Femtosecond Time Regime Research Articles

Photoinduced Electrooptics in Pyrene Molecular Crystallites Incorporated within Polymer Matrices

Photoinduced linear electrooptics effect in pyrene crystallites of the sizes 10-15 nm incorporated within oligoether acrylate matrices is revealed for the first time. A correlation between the number of macrovacancies measured by positron annihilation and the EOE tensors indicates an appearance of additional non-centrosymmetry in the charge density distribution caused by the defects. Simultaneously we demonstrate changes of kinetics of the mentioned phenomena in femtosecond time regime. The later confirms an essential role of electron-phonon subsystem in the observed electrooptics effect.

Ultrafast Excited State Deactivation of Triphenylmethane Dyes

We have carried out ultrafast pump−probe measurement of four TPM dyes, malachite green (MG), brilliant green (BG), crystal violet (CV), and ethyl violet (EV), with a time resolution of 30 fs. The pump−probe signal showed that solvent dependence arose first in the femtosecond time regime, e.g., the decay of n-butanol solution was clearly slower than the methanol solution just 50 fs after the initial photoexcitation. The signal decays in a multiexponential manner and the slower components showed stronger linear dependence on the solvent viscosity than did the faster components. We have also carried out temperature-dependent measurement of ethanol solution and calculated the activation energies from the Arrhenius plots of each components. The activation energies and effective volumes were larger for slower decays. The activation energy of the viscosity of ethanol was larger than that of the decay components of TPM dyes. These observations are explained with a combined effect of microviscosity and intramolecu...

Dynamics of Photoexcited High Density Carriers in ZnO Epitaxial Thin Films

Time-resolved luminescence spectra of ZnO epitaxial thin films were measured by the optical Kerr gate method in femtosecond time regime under resonant excitation of the excitonic state. We observed the relaxation of hot carriers until the buildup of luminescence due to an electron-hole plasma (EHP) state and recovery of the renormalized bandgap due to radiative recombination of electrons with holes in the quasi-thermalized EHP state. The relaxation of hot carriers mainly comes from carrier-carrier collisions but might include in part carrier-LO phonon scattering.

Dynamics of Photoexcited High Density Carriers in ZnO Epitaxial Thin Films

Time-resolved luminescence spectra of ZnO epitaxial thin films were measured by the optical Kerr gate method in femtosecond time regime under resonant excitation of the excitonic state. We observed the relaxation of hot carriers until the buildup of luminescence due to an electron–hole plasma (EHP) state and recovery of the renormalized bandgap due to radiative recombination of electrons with holes in the quasi-thermalized EHP state. The relaxation of hot carriers mainly comes from carrier–carrier collisions but might include in part carrier–LO phonon scattering.

Investigation on relaxation dynamics of photoexcitations and spectral narrowing of photoluminescence in PPV films

Photoinduced absorption spectral feature as well as their temporal profiles on PPV thin film in femtosecond time regime exhibit a strong pump power density dependence. The increase in pump power density also results in the spectrally narrowed emission (SNE) band superimposed to the well-known photoluminescence from PPV. The dependencies on both pump- and probe densities denote much lower saturation intensity in photoinduced absorption as compared with that in stimulated emission from the identical primary photoexcitations, the singlet excitons. We have also discussed the nature of SNE, which has a correlation with an appearance of ultrafast decay component (τ1 ps) in temporal decay profile of the photoinduced absorption signal.

Circuit analysis of an ultrafast junction mixing scanning tunneling microscope

A lumped element circuit model for the operation of a junction mixing scanning tunneling microscope (JM-STM) is presented. Fits from this model show excellent agreement with experimental results in the picosecond time regime. The tip sample capacitance employed in the model was calculated to be 33 fF, using the method of images. By varying the capacitance, various tip/sample geometries can be investigated. Testing the response of the model tunnel junction, for faster electrical pulses, suggests how the JM-STM can be pushed into the femtosecond time regime.

Laser–solid interaction in the femtosecond time regime

Modification of the surface structure of solid materials by laser radiation involves a complex chain of processes. The first step is the deposition of a certain amount of optical energy in the material. The character of the material excitation is strongly dependent on the laser pulse duration. With the use of ultrashort laser pulses non-equilibrium energy distributions with large excess population in the excited states can be produced. The distinct physical processes which come into play in laser–solid interaction on the ultrafast time scale open new routes of modifying the structure and the morphology of materials and offer interesting perspectives in laser materials processing.

Femtosecond time-resolved spectroscopy of self-trapping processes of holes and electron-hole pairs in alkali bromide crystals.

