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Time-integrated and spectrally resolved four-wave mixing (FWM) has been applied to study the impact of the k-linear term on nonlinear optical response of the C-exciton manifold in ZnO. From the excitation wavelength dependence of the FWM signal, we have found possible evidence that the mixing of the Γ1 and Γ5L states leads to a three-branch excitonic polariton dispersion but the oscillator strength transfer is small. The energy separation between the lower polariton branch and the extra branch due to the k-linear term has been found to be 3.5 meV. In addition, a binding energy of biexcitons consisting of two C excitons of 1.4 meV has been obtained.

On the basis of an optical Kerr-gate method, we have constructed a high performance system for femtosecond spectral snapshots to measure ultrafast behavior of time-dependent spectrum of weak luminescence. Optical components and their configurations have been investigated to optimize the system. We have measured nonlinear refractive indices and two-photon absorption coefficients of cubic crystals and glasses by the z-scan technique in order to evaluate their performances for an optical Kerr gate. SrTiO3 has been found to be highly efficient and suitable as a Kerr-gate material. Simulations based on the Gaussian-beam propagation were performed to find appropriate configurations to obtain high time resolution. Observation for time-resolved luminescence spectra of β-carotene has revealed that time resolution of 200 fs is achieved in a wide range of wavelengths, which is limited by the pulse width of the gating light.

The factorized time correlation (FTC) diagram analysis of Raman induced Kerr effect spectroscopy from noisy light is presented. The complete set of FTC diagrams is given. This process requires a more sophisticated FTC diagram analysis than is typical because of the important role color-locking plays for this spectroscopy. A physical understanding of the terms that give rise to the slow decaying asymmetric interferograms seen in the experimental literature can be gleaned from these FTC diagrams. The interferograms carry information about the decay of the librational coherences.

The grating translation technique (GTT), which employs a momentary shift of light fringes during holographic recording, is a powerful tool for characterization of nonlinear materials. Earlier, it was used in the transmission geometry. We formulate the main relations of GTT for the reflection case. These allow us to determine in situ the real and imaginary parts of the coupling strength and to detect the formation of refractive-index and absorption gratings. We apply GTT to determine the photorefractive characteristics of two-wave coupling in z-cut iron-doped lithium niobate crystals for an extremely small grating spacing (Λ≃105 nm) and extremely large externally applied electric fields (up to 650 kV/cm). Stabilization of the light fringes, crucially important in the reflection geometry, has been achieved with the use of an electronic feedback loop. We prove that the feedback introduces a frequency detuning between the light beams and modifies strongly the nonlinear response.

The effect of collisions on the magnitude of polarization spectroscopy (PS) signals from the OH radical on the A 2Σ+–X 2Π (0,0) band has been studied. OH was produced by the 266-nm photolysis of H2O2 and detected by one-color degenerate PS, using ≈5-ns pulses from a Nd:YAG pumped dye laser. Spectra with both circular and linear pump polarizations are reported, together with signal dependence on OH number density and pump pulse fluence. The relative line intensities in the spectra and measured square dependence on OH number density are consistent with a description of PS as a variant of four-wave mixing spectroscopy. The pump pulse fluence dependence is fitted well by literature saturation curves. The collisional dependence of the PS signal was investigated by adding increasing pressures of He, Ar, or N2 collider gases for fixed overlapping pump and probe pulses. The principal finding is the very rapid loss of the PS signal with increasing collider pressure. The resulting phenomenological rate constants ar...

A novel two-dimensional (2D) circularly polarized (CP) pump–probe (PP) spectroscopy is theoretically studied and proposed. Utilizing circularly polarized pump field, one can measure both the left- and right-CP PP spectra in the 2D frequency space spanned by the pump and probe field frequencies. Although the dominant contribution to the measured signal is all electricdipole-allowed four-wave-mixing term, it can be removed by taking the difference between the left- and right-CP PP signals. Taking the first-order terms with respect to either magnetic dipole- or quadrupole-pump field interaction, we find that two distinctively different contributions to the third-order CP-PP polarization are important. However, by controlling the crossing angle between the pump and probe field propagation directions to be the magic angle, θ=tan−1(1/√), it is shown that the magnetic dipole–field interaction contribution to the third-order polarization can be selectively measured. It is observed that the relationship between th...

