Subpicosecond Range Research Articles

Observation of formation dynamics of solvated aromatic cation radicals following photoionization

We have studied the ultrafast relaxation dynamics of aromatic cation radicals following photoionization by time-resolved absorption and Raman spectroscopic techniques. In nonpolar solvents, the absorption due to the cation radical decreases in subpicoseconds, which arises from the geminate recombination of the ejected electrons with their parent cations. In polar solvents, on the other hand, the absorption of the cation radical first decreases in the subpicosecond range, and then increases in tens of picoseconds. The subpicosecond decay is ascribed to the geminate recombination of the unsolvated cation radical in the polar solvents. The picosecond rise is observed in all the cation radicals treated (naphthalene, 1,4-diphenylbutadiene, and biphenyl), suggesting that the picosecond relaxation process increasing the absorption intensity occurs after the photoionization of the aromatic molecules in the polar solvents. The observed picosecond relaxation is discussed in terms of the thermal excitation of the neighboring solvent molecules due to intermolecular vibrational relaxation.

Cage Dynamics in the Third-Order Off-Resonant Response of Liquid Molecules: A Theoretical Realization

Abstract It is generally believed that the ultrafast initial spectroscopic response from the molecules in the condensed phase originates from small amplitude inertial motions within the cage formed by the nearest-neighbor solvent molecules surrounding the probe, or the cage effect. However, no quantitative estimate of this dynamics has been available for the currently popular experiments which measure the third-order off-resonant response. In this work, we fill this gap by a microscopic approach and demonstrate that the cage dynamics alone can produce the initial rise in the subpicosecond (ca. 200 fs) range in the third-order response. A simple analytical expression for the initial Gaussian time constant relevant to various kinds of the third-order off-resonant experiments is presented, which is found to be rather strongly dependent on the temperature. Connection with the non-polar solvation dynamics is also discussed.

Ultrashort-laser-pulse damage threshold of transparent materials and the role of incubation

We present investigations of the surface damage threshold for transparent materials, e.g. a-SiO2, CaF2 and LiF, after single- and multiple-laser-pulse irradiation at 800 nm in the picosecond and sub-picosecond duration range. Our study shows clearly that the surface damage threshold drops dramatically during multiple-laser-shot irradiation, due to material-dependent incubation effects. This has important consequences for applications such as laser machining and for the lifetime of optical components. Different processes that can reduce the surface damage threshold with increasing laser shots are evaluated, such as sub-surface damage and defect formation. The mechanism of laser-induced defect formation, e.g. color centers, is believed to be mainly responsible for the observed reduction in the threshold for surface damage with increasing laser-shot numbers.

Behavior of the soliton pulse envelope in a nonlinear medium in the subpicosecond range

The impact of additional higher dispersion and nonlinear terms in the model equation on the properties of short pulses is studied. The analysis is based on two mutually complementary asymptotic approaches. We found explicit expressions describing distortions in the amplitude and phase of the signal envelope. The conditions under which the additional terms compensate for each other are examined.

Modeling of primary chemical processes of water radiolysis and simulation by spur diffusion model

Chlorite ion is selected as a solute reactive toward e aq − and OH. Experimentally observed G(ClO 2) is reproduced by the conventional spur diffusion model in even concentrated region without considering any reactions within 10 −12 s. For the solutes reactive toward the quasi-free electron, an extended spur diffusion model including in sub-picosecond time range is proposed by taking account of the experimental data of picosecond pulse radiolysis, femtosecond laser photolysis and positronium results are compared with corresponding experimental data.

Ultrashort orientational dynamics of liquid crystals in the smectic-A phase

A femtosecond time-resolved optical Kerr effect experiment was carried out to study the reorientational motion of a series of cyanobiphenyl molecules around their long axis in the smectic-A phase. The Kerr effect was found to mainly result from the reorientation of the cyanobiphenyl core of the liquid crystal molecule. Three relaxation times were observed in the subpicosecond and picosecond range. They appear to be associated with libration and diffusive reorientations of chromorphores of individual and paired molecules. The analysis on several temporal scales reveals the existence of strong molecular interactions within the smectic-A layers.

Radiation generation by photoswitched, periodically biased semiconductors.

A laser pulse, propagating nearly parallel to the surface of a planar semiconductor wafer, will generate electron-hole pairs. If the semiconductor is spatially biased with a static electric field of period ${\ensuremath{\lambda}}_{0}$, the laser pulse acts as a fast switch and generates a periodic current. The rapid switching of the current generates radiation, which propagates along the surface and can be confined by a conducting wall placed parallel to the wafer. The wavelength of the radiation can be tuned by adjusting ${\ensuremath{\lambda}}_{0}$, the wafer-wall separation, and/or the carrier density. In the absence of collisional damping, ${\mathit{N}}_{0}$ periods of the static bias electric field will generate ${\mathit{N}}_{0}$ periods of radiation. Under idealized conditions, the maximum electric field of the radiation is equal to the applied static field and the maximum efficiency of converting the static electric field energy to electromagnetic energy is 30%. In practice for typical parameters, tunable electromagnetic radiation can be generated with wavelengths in the 50--500-\ensuremath{\mu}m range, pulse durations in the picosecond or subpicosecond range, and peak powers on the order of 100 W. \textcopyright{} 1996 The American Physical Society.

