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Showing papers on "Femtosecond published in 1994"


Journal ArticleDOI
TL;DR: Optical thin-film structures exhibiting high reflectivity and a nearly constant negative group-delay dispersion over frequency ranges as broad as 80 THz are presented, making these coatings well suited for intracavity dispersion control in broadband femtosecond solid-state lasers.
Abstract: Optical thin-film structures exhibiting high reflectivity and a nearly constant negative group-delay dispersion over frequency ranges as broad as 80 THz are presented. This attractive combination makes these coatings well suited for intracavity dispersion control in broadband femtosecond solid-state lasers. We address design issues and the principle of operation of these novel devices.

675 citations


Journal ArticleDOI
TL;DR: In this paper, an IR pump beam is used to heat the electron distribution and changes in electron temperature are measured with a visible probe beam at the d band to Fermi-surface transition, showing that the subpicosecond optical response of gold is dominated by delayed thermalization of the electron gas.
Abstract: Femtosecond electron thermalization in metals was investigated using transient thermomodulation transmissivity and reflectivity. Studies were performed using a tunable multiple-wavelength femtosecond pump-probe technique in optically thin gold films in the low perturbation limit. An IR pump beam is used to heat the electron distribution and changes in electron temperature are measured with a visible probe beam at the d band to Fermi-surface transition. We show that the subpicosecond optical response of gold is dominated by delayed thermalization of the electron gas. This effect is particularly important far off the spectral peak of the reflectivity or transmissivity changes, permitting a direct and sensitive access to the internal thermalization of the electron gas. Using a simple rate-equation model, line-shape analysis of the transient reflectivity and transmissivity indicates a thermalization time of the order of 500 fs. At energies close to the Fermi surface, longer thermalization times \ensuremath{\sim}1--2 ps are observed. These results are in agreement with a more sophisticated model based on calculations of the electron-thermalization dynamics by numerical solutions of the Boltzmann equation. This model quantitatively describes the measured transient optical response during the full thermalization time of electron gas, of the order of 1.5 ps, and gives new insight into electron thermalization in metals.

642 citations


Journal ArticleDOI
21 Oct 1994-Science
TL;DR: The observation of coherent vibrational motion of the photoproduct supports the idea that the primary step in vision is a vibrationally coherent process and that the high quantum yield of the cis-->trans isomerization in rhodopsin is a consequence of the extreme speed of the excited-state torsional motion.
Abstract: Femtosecond pump-probe experiments reveal the impulsive production of photoproduct in the primary event in vision. The retinal chromophore of rhodopsin was excited with a 35-femtosecond pulse at 500 nanometers, and transient changes in absorption were measured with 10-femtosecond probe pulses. At probe wavelengths within the photo-product absorption band, oscillatory features with a period of 550 femtoseconds (60 wavenumbers) were observed whose phase and amplitude demonstrate that they are the result of nonstationary vibrational motion in the ground state of the photoproduct. The observation of coherent vibrational motion of the photoproduct supports the idea that the primary step in vision is a vibrationally coherent process and that the high quantum yield of the cis-->trans isomerization in rhodopsin is a consequence of the extreme speed of the excited-state torsional motion.

563 citations


Journal ArticleDOI
TL;DR: Direct measurement of hot-electron temperatures and relaxation dynamics for peak electron temperatures between 3400 and 11000 K utilizing two-pulse-correlation femtosecond thermionic emission is reported.
Abstract: We report direct measurement of hot-electron temperatures and relaxation dynamics for peak electron temperatures between 3400 and 11 000 K utilizing two-pulse-correlation femtosecond (fs) thermionic emission The fast relaxation times (15 ps) are described by extending RT characterizations of the thermal conductivity, electron-phonon coupling, and electronic specific heat to these high electron temperatures

318 citations


Journal ArticleDOI
TL;DR: A new pulse-shaping technique, using an acousto-optic modulator as a spatial modulator in a zero-dispersion delay line that simplifies optical alignment and dramatically improves update rates.
Abstract: We demonstrate a new pulse-shaping technique, using an acousto-optic modulator as a spatial modulator in a zero-dispersion delay line. Compared with existing techniques, this approach simplifies optical alignment and dramatically improves update rates. It should also improve flexibility for generating complex waveforms.