The broadband multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy in the frequency domain was used to study six-wave mixing processes of simple aromatic molecules such as benzene and toluene. The six-wave mixing CARS signals were observed to have similar spectral profiles to those of the four-wave mixing CARS signals, but the peak positions were located in the overtone and the combination regions. By examining the concentration dependence, the six-wave mixing CARS signals were attributed to the so-called parallel-cascading third-order process. It is shown that the parallel-cascading process is useful for differentiating the contribution of the inhomogeneous broadening to the spectral profile.

We carried out a wavelength-dependent homodyne and heterodyne transient grating (TG) studies on a resonant system, rhodamine 640 in dimethylsulfoxide, with a simplified diffractive optics method. The real and imaginary components of the third-order molecular polarization at different excitation wavelengths have a similar longtime decay (>200 fs), while the amplitude of the real component is strongly wavelength dependent. The nonresonant solvent and the resonant solute contributions to the third-order polarization are found to have a similar magnitude in a dilute solution. The interference of the wavelength-dependent resonant solute contributions with the nonresonant solvent contribution is found to be crucial to properly understand the wavelength dependence of the initial dynamics in homodyne TG and other four-wave mixing techniques. Such an interference effect is further confirmed by concentration-dependent homodyne TG measurements.

A method for directly monitoring two-beam coupling (2BC) in photorefractive materials is presented. This method allows accurate measurement of the gain and loss in the probe and pump beams, respectively, even in situations where substantial power is diffracted into higher order beams (i.e., for large grating spacings). Modulation of the pump beam at a frequency significantly greater than the grating decay rate, coupled with synchronous detection of the modulated probe beam signal, provides this information, automatically correcting for diffractive losses. The probe beam gain and pump beam loss are obtained without further correction. 2BC measurements are made on dye-doped photorefractive polymer-supported liquid crystal films to demonstrate the advantages of this method. The measurements are made for small grating spacings, in which little high-order diffraction is observed, and for large grating spacings for which there are several diffraction orders.

This work exploits the passive phase stabilization of diffractive optics to implement heterodyne detection of the complete χ(5) tensor of liquid CS2 as an example of a simple liquid. This approach permits the use of two different colors for the excitation, probe, and detection beam protocols and enables full optimization of the signal with respect to discrimination against lower order cascaded third-order responses. This work extends the previous study of polarization selectivity, in combination with heterodyne detection, to all independent polarization components to provide further insight into the origins of the fifth-order response and its connection to the multitime correlation of the liquid dynamics. The characteristic feature that clearly distinguishes the direct fifth-order response from lower order cascades is the pronounced ridge along the τ4 axis (probe pulse delay) with very rapid decay along the τ2 axis (excitation pulse delay). This observation is in contrast to recent related work using one-...

An effective core-exciton Hamiltonian is constructed for nitroanilines which includes 1s core hole transitions of both nitrogen atoms. The wavevector and frequency dependent third order susceptibility χ(3) is calculated and used to predict the frequency-domain pump–probe spectra which show both photobleaching and excited state absorption components. Signatures of electron delocalization and differences among the para-, meta-, and ortho-isomers are discussed.

The effects have been studied of increasing laser bandwidth and intensity on temperatures derived from broadband four-wave mixing (FWM) spectra. A simple model of optical dephasing arising from pumping of coupled transitions is presented and validated by experimental measurements. The model is found to predict qualitatively the effect on relative spectral intensity in broadband FWM of increasing bandwidth and input intensity. The implications for thermometry are discussed to indicate the circumstances in which accurate and precise temperatures may be derived from broadband FWM spectra.