Optical ablation by high-power short-pulse lasers

Laser-induced damage threshold measurements were performed on homogeneous and multilayer dielectrics and gold-coated optics at 1053 and 526 nm for pulse durations τ ranging from 140 fs to 1 ns. Gold coatings were found, both experimentally and theoretically, to be limited to 0.6 J/cm2 in the subpicosecond range for 1053-nm pulses. In dielectrics, we find qualitative differences in the morphology of damage and a departure from the diffusion-dominated τ1/2 scaling that indicate that damage results from plasma formation and ablation for τ ≤ 10 ps and from conventional heating and melting for τ > 50 ps. A theoretical model based on electron production by multiphoton ionization, joule heating, and collisional (avalanche) ionization is in quantitative agreement with both the pulse-width and the wavelength scaling of experimental results.

Subpicosecond pulse amplification in semiconductor laser amplifiers: theory and experiment

We present a phenomenological model of subpicosecond pulse evolution in semiconductor laser amplifiers, which includes carrier heating effect, gain dispersion, gain saturation, and three kinds of self-phase modulations (SPM). The results obtained from this model are applied to and found to agree very well with experimental measurements of spectral distortions and time-resolved gain on a semiconductor laser amplifier with 460-fs and 2-ps input pulses. The device parameters that are used to match the experimental results are within a reasonable range, either suggested by previous experiments or by published calculations, The results show that the evolution of the pulses and their spectra is sensitively dependent on the input pulse shape and on a variety of time domain and frequency domain amplitude and phase shaping effects. Matching the theory to the experimental data suggests that among the possible causes of the carrier heating two-photon absorption (TPA) is the most important. This effect of TPA needs to be included to properly account for the observed dependence of the carrier heating gain reduction on the pulse length. By examining separately the contribution of the corresponding terms, we also prove the importance of SPM due to the carrier heating and to the instantaneous nonlinear index in shaping the output spectrum. We compare our model with Agrawal's theory, which is valid for pulses of tens of picosecond duration, and find large differences for subpicosecond pulses. For 2-ps pulses, on the other hand, the differences are fewer and less dramatic. The good agreement of the new model with experiments will now allow diagnostics and predictions of pulse evolution in semiconductor optical amplifiers from the picosecond to the subpicosecond range.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A 2 to 4 nm high power FEL on the SLAC linac

We report the results of preliminary studies of a 2 to 4 nm SASE FEL, using a photoinjector to produce the electron beam, and the SLAC linac to accelerate it to an energy up to 10 GeV. Longitudinal bunch compression is used to increase ten fold the peak current to 2.5 kA, while reducing the bunch length to the subpicosecond range. The saturated output power is in the multi-gigawatt range, producing about 10 14 coherent photons within a bandwidth of about 0.2% rms, in a pulse of several millijoules. At 120 Hz repetition rate the average power is about 1 W. The system is optimized for X-ray microscopy in the water window around 2 to 4 nm, and will permit imaging a biological sample in a single subpicosecond pulse.

Temperature-dependent ultrafast solvation dynamics in a completely nonpolar system

Transient hole burning measurements on dimethyl-s-tetrazine in n-butylbenzene are reported from the low-viscosity room-temperature liquid down to the low-temperature glass. The results give a detailed picture of the solvation of a nonpolar solute in a nonpolar solvent. The dynamics separate into a phonon modulated and a structural component, as was found previously for polar solvents. The structural component is frozen in the glass, but its relaxation rate increases into the subpicosecond range with increasing temperature. The time decay of the structural relaxation is highly nonexponential. The coupling of the solute electronic state to the structural coordinates is close to linear, but the coupling to the phonon coordinates cannot be accounted for by simple linear or quadratic coupling models. Effects are also found that are attributed to changes in coupling constants with changing density.

A view of fast charge separation in the bacterial photosynthetic reaction centre involving weak environmental damping of a populated auxiliary chlorophyll state

The initial fast electrical charge separation in the bacterial photosynthetic reaction centre involves electron transfer (ET) from the photo-excited special pair chlorophyll to pheophytin. The ET is assisted by the auxiliary chlorophyll monomer which may be populated in the sub-picosecond time range. We provide a new theoretical description of this process which rests on three-level coherent ET theory, incorporating environmental damping and partial vibrational relaxation in the intermediate state of auxiliary chlorophyll population. If damping is not too strong, this view offers a clue to the ultrafast, activationless and possibly coherent character of both steps, also in the limit when the electronic coupling between the three molecular entities is weak.

Dynamics of three-body reactions in ICN 2+ and related molecules

Correlations between the initial momenta of fragments from atomisation of ICN 2+ and related molecules are examined by the photoelectron—photoion—photoion coincidence technique. The shapes of peaks in the spectra show that some fragmentation reactions are stepwise but faster than rotation of the intermediates; lifetimes in the subpicosecond range are estimated. Deviations of the peak shapes from predictions based on slow stepwise reaction are attributed both to reaction within the Coulomb repulsion zone and to energy release in decay of aligned intermediates. Suppression of rotation in intermediate ions by the strong Coulomb field is important in interpretation of the mechanisms.

