Time-resolved Optical Techniques Research Articles

Ultrafast third-order optical non-linearity of 0.56GeS 2–0.24Ga 2S 3–0.2KX(X = Cl, Br, I) chalcohalide glasses by femtosecond Optical Kerr Effect

0.56GeS 2–0.24Ga 2S 3–0.2KX(X = Cl, Br, I) chalcohalide glasses were prepared and their third-order optical non-linearities χ (3) have been studied systematically using the femtosecond time-resolved Optical Kerr Effect technique at wavelength of 800 nm. In this system, 0.56GeS 2–0.24Ga 2S 3–0.2KCl glass shows the largest χ (3) (1.82 × 10 −13 esu), but 0.56GeS 2–0.24Ga 2S 3–0.2KBr glass has the fastest optical non-linear response time in subpicosecond domain (about 340 fs), which is due to the ultrafast distortion of the electron clouds surrounding the balanced position of Ge, Ga, S, K and Br atoms. The local hyperpolarizability determining non-linear optical response are determined by the partially ionic bonds originating from Ge(Ga)–S bonds and halogen valence bonds. They show great potential applications on the glass-based optoelectronic devices like optical switching.

Bundling influence on ultrafast optical nonlinearities of single-walled carbon nanotubes in suspension and composite film

Nonlinear optical characteristics of single-walled carbon nanotubes (SWCNTs) dispersed in dichlorobenzene and imbedded in polymer were investigated at 800 nm using the time-resolved optical Kerr gate technique. For systematic study of the influence of SWCNT bundling on optical nonlinearities, SWCNT solutions with different concentrations and a series of SWCNT/polymer composites deposited on glass substrates with different concentrations and thicknesses were prepared. The nonlinear response was comparable to the pulse duration of the laser used (∼90 fs) both in SWCNT solutions and SWCNT/polymer composites. Over three orders of magnitude enhancement was observed in the third-order nonlinear susceptibility of SWCNT/polymer composite film compared with that of SWCNT solution. An appreciable reduction of microscopic and macroscopic nonlinearities was observed with increasing SWCNT concentrations due to stronger bundling of SWCNTs.

Ultrafast polarization and magnetization response of multiferroicGaFeO3using time-resolved nonlinear optical techniques

We have investigated the ultrafast electric and magnetic response of multiferroic ${\text{GaFeO}}_{3}$ induced by irradiation of a femtosecond laser pulse. By using the polarization selection rules of crystallographic and magnetic second-harmonic generation, the dynamics of electric polarization $P$ and toroidal moment $T$ were simultaneously and separately detected on a femtosecond time scale. The magnetization dynamics was derived by comparing the dynamics of $P$ and $T$. We demonstrate the availability of time-resolved nonlinear optical techniques for the dynamical probe of multiple degrees of freedom in multiferroics.

Annealing effect on the ultrafast dynamics of Ag nanoparticles embedded in soda-lime silicate glasses

Ag nanoparticles embedded in soda-lime silicate glasses were fabricated by the ion-exchange technique. Effects of thermal treatment on the optical nonlinearity and ultrafast dynamics of Ag nanoparticles were investigated by applying time-resolved optical Kerr effect and pump-probe techniques. The results indicate that thermal treatment is an efficient method to improve the nonlinear optical performance of this kind of material.

Ultrafast polarization and magnetization dynamics in a multiferroic GaFeO3

We have investigated the ultrafast dynamical behaviors of the electric polarization and the magnetization in a multiferroic GaFeO3. We show that the time-resolved nonlinear optical technique is a powerful tool for the probe of the ultrafast dynamics of multiferroic properties.

Optical spectroscopy of carrier relaxation processes in Si nanocrystals

We have investigated thermalization, trapping and recombination processes of carriers in Si nanocrystals embedded in a SiO 2 matrix. The study has been performed using time-resolved optical techniques of photoluminescence and induced absorption, allowing for 17 ps and 150 fs resolution, respectively. Based on these results, the possible relaxation processes are discussed. Special attention is paid to cooperative processes circumventing the effect of phonon bottleneck theoretically expected for Si nanocrystals.