A technique hatched from concepts in acousto-optics and phase conjugation could be ideal for biomedical imaging and therapy.

We investigate nonlinear wave mixing in two-level atoms wherein, using a dressed state formalism, it is shown that the signal arises as a consequence of quantum interference between relevant probability amplitudes for photon emission. We then report a theoretical study of the modifications that are induced in the four-wave mixing signals when the transition frequency of the atoms is stochastically modulated. Such modulations, which arise from perturbations in the immediate environment of the atoms, are shown to cause significant alterations in the nonlinear response of the atoms. We also describe how one can, via an investigation of the four-wave mixing signal, differentiate between Markov and non-Markov fluctuations in the transition frequency of the two-level system. The calculations rely on a recently developed Monte Carlo procedure that can be used to obtain exact solutions to a wide variety of spectroscopic problems, with minimal restrictions on atom, field, and noise parameters.

Using narrow-bandwidth vacuum-ultraviolet radiation generated by difference-frequency four-wave mixing in Xe, the O2 photoabsorption cross section in the Schumann–Runge system B 3Σu−←X 3Σg−, near the dissociation limit of the B state, is measured with a resolution of ∼0.1 cm−1 full-width at half-maximum. The rotational structure of B−X(22,0) is assigned for the first time, and evidence is found for transitions into the F1 levels of B(v=23). Experimental values for the triplet-splitting constants λ and γ for B(v=22) are found to be inconsistent with the rising trend observed for v⩽19, due to a change in the sense of the perturbation of the B-state levels by the C′ 3Πu levels near B(v=20). A new value for the dissociation energy of the B state, D(B)=57 136.2±0.3 cm−1, determined from an analysis of the observed and missing lines near the dissociation limit, is in excellent agreement with previous values arising from the observation of rotational thresholds in the Schumann–Runge continuum. © 2001 American In...

We have grown Bi12GeO20 and Bi12SiO20 single crystals from melts with different GeO2 and SiO2 contents, respectively, to investigate the influence of an intrinsic defect, the antisite defect (Bi on Ge or Si site), on the light-induced charge transport. The optical absorption and the effective trap density of the crystals increase with decreasing GeO2 and SiO2 content in the melt. Furthermore, a variation of the photoconductivity is observed. Our results can be described by a one-center model with the antisite defect Bi3+/4+ on Ge or Si site as dominant photorefractive center.

The photorefractive properties of nominally pure near-stoichiometric LiNbO3 crystals grown from congruent melts containing 11 mol % K2O are studied, and some photorefractive properties which are very different from those in congruent LiNbO3 crystals are observed. Large stationary energy transfer (Γmax=25 cm−1) exists in near-stoichiometric crystals though the photovoltaic field is dominant. The light-induced scattering fanning is markedly asymmetric in the crystals, which is very different from the symmetric fanning observed in congruent ones. Our analysis shows that these differences are caused by the large photovoltaic field and the relatively small density of filled traps in as-grown near-stoichiometric LiNbO3.

The abnormal behavior of CO E 1Π(v=0)←X 1Σ+(v=0) three-photon resonance enhanced multiphoton ionization (REMPI) spectroscopy is studied, including the anomalous band structures and adverse pressure effects on the spectral intensity. One- and two-color experiments with laser beams in co- and counterpropagations have been performed to elucidate the mechanisms. It is demonstrated that the abnormalities originate from two mechanisms, i.e., the reabsorption of the resultant third harmonic generation (THG) in the one-color case or four wave mixing sum frequency generation (FWSFG) in the two-color case, and the interference cancellation between the one-photon excitation channel induced by THG/FWSFG reabsorption and the three-photon direct excitation channel. The former mechanism dominates in the wing region of a resonant transition profile, while the latter is restricted within the kernel portion of the transition.

