Four-wave Mixing Technique Research Articles

Isomerization Dynamics of 1,1’-Diethyl-4,4’-Cyanine (1144C) Studied by Different Third-Order Nonlinear Spectroscopic Measurements

The isomerization of 1,1’-diethyl-4,4’-cyanine (1144C) in two solvents, ethanol and hexanol, was studied by three different third-order four-wave mixing techniques: three-pulse photon echo peak shift (3PEPS), transient grating (TG), and pump−probe (PP) to provide complimentary information on different aspects of the reaction dynamics. A double-sided Feynman diagram analysis was used to assist in the formulation of the kinetic behavior in the TG and PP signals. The ground-state recovery process can be described as a single exponential process which is dominated by the bond-twisting motion on the excited-state surface. The rate is strongly viscosity and temperature dependent. A larger yield of the isomer is obtained in less viscous solvents or with greater excess excitation energy. Apparently, contradictory wavelength-dependent decay behavior was found in the TG and PP signals. This can be understood by considering the different real and imaginary contributions to the signals from hot ground-state absorpti...

Novel nonlinear optical organic materials: Dithienylethylenes

In this paper, we report the measurements of the third-order susceptibilities of new dithienylethylenes in solutions using the degenerate four-wave mixing technique at λ=532 nm in the picosecond regime. From these measurements we deduced the second-order hyperpolarizabilities γ values, which are about 103 larger than the γ value of CS2 and are comparable with the corresponding values for the new synthesized diphenylpolyene derivatives. We also determined the optical power limiting properties of the mentioned compounds using nanosecond laser pulses in the visible range. The dispersion of nonlinear absorption coefficients is obtained in the framework of Gaussian collimated beam propagation in sample. Kerr ellipsometry measurements are responsible for the excited absorption cross section and are spectrally shifted comparing with the optical limiting processes (up to 50 nm). The appropriate quantum chemical calculations are done. The contributions of electronic and vibration subsystems as well as contribution of the particular parts of each molecule into the output third nonlinear optical response are given.

Nonlinear transportgoverned enhancement of electro-optic nonlinearity by a strong-microwave field

Nonlinear hot carrier transport features and relatedoptical nonlinearities are investigated under nonuniformillumination and carrier heating in microwave (mw)-biased high-resistivity GaAs crystals. The dynamics of light-interferencepattern-induced nonequilibrium carriers and evolution of internal electric field are studied in mw fieldsextending above a region of negative differential conductivity.We analysed the peculiarities of nonlinear transport in a spatiallymodulated structure at various photoexcited carrierconcentrations, which was kept below its threshold for mw-induced high-field Gunn-domain grating formation. Atlow-illumination levels, we found a three-fold increase in thespace-charge (SC) field due to electron gas heating. At higher lightintensities, enhancement of the internal field is determined bya fast screening of the external mw field and the transportof nonuniformly heated carriers. Hot carrier transport effectsin a region of negative differential conductivity, even atnearly homogeneous but high illumination, lead to SC-wave formation with amplitudes, which may exceed the diffusiveSC-field amplitude by 100 times. The latter effectwas confirmed experimentally using a four-wave mixingtechnique in mw-biased GaAs crystals and short laserpulses.

Comparative Study of Degenerate Four-Wave Mixing and Laser-Induced Fluorescence Techniques as Diagnostic Methods for Laser Ablation Plume

The degenerate four-wave mixing (DFWM) technique has been applied for the first time to the diagnostics of the laser ablation plume in order to overcome some difficulties encountered in the laser-induced fluorescence (LIF) diagnostic method, such as the interference with the intense plasma background light from the plume, the self-absorption and the collisional quenching in LIF signals due to the high density of the plume. A comparative study between the LIF and DFWM techniques is presented.

Ultralong dephasing time in InGaAs quantum dots.

We measure a dephasing time of several hundred picoseconds at low temperature in the ground-state transition of strongly confined InGaAs quantum dots, using a highly sensitive four-wave mixing technique. Between 7 and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian line shape with a lifetime-limited zero-phonon line and a broadband from elastic exciton-acoustic phonon interactions.

