Showing papers on "Femtosecond published in 1987"

Femtosecond studies of nonequilibrium electronic processes in metals.

Abstract: High-intensity femtosecond laser pulses are used to induce non- equilibrium electron heating in gold metal. The thermal relaxation of the electronic distribution is studied through pump and continuum probe measurements of transient reflectivity. Measurements are performed for different heating-pulse fluences and probe-photon energies. The observed reflectivity line shape demonstrates the generation of nonequilibrium electron temperatures which cool to the lattice on a 2--3 ps time scale.

Femtosecond electronic heat-transport dynamics in thin gold films.

Abstract: We have observed ultrafast heat transport in thin gold films under femtosecond laser irradiation. Time-of-flight (from-pump back-probe) measurements indicate that the heat transit time scales linearly with the sample thickness, and that heat transport is very rapid, occurring at a velocity close to the Fermi velocity of electrons in Au.

Excess electrons in liquid water: First evidence of a prehydrated state with femtosecond lifetime

Abstract: The localization and solvation of excess electrons in pure water have been resolved at the femtosecond time scale. Before it becomes solvated, the electron thermalizes and reaches in 110 fs a localized state absorbing in the infrared. This transient species with lifetime 240 fs has been postulated to exist but has not been observed previously in liquid water.

Femtosecond UV excimer laser ablation

Abstract: Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient αeff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.

Single-shot measurement of a 52-fs pulse.

Abstract: We demonstrate the possibility of a 52-fs pulse single-shot measurement by using the spatial analysis of the second harmonic beam produced in an optical autocorrelator. We show that this method gives a reliable means of optimizing low repetition rate femtosecond amplifiers. Since the introduction of femtosecond laser systems in 19811 much progress has been made toward reducing the duration of the produced pulses. These lasers are able to produce pulses shorter than 70 fs and with a peak power up to 10 GW. 2 3 Moreover, one always uses the classical background-free autocorrelation scheme 4 (modified Michelson interferometer with a scanning arm) to measure the pulses. There have not been significant advances in obtaining a real time measurement (i.e., made with a single pulse) of femtosecond pulses. In the picosecond domain, streak cameras with 0.4-ps rise time exist, but they cannot be used for 100-fs pulses. At this time, there is much theorizing about new measurement systems for femtosecond pulses 5 - 7 but few experimental demonstrations of their capabilities. The problem is to transform the temporal information (shape of the pulse) into a more convenient form (spatial or slowly varying signal). In a classical autocorrelator the delay between the two pulses is slowly swept, and the autocorrelation function is built from a large number of successive pulses. In the picosecond domain, single-shot measurements have been demonstrated using two-photon fluorescence (TPF) 8 or second harmonic generation. 9 In a recent paper, Sperber and Penzkofer 8 show that TPF traces could be decorrelated to determine the approximate temporal shape of modulated pulses. Nevertheless, the continuous background signal produced by each individual beam gives a maximum contrast ratio of 3:1 between the maximum value of the autocorrelation function and the background level. In 1981, Wyatt and Marinero 9

Femtosecond Studies of Coherent Transients in Semiconductors

Abstract: The coherent interaction of femtosecond laser pulses and thin CdSe and GaAs samples is investigated experimentally and theoretically Oscillatory structures in the differential probe transmission around the exciton resonance and around the pump frequency are observed when the probe pulse precedes the pump Comparison with theory attributes the oscillations to the coherent coupling between the light field and the electron-hole transitions in the semiconductor For nonresonant excitation, the oscillatory structures around the exciton are identified as the early stages of the optical Stark effect

Oscillatory femtosecond relaxation of photoexcited organic molecules

Abstract: The ultrafast relaxation of large organic molecules in solution has been investigated with the transmission correlation technique. The existence of damped sinusoidal decays after photoexcitation has been verified for several organic dyes. This behavior (the first such decay observed on a femtosecond time scale for any material) is explained as quantum beats. For all molecules examined, we have also observed fast initial exponential decays (<100 fs) in addition to previously known picosecond processes.