An improved phase demodulator for the detection of ultrasound based on two-wave mixing (TWM) in the diffusion regime in a semi-insulating photorefractive GaAs crystal is presented. A new optical layout is proposed in which the total laser power is injected into the crystal to reduce the grating buildup time. The device is then less sensitive to ambient vibrations or motion of the inspected part. Another new feature of the device is a balanced receiver based on large area InGaAs detectors. The measured rejection ratio of this balanced receiver is 50 dB. However, in this new optical configuration the signal and pump beam paths cannot be made equal which results in some sensitivity to high frequency laser phase noise. The performance of the device operated with a cw and a pulsed laser source is described. A comparison with the conventional confocal Fabry–Perot in the transmission mode is also presented. As expected, the GaAs-based TWM photorefractive system is less sensitive than the Fabry–Perot at high frequencies but becomes more sensitive below some frequency value. For the 30 W pumping level used into the crystal and a 1 m long Fabry–Perot with 85% reflectivity mirrors, this value was found to be about 1.5 MHz. This feature is known to be useful in the cases where low frequencies have to be detected, higher frequencies being attenuated by the material. This is the case of coarse microstructure materials or viscous or mushy materials. This photorefractive configuration is also attractive like all photorefractive systems for compactness and the absence of any need for active stabilization. To obtain a sensitivity equal or better than that of the confocal Fabry–Perot in the transmission mode, this new setup can be used with an InP:Fe or a CdTe:V photorefractive crystal under an applied field.

The field-dependence of the charge photogeneration efficiency reduces the holographic contrast in an organic photorefractive composite. In this article, the Braun model for field-dependent photogeneration efficiency is incorporated into the standard model of photorefractivity. Using this formalism, the effect on the space-charge field, and thereby holographic index contrast, is calculated directly without the need for empirically found parameters. The dependence of the space-charge field on experimental geometry is also analyzed. A powerful description of the effective space-charge field is thus derived and is experimentally verified. This analysis is then used to compare the observed holographic contrast to that predicted from transmission ellipsometry measurements.

The nonlinear cross talk between two photorefractive gratings recorded in a BaTiO3 crystal has been investigated. Besides the photorefractive response described by the material equations, our theoretical analysis also includes the effect of beam coupling through the coupled-wave equations. We show that beam coupling has a strong influence on the nonlinear cross talk in BaTiO3. Depending on the initial conditions, it may lead to an increase or decrease of the nonlinear grating cross talk. Moreover, our experimental results provide experimental evidence of nonlinear cross talk in BaTiO3. Significant cross talk values, up to ∼20%, have been found experimentally. Furthermore, the experimental data are in good agreement with the theoretical predictions. The results have relevant implications in practical situations such as sequential recording of holograms for holographic storage.

We have constructed a cw dye master oscillator/dye pulse-amplifier system that generates 4 ns, 100 mJ pulses with a repetition rate of 30 Hz. The output pulse has a bandwidth of 275 MHz. Backward-stimulated Brillouin scattering is used to control the growth of amplified spontaneous emission (ASE). The content of ASE in the final output is under our detection limit (

Optical nonlinearity and dynamic response of a novel inorganic metal cluster (μ3-MoSe4)Ag3(PPh3)3Cl were studied. Large optical limiting effect was observed with the inorganic metal cluster (μ3-MoSe4)Ag3(PPh3)3Cl dissolved in CH2Cl2. We also proved that nonlinear scattering contributes much to its optical limiting performance, which counts for nearly 75% of the total nonlinear energy loss used. The dynamic response of the cluster when pumped at high fluence was tested by a standard time-resolved pump-probe technique. The effective third-order susceptibility χ(3) for this sample was determined in a standard backward degenerate four wave mixing experiment with pump fluence lower than limiting threshold Fth. The conjugate signal intensity tends to saturate when the incident fluence rolls over this value (Fth) in spite of the fact that large optical limiting effect was observed in this fluence region, which implies that the nonlinearity mechanism is different for these two regions.

