Showing papers on "Femtosecond published in 1992"

Programmable shaping of femtosecond optical pulses by use of 128-element liquid crystal phase modulator

Abstract: Programmable shaping of femtosecond pulses by using a 128-element liquid crystal modulator to manipulate the phases of optical frequency components which are spatially dispersed within a grating-and-lens pulse shaping apparatus is described. This apparatus makes possible gray-level control of the spectral phases and allows modification of the pulse shape on a millisecond time scale under electronic control. Refinements in the design of the multielement modulator result in pulse shaping fidelity comparable to that which can be achieved with microlithographically fabricated masks. Several examples of pulse shaping operation, including pulse position modulation, programmable pulse compression, and adjustable cubic phase distortion, are described. >

Optoelectronic measurement of semiconductor surfaces and interfaces with femtosecond optics

Abstract: The basic concepts and preliminary applications of optically induced electromagnetic radiation from semiconductor surfaces and interfaces by using femtosecond optics are discussed. This submillimeter‐wave radiation provides a novel optoelectronic technique to study semiconductor electronic surface and interface properties with a contactless approach. The amplitude and phase of the electromagnetic radiation from the semiconductor surfaces depend on carrier mobility, impurity doping concentration, and strength and polarity of the static internal field. A large selection of bulk, epitaxial layer and superlattice samples from III‐V, II‐VI and group‐IV semiconductors has been tested. The orientation and strength of the static built‐in fields of a wide range of semiconductor surfaces, such as surface depletion, metal/semiconductor Schottky, p‐n junction and strain‐induced piezoelectric fields, can be determined and estimated.

Terahertz beam generation by femtosecond optical pulses in electro‐optic materials

Abstract: We have observed subpicosecond pulses having terahertz bandwidths generated from electro‐optic materials by using femtosecond optical pulses. The time‐resolved radiation signal clearly indicates that the terahertz radiation comes from the regions within the coherence length near the surfaces of electro‐optic materials under the phase‐mismatched condition. When the high‐energy femtosecond laser pulses are used, this method provides a new way to generate large amplitude broad bandwidth terahertz radiation efficiently.

Femtosecond solid-state lasers

Abstract: The emergence of new ultrafast optical modulation techniques has opened the way towards a new femtosecond laser technology based on solid-state gain media. The authors address the requirements for stable ultrashort pulse generation in these novel femtosecond sources. The theoretical considerations are backed up by experimental results obtained with a number of different laser systems. The conclusions drawn from the presented theoretical and experimental investigations provide general guidelines for the design and optimization of a wide range of femtosecond solid-state laser oscillators. >

Femtosecond laser control of a chemical reaction

Abstract: The critical stage in a chemical reaction — the progression through the transition state from reagents to products — occurs in less than a picosecond (10^(−12)s). Using laser pulses of femtosecond (10^(−15)s) duration it is possible to probe the nuclear motions throughout formation and break-up of the transition state. The coherence and very short duration of these femtosecond pulses provides a means to influence the course of the reaction during this stage if the time resolution is made sufficiently short. Here we describe a demonstration of such control of a chemical reaction on the femtosecond timescale. Using two sequential coherent laser pulses, we can control the reaction of iodine molecules with xenon atoms to form the product XeI by exciting the reactants through the transition state, in a two-step process. The yield of product XeI is modulated as the delay between the pulses is varied, reflecting its dependence on the nuclear motions of the reactants.

Analysis of Femtosecond Dynamic Absorption Spectra of Nonstationary States

Abstract: time-dependent wave­ packets, transition state spectroscopy, hole-burning, reaction dy­ namics, resonant impulsive Raman

Femtosecond real‐time probing of reactions. IX. Hydrogen‐atom transfer

Abstract: The real‐time dynamics of hydrogen‐atom‐transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular ‘‘proton’’ transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl‐group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen‐transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of ∼7200 cm−1. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O–H—O reaction coordinate. The vibrational periods (2π/ω) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low‐frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational‐energy redistribution dynamics at longer times are important to the hydrogen‐bond dissociation and to the nonradiative decay of the hydrogen‐transferred form.

Direct observation of fast proton transfer: femtosecond photophysics of 3-hydroxyflavone

Abstract: We report the first investigation of the fast excited-state intramolecular proton transfer of 3-hydroxyflavone. Picosecond stimulated emission and transient absorption spectroscopy demonstrate that the tautomer formation can be monitored by 620-nm transient absorption. The excited-state proton transfer was studied at 620 nm with femtosecond time resolution, and determined to be 240±50 fs in a nonpolar solvent environment. The fast excited-state proton transfer in methanol solution, however, was faster than the 125-fs instrument response

Theory of dynamic absorption spectroscopy of nonstationary states. 4. Application to 12-fs resonant impulsive Raman spectroscopy of bacteriorhodopsin