Signature of the Overhauser field on the coherent spin dynamics of donor-bound electrons in a single CdTe quantum well

We have studied the coherent spin dynamics in an oblique magnetic field of electrons localized on donors and placed in the middle of a single CdTe quantum well, by using a time-resolved optical technique: the photo-induced Faraday rotation. We showed that this dynamics is affected by a weak Overhauser field created via the hyperfine interaction of optically spin-polarized donor-bound electrons with the surrounding nuclear isotopes carrying non-zero spins. We have measured this nuclear field, which is on the order of a few mT and can reach a maximum experimental value of 9.4 mT. This value represents 13 % of the maximal nuclear polarization, and corresponds also to 13 % of maximal electronic polarization.

Intra-molecular mobility of charge carriers along oligogermane backbones studied by flash photolysis time-resolved microwave conductivity and transient optical spectroscopy techniques

Time-resolved microwave conductivity (TRMC) measurement has been performed for fullerene-doped thin films of oligo (dimethylgermane) at different excitation energies to evaluate the intra-molecular mobility of holes along their Ge backbones. Photo-induced electron transfer reaction between oligogermane and fullerene has been observed in the solution with a variety of solvent polarity using transient optical spectroscopy (TOS). The transient spectrum at 391 nm can be attributed to the radical cation of the oligomer under an excitation at 532-nm light, whereas the same spectrum (391 nm) is the overlapping of absorptions of radical cations and neutral radicals of oligogermanes upon exposure of 355-nm light in polar solvent. A combined TRMC and TOS experiments on the solutions of oligomer confirms the conductive transients originate from the radical cations on the backbone chains.

SbS 3 enhanced ultrafast third-order optical nonlinearities of Ge–S chalcogenide glasses at 820 nm

GeS 2 based multi-component chalcogenide glasses, Ge–S–Sb(Cd), were prepared in series. With femtosecond time-resolved optical Kerr gate technique, we investigate their ultrafast third-order nonlinear optical (NLO) properties. The strongest NLO response comes from 0.70GeS 2–0.30Sb 2S 3. Its χ (3), 1.8 × 10 −12 esu, is more than three times to GeS 2 glass. Their instant NLO responses are within 120 fs pulse duration. We suggest that lone-pair electrons from highly polarizable Sb atom in [SbS 3] structure unit play an important role for the enhanced NLO response. This enhancement is much stronger to the contribution from S–S covalent bond.

Femtosecond time-resolved dynamics in electron-doped high-Tc superconductor La2-xCexCuO4

The quasiparticle dynamics in the electron-doped high-Tc superconductor La2-xCexCuO4 (LCCO) was investigated by ultrafast time-resolved optical pump-probe technique. We present results in three temperature regions: namely the low temperature (TTc) region, that around Tc(0.7Tc≤T≤Tc) and the high temperature (T>Tc) region. In general, there was a slowdown of the relaxation as the temperature decreases. However, the quasiparticle lifetime was found to exhibit a quasi-divergence as the temperature approached Tc from below. The relaxation time in normal state was much longer than what was expected for metallic relaxation, indicating the presence of a T-independent pseudogap. We have also observed picosecond rise component of photoinduced change in reflectivity ΔR/R in the electron-doped LCCO, which is attributed to the Cooper pair-breaking dynamics. The rise time became shorter as the temperature increased. The experimental results are analyzed by Rothwarf-Taylor model.