A differential equation and boundary conditions are derived for the amplitude of the first harmonic of the space-charge field in the case of a lossy thin photorefractive film with anisotropic conductivity that is illuminated by a moving interference pattern. It is shown that for certain values of the anisotropy parameter, detuning, and film thickness, a new resonance phenomenon may occur. This resonance is distinct from and additional to the one that relies on the agreement between the speed of the interference pattern and that of the space charge wave, and has been known for a long time.

Photorefractive damage (PRD) of LiNbO3 quasiphase matched (QPM) wavelength converters is studied by a novel two-beam method, in which the second-harmonic generation (SHG) tuning curve of a broadband probe beam is monitored. The QPM condition and wavelength conversion efficiency, which are characterized by the peak wavelength and peak intensity of the SHG tuning curve respectively, are investigated in detail with respect to the irradiation power and irradiation time. It is found that the QPM condition and wavelength conversion efficiency change markedly even for low irradiation powers (

In this article, we studied the photorefractive properties of Cu-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 (Cu:KNSBN) crystals with different doping levels. The properties include two-wave mixing and self-pumped phase conjugation, absorption properties, and the growth conditions as well. We also studied the effect of the dimensions on the photorefractive properties. The mechanism of Cu-doped KNSBN and the influence of the different sites occupied by Cu+ on photorefractive properties are also discussed. All the experimental results showed that doping levels have much effect on the photorefractive properties. The crystal with a higher doping level has a higher effective charge carrier density, a larger coupling coefficient, and a faster response rate, which is suitable for fast optical information processing. Dimension has much influence on the photorefractive properties. The thinner crystal with the same doping level has a larger coupling gain coefficient, and a shorter response time.

We experimentally demonstrate that the temporal shape of the four-wave mixing (FWM) signal in thick semiconductors is significantly influenced by the absorption of the generated FWM signal during its propagation through the sample as well as the pulse broadening effect, by performing the time-integrated (TI) and time-resolved (TR) FWM experiments, and by measuring the pulse shape of the transmitted probe beam in the presence of a pump beam for a 500- and a 100-μm-thick undoped GaAs. We find that the temporal shape of the TR-FWM signal in the vicinity of the exciton resonance depends on the sample thickness and time delay between two incident pulses. As the excitation laser is tuned far below the exciton resonance, however, the propagation effect of the FWM signal itself through the sample becomes dramatically weaker and the temporal shape of the FWM signal resembles the distorted probe pulse shape. Furthermore, the peak intensities of the TI- FWM signals near the exciton resonance are smaller in the thick...

We investigated highly predissociative transitions D 2Π–B 2Σ−(0-0) of CH in an oxyacetylene flame with two-color resonant four-wave mixing, in which two grating beams are in resonance with the D–B transition and the pump (and signal) beam is resonant with a selected B–X transition. A total of 86 predissociative lines in six branches are observed, corresponding to excitation of the D state up to N′=16; in previous experiments lines up to only N′=6 were observed. Observed rovibronic transitions of the D–B(0-0) band are fitted to known Hamiltonians to yield improved spectral parameters of the D 2Π (v=0) state. The parameters for centrifugal distortion (D,H) and Λ doubling (p,q,qD) of the D 2Π (v=0) state are deduced for the first time. The predissociative line width is about 2 cm−1 for the rotational level with N′=1 and it gradually increases with N′; for N′=15 the width is about 7 cm−1. Predissociation mechanisms of the D 2Π state are discussed. A barrier height ∼5600 cm−1 for the D state is estimated based...

Photorefractive barium-calcium titanate crystals with the congruently melting composition (Ba0.77Ca0.23TiO3) doped with iron are annealed in a hydrogen atmosphere at various temperatures. The samples are studied with electron paramagnetic resonance (EPR) and optical absorption, both performed separately as well as simultaneously, before and after illumination. The reduced samples are electron conductive and the charge transport at room temperature is governed by only one level identified by photo-EPR as Fe2+/3+. The optical absorption of Fe2+ is found to be peaked at 1.9 eV.