277 citations


Journal ArticleDOI
TL;DR: In this article, a thorough experimental investigation of the phenomenon of soliton resonance sideband generation in femtosecond fiber lasers is presented, and the dependence of the sideband wavelengths on the dispersion and pulse length is confirmed.
Abstract: We present a thorough experimental investigation of the phenomenon of soliton resonance sideband generation in femtosecond fiber lasers. The dependence of the sideband wavelengths on the dispersion and pulse length is confirmed. Third order dispersion is found to play a significant role in determining the sideband spectrum. We show that the minimum pulse length obtained in a fiber laser is determined by the cavity dispersion and relate this to loss into the sidebands. We show how the sideband spectrum can be used as a diagnostic of the fiber parameters and of the formation of ultrashort pulses in the laser. >

251 citations


Journal ArticleDOI
TL;DR: In this article, a femtosecond photon-echo technique was used to probe the dynamics of quantum-confined excitons in nanocrystals of CdSe.
Abstract: Femtosecond photon-echo techniques are used to probe the dynamics of quantum-confined excitons in nanocrystals of CdSe. Using three-pulse photon echoes, the modulation of the echo signal from the LO-phonon mode is effectively suppressed, and both the electronic dephasing and the couplings to lattice vibrations are probed directly. Detailed measurements are reported as a function of both nanocrystal size and temperature. The dephasing times vary from 85 fs in nanocrystals of 20-\AA{} diameter to 270 fs in 40-\AA{} crystals. These rates are determined by several dynamical processes, all of which depend sensitively on the size of the nanocrystal. The time scale of the trapping of the electronic excitation to surface states increases with increasing size. The coupling of the excitation to low-frequency vibrational modes is strongly size dependent as well, in accordance with a theoretical model. The photon echo also gives information on the polar coupling between the electronic state and the LO-phonon mode. This coupling is found to peak at an intermediate size. This phenomenon is interpreted as a manifestation of coupling between the interior confined excitons and localized surface states, which destroys the spherical symmetry of the excited state. Using these data, all of the important contributions to the size-dependent homogeneous linewidths can be enumerated.

248 citations


Journal ArticleDOI
TL;DR: In this article, the nonlinear index of refraction and the two-photon absorption coefficient of CdS, ZnSe, and ZnS were measured using the Zscan technique with 100 femtosecond pulses at 610 nm, 780 nm, and 1.27 μm.
Abstract: The nonlinear index of refraction and the two‐photon absorption coefficient of CdS, ZnSe, and ZnS were measured using the Z‐scan technique with 100‐femtosecond pulses at 610 nm, 780 nm, and 1.27 μm. The results are in reasonable agreement with measurements made at similar wavelengths with longer pulses. A simple theoretical model accounts for the dispersion of the nonlinearities and confirms their electronic nature.

242 citations


Journal ArticleDOI
TL;DR: In this paper, the authors measured the sub-picosecond optical response of a solid-state, semiconductor-to-metal phase transition excited by femtosecond laser pulses.
Abstract: We have measured the subpicosecond optical response of a solid‐state, semiconductor‐to‐metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of VO2 thin films by making pump‐probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties of the high‐temperature metallic state being attained within 5 ps. The ultrafast change in complex refractive index enables ultrafast optical switching devices in VO2.

237 citations


Journal ArticleDOI
TL;DR: In this article, the time-resolved difference absorption spectra of J-aggregates were measured by femtosecond pump-probe spectroscopy and the induced absorption near the J-band at 20 K was assigned to the transitions from (n + 1)-exciton states (n⩾1).

210 citations


Journal ArticleDOI
Oliver B. Wright1
TL;DR: In this article, the dynamics of coherent phonon generation by femtosecond optical pulses in thin gold and silver films was studied using a pump and probe scheme, which was achieved by monitoring ultrafast surface vibrations in real time using laser-beam deflection.
Abstract: The dynamics of coherent phonon generation by femtosecond optical pulses in thin gold and silver films is studied using a pump and probe scheme. Detection is achieved by monitoring ultrafast surface vibrations in real time using laser-beam deflection. The phonon strain pulse shapes can be explained through the nonequilibrium coupling of the electron and phonon distributions, suggesting a new method for measuring the electron-phonon coupling constant.

