cavity containing this Cs cell is demonstrated.