The effect of laser line shape and bandwidth on the signal detected in two-color, resonant four-wave mixing (TC-RFWM) spectroscopy is determined by means of an ab initio calculation of the third-order polarization based on diagrammatic perturbation theory. Modifications to the approach previously used for the case of δ-function laser line shapes are made by introducing a different treatment of the rotating wave approximation and phase-matching conditions. A three-level excitation scheme for double-resonance spectroscopy of bound and quasibound states is analyzed. In the case of Lorentzian laser line shapes, analytic expressions for the signal line profile are obtained for each excitation scheme. Analytic approximations of the signal line profile are also obtained in the case of Gaussian laser line shapes.

In Bi12TiO20 with an applied ac voltage the coupling for two-wave mixing can reach an extreme value at the transient moment tp when the internal electric field is zero. This is because of the changing polarization state of the pump wave during field reversal. The internal field is found to be the sum of the applied field and a time developing screening field arising from charge redistribution. Therefore, at the moment tp the amount of the screening field is equal to the value of the applied field and thus can be measured. Also tracking of the temporal course of the electric screening field is possible. The buildup of screening fields leads to an intensity dependence of tp for a given shape of the ac voltage.

Degenerate four-wave mixing in the sub-Doppler phase conjugate geometry was used to record high resolution spectra of the d3Πg-a3Πu (0-0) Swan band of C2 produced in an oxy-acetylene flame. The line positions of isolated transitions were measured to an accuracy of ∼3×10-3cm-1 and calibrated using a Fizeau interferometer system. The data obtained from these spectra was used to calculate rotational constants and lambda doubling parameters for the 3Π states from which the line positions for the whole band were calculated. Noticeable improvements between experimental and calculated line positions are seen when compared to previously published values. The effect of inaccuracies in line positions on the simulation of degenerate four-wave mixing spectra is discussed and some examples of the improvement in simulation using the newly calculated line positions are presented. © 1999 American Institute of Physics.

The theory underlying a six-wave mixing experiment is developed using the methods of molecular quantum electrodynamics. This general theory allows the intensity of the second harmonic radiation generated by the six-wave process to be found for arbitrary arrangements of the generating laser beams. Several different polarization geometries are treated in detail, and comparison is made to experiments performed using near-resonant conditions. The agreement is good in all cases and allows detailed information pertaining to the six-wave tensor to be extracted. The information thus obtained provides evidence of a marked departure from Kleinman symmetry.

We have demonstrated a novel optical interferometric technique for a highly scattering environment, using photorefractive coherence gating. A two-wave mixing process in a photorefractive barium titanate crystal effectively reduces scattering noise, and we have successfully detected interferometric signals through scattering media of up to 17 mfp, with a high visibility. We have also demonstrated a two-dimensional thickness measurement of an optically scattering thin film using our new interferometry.

Experimental results show that the wavelength selectivity and the shape of the detuning curve of the complex grating structure formed in a photorefractive self-pumped BaTiO3 phase conjugator depend strongly on the coherence length of the incident beam. It is observed that the wavelength selectivity becomes larger as the coherence length of the incident beam is decreased. With a coherence length much longer than the dimensions of the conjugator crystal a wavelength selectivity of 0.026 nm is measured. It is shown that this very small wavelength selectivity and the experimental detuning curves are in agreement with a simple model that takes into account the interference between light reflected in 2k gratings and light that has traveled one round trip in the internal loop of the conjugator crystal. The findings explain how the optical feedback generated by the dynamic gratings in the phase conjugator can increase the coherence length of a multimode laser.