Journal ArticleDOI
TL;DR: In this paper, the authors extend their previous analysis of the ultrafast laser-induced instability of the diamond structure of semiconductors by including longitudinal optical-phonon distortions in addition to the instability of transverse acoustic phonons.
Abstract: We extend our previous analysis of the ultrafast laser-induced instability of the diamond structure of semiconductors by including longitudinal optical-phonon distortions in addition to the instability of the transverse acoustic phonons. Generally, longitudinal optical distortions enhance the instability of the transverse acoustic phonons, increasing the kinetic energy of the atoms and the final lattice temperature. These phonons make a transition to a centrosymmetric structure of GaAs with metallic properties possible. We present results for the time dependence of the instability of Si for the case where 15% of the valence electrons have been excited into the conduction band. Thus, already 100 fsec after the excitation of the plasma the atoms are displaced about 1 \AA{} from their equilibrium position and their kinetic energy has increased to approximately 0.4 eV. Collisions between the atoms then lead to a rapid melting of the crystal. These results are in good agreement with recent experiments performed on Si and GaAs.

Journal ArticleDOI
TL;DR: In this paper, the authors performed femtosecond thermoreflectivity experiments to investigate energy deposition and transport during the very early period of short-pulsed laser heating of gold and chromium multi-layer metal films.

Journal ArticleDOI
TL;DR: In this paper, femtosecond infrared pulses from an ultrashort high-peak power Ti:sapphire laser were successfully utilized for ablation of polymer films such as polytetrafluoroethylene, polyimide, tetrafluorethylene and tetraflamethylene copolymer.
Abstract: Femtosecond infrared pulses from an ultrashort high‐peak‐power Ti:sapphire laser were successfully utilized for ablation of polymer films such as polytetrafluoroethylene, tetrafluoro‐ ethylene‐hexafluoropropylene copolymer, and polyimide. Good agreement between experimental data and calculations from a model suggested that simultaneous absorption of three photons for polyimide and five photons for tetrafluoroethylene‐hexafluoropropylene copolymer predominated over the other absorptive channels and that such a multiphoton absorption was induced efficiently.

Journal ArticleDOI
TL;DR: A frequency-domain interferometer for probing the variations of the dielectric constant of a plasma with sub-100- fs temporal resolution and lambda/2000 phase resolution is described.
Abstract: A frequency-domain interferometer for probing the variations of the dielectric constant of a plasma with sub-100- fs temporal resolution and lambda/2000 phase resolution is described. Imaging the plasma on the entrance slit of a spectrograph provides spatial resolution along a diameter of the focal spot. The technique is used to map out the expansion of the critical density surface of a femtosecond laser-produced plasma with subnanometer spatial resolution along the laser axis.

Journal ArticleDOI
TL;DR: The generation of highly stable optical pulses as short as 11 fs from a Kerr-lens mode-locked Ti:sapphire laser containing no intracavity prisms is demonstrated and novel dielectric mirrors provide broadband dispersion control for solitonlike pulse formation.
Abstract: The generation of highly stable optical pulses as short as 11 fs from a Kerr-lens mode-locked Ti:sapphire laser containing no intracavity prisms is demonstrated. In the femtosecond oscillator design reported, novel dielectric mirrors provide broadband dispersion control for solitonlike pulse formation.

Journal ArticleDOI
TL;DR: Femtosecond pump-probe experiments with a time resolution of 100 fs have been performed for copper particles with a radius of 4 nm as mentioned in this paper, showing that the broadening of the absorption band due to the surface plasmon can be explained by the usual electron-phonon coupling model.
Abstract: Femtosecond pump‐probe experiments with a time resolution of 100 fs have been performed for copper particles with a radius of 4 nm Differential absorption spectra for a pump centered at 205 eV indicates the broadening of the absorption band due to the surface plasmon The nonlinear response time derived from the recovery time of the nonlinear absorption is dependent on the pumping laser fluences, and is as short as 07 ps for 210 μJ/cm2 The relaxation dynamics of nonequilibrium electrons can be described by the usual electron‐phonon coupling model

