Wave Mixing Spectroscopy Research Articles

A simple model to describe thermal grating effects in degenerate four wave mixing spectroscopy

The pressure dependence of the resonant degenerate four wave mixing (D4WM) spectra of NO2 recorded between 0 and 900 Torr of N2, Ar and He are reported. The results are interpreted in terms of a transient grating model that incorporates the effects of thermalising collisions between the optically pumped molecule and the bath gas. The observed pressure dependence demonstrates that two transient gratings contribute to the total signal. The signal observed at low pressure is that due to a laser induced population grating. After an initial saturation region where the D4WM signal is essentially pressure-independent, a sharp fall-off due to the collisional removal of energy from the population grating is observed. At pressures above about 100 Torr, depending on the buffer gas, a second mechanism, with a near-quadratic pressure dependence, becomes important. This contribution is attributed to a thermal grating with local heating caused by thermal relaxation of the population grating. A simple model that includes both of these contributions is developed. This model is able to reproduce the observed behaviour. Confidence in the model is reinforced by its ability to predict the behaviour of argon from the results obtained for nitrogen with no additional assumptions or free parameters. The experiments were repeated under identical conditions with an optical arrangement involving cross-polarised laser beams. The secondary rise in D4WM signal is not observed in this experiment. A decay of D4WM signal with pressure was however still observed and can be accounted for by the re-orientation and re-alignment of molecules within a polarisation grating by collisions with other gas phase species.

Coherent anti-Stokes Raman scattering: from proof-of-the-principle experiments to femtosecond CARS and higher order wave-mixing generalizations

A review of CARS evolution from proof-of-the principle experiments to recent CARS developments, including femtosecond CARS and higher order wave-mixing spectroscopy generalizations, is given, with the main emphasis placed on the discussion of physically new ideas and promising directions of CARS studies. Copyright © 2000 John Wiley & Sons, Ltd.

Singly vibrationally enhanced infrared four wave mixing spectroscopy

We describe a new nonlinear, nonparametric process that provides a direct measure of vibrational enhancement in four wave mixing. This process has 14 coherent pathways that create the final coherence. In the limit of no pure dephasing and a single vibrational resonance, the 14 pathways combine into a single process that is vibrationally enhanced. The frequency and concentration dependence expected for this process matches that observed in experiments with the methyl- and methylene C–H stretch vibrations in hexane samples. The third-order nonlinear susceptibility for the C–H vibrations was measured by the interferometric method of Levenson and Bloembergen. The vibrational nonlinearity was observed as a difference between the nonresonant electronic contributions and the contribution that depended on resonance with the vibrational absorption transition. The measurements show that the vibrational enhancement is 50 times larger than the nonresonant electronic contribution and it shows that spectroscopic methods based on resonant nuclear nonlinear polarizations are observable above the electronic nonlinear polarization background.

Measurement of orientational relaxation times of OH (A 2ΣX 2Π) transitions in atmospheric pressure flames using picosecond time-resolved nonlinear spectroscopy

Time-resolved picosecond pump-probe polarization and degenerate four wave mixing (DFWM) spectroscopy have been applied for the observation of coherently excited states in the (0,0) band of the electronic A 2ΣX 2Π transition of OH in atmospheric pressure CH 4 air and H 2 O 2 flames. From the exponential decays of the signal after excitation with a 20 ps pump pulse the decay of orientation and alignment through dephasing collisions and rotational energy transfer takes place within 150–370 ps depending on the rotational state quantum number. For simultaneously excited transitions within the bandwidth of the picosecond laser pulse, oscillations in the transient DFWM signal superimposed on the exponential decay have been observed for cross-polarized pump and probe laser beams.

Wave mixing spectroscopy for surface studies

Optical second-harmonic generation and sum-frequency generation are intrinsically surface-specific. They can therefore be used as effective surface probes. We briefly review the wide range of applications found for these probes in recent years. Many of them have opened the door for exciting new research opportunities.