Using a pulsed supersonic slit nozzle, the nonfluorescing π*←n transition of pyridazine was investigated. The degenerate four wave mixing (DFWM) spectra showed numerous vibrational bands over a 1200 cm−1 region. Most of these bands were parallel transitions with a strong Q branch and weaker but observable P and R branches. Based on our previous model [H. Li and W. Kong, J. Chem. Phys. 107, 3774 (1997)], these transitions were simulated with success. The polarization dependence of the rotational branching ratios suggested that primary contributions to the DFWM signal were from large spaced gratings formed by ground state molecules. The lack of contributions from excited state gratings and small spaced gratings was attributed to the fast internal conversion process on the S1 surface of pyridazine (0.3–3 ns), the wash-out time due to movements of the sample in a molecular beam, and the duration time of the excitation laser (7 ns). Two vibrational bands showed unexpected enhancement in the P or R branch, but ...

Accounting for both the long- and short-wavelength density gratings by considering the polarizations of all four electromagnetic waves parallel to each other and a finite tilt angle between the signal and pump waves, the steady-state phase conjugation in the low reflectivity regime by nearly degenerate four-wave mixing in a homogeneous plasma is investigated. The response of the density gratings caused by the beating of the signal and pump waves is considered from the theory of stimulated Brillouin scattering in a two-component plasma. The expression for the power reflectivity of the conjugate wave for the arbitrary difference frequency between the signal and pump waves and that for resonant four-wave mixing are obtained. Numerical results have been discussed in light of the experiment reported in the literature. It is noted that the measured values of the difference frequency corresponding to either density grating resonance, and the corresponding maximum reflectivity of the conjugate wave, provide a diagnostic tool for the electron temperature and the electron–ion temperature ratio, respectively.

Degenerate four-wave mixing (DFWM) spectroscopy is shown to provide a facile means for probing angular momentum (or rotational) anisotropy in nonequilibrated ensembles of gaseous molecules, with judicious selection of experimental conditions permitting quantitative determination of population distributions and Zeeman coherences for magnetic sublevels of the target species. A theoretical description of the nonlinear response induced under such circumstances is obtained by incorporating a state multipole expansion of the zero-order density operator into a perturbative (weak-field) treatment for the DFWM interaction. Aside from allowing the effects of incident field polarizations and phase-matching geometries to be considered in detail, this compact spherical tensor formalism provides guidelines for the extraction of spatial information from rovibronically resolved spectral data. Furthermore, these analyses have identified unusual polarization schemes that lead to signal generation only in the presence of ro...

Based on the two-center model and the multi-three-wave mixing model, we theoretically study the possible origins of the threshold effect of the incident light intensity for the photorefractive light-induced scattering in LiNbO3:Fe,M, (M=Mg2+, Zn2+, In3+) crystals. Our results show that the value of the threshold intensity of incident light is strongly dependent on the dark conductivity of the crystal, and the maximum total fanning noise is dependent on the concentration of Fe ion on the Li site. These two factors together determine the threshold behavior of incident light intensity for the photorefractive light-induced scattering in LiNbO3:Fe,M crystals when the incident light intensity is in moderate range.

A sensitive near-resonant four-wave mixing technique based on two-photon parametric four-wave mixing has been developed. Seeded parametric four-wave mixing requires only a single laser as an additional phase matched “seeder” field is generated via parametric four-wave mixing of the pump beam in a high gain cell. The seeder field travels collinearly with the pump beam providing efficient nondegenerate four-wave mixing in a second medium. This simple arrangement facilitates the detection of complex molecular spectra by simply scanning the pump laser. Seeded parametric four-wave mixing is demonstrated in both a low pressure cell and an air/acetylene flame with detection of the two-photon C 2Π(v′=0)←X 2Π(v″=0) spectrum of nitric oxide. From the cell data a detection limit of 1012 molecules/cm3 is established. A theoretical model of seeded parametric four-wave mixing is developed from existing parametric four-wave mixing theory. The addition of the seeder field significantly modifies the parametric four-wave m...

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.