Journal ArticleDOI
TL;DR: The dynamics of carrier-induced absorption changes in CdSe quantum dots are investigated with femtosecond spectroscopy and a redshift of the lowest optical transition is observed in the initial phase of carrier relaxation.
Abstract: The dynamics of carrier-induced absorption changes in CdSe quantum dots are investigated with femtosecond spectroscopy. After excitation with 4-eV photons a redshift of the lowest optical transition is observed in the initial phase of carrier relaxation. This shift is attributed to a biexciton effect where two electron-hole pairs interact via the Coulomb potential.

Journal ArticleDOI
TL;DR: A near-diffraction-limited transform-limited multiterawatt laser system that produces approximately 30-fs 125-mJ 800-nm pulses at a repetition rate of 10 Hz has been constructed.
Abstract: A near-diffraction-limited transform-limited multiterawatt laser system that produces approximately 30-fs 125-mJ 800-nm pulses at a repetition rate of 10 Hz has been constructed. Methods for the control of femtosecond time-scale phase and amplitude distortions have been developed and implemented.

Journal ArticleDOI
TL;DR: In this paper, a temporal hologram of a short optical pulse with a continuous-wave diode laser is read out and the shape information is converted into a spatial pattern that can be viewed with a camera.
Abstract: We report time-to-space mapping of femtosecond light pulses in a temporal holography setup. By reading out a temporal hologram of a short optical pulse with a continuous-wave diode laser, we accurately convert temporal pulse-shape information into a spatial pattern that can be viewed with a camera. We demonstrate real-time acquisition of electric-field autocorrelation and cross correlation of femtosecond pulses with this technique.



Journal ArticleDOI
TL;DR: In this paper, a β-barium borate (BBO) femtosecond optical parametric oscillator (OPO) pumped by the second-harmonic of a modelocked 82 MHz Ti:sapphire laser is described.

Journal ArticleDOI
TL;DR: In this article, the authors present a detailed description of the Caltech apparatus, which consists of a femtosecond laser system, a picosecond electron gun, and a two-dimensional charge-coupled device detection system.
Abstract: This paper, the fifth in a series, is concerned with the experimental description of ultrafast electron diffraction and its application to several isolated chemical systems. We present a detailed description of the Caltech apparatus, which consists of a femtosecond laser system, a picosecond electron gun, and a two-dimensional charge-coupled device ( CCD) detection system. We also discuss the analysis of the scattering patterns. Ultrafast diffraction images from several molecules (CCl_4, I_2, CF_3l, C_2F_4I_2) are reported. For our first study of a chemical reaction in a molecular beam, we show the change in the radial distribution function following the formation of CF_3 radical after dissociation of CF_3l. The total experimental temporal resolution is discussed in terms of the electron pulse width and velocity mismatch. The electron pulse was characterized temporally with a streaking technique that yielded the width as a function of the number of electrons per pulse. Experimental results show that the electron source produces picosecond (or less) pulses at densities of 100 electrons per pulse and 10-ps pulses at 1000 electrons per pulse. We also report our observation of a novel photoionization-induced lensing effect on the undiffracted electron beam, which we have used to establish time zero for UED when reactions are initiated by a laser pulse.

Journal ArticleDOI
TL;DR: In this paper, the dynamics of photoinduced electrons at the liquid-solid interface of aqueous CdS colloids on the femtosecond time scale were investigated and it was shown that electron trapping due to surface states or defect sites occurs in less than 100 fs.
Abstract: We report the direct measurements of the dynamics of photoinduced electrons at the liquid-solid interface of aqueous CdS colloids on the femtosecond time scale. The observed transient absorption is attributed to electrons trapped at the liquid-solid interface. We show that electron trapping due to surface states or defect sites occurs in less than 100 fs. The trapped electrons then decay by a double exponential with time constants of 2-3 ps and 50 ps at high excitation intensities, while a single-exponential decay (50 ps) is observed at low intensities. The slow, 50-ps decay is attributed primarily to geminate electron-hole recombination, which dominates the decay dynamics at low excitation intensities