Measurement of Zero Dispersion Wavelength in an Optical Fiber Using the Oscillatory Behavior of Four-Wave Mixing Efficiency

Non-destructive measurement of zero-dispersion wavelength variation in a dispersion shifted fiber by four-wave mixing technique is carried out. The oscillatory behavior of the four-wave mixing efficiency is utilized for the measurement of the linear dispersion slope and zero-dispersion wavelength. A simple formula useful for engineering estimation of the characteristics of fiber four-wave mixing efficiency is presented.

Synthesis and third-order optical nonlinearity of some salts comprising planar pomplex cation and anion

Third-order optical nonlinearity of some new salts, which consist of planar complex cation [Cu(L)(2)](2+) (L=2,2 ' -bipyridine or phenanthroline) and planar complex anion [M(mnt)(2)](2-) (M = Ni, Co, Cu or Pd: mnt = maleonitriledithiolate). were studied by degenerate four-wave mixing (DFWM) technique. It was found that these compounds exhibit large non-resonant nonlinearity at 1064 nln (gamma (1111) in the order of 10(-31)-10(-30) esu). and that among the molecules of the same series. the molecule with metallic ion of larger atomic number possesses larger nonlinear hyperpolarizability gamma (1111). The results also indicate that the third-order optical nonlinearity of these salts mainly comes from the part of complex cation.

Stimulated effects in one-photon resonant interferometric four-wave mixing with incoherent light

The interferometric four-wave mixing technique with broadband light is extended to measurements of population relaxation. For what is believed to be the first time, the population relaxation rate in the presence of a pump field is measured. It is shown that stimulated emission can significantly affect the results when one is dealing with resonant transient wave mixing, and, consequently, in most cases stimulated processes should be taken into account.

ULTRAFAST NONLINEAR PROCESSES IN QUANTUM-DOT OPTICAL AMPLIFIERS

Ultrafast gain dynamics in quantum-dot optical amplifiers has been studied by using the pump-probe and four-wave mixing (FWM) techniques. It was found that there are at least three nonlinear processes, which are attributed to carrier relaxation to the ground states, phonon scattering, and carrier capture from the wetting layers into the quantum dots (QDs). The relevant time constants were evaluated to be ∼90 fs, ∼260 fs, and ∼2 ps, respectively, under a 50 mA bias condition. The compressed gain recovered to 3% of its initial value in 4 ps, and no recovery component slower than 2 ps could be seen in the temporal range tested. This is quite different from the feature in quantum wells, where a very slow component (> 50 ps) exists. This suggests a possibility of enhancing the operation speed of semiconductor optical amplifiers by using QDs as an active layer. The third-order optical susceptibility (χ(3)) has been evaluated by means of both nonlinear transmission and FWM experiments. The results show that the nonlinearity expressed by χ(3)/g 0 is quite similar to that of bulk and quantum wells, which can be explained by the similar relaxation times.

Femtosecond photon echo and virtual echo measurements of the vibronic and vibrational coherence relaxation times of iodine vapor

Femtosecond three-pulse four-wave mixing (FWM) techniques are used to sort and measure the processes that contribute to coherence relaxation. We compare the observed relaxation times for vibronic coherence using photon echo (PE) and reverse transient grating (RTG) measurements at different temperatures to isolate inhomogeneous and homogeneous components. Different pulse sequences are used to select ground or excited state vibrational coherences. Measurements of ground and excited state wave packet spreading times due to anharmonicity, a process that does not involve energy dissipation or phase relaxation, are also presented.