NONLINEAR POLARIZATION SPECTROSCOPY (FREQUENCY DOMAIN) STUDIES OF EXCITED STATE PROCESSES: THE B800–850ANTENNA OF RHODOBACTER SPHAEROIDES

AbstractNonlinear polarization spectroscopy in the frequency domain allows rate constant determinations of fast electronic energy and phase relaxations together with characterization of the type of line broadening. Application of this method to the B850 component of the isolated B800–850antenna ofRhodobacter sphaeroides at room temperature shows that B850 is inhomogeneously broadened, with homogeneous widths between 30 and 200 cm−1, depending on the spectral position of the subforms. The corresponding phase relaxation times are clearly in the subpicosecond range. There is also indication of an up‐to‐now unspecified1–5 ps energy relaxation channel per subunit.

Experimental Results in Picosecond and Subpicosecond Range of IIa Type Diamond Detector in X-UV, Visible and Ir Fields

ABSTRACTType IIa diamonds present photoconductive properties as well in X-ray detection range, as in near UV, visible, IR light range, in spite of the large bandgap of the material. Experimental results in these four fields, time measurements in picosecond and subpicosecond ranges are presented.

Caustics of a Light Beam Passing Through a Time-varying Dielectric Wedge

Abstract A caustic of cusp type in space-time is found to arise in a laser beam deflected by a wedge with time-varying refractive index. Asymptotic formulae describing the wave field under the assumption that the refractive index varies slowly compared with the oscillations of the light wave are given. The intensity of the wave field at the cusp point can be, say, 103 times greater than the initial one. The temporal variation of the field at the cusp gives rise to an ultrashort pulse. The lengths of these pulses can be 105 times less than the characteristic time of modulation of the refractive index, i.e., of subpicosecond range.

Modeling the subpicosecond pulse compression in the vacuum ultraviolet and the extreme ultraviolet

A method for pulse compression to the subpicosecond range in the VUV and the XUV is investigated theoretically. The induced phase modulation on a signal by a pump pulse near two-photon resonance in inert gases is analyzed, including group-velocity dispersion, pulse walk-off, initial delay, shift of the signal peak, and linear and nonlinear chirp rates for Gaussian and hyperbolic secant pulses. A reasonable agreement is found with the theoretical and experimental results of other authors.

Above-threshold ionization spectra for hydrogen atoms exposed to ultra-intense linearly polarized high-frequency laser pulses

The time evolution of H (1s) atoms in very intense (Io approximately=3.5*1018 W cm-2) high-frequency ( omega =1 au) laser fields is calculated by numerical integration of the time-dependent Schrodinger equation in the Kramers-Henneberger frame. ATI spectra are analysed as a function of pulse duration for a sin2 pulse shape and pulse durations in the sub-picosecond range. A prominent feature in these spectra is a combination of peak broadenings due to interaction time and laser power. For short pulses the spectra are dominated by ionization early during the pulse. For the longer pulses find an additional series of peaks that correspond to ionization from the Stark-shifted H(nl) levels (n)1) near the peak of the laser pulse. Such peak positions (for n=1) were predicted before by Pont et al. (1990) using a high-frequency Floquet theory,but have been calculated for the first time in an ab initio approach for finite laser pulses.

Feasibility of optical pulse compression by stimulated Brillouin scattering in a plasma

High-power ultrashort optical pulses are obtained by compression under conditions of stimulated scattering in nonlinear media, although the radiation intensities used are generally limited by the possible breakdown of the medium. The feasibility of compression of a high-power optical pulse by stimulated Brillouin scattering in a laser plasma under conditions of amplification of a Stokes pulse from the thermal noise level, allowing for optical heating of the plasma, is studied for the first time. It is demonstrated that a laser plasma may be a convenient medium for appreciable compression of high-power long-wavelength subnanosecond pulses to durations of a few tens of picoseconds and for compression of short-wavelength pulses from a few tens of picoseconds to the subpicosecond range.

New concepts in the physics and chemistry op solid polymers fracture

AbstractElementary events in polymer fracture at the molecular level are the breakdowns of single polymeric chains. Breakdown of a stretched macromolecule is accompanied by its mechanical degradation. Comparison of experimental data on the molecular products of thermal and mechanical degradation points to significant distinctions between these two processes. Generation of highly excited (“hot”) molecular products of mechanical degradation by the growing crack cannot be explained in terms of the kinetic concept of polymer fracture. Elaborate theoretical analysis of polymeric chain breakdown and generation of “hot” molecular products in subpicosecond time range is made by numerical molecular dynamic simulation with a computer. The mechanisms of primary and secondary chain breakdowns are established. The principal initiators of breakdowns are localized nonlinear modes of the lattice soliton type. Generation of highly excited (“hot”) molecular products occurs at the initial stage of mechanical degradation (during≈1 picosecond after the first breakdown).