Generation of high-peak-power tunable infrared femtosecond pulses in an organic crystal: application to time resolution of weak infrared signals

Abstract: High-intensity femtosecond pulses tunable in the 0.8–1.6-μm range have been generated by parametric amplification of a continuum white light in a new organic crystal, N-(4-nitrophenyl)-L-prolinol (NPP). The traditional concept of noncritical phase matching was revised in view of requirements linked to the observation of ultrafast subpicosecond nonlinear phenomena. The notions of θ (noncritical) and λ (noncritical) phase matching are introduced together with their applications. An experimental determination of phase-matching curves for both second-harmonic generation and three-wave mixing has been carried out. A θ noncritical phase-matching configuration for second-harmonic generation at 1.15 μm and a quasi-λ noncritical phase-matching configuration in the near IR for three-wave mixing were evidenced. Frequency and pump-intensity dependences of the gain have also been studied. Parametric emission at degeneracy was observed, with the emitted bandwidth extending from 1.0 to 1.4 μm. Time resolution of the amplified signal has been carried out by cross correlating the pump with the incoming signal, evidencing a reduced time broadening of the interacting pulses; a new spectroscopic method with subpicosecond time resolution is derived from the previous nonlinear optical characterization experiments by replacing the IR continuum from the water cell by any sample emitting in the same frequency range. This method, termed parametric amplification and sampling spectroscopy, was used for temporal analysis of amplified and emitted infrared signals generated in an NPP crystal.

Femtosecond carrier dynamics in GaAs

Abstract: Femtosecond carrier dynamics in GaAs and Al0.3Ga0.7As are investigated using pump probe measurements of transient absorption saturation. Pulses of 35 fs duration are used both to excite carriers and to investigate their subsequent scattering out of their initial optically excited states. A two‐component ultrafast relaxation is observed. In GaAs the initial rapid relaxation occurs on a time scale of 10–35 fs. Measurements performed in Al0.3Ga0.7As indicate that this initial process slows significantly to 130–170 fs for increasing band gap.

Femtosecond laser-tissue interactions: Retinal injury studies

Abstract: 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 50 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 50-100 \mu 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.

Picosecond and femtosecond Fourier pulse shape synthesis

Abstract: 2014 We review our work on the synthesis of arbitrarily shaped, coherent optical pulses, by spatial filtering in a grating pulse compressor. We describe picosecond pulses shaped using a fiber and grating pulse compressor and femtosecond pulses shaped using a nondispersive grating compressor. The discussion includes generation of coherent square pulses and abruptly phase-modulated « odd » pulses, two slit temporal interference measurements of the optical phase spectrum, and ultrashort pulse encryption and decoding by phase scrambling. We conclude with a proposal for an ultrafast optical self-routing switch based upon frequency domain phase coding of ultrashort pulses. Revue Phys. Appl. 22 (1987) -1628 DÉCEMBRE 1987, PAGE 1619

Evolution of femtosecond pulses in single-mode fibers having higher-order nonlinearity and dispersion.

Abstract: Using computer simulations, we examine the effects of higher-order dispersive and nonlinear propagation processes on the spectral and time development of ultrashort, high-intensity pulses propagating in single-mode optical fibers having normal dispersion. Our results indicate that both the cubic-dispersion term and the shock term of the nonlinear Schrodinger equation contribute to asymmetry in the pulse power spectrum and cause highly nonlinear chirp.

Femtosecond hot‐carrier energy relaxation in GaAs

Abstract: Excited carrier dynamics in GaAs and Al0.2Ga0.8As are investigated using femtosecond pump and continuum probe techniques. We observe absorption spectral hole burning arising from excited carriers generated by transitions from the split‐off band as well as the heavy‐ and light‐hole bands. Transient absorption saturation measurements indicate that the initial nonthermal carrier distribution thermalizes on a time scale of several tens of femtoseconds.

Femtosecond dephasing in a polydiacetylene film measured by degenerate four-wave mixing with an incoherent nanosecond laser

Abstract: Dephasing times in a polydiacetylene (poly-3BCMU) film were resolved for the first time at two wavelengths by degenerate four-wave mixing using incoherent light. The dephasing times, 30 fs at 648 nm and 130 fs at 582 nm, correspond to excitons in chains of the polymer with different conjugation lengths.

Decay of femtosecond higher-order solitons in an optical fiber induced by Raman self-pumping.

Abstract: We have numerically solved an extended version of the nonlinear Schrodinger equation, taking into account higher-order dispersion, the shock (self-steepening) term, and a term describing the Raman self-pumping of an ultrashort pulse. It is shown that the Raman effect is dominant on a femtosecond time scale and leads to the decay of higher-order solitons. For the case of the N = 2 soliton an intense pulse at a distinctly Stokes-shifted frequency is created. This pulse eventually shapes into a fundamental soliton, and its further evolution is governed by the combination of dispersion, self-phase modulation, and the soliton self-frequency shift.

