Showing papers on "Femtosecond published in 1990"
TL;DR: In this paper, a bare semiconductor wafer was illuminated by femtosecond optical pulses, and electromagnetic waves radiated from the surface and formed collinear diffraction-limited electromagnetic beams in the inward and outward directions.
Abstract: We have generated electromagnetic beams from a variety of semiconductors. When a bare semiconductor wafer was illuminated by femtosecond optical pulses, electromagnetic waves radiate from the surface and form collinear diffraction‐limited electromagnetic beams in the inward and outward directions. The amplitude and phase of the radiated field depend on carrier mobility, the strength and polarity of the static internal field at the semiconductor surface.
457 citations
TL;DR: Optical control over elementary molecular motion is enhanced with timed sequences of femtosecond pulses produced by pulse-shaping techniques, analogous to repetitively pushing a child on a swing with appropriate timing to build up a large oscillation amplitude.
Abstract: Optical control over elementary molecular motion is enhanced with timed sequences of femtosecond (10-15 second) pulses produced by pulse-shaping techniques. Appropriately timed pulse sequences are used to repetitively drive selected vibrations of a crystal lattice, in a manner analogous to repetitively pushing a child on a swing with appropriate timing to build up a large oscillation amplitude. This process corresponds to repetitively "pushing" molecules along selected paths in the lattice. Amplification of selected vibrational modes and discrimination against other modes are demonstrated. Prospects for more extensive manipulation of molecular and collective behavior and structure are clearly indicated.
417 citations
TL;DR: This work reports programmable shaping of femtosecond optical pulses by use of a multielement liquid-crystal modulator to manipulate the phases of spatially dispersed optical frequency components.
Abstract: We report programmable shaping of femtosecond optical pulses by use of a multielement liquid-crystal modulator to manipulate the phases of spatially dispersed optical frequency components. Our approach provides for continuously variable control of the optical phase and permits the pulse shape to be reconfigured on a millisecond time scale. We use the apparatus to demonstrate femtosecond pulse-position modulation as well as programmable compression of chirped femtosecond pulses.
378 citations
TL;DR: Coherent LO phonons are excited at the surface of bulk GaAs with femtosecond laser pulses through electro-optic modulations of the transient reflectivity through which the components of the coherent phonon states dephase relative to each other in the order of picoseconds depending on the density of optically excited electron-hole pairs.
Abstract: Coherent LO phonons are excited at the surface of bulk GaAs with femtosecond laser pulses. They are observed for the first time through electro-optic modulations of the transient reflectivity. The corresponding signal oscillations are superimposed upon an additional longitudinal polarization feature which decays exponentially and whose rise time is sufficiently short to act as a driving force for the coherent phonons. The components of the coherent phonon states dephase relative to each other in the order of picoseconds depending on the density of optically excited electron-hole pairs.
345 citations
TL;DR: In this paper, the authors used femtosecond nonlinear optical measurements to probe molecular vibrations and elementary photophysical and photochemical events in real-time, using pump-probe spectroscopy.
Abstract: Ultrafast optical spectroscopy using laser pulses as short as 6 femtoseconds provides a novel probe for molecular nuclear motions ( l , 2). Since the duration of such pulses is comparable to or shorter than typical molecular vibrational periods ( 1 0 fsec is the period of a 3300 cm -1 vibration), these pulses make it possible to probe molecular vibrations and elementary photophysical and photochemical events in real time. Femtosecond dynamics and relaxation studies have been carried out in solutions (39a,b), neat liquids ( 1 01 3), conjugated polymers ( 1 41 5), proteins and biological systems ( 1 6-1 7), crystals (I 8a-c) , surfaces ( 1 9a,b), semi conductors (20-21 ), molecular aggregates (22-23), the hydrated electron (24a-c), isolated molecules in supersonic beams (25-26), and the gas phase (27a,b). Femtosecond nonlinear optical measurements have numerous unique advantages. Consider, for example, the simplest ultrafast spectro scopic technique: pump-probe spectroscopy (4-6, 25a-c). Tn this technique the system is subjected to two short pulses separated by a time delay r. The first pulse (the pump pulse) has a frequency OJ 1 and the absorption of
243 citations
TL;DR: In this paper, a tunable, singley-resonant optical parametric oscillator provides femtosecond light pulses in the infrared wavelength band, including an internally mounted thin crystal of KTiOPO 4 which is synchronously pumped by femto-cond pulses from a colliding-pulse passively mode-locked dye laser.
Abstract: A tunable, singley-resonant optical parametric oscillator provides femtosecond light pulses in the infrared wavelength band. The oscillator includes an internally mounted thin crystal of KTiOPO 4 which is synchronously pumped by femtosecond pulses from a colliding-pulse passively mode-locked dye laser. Rotation of the crystal varies the wavelength of the oscillator. Prisms within the oscillator cavity control group velocity dispersion to limit pulse width, and the oscillator as stabilized by a feedback network which varies the length of the cavity in response to spectral changes.