Novel materials from bis(arene)metal-containing polyacrylonitrile

Novel bis(arene)metal-containing polyacrylonitrile materials have been prepared by the polycyanoethylation reaction between acrylonitrile and (arene) 2 M (M = Cr or V; arene = PhH, C 6 H 4 Et 2 or mesitylene) in the absence of solvent. The resulting star-shaped molecules consist of a central (arene) 2 M species with up to four polyacrylonitrile arms covalently bonded to the arene ligands. The materials are readily soluble and films can be cast from solutions in acetonitrile. The IR and solid state 13 C NMR spectra (or EPR spectrum for the oxidized chromium-containing polymer) are consistent with the presence of a metal-arene bond and confirm the persistence of the sandwich structure. The properties of the thermolysed materials are consistent with the formation of conjugated naphthyridine-type structures. The value of |n 2 | determined by the degenerate four-wave mixing technique at 1064 nm with a 6 ns pulse duration for a solution in conc. H 2 SO 4 (1 g l -1 ) of the chromium-containing polymer pyrolysed at 350 °C was found to be 0.8 × 10 -13 cm 2 W -1 corresponding to |Re X (3) | = 0.4 x 10 -11 esu.

The 3rd-order nonlinearity of bacteriorhodopsin by four-wave mixing

The 3rd-order nonlinear optical susceptibility X(3) and the response time of the light-transducing biomolecule bacteriorhodopsin were measured with the four-wave mixing technique and a picosecond frequency-doubled Nd: YAG laser (532 nm). TheX 3) and the response time measured are 10-9 esu and 20 ps, respectively. The possible mechanism for generating the 3rd-order nonlinear optical susceptibility X(3) and response time were discussed.

Characterization of Proton-Irradiated InGaAs/GaAs Multiple Quantum Well Structures by Nonresonant Transient Four-Wave Mixing Technique

Effects of proton irradiation and thermal annealing on free carrier lifetime and transport in a InGaAs/GaAs multiple quantum well (MQW) structure and its substrate have been studied by using the nonresonant transient four-wave mixing technique. Under the excitation of the surface region of the structure by 0.53 µm wavelength, we determined the carrier lifetime τ=150–600 ps, bipolar diffusion coefficient D=24–26 cm2 s-1, and mobility-lifetime product µτ≈(2–6)×10-7 cm2 V-1 in 3 MeV proton irradiated and annealed MQW samples. At excitation by 1.06 µm wavelength, we studied the full proton penetration range, including the substrate. We found an unexpected twofold increase in carrier bulk lifetime with increasing irradiation dose. Features of this effect at low photoexcitation and its absence at high photoexcitation are attributed to carrier separation by potential fluctuations in the vicinity of irradiation-created deep defects. These features indicate that proton-irradiated semi-insulating GaAs exhibits properties of mesoscopic pointlike defects, similar to those previously reported in nonstoichiometric crystals of GaAs. This result is important to understand the mechanism of carrier compensation in proton-irradiated semiconductors.

Nonlinear optical properties of GaSb and GaInAsSb and their application for phase conjugation in degenerate four-wave mixing

The nonlinear optical properties of, and phase conjugation in, the bulk semiconductor GaSb were investigated at 2.1 µm by use of the Z-scan and the degenerate four-wave mixing techniques. Measurements were also carried out near the fundamental bandgap of the quaternary compound Ga0.87In0.13As0.11Sb0.89. Z-scan measurements as a function of sample temperature, in conjunction with theoretical modeling, identified the predominant sources of the medium nonlinearity to be nonequilibrium free carriers generated through two-photon absorption in GaSb, while it is saturation of real transitions near the fundamental band edge in Ga0.87In0.13As0.11Sb0.89 that was identified as the primary cause of this nonlinearity. Degenerate four-wave mixing phase-conjugate reflectivity of as much as 14% has been achieved in GaSb.

Coherence Switching in a Four-Level System: Quantum Switching

Dark resonance switching among three-laser interactions in a four-level system is observed by using an enhanced nondegenerate four-wave mixing technique. This coherence switching mechanism is based on simultaneous suppression and enhancement of two-photon absorption and has a novel application to high-speed optical switches.