Direct imaging of carrier motion in organic transistors by optical second-harmonic generation

Interest in the dynamic behaviour of carriers in organic materials is motivated by possible applications that include organic thin-film transistors, organic electro-luminescent devices and organic photoconductors. It can also provide an insight into the modelling of carrier transport and trapping in organic semiconductors and insulators. Here, we use an advanced second-harmonic generation technique to probe and visualize real carrier motion in organic materials. This is a time-resolved microscopic optical second-harmonic generation technique that allows direct and selective probing of dynamic carrier motion in organic materials. Experiments making use of this technique and using pentacene field-effect transistors have revealed dynamic changes of second-harmonic-generation intensity profiles arising from pentacene. Carrier velocity in organic solids is thus determined from the visualized carrier motion. We anticipate that this direct visualization technique will find wide application in the illustration of space-charge field formation in organic and inorganic materials, including biomaterials and polymers.

Ultrafast third-order optical nonlinearities of vertically-aligned ZnO nanorods

Vertically-aligned ZnO nanorods with different rod diameters of 50 nm, 110 nm and 240 nm were synthesized by a low-temperature wet-chemical method. Morphologies and structures of ZnO nanorods were examined by scanning electron microscopy and X-ray diffraction. Third-order optical nonlinearities such as nonlinear refraction and absorption were determined by closed- and open-aperture z-scan measurements at 815 nm using femtosecond laser pulses. Enhanced optical nonlinearities were observed with increasing rod diameters and aligned degrees of ZnO nanorods along the c-axis. Nonlinear response time, measured by using a time-resolved optical Kerr-gate technique, was less than 95 fs.

Generation and detection of coherent longitudinal acoustic phonons in the La0.67Sr0.33MnO3 thin films by femtosecond light pulses

The authors report on an anomalous first-to-zero sound crossover in the colossal magnetoresistance compound La0.67Sr0.33MnO3 (LSMO) using ultrafast time-resolved optical techniques. Two-color pump-probe setup was employed to record the photoinduced reflectance oscillations due to excitations of longitudinal acoustic phonons. By changing the sample thickness and probe wavelength, they determine the longitudinal sound velocities of both the thin film and substrate . The values of the sound velocity in LSMO at gigahertz frequencies are more than 20% larger than those from ultrasound experiments in the megahertz regime near TC. The results show the possible existence of dynamic polaron correlations in LSMO.

Transport properties of ground-state aluminium atoms in hollow cathode gas discharge plasma

For investigation of Al atoms transport properties a pulse-periodic longitudinal hollow cathode gas discharge in cylindrical geometry is used. The cathode represents a 20 cm in length and 1.8 cm in inner diameter hollow aluminium cylinder. During the discharge Al atoms are sputtered from the wall into the cathode volume, while in the afterglow phase their concentration decreases due to diffusive losses back to the wall surface. The diffusive loss rates at different buffer gas pressures are measured by using a double-channel time-resolved optical absorption technique with gated photon counting. By relating the experimental loss rates with a diffusion model based on the experimental geometry and a boundary condition of the third kind, the diffusion coefficient of ground-state Al atoms in argon, as well as the reflection coefficient of these atoms from the cathode surface are obtained. The experimental value for the diffusion coefficient at 313 K temperature and 133 Pa argon pressure is D0 = 160 ± 30 cm2s−1. The reflection coefficient was found to be in the range: 0.2 < ρ < 0.82. The result for the diffusion coefficient is compared with theoretical results based on 12-6 Lenard-Jones and rigid-sphere inter-atomic interaction model.

Large and Ultrafast Third-Order Nonlinear Optical Propertiesof Ge-S Based Chalcogenide Glasses

We report ultrafast third-order nonlinear optical (NLO) properties ofseveral chalcogenide glasses GeSx (x = 1.8, 2.0, 2.5)measured by femtosecond time-resolved optical Kerr gate technique at820 nm. The third-order nonlinear susceptibility of GeS1.8 glassis determined to be as large as 1.41×10−12 esu, which is themaximum value of the third order nonlinear susceptibility χ(3)for the three compositions investigated. The symmetric Gauss profiles ofoptical Kerr signals reveal the nature of ultrafast nonlinear responseof these samples, which are originated from the ultrafast polarizationof the electron clouds. By detailed microstructural analysis of theseglasses based on the chain-crossing model (CCM) and therandom-covalent-network model (RCNM), it can be concluded thatχ(3) value of GeSx glasses can be enhanced greatly by S–Scovalent bonds or S3Ge–GeS3 ethane-like units.