LOW FREQUENCY MODES PROBED BY TIME-DOMAIN OPTICAL KERR EFFECT SPECTROSCOPY

The principle and application of ultrafast optical Kerr effect (OKE) spectroscopy have been reviewed. This spectroscopy is shown to be very useful to investigate low frequency modes in disordered materials and the obtained data are directly comparable with frequency-domain light scattering spectroscopy. Experimental study to show the consistency between the time- and frequency-domain spectroscopy has been performed for liquid nitrobenzene and the excellent agreement is attained over three orders of magnitude in frequency range. It is also shown that the result obtained by the OKE measurement is consistent with that obtained by four wave mixing spectroscopy. Combination of these spectroscopic techniques is particularly suited for the investigation of low frequency modes because a wide frequency range is covered with great accuracy. Several remarks concerning the OKE spectroscopy are presented such as the breakdown of Debye relaxation model and various interference effects which may distort the time-domain data.

Infrared degenerate four wave mixing spectroscopy of jet-cooled C 2H 2

We report the application of infrared degenerate four-wave mixing (IR DFWM) spectroscopy to probe rotational-vibrational transitions in the v 3/ v 2 + v 4 + v 5 Fermi dyad region of jet-cooled C 2H 2. The spectral intensities show a quadratic population dependence, and are well reproduced by a Boltzmann distribution of J state populations at T rot = 25 K. Intensity dependence measurements and lineshape analysis show a cubic dependence of the DFWM signal on laser intensity, as expected from theory in the weak field limit ( I ⪡ I sat). We compare DFWM signals in jet versus bulk experiments.

Femtosecond four wave mixing spectroscopy of conjugated polymers

Time-resolved four wave mixing (4WM) signals of conjugated polymers are calculated using a density matrix approach and the Hartree–Fock approximation. The influence of the Coulomb interaction as well as size effects on the nonlinear optical response are examined. For the Hückel model (no Coulomb interaction), quantum beats due to the coherent excitation of several band states are found for small sizes, and photon echo-like behavior is obtained for infinite size. The Coulomb interaction changes the nature of the response and gives rise to a broad delayed signal whose temporal profile is determined by the dephasing time.

Resonant degenerate four wave mixing detection of HCO

The HCO radical has been detected by degenerate four wave mixing (D4WM) spectroscopy following 308 nm photolysis of CH 3CHO. Spectra have been obtained for the (0, 11, 0) ← (0, 0, 0) and (0, 10, 0) ← (0, 0, 0) vibronic transitions of the à 2A″←X̃ 2A′ band system, and the effects of Renner—Teller rovibronic coupling to the dissociative X̃ state have been investigated. The most important observation is the enhanced spectral resolution that is obtained in D4WM spectroscopy, which results from the Lorentzian cubed line profile that is predicted for degenerate four wave mixing spectroscopy in the limit of extreme homogeneous broadening (stationary atom limit). We estimate the detection limit to be approximately 1 × 10 12 cm −3 quantum state −1.

Vibronic spectroscopy of amorphous materials using higher order, multiresonant coherent anti-Stokes Raman spectroscopya)

Multiresonant coherent anti-Stokes Raman spectroscopy is performed with three tunable lasers on perylene doped polymethylmethacrylate (PMMA). Sharp vibronic features can be observed in vibronic scans at constant energy from the parent electronic transition when resonance is established within the inhomogeneously broadened electronic band. These features are attributed to the nonlinear line narrowing predicted by Ouellette and Denariez-Roberge for a higher order saturated coherent anti-Stokes Raman process since line narrowing should be absent for four wave mixing coherent anti-Stokes Raman spectroscopy. It is shown that the features are sharply dependent on the presence of a simultaneous vibrational resonance as is also predicted by the higher order coherent anti-Stokes Raman model. Excited state coherent anti-Stokes Raman spectroscopy with resonance enhancement from higher singlet states does not contribute to the narrow features since such a process would not have vibrational resonances. Conventional two laser coherent anti-Stokes Raman shows only a weak line at the vibronic transition.