Abstract: A time-dependent theory for femtosecond dynamic absorption spectroscopy is used to describe the creation and observation of molecular ground-state vibrational coherence through the resonance impulsive stimulated Raman mechanism. Model calculations show that the oscillatory absorption signal that arises from this ground-state coherence is maximized for a limited range of pulse lengths and that there is a complex relationship between the probe wavelength and the strength of the spectral oscillations. The generalized time-dependent linear susceptibility of the nonstationary system created by the impulsive pump pulse is defined and used to discuss the strong dependence of the measured signals on the properties of the probe pulse. Finally, calculations are presented to analyze the high-frequency oscillations ({approximately}20-fs period) recently observed in the transient absorption spectra of light-adapted bacteriorhodopsin (BR{sub 568}) following excitation with a 12-fs optical pulse. At the probe wavelengths used in this experiment, the contribution of stimulated emission is negligible at long times because of the extremely rapid excited-state isomerization; as a result, the spectral oscillations observed after this time are due to the impulsive excitation of coherent vibrations in the ground state. The transient response observed for BR{sub 568} is calculated using a 29-mode harmonic potential surface derived from amore » prior resonance Raman intensity analysis. Both the oscillatory signals and their dependence on the probe wavelength are satisfactorily reproduced. 68 refs., 11 figs.« less

Space - time focusing: breakdown of the slowly varying envelope approximation in the self-focusing of femtosecond pulses.

Abstract: Numerical analysis of the self-focusing of femtosecond optical pulses reveals that the usually invoked slowly varying envelope approximation breaks down long before the temporal structure reaches the time scale of an optical cycle. This breakdown leads to a dramatic departure from the recently predicted symmetric development of the self-focusing pulse. By properly including the linear space–time focusing properties of short pulses, it is found that the trailing half of the self-focusing pulse is greatly enhanced and culminates in the formation of a trailing shock edge.

Femtosecond pulse amplification at 250 kHz with a Ti:sapphire regenerative amplifier and application to continuum generation

Abstract: Ultrashort optical pulses have been amplified to 1.7 μJ of energy at a repetition rate of 250 kHz with a novel cw argon-pumped Ti:sapphire regenerative amplifier. Acousto-optic switching of the amplifier is used to achieve the high repetition rate. Chirped-pulse amplification is used to avoid nonlinear effects in the amplifier. After recompression, 1-μJ, 130-fs pulses are obtained and are used to generate a white-light continuum in an ethylene glycol jet.

Pulse splitting during self-focusing in normally dispersive media.

Abstract: The self-focusing of femtosecond optical pulses in a normally dispersive medium is studied numerically. This situation represents a general problem that may be modeled by a 3 + 1-dimensional nonlinear Schrodinger equation, where two dimensions are self-focusing and the third is self-defocusing. The numerical simulations show that the dispersion causes the splitting of a pulse before it self-focuses into two temporally separated pulses, which then continue to self-focus and compress rapidly. The calculated behavior results in periodic modulation of the generated continuum spectrum, as was recently observed in continuum generation by focused femtosecond pulses in gases.

THz time-domain spectroscopy of nonpolar liquids

Abstract: A newly developed terahertz time-domain spectrometer based on photoconductive dipole antennas driven by femtosecond laser pulses was used to measure the absorption and index of refraction for benzene, carbon tetrachloride, and cyclohexane in the far infrared. The spectra cover the region of 3-66 cm/sup -1/, where the dielectric response of the nonpolar liquids is dominated by collision induced dipole moments. With terahertz time-domain spectroscopy, information on both the real and imaginary part of the frequency response is obtained in a range difficult to access by standard techniques. >

Generation of coherent phonons in condensed media

Abstract: The generation of coherent phonons by absorption of femtosecond laser pulses through interband transitions in different materials is summarized. Starting with layered III-VI semiconductors, where optical excitation is performed with amplified pulses, the generation mechanism via strong interband transitions is investigated. In Ge, stimulated resonant Raman scattering is found to be the decisive driving force. In GaAs, however, the ultrafast screening of surface space charge fields launches LO-phonons via electrostatic coupling. In the metallic state of high-temperature superconductors, the generation of highly symmetric A/sub g/ modes is assigned to displacive processes induced by a nonequilibrium electronic carrier distribution. In the superconducting state the amount of coherent displacement is strictly correlated to the number of optically broken superconducting pairs. In addition to the comparison of various generation processes, the dephasing of coherent phonons is addressed. >

Spatial and temporal transformation of femtosecond laser pulses by lenses and lens systems

Abstract: A Fourier-optical analysis of the transformation of ultrashort light pulses by lenses is given. Inserting the material dispersion up to second order, we find a coupling between the temporal, spectral, and spatial properties of the light pulse. In general, this coupling leads to a drastic increase in pulse duration and width of the spatial intensity distribution in the focal plane of the lens, which can be avoided with the use of achromatic lens doublets. The results are discussed for Gaussian-shaped input pulses.

Frequency-domain interferometer for femtosecond time-resolved phase spectroscopy

Abstract: A new femtosecond time-resolved interferometer was developed that utilizes interference fringes in the frequency domain to obtain simultaneously difference phase spectra (DPS) and difference transmission spectra with a multichannel spectrometer. For the first time to our knowledge, transient oscillations were observed in DPS and the spectral shift of a probe pulse was time resolved together with the rise in DPS, which is clear evidence for induced phase modulation in absorptive materials.