Acoustic phenomena in filled porous glasses by time-resolved spectroscopy

We studied the propagation of ultrasonic acoustic waves in two porous glasses characterized by different pore diameters (4 and 200 nm) filled with carbon tetrachloride. Using a time-resolved optical spectroscopic technique (Transient Grating) we can measure the acoustic dynamics of these systems. The sound velocities, measured in both the samples, show high values compared with the sound velocity of bulk carbon tetrachloride; they cannot be predicted by the simple effective medium model, but are in good agreement with the estimate obtained from the Biot theory.

Synthesis, characterization and nonlinear optical properties of plasma-prepared poly(4-biphenylcarbonitrile) thin films

A novel conjugated polymer, poly(4-biphenylcarbonitrile) (PBPCN), was prepared using a plasma polymerization technique. The effect of the discharge power on the chemical structure and surface compositions of PBPCN thin films was investigated using Fourier transform infrared, UV-visible absorption and X-ray photoelectron spectroscopies. A femtosecond time-resolved optical Kerr effect technique was applied to investigate the third-order nonlinearity of the obtained plasma PBPCN thin films. For the first time, a non-resonant optical Kerr effect and ultrafast response for the PBPCN thin films were observed. Copyright © 2006 Society of Chemical Industry

Gigahertz optical spin transceiver

We present a time-resolved optical technique to measure electron spin dynamics with GHz dynamical bandwidth, transform-limited spectral selectivity, and phase-sensitive (lock-in) detection. Use of a continuous-wave (CW) laser and fast optical bridge enables greatly improved signal-to-noise characteristics compared to traditional optical sampling (pump-probe) techniques. We demonstrate the technique with a measurement of GHz-spin precession in n-GaAs. This approach may be applicable to other physical systems where stroboscopic techniques cannot be used because of either noise or spectral limitations.

Laser ablation of optically thin absorbing liquid layer predeposited onto a transparent solid substrate

Ablation of optically thin liquid 2-propanol layers of variable thickness on IR-transparent solid Si substrate by a nanosecond CO2 laser has been experimentally studied using time-resolved optical interferometric and microscopy techniques. Basic ablation parameters—threshold fluences for surface vaporization and explosive homogeneous boiling of the superheated liquid, ablation depths, vaporization (ablation) rates, and characteristic ablation times versus laser fluence—were measured as a function of alcohol layer thickness. The underlying ablation mechanisms, their thermodynamics, and microscopic details are discussed.

Diffusion of Sputtered Ground-State Tungsten Atoms in Krypton

The work is dedicated to investigations of the diffusive transport of sputtered ground-state tungsten atoms in buffer gas krypton at ambient temperatures. For sputtering of the metal, a pulse-periodic longitudinal hollow cathode gas discharge in cylindrical geometry is used. The cathode represents a 20 cm in length and 2.6 cm in diameter hollow copper cylinder, the inner surface of which is coated with a thin tungsten layer. During the discharge W atoms are sputtered from that layer into the cathode volume, while in the afterglow phase their density decreases due to diffusive losses back to the surface. The diffusive loss rates are measured by using a time-resolved double-channel optical absorption technique. By relating the experimental loss rates with a diffusion model based on the experimental geometry and a boundary condition of the third kind, the following value for the diffusion coefficient of W atoms in krypton is obtained: D0 = 130+40−30 cm2s−1 at 133 Pa buffer gas pressure and 315 K temperature. The reflection coefficient of these atoms at the cathode surface is obtained within the range 0 ⩽ ρ ⩽ 0.8. The experimental results for the diffusion coefficient are compared with theoretical calculations based on model interaction potentials.