237 citations
TL;DR: Theoretical and experimental results are reported which provide the first evidence for biexciton states in semiconductor quantum dots, observed as pronounced induced absorption features on the high-energy side of the bleached exciton resonances in femtosecond and nanosecond pump-probe experiments of quantum dots in glass matrices.
Abstract: Theoretical and experimental results are reported which provide the first evidence for biexciton states in semiconductor quantum dots. The theory predicts an increasing biexciton binding energy with decreasing dot size. Unlike bulk semiconductors, quantum dots have excited biexciton states which are stable. These biexciton states are observed as pronounced induced absorption features on the high-energy side of the bleached exciton resonances in femtosecond and nanosecond pump-probe experiments of quantum dots in glass matrices.
230 citations
TL;DR: In this article, the authors report real-time observations of ultrafast molecular vibrations and rotations using femtosecond laser pulses, where the system is prepared in the particular state(s) of interest by coherent excitation with an initial femtocond laser pulse, and the subsequent motions are probed with successive femto-cond pulses.
Abstract: Ultrafast molecular vibrations and rotations are the fundamental motions that characterize chemical bonding and determine reaction dynamics at the molecular level. The timescales for these motions are typically 10^(−10) s for vibrations and 10^(−13) s for rotations. For decades, time-integrated (frequency-resolved) spectros-copy has provided a powerful tool for probing the dynamics of motion, but the motions themselves are not 'seen' directly in real-time. With femtosecond laser techniques it is now possible to follow the motions of isolated molecular systems as they occur. The requirement is that the system is excited (for vibration) and aligned (for rotation) on a timescale shorter than the vibrational and rotational periods. Here we report real-time observations of these molecular motions. The system—in this case, molecular iodine—is prepared in the particular state(s) of interest by coherent excitation with an initial femtosecond laser pulse, and the subsequent motions are probed with successive femtosecond pulses. The probe monitors changes in the interatomic distance (vibration) or molecular orientation (rotation), so that the measured signal provides direct 'snapshots' of the molecular motions.
171 citations
TL;DR: In this article, the authors studied ultrafast gain dynamics in InGaAsP optical amplifiers by means of pump-probe and cross-correlation measurements using 180 fs optical pulses.
Abstract: We have studied ultrafast gain dynamics in InGaAsP optical amplifiers by means of pump‐probe and cross‐correlation measurements using 180 fs optical pulses. The data show strong gain nonlinearities due to nonequilibrium carrier distributions and differ significantly from those observed in AlGaAs amplifiers.
160 citations
TL;DR: In this article, the Fourier transform four-wave mixing (FT 4WM) formalism is applied to femtosecond optical-heterodyne detected optical Kerr effect data for pyridine liquid.
150 citations
TL;DR: A simple linear filtering technique for transforming individual femTosecond light pulses into terahertzrepetition-rate bursts of femtosecond pulses achieves high efficiency and pulse repetition rates approaching 6 THz.
Abstract: We describe a simple linear filtering technique for transforming individual femtosecond light pulses into terahertzrepetition-rate bursts of femtosecond pulses. By using phase-only filtering, high efficiency is achieved. Pulse repetition rates approaching 6 THz are obtained.
TL;DR: In this article, the use of femtosecond temporal spectroscopy (FTS) was shown to yield accurate spectral information (vibrational and rotational) and to invert directly to the potential energy curve.
TL;DR: In this article, a highly efficient frequency doubler for femtosecond pulses was developed, and the design aspects of the doubler were discussed, as well as experimental results testing with 300 fs, 496 nm pulses showed no spectral narrowing.
Abstract: A highly efficient frequency doubler is developed for femtosecond pulses. Design aspects of the device are discussed. Doubling bandwidth calculations have shown, that with one millimeter long crystals pulses as short as 10 fs can be frequency doubled. Experimental results testing the doubler with 300 fs, 496 nm pulses are presented showing no spectral narrowing.
TL;DR: In this article, femtosecond pulsed laser techniques are used to follow the evolution in space and time of particle trajectories during the breakage of a chemical bond in the dissociation of sodium iodide.
Abstract: IN chemical reactions, the dynamics of the transition from reagents to products can be described by the trajectories of particles (or rigorously, of quantum mechanical wave packets) moving on a potential-energy surface. Here we use femtosecond pulsed laser techniques to follow directly the evolution in space and time of such trajectories during the breakage of a chemical bond in the dissociation of sodium iodide. The bond breakage can be described in terms of the time evolution of a single reaction coordinate, the internuclear separation. As the velocities of the separating fragments are typically of the order of a kilometre per second, a time resolution of a few tens of femtoseconds is required to view the motions on a molecular distance scale of less than an angstrom. The resolution obtained here permits the direct visualization of the wave packet's motion and provides snapshots of the trajectories along the reaction coordinate.