Reflection by the thermal phase grating formed in methanol containing small Pd metal particles has been measured by using a degenerate four‐wave mixing optics equipped with a nanosecond Nd:YAG laser and a streak camera. Although the integrated intensity of the reflection depended on the cubic power of the laser intensity, the temporal profile of the reflection in a pulse duration showed the delayed response, the theoretical profile of which has been formulated by considering the formation of thermal phase grating. The theoretical calculation based on heat conduction equation was in good agreement to the experimental results with respect to the shape of the profile, the intensity, and the polarization dependence.

We report the exploitation of a one-laser-shot measurement technique to study photo-induced effects in semiconductor-doped glasses. The technique, based on the use of a phase object in a 4f coherent imaging system, allows one to characterize the kinetics of the nonlinear refractive index of samples versus the light exposure time.

We have experimentally investigated the linear and nonlinear characteristics of AlGaAs multimode waveguides. By varying the angle of incidence and∕or the intensity of the input beam, it is possible to change the multipeak intensity distributions at the output facet in a controllable fashion. Numerical simulations have reproduced the main features of the intensity profiles observed at the waveguide output over a range of intensities and angles.

We report on photorefractive polymer composites that exhibit stable dynamic properties under continuous operation. These materials are based on a bis-triarylamine side-chain polymer matrix with a low ionization potential. The evolution of the response time for exposures up to 4 kJ/cm2 was studied and compared with that obtained in poly(n-vinylcarbazole) (PVK) based composites. In the composites, operational stability is combined with video-rate compatible dynamics, large dynamic range at moderate fields, and long shelf lifetimes.

A family of photorefractive polymer composites has been developed that enable high-performance device operation at a wavelength of 975nm. This constitutes a major extension into the near-infrared spectral region for the operation of all-organic photorefractive devices. Utilizing our photorefractive materials, we demonstrate large net two-beam coupling gain of more than 100cm−1, 60% diffraction efficiency in four-wave mixing experiments, and a fast response time of 33ms, at an irradiance of 1W∕cm2.

The characteristics of nondegenerate four-wave mixing (FWM) in an electroabsorption modulator are reported. The input power dependence of FWM intensity agrees well with theoretical results based on third-order nonlinearity over a wide input power range of 16dB. The detuning characteristics of FWM have a −6dB∕octave response based on the carrier-density pulsation effect. The experimental results show that a higher reverse voltage brings about a faster carrier recombination. The asymmetricity of positive and negative detuning is the reverse of that of the semiconductor optical amplifier and is explained through a rate equation simulation.

We propose a scheme of multiplex nonresonant pump four-wave mixing (NRP-FWM) process which is highly immune to saturation problems. The process is accomplished with two high-intensity pump beams detuned far from the resonance of the sample under investigation and a resonant probe beam as weak as possible not to give rise to nonlinear absorption. Only a single-broadband probe beam is used for the multiplex experiment to detect the electronic swan band spectrum of C2 molecules in a premixed acetylene–oxygen flame. Comparing the signal of the degenerate four-wave mixing (DFWM) experiment on the same sample, we found that the signal level at which the NRP-FWM starts to show saturation is about 250 times higher than in the case of the DFWM.

We report full-field, retroreflective holographic imaging through turbid media using a photorefractive polymer composite as a coherence gate. A four-wave mixing geometry was used to record and reconstruct two-dimensional images of test objects through 6.5 scattering mean-free-paths in real-time. Images with a transverse spatial resolution better than 42μm were acquired in a few seconds using a 6.3mW helium neon laser at 633nm. The photorefractive devices used are based on a poly (N-vinylcarbazole) (PVK):2,4,7-trinitro-9-fluorenone dimalenitrile (TNFDM) charge transport network, doped with the electro-optic chromophore 1-(2′-ethylhexyloxy)-2,5-dimethyl-4-(4″-nitrophenylazo)benzene (EHDNPB).

Conversion of digital- and analog-modulated optical signals from the 1550nm band to the 1300nm band is demonstrated in silicon waveguides. The conversion is based on parametric Stokes to anti-Stokes coupling using the Raman susceptibility of silicon.