High resolution nonlinear laser spectroscopy

High resolution laser techniques were developed that address the problems listed above. The techniques are collectively called site selective laser spectroscopy. They relied upon the idea that a narrow band laser could be tuned to excite selectively an absorption line of a specific component or site within a sample so the resulting fluorescence spectrum came only from the site or component excited. One could simplify spectral congestion with this approach. One could also eliminate broadening that was caused by inhomogeneities in the samples because excitation within a broadened line would only excite components that had energy states resonant with the laser so the resulting fluorescence spectrum would not reflect the inhomogeneities. These methods were applied to matrix isolation, low temperature organic glasses, Shpol'skii systems, inorganic analysis using precipitates, and supersonic jet spectroscopy to measure a variety of inorganic and organic materials at ultra-trace levels. The methods all relied upon sample fluorescence and they failed if the samples were non-fluorescent. We have recently shown that there is a new family of high resolution laser spectroscopies that have the same capabilities but do not require a fluorescent sample. These spectroscopies are based upon nonlinear mixing where several tunable lasers are focused into a material and new frequencies are formed at all of the sums and differences of the original laser frequencies. This nonlinear mixing is resonantly enhanced when some of the laser combinations match resonances of components in the sample. The nonlinear mixing can be used to perform atomic spectroscopy and molecular spectroscopy. We will concentrate on molecular spectroscopy in this discussion. One can perform component selection by tuning the lasers to match resonances on one specific component in the sample. One would then expect to have that component contribute dominantly to the nonlinear mixing. One can also eliminate inhomogeneous broadening by tuming the lasers to match the resonances of specific sites within the inhomogeneously broadened line. Again, one would expect that those sites would contribute dominantly to the mixing and the nonresonant sites would be discriminated against. We have tested these ideas in several model systems using four wave mixing spectroscopy. The two model systems are pentacene doped into p-terphenyl crystals and pentacene doped into benzoic acid crystals where p-terphenyl was added in small amounts to introduce controlled amounts of inhomogeneous broadening. The experiments were done at 2 K to eliminate thermal effects. There are four schemes that one can use to establish resonances with the pentacene molecules. In all of them, one establishes resonances with the vibrational levels, the excited electronic states, and the vibrational levels of the excited electronic state (which we will call vibronic states). The four schemes differ in which states are involved in the resonance associated with the emitted light. If the emitted light involves transitions between two levels that are not initially populated, the technique is classified as a nonparametric process. If one of the levels were initially populated, the technique is a parametric process. Theories for the ideas predict that each possibility will have a different ability to provide selectivity in the measurement. The pentacene in p-terphenyl system was studied first. Pentacene has four different crystallographic sites in this crystal, some of which differ only slightly from each other. Conventional spectroscopy shows that the transitions from each

Four wave mixing spectroscopy of mixed crystals using three input frequencies

Unambiguous three input frequency scanning methods are described for pentacene in several host crystals, specifically defining ground or excited state vibrations. Implications for site selective experiments are discussed.

Line Narrowing Effect in Fully Resonant Four Wave Mixing Spectroscopy in Solids

We explain the utility of third-order nonlinear susceptibility, χ (3) (ω 2 ; ±ω 1 , \({\mp}\omega_{1}\), ω 2 ), for the observation of resonances with homogeneous widths from inhomogeneously broadened distributions in three-level systems related to Raman or two-photon allowed electronic transitions. The line narrowing and high resolution capabilities are demonstrated theoretically using a model for inhomogeneous distributions of transitions in which we regard an absorption spectrum as the composition of some inhomogeneously broadened substructures. The experimental methods to separate a signal having frequency ω 2 from ingoing beam at the same frequency are outlined.

Four wave mixing spectroscopy and additional boundary conditions: A proposal

A nonlinear spectroscopic method for the direct measurement of amplitude ratios among various polariton modes in spatially dispersive media is proposed and theoretically analyzed. In this method four wave mixing among three laser beams is used. Two beams have the same frequency higher than the longitudanal exciton, but different angles of incidence, and excite various polariton modes in the crystal. The third is the probe beam whose frequency can be chosen in the transparent region of the crystal. The probe beam is elastically diffracted from the grating generated by the interference between some appropriate pair of excited modes. The phase matching condition and hence the exit angle of the diffracted beam depend sensitively on the wave-vector of the grating. Thus the contribution from each polariton branch can be measured separately, by selectively tuning the angle of incidence of the probe beam; from the measured ratio the amplitude ratio, and hence the ABC can be obtained.