Generation and kilohertz-rate amplification of femtosecond optical pulses around 800 nm

Abstract: The generation of optical pulses of less than 100-fsec duration in the near-infrared region at wavelengths around 800 nm is reported. These pulses were amplified to microjoule energies at a 6-kHz repetition rate, producing focusable power densities of greater than 1013 W/cm2. A white-light continuum extending into the ultraviolet and infrared regions results when these pulses are focused into a jet of ethylene glycol, opening new possibilities for femtosecond spectroscopic investigations.

Femtosecond dynamics of highly excited carriers in AlxGa1−xAs

Abstract: Femtosecond transient absorption saturation measurements are used to investigate the scattering of optically excited carriers in AlGaAs. With pulses as short as 35 fs at 1.98 eV, scattering times ranging from 13 to 330 fs are observed in samples of AlxGa1−xAs with x=0, 0.1, 0.2, 0.3, and 0.4. A dramatic decrease in the rate of carrier scattering out of the initial optically excited states is observed with increasing Al concentration.

Generation and kilohertz-rate amplification of femtosecond optical pulses around 800 nm

Abstract: The generation of optical pulses of less than 100-fsec duration in the near-infrared region at wavelengths around 800 nm is reported. These pulses were amplified to microjoule energies at a 6-kHz repetition rate, producing focusable power densities of greater than 1013 W/cm2. A white-light continuum extending into the ultraviolet and infrared regions results when these pulses are focused into a jet of ethylene glycol, opening new possibilities for femtosecond spectroscopic investigations.

Femtosecond streak tube

Abstract: A new design concept of a streak tube is conceived to obtain femtosecond temporal resolution. It is based on four factors as follows. First, in order to decrease the photoelectron transit time spread, one must investigate how high electric field is practically usable near the photocathode. It is found that the value can be increased up to ∼6 kV/mm and this is used as a design value. Second, as the other method to decrease the photoelectron transit time spread, an electromagnetic focusing method must be adopted because there is no region similar to the focusing section of an electrostatic focusing type where the electric potential is low. Third, the focusing magnetic field should be located in a limited region to obtain a very high sweep speed of ∼2×108 m/s. And fourth, in the operation, a special readjustment of the focusing magnetic field should be performed to compensate the photoelectron beam defocusing caused by the very rapid transient deflection field. Based on the above, a new streak tube has been designed, manufactured, and tested. The limiting temporal resolution of ∼400 fs has been predicted theoretically and the value less than ∼500 fs has been experimentally produced.

Femtosecond synchronously pumped Pyridine dye lasers.

Abstract: Optical pulses as short as 103 fsec at 695 nm and 263 fsec at 733 nm have been generated in synchronously pumped hybrid Pyridine 1 and Pyridine 2 cw dye lasers, respectively, both using the saturable absorber 1,1′-diethyl-2,2′-dicarbocyanine iodide (DDI). These results combine with other data from the same linear cavity to give direct femtosecond generation over the spectral range 560–840 nm.

Photomultiplier tube having a transmission strip line photocathode and system for use therewith

Abstract: A photomultiplier tube which may be used in time resolving a luminiscence profile emitted from a sample with picosecond resolution using short (picosecond) electrical pulses as a probe and in time resolving an electrical pulse profile produced by fast electronic or optoelectronic devices with femtosecond resolution, using short (femtosecond) laser pulses as the probe is disclosed. The photomultiplier tube includes a photocathode for receiving light and producing emission of electrons in proportion to the intensity of the light, said photocathode having a transmission strip line configuration, accellerating means for accellerating electrons emitted by said photocathode, electron multiplication means for performing electron multiplication on the electrons emitted from the accellerating means, anode means for receiving electrons from the electron multiplication means and producing an analog electrical signal output, means for causing electrons emitted by the photocathode to move through the accellerating means and the electron multiplication means and then impinge on the anode means, and means connected to said photocathode for receiving an ultrafast voltage signal.

Effects of self, induced and cross phase modulations on the generation of picosecond and femtosecond white light supercontinua

Abstract: 2014 The effects of self, induced, and cross phase modulations on the generation of picosecond and femtosecond « white » laser pulses are reviewed. Recent measurements of cross and induced phase modulations are reported using femtosecond and picosecond laser pulses. Revue Phys. Appl. 22 (1987) -1694 DÉCEMBRE 1987, PAGE 1677