Subpicosecond degenerate four-wave mixing and optical Kerr effect of organic dyes in excited states

The third-order optical nonlinearities of electronically excited states were measured for three liquid scintillation dyes such as bis-MSB by means of subpicosecond degenerate four-wave mixing (DFWM) and optical Kerr effect (OKE) techniques. Molecular hyperpolarizability γ DFWM of the first excited singlet state of bis-MSB in tetrahydrofuran was estimated to be ca. 7×10 −56 A s m 4 V −3 at 780 nm. The enhancement factor of bis-MSB on UV excitation was as large as ca. 1.5×10 4. Temporal profiles of DFWM signal were measured with time resolution of 0.3 ps, and were found to consist of two components, the coherent instantaneous nonlinear response and the delayed response with decay time constants of ca. 9 ps. Though the profiles of OKE lacked or had very little coherent instantaneous component, delayed response had decay time constants almost same as those of DFWM. This delayed response can be explained in terms of molecular reorientation, the laser beam-induced reorientation of molecules in the direction of the laser beam polarization.

Study on dynamic optical nonlinearities of excited state proton transfer in ethanol solution of 7-hydroxyquinoline

The dynamic process of 7-hydroxyquinoline with excited state proton transfer has been studied by the pump–probe method in which a three-dimensional degenerate four-wave mixing technique is employed for the probe. The molecular hyperpolarizability and lifetime of the excited state involved in the excited state proton transfer process of 7-hydroxyquinoline are obtained in our experiment. The decay time of the cation excited state is 35 ps and the decay time of the anion excited state is 250 ps. The molecular hyperpolarizability of 7-hydroxyquinoline in the enol form ground state, cation excited state and anion excited state are 1.3±0.2×10 −31, 8.0±1.0×10 −31 and 5.8±0.8×10 −30 esu, respectively.

Nonlinear Optical Responses of Spin-Coated Vanadium Oxide Films

AbstractWe report on the third-order nonlinear optical responses of V2O5 films prepared by pyrolysis of spin-coated films of vanadium 2-ethylhexanoate. Temporal behavior of third-order optical nonlinearity of the spin-coated V2O5 films was examined at a wavelength of 532 nm by the degenerate four-wave mixing (DFWM) technique using a mode locked Nd/YAG laser (35 ps fwhm). Fast relaxation with a lifetime of 25 ps and slow relaxation with a lifetime of 150 ps were observed. The nonlinear transmission experiment on the spin-coated V2O5 films using the same laser source showed that the transmittance decreased with increasing intensity of the incident light. The intensity-dependent absorption coefficients (β) were roughly estimated to be 3.5×10−5 cm/W. Transient absorption properties of the V2O5 film were measured by use of the second and third harmonics of a Q switched Nd/YAG laser (10 ns fwhm). The results observed for the nonlinear transmission and the transient absorption suggest that induced absorption would contribute to the third-order optical nonlinearity of the V2O5 films with fast response times, and also suggest that the V2O5 films have potentiality for use as optical power limiting materials.

Contactless Characterization of Porous Silicon Structures by Four-Wave Mixing and Microwave Techniques

We apply optical contactless techniques, namely the four-wave-mixing and microwave harmonics generation for the characterization of nanocrystalline free-standing films and platelets of microcrystalline porous silicon. We observe (i) full carrier localization and significant lifetime shortening in free-standing films, which is thought to be a manifestation of their low-dimensional confinement, (ii) increased carrier lifetime in microcrystalline porous silicon, presumably originating from passivated surface states at the surface of pores.

Third-order optical nonlinearities of near-infrared dyes

Near-infrared dyes – di- and tri-indocarbocyanine dyes and nickel dithiolenes – were synthesized and their third-order optical nonlinearities in solutions were investigated by using the degenerate four-wave mixing technique. Large third-order nonlinear optical susceptibilities with γ values of up to 10 −29 esu were observed. The γ value of the indocarbocyanine dyes increases with increase of conjugation length of the polymethine chain. The central ring structure in the chromophore lowers the γ value of the indocarbocyanine dyes. With change of the substituents on the benzene ring an increase in γ value is observed for all the asymmetric nickel dithiolenes. The order of increase of nonlinear hyperpolarizability is as follows: H<Cl<CH 3<C 2H 5<CH(CH 3) 2.