The dynamics of the lattice relaxation of holes and electron-hole ($e\ensuremath{-}h$) pairs generated by the band-gap excitation in alkali bromide crystals has been studied in femtosecond time regime at temperature range from 6 to 300 K. In KBr including electron-trapping impurities, where relaxation of holes takes place without interaction with electrons, it is found that the self-trapped hole in the form of a halogen-molecular ion, the ${V}_{K}$ center, is generated via a transient localized-hole center as a precursor. This transient hole center shows broad optical absorption bands in the visible region and has a lifetime of about 3 ps at 273 K. It is found that in pure KBr and RbBr, the relaxation processes of ($e\ensuremath{-}h$) pairs have two distinct stages. In the first stage, which terminates within 6 ps after excitation, the transient hole centers are generated as in doped specimens, and their interaction with electrons results in the fast formation of a Frenkel pair consisting of an $F$ center, a halogen vacancy trapping an electron, and an $H$ center, an interstitial halogen atom. Also, an intermediate state is formed in the first stage, and the state is ascribed to the self-trapped exciton (STE) with the on-center configuration on the basis of its spectroscopic features. The second stage of the relaxation, which lasts over 100 ps at low temperatures, comprises the off-center relaxation of the on-center STE formed in the first stage into Frenkel pairs and STE's with the off-center configuration. Relaxation pathways and characteristics including temperature dependence are discussed based on the experimental results and their analysis in terms of a rate-equation model.

A theoretical investigation of carrier-carrier effects in ultrafast experiments

We present a theoretical analysis of the effects of carrier-carrier interaction in polar semiconductors. Though a Monte Carlo simulation we show that carrier-carrier scattering is the dominant scattering mechanism in the femtosecond time regime after a laser photoexcitation. We compare our theoretical results with both ultrafast absorption saturation studies and up-conversion luminescence experiments. The agreement with all types of experiments is extremely good and shows the accuracy of our simulations. We also verify that pump and probe experiments are the ideal tool for the study of the strength of carrier-carrier interaction.

Monte Carlo investigation of carrier-carrier effects in femtosecond pump and probe experiments

We present a Monte Carlo simulation of ultrafast absorption saturation studies of hot carriers in GaAs. We show that carrier-carrier scattering is the dominant scattering mechanism in the femtosecond time regime and that pump and probe experiments are the ideal tool for the study of the strength of this interaction. Three different approaches are used to study carrier-carrier scattering and the best agreement with experimental results is obtained by molecular dynamics.

Transport properties of optically generated free carriers in hydrogenated amorphous silicon in the femtosecond time regime.

We present spectral- and time-resolved reflectivity and transmission measurements on hydrogenated amorphous silicon, a-Si:H, with 50 fs time resolution. Electron-hole pairs are photoexcited into the extended states of a-Si:H by an ultrashort pump pulse at a photon energy of 2 eV. The optical response of photogenerated carriers is studied on an ultrashort time scale, \ensuremath{\Delta}t\ensuremath{\sim}100 fs, after the excitation process where recombination and trapping can be neglected. The spectral dependence of the optical response is analyzed in terms of a hopping model for the transport of the photogenerated carriers in the extended states of a-Si:H.

Analytical chemistry--feeding the environmental revolution?

There can be no question in anyone's mind that our ability to analyze the structure and composition of materials in all media has grown in amazingly exponential ways in the last several decades. Our detection limits have gone from micrograms to the subpicogram level. A millimeter-sized sample presented a challenge in 1950; but today we look at submicron units, in fact, at single atoms resolved in analytical electron microscopes in an almost routine manner. Analysis time has changed from hours or sometimes even days for a complete qualitative and quantitative analysis on a sample to modern capabilities which include real-time spectroscopy on in situ experiments which give us data on molecular dynamics in the femtosecond time regime. Trace analysis, the detection and determination of impossibly tiny quantities of elements and molecules in increasingly complex matrices, is readily accomplished today. These are exactly the types of samples usually involved in environmental analysis. Yet, there is a tendency among some groups to view analytical advances as a negative factor in environmental studies because the ability to measure extremely small quantities can focus regulators on placing restrictions on materials not previously detectable. If so, that is a rare instance where increased knowledge can be considered a setback.

Femtosecond cascade-excitation spectroscopy for nonradiative deexcitation and lattice relaxation of the self-trapped exciton in NaCl.

Optical-absorption change following photoexcitation of the lowest triplet self-trapped exciton (STE) in NaCl has been measured in the femtosecond time regime. It is found that a short-lived intermediate state is generated within 1 ps and decays accompanied by restoration of the lowest STE. An oscillation in the population of the lowest STE and the intermediate state with a period of 1.1 ps was observed. Formation of Frenkel pairs has been found to occur not at the initial stage of relaxation but as a result of the relaxation leading to the lowest STE.

Femtosecond laser-tissue interactions: Retinal injury studies

We report the first study of laser-tissue interaction in the femtosecond time regime. Retinal damage thresholds and mechanisms produced by exposure to high-intensity femtosecond laser pulses were investigated in chinchilla grey rabbits. Exposures were performed using single laser pulses of 80 fs duration at 625 nm. ED <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</inf> injury thresholds of 0.75 and 4.5 μJ were measured using fluorescein angiographic and ophthalmoscopic visibility criteria evaluating 204 laser exposures. Ultrastructural studies including light and electron microscopy were performed on selected lesions. Results suggest that the primary energy deposition in the retina occurs in melanin, However, in contrast to laser injuries produced by longer pulses, exposures of more than 100 × threshold in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50-100 \mu</tex> J range did not produce significantly more severe lesions or hemorrhage. This suggests the presence of a nonlinear damage limiting mechanics in tissue exposed to femtosecond laser pulses.