Femtosecond spectral holography

Abstract: Storage, recall, and processing of shaped femtosecond waveforms are achieved by performing spectral holography within a femtosecond pulse shaping apparatus. Time reversal, as well as correlation and convolution, of femtosecond temporal signals is demonstrated. Applications of this technique to matched filtering, dispersion compensation, encryption and decoding and femtosecond waveform synthesis are also discussed. The work extends the powerful principles of holographic signal processing, which have been used extensively for pattern recognition and filtering of two-dimensional spatial signals, to the femtosecond time domain. >

Femtosecond resolution of soft mode dynamics in structural phase transitions.

Abstract: The microscopic pathway along which ions or molecules in a crystal move during structural phase transition can often be described in terms of a collective vibrational mode of the lattice. In many cases, this mode, called a 'soft' phonon mode because of its characteristically low frequency near the phase transition temperature, is difficult to characterize through conventional frequency-domain spectroscopies such as light or neutron scattering. A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals. The low-frequency lattice dynamics of KNbO3 and BaTiO3 are clarified in a manner that permits critical evaluation of microscopic models for their ferroelectric transitions. The results illustrate the advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.

Ultrafast photodissociation of I3− in solution: Direct observation of coherent product vibrations

Abstract: We report the observation of terahertz oscillations in transient transmission measurements following femtosecond UV photolysis of I−3 in ethanol solutions. Transmission signals at 620 nm and at 880 nm, which are above and below the λmax of the known absorption of I−2 oscillate at precisely opposite phase. This and other results presented indicate that though the oscillations we are observing coherent vibration of the I−2 photofragment.

Generation of sub-100-fs pulses from a cw mode-locked chromium-doped forsterite laser

Abstract: Generation of femtosecond pulses from a continuous-wave mode-locked chromium-doped forsterite laser is reported. The forsterite laser was actively mode locked by using an acoustooptic modulator operating at 78 MHz with two Brewster high-dispersion glass prisms for intracavity chirp compensation. Transform-limited sub-100-fs pulses were routinely generated in the TEM(00) mode with 85 mW of continuous power (with 1 percent output coupler), tunable over 1230-1280 nm. The shortest pulses measured had a 60-fs pulse width.

Control of HOD photodissociation dynamics via bond‐selective infrared multiphoton excitation and a femtosecond ultraviolet laser pulse

Abstract: A scheme for controlling the outcome of a photodissociation process is studied It involves two lasers—one intense laser in the infrared region which is supposed to excite a particular bond in the electronic ground state, and a second short laser pulse in the ultraviolet region which, at the right moment, excites the molecule to a dissociative electronic state We consider the HOD molecule which is ideal due to the local mode structure of the vibrational states It is shown that selective and localized bond stretching can be created in simple laser fields When such a nonstationary vibrating HOD molecule is photodissociated with a short laser pulse (∼5 fs) complete selectivity between the channels H+OD and D+OH is observed over the entire absorption band covering these channels

Generation of 33-fs optical pulses from a solid-state laser.

Abstract: Using the results of recent theoretical research, we present practical guidelines for the optimization of femtosecond solid-state oscillators and demonstrate reproducible sub-40-fs pulse generation in a synchronously pumped Ti:sapphire laser.

Ultrafast reversible phase change in GeSb films for erasable optical storage

Abstract: Amorphous‐to‐crystalline and crystalline‐to‐amorphous transformations are triggered in GeSb thin films by irradiation with femtosecond and picosecond laser pulses. Phase changes are accompanied with optical contrast and therefore the feasibility of phase‐change optical recording at ultrafast rates is demonstrated for the first time. The phase reversal by ultrashort pulses seems to be related to the dependence of the degree of undercooling prior to solidification on the irradiation energy density.

Picosecond and femtosecond pulse generation in a regeneratively mode-locked Ti:sapphire laser

Abstract: The authors have produced transform-limited pulses ranging from 100 ps to 40 fs duration from a Ti:sapphire laser. Output powers in excess of 1 W and peak powers of 0.5 MW have been observed. They describe the technique of regenerative mode locking and present evidence that a transient with a peak power of more than 10 kW is required to initiate mode locking. The role of group velocity dispersion is highlighted and a value of -750 fs/sup 2/ is measured for the group delay dispersion in an operating laser. The authors describe the limits on both the power and pulsewidth obtainable from this laser and present pulse compression experiments which produce 17 fs pulses with 70 mW of average power. >

Laser excitation of ultrashort acoustic pulses: New possibilities in solid-state spectroscopy, diagnostics of fast processes, and nonlinear acoustics

Abstract: The experimental results in the field of laser generation of acoustic pulses of duration less than 1 ns are reviewed. The various physical mechanisms of optoacoustic conversion are analyzed theoretically. Possibilities are shown for shortening the duration of optoacoustic pulses by increasing the intensity of the laser exposure. The prospects of initiating ultrashort, strong shock pulses with high-power femtosecond light pulses, are discussed.