Proceedings ArticleDOI
07 Sep 1994
TL;DR: A femtosecond and a nanosecond dye laser with pulse durations of 300 fs (< 200 (mu) J) and 7 ns (< 10 mJ), respectively, and center wavelengths at 612 and 600 nm, respectively, have been applied either to absorbing substrates, like polycrystalline gold, silicon (111), aluminum nitride ceramics, or transparent materials, like synthetic and human dental hydroxyapatite composites, bone material, and human cornea transplants as mentioned in this paper.
Abstract: Production of holes and grooves of > 1012 W cm-2, (tau) < 1 ps) opens up possibilities for materials processing by cold plasma generation and ablation of metals, semiconductors, ceramics, composites, and biological materials A femtosecond and a nanosecond dye laser with pulse durations of 300 fs (< 200 (mu) J) and 7 ns (< 10 mJ), and center wavelengths at 612 and 600 nm, respectively, both focused on an area of the order of 10-5 cm2, have been applied either to absorbing substrates, like polycrystalline gold, silicon (111), aluminum nitride ceramics, or transparent materials, like synthetic and human dental hydroxyapatite composites, bone material, and human cornea transplants The fs-laser generates its own absorption in transparent materials by a multiphoton absorption process, and thus forces the absorption of visible radiation Because the time is too short (< ps) for significant transport of mass and energy, the beam interaction generally results in the formation of a thin plasma layer of approximately solid state density Only after the end of the subpicosecond laser pulse, it expands rapidly away from the surface without any light absorption and further plasma heating Therefore, energy transfer (heat and impulse) to the target material, and thermal and mechanical disruption are minimized In contrast to heat- affected zones (HAZ's) generated by conventional nanosecond pulse lasers of the order of 1 - 10 micrometers , HAZ's of less than 002 micrometers were observed

Journal ArticleDOI
TL;DR: A tunable regenerative Ti:sapphire amplifier system working at a 1-kH(z) repetition rate pumps an optical parametric generator providing near-infrared pulses tunable in the wavelength range from 1 to 2.5 microm, as directly determined from cross-correlation measurements.
Abstract: A tunable regenerative Ti:sapphire amplifier system working at a 1-kH(z) repetition rate pumps an optical parametric generator providing near-infrared pulses tunable in the wavelength range from 1 to 2.5 microm. The signal and idler pulses with a duration below 100 fs are mixed in a AgGaS(2) crystal to generate pulses at the difference frequency. The midinfrared output is continuously tunable between 3.3 and 10 microm, with pulse energies of as high as 50 nJ and durations of 160 fs, as directly determined from cross-correlation measurements.

Journal ArticleDOI
TL;DR: In this paper, a femtosecond control pulse intercepts the predissociative system at a fixed time and internuclear separation, both determined by the "window" of the laser pulse.

Journal ArticleDOI
TL;DR: In this article, the authors proposed Thomson scattering of short pulse laser beams by low energy electron beams at a right angle for generation of femtosecond X-rays, based on the observation that low emittance electron beam can be focussed much more tightly in a transverse dimension than in the longitudinal dimension.
Abstract: We propose Thomson scattering of short pulse laser beams by low energy electron beams at a right angle for generation of femtosecond X-rays. The basic idea is the observation that a low emittance electron beam can be focussed much more tightly in a transverse dimension than in the longitudinal dimension. Therefore much shorter pulses of X-rays can be generated (in the direction of the electron beam) by arranging the laser beam to meet the electron beam at a right angle rather than head on as in the Thomson backscattering configuration. Simple analysis of the process is presented by noting the similarity between the Thomson scattered radiation and the well-understood undulator radiation. Using the parameters of the recently developed femtosecond visible lasers and the high brightness electron guns, it is shown that 1 A X-ray pulses, of 300 fs duration, containing several 10 5 photons within 10% bandwidth per collision, can be generated.

Journal ArticleDOI
TL;DR: Pump-probe studies of I 2 in matrix Ar, with a time resolution of 180 fs, are reported in this paper, where the experiments are simulated using classical molecular dynamics, and the recombinant molecule vibrates coherently even after extensive energy loss.