TL;DR: In this paper, the femtosecond laser beam from a semiconductor was used to determine impurity doping concentration, carrier mobility, sign, and strength of the depletion field near the semiconductor surface.
Abstract: Ultrafast electromagnetic radiation induced by a femtosecond laser beam from a semiconductor provides determination of the impurity doping concentration, carrier mobility, sign, and strength of the depletion field near the semiconductor surface.
Proceedings Article•
21 May 1990
TL;DR: This work has shown that femtosecond thermo-modulation experiments performed to study the relaxation dynamics of electrons in metals results in a nonexponential decay in electrons heated to a high nonequilibrium temperature.
Abstract: In recent years, femtosecond thermo-modulation experiments1–3 have been performed to study the relaxation dynamics of electrons in metals. The electrons were heated to a high (>1000 K) nonequilibrium temperature. In these extreme conditions, the dynamics depends strongly on the electron temperature resulting in a nonexponential decay.
TL;DR: In this article, a time-dependent wavepacket theory for dynamic absorption spectroscopy is used to analyze the evolution of the transient absorption spectrum of nile blue following excitation with a 6 fs pulse.
TL;DR: In this article, the wavelength dependence of the nonresonant nonlinear index of refraction, n2, in AlGaAs waveguides at room temperature was measured using time division interferometry with 430 fs tunable laser pulses.
Abstract: Time‐division interferometry with 430 fs tunable laser pulses is used for direct femtosecond measurements of the wavelength dependence of the nonresonant nonlinear index of refraction, n2, in AlGaAs waveguides at room temperature. Below band‐gap n2 values of ∼10−12 cm2/W are observed with resonant enhancement as the laser wavelength is tuned toward the band edge.
TL;DR: In this paper, the proton transfer in 2-(2′-hydroxy-5′-methylphenyl)benzotriazole is studied in nonpolar solvents, where two major ground state conformers are identified.
15 Dec 1990
TL;DR: In this paper, the femtosecond investigations of the very primary processes occurring in pure liquid water following a photoionization of solvent molecules by ultraviolet femtocond pulses were conducted.
Abstract: The elucidation of detailed mechanisms of primary events in molecular dynamics, charge transfer or reaction dynamics have been made possible by advances in spectroscopic techniques using ultrashort laser pulse generation. In this paper we will center on femtosecond investigations of the very primary processes occurring in pure liquid water following a photoionization of solvent molecules by ultraviolet femtosecond pulses. We have observed in the near ultraviolet region (460, 410 nm), an instantaneous transient absorption with ultrashort lifetime which appears during the initial energy deposition. This induced absorption rises within the pulse, i.e. in less than 100 fs and faster than the precursor of the fully hydrated electron. Its relaxation can be described by a monoexponential law. This ultrashort transient absorption is tentatively assigned to the water cation H2O+. The relaxation would then correspond to the ion-molecule reaction H2O+ + H2O → H3O+ + OH for which the cleavage rate constant is measured to be 1013 s−1 at 294 K. An H/D isotope effect on the dynamics of the ion-water molecule reaction has been observed. These results are analyzed considering the dynamical properties of the protic solvent. The influence of the local dynamical molecular structure of the fluid on the primary reactions involving an ultrafast geminate recombination (e−hyd…X3O+; e−hyd…OX with X = H or D) will be discussed.
TL;DR: In this paper, femtosecond single-pulse and time-resolved absorbance in an optically thin silicon film is used to extract the direct two-photon absorption coefficient βTPA of silicon over a wide spectral range; it is also used to distinguish other nonlinear absorption channels, even with the fundamental pulse far above the indirect gap.
Abstract: Measurement of femtosecond single-pulse and time-resolved absorbance in an optically thin silicon film is used to extract the direct two-photon absorption (TPA) coefficient βTPA of silicon over a wide spectral range; it is also used to distinguish other nonlinear absorption channels, even with the fundamental pulse far above the indirect gap. We find that βTPA varies as follows: 15 cm/GW < βTPA < 36 cm/GW ± 6 cm/GW over the two-photon energy range 4.0 eV < 2hν < 4.5 eV, consistent with available calculated values. The spectral structure of the results is related to the underlying band structure. The results show that second-order absorption nonlinearities dominate for fluences up to 0.07 J/cm2, just below the single-shot melting threshold. At fluences above this level a much stronger nonlinearity dominates.
TL;DR: Femtosecond probing of nuclear motion in an alkali halide (NaI) reaction reveals a persistent wave packet, recurring at long times up to 40 ps as mentioned in this paper, indicating non-exponential dynamics through the crossing zone of the covalent and ionic potential energy curves.
TL;DR: In this article, femtosecond laser pulses are used to probe nonequilibrium carrier distributions in high-Tc superconducting films and an increase in the relaxation time and peak fractional transmissivity change was observed when the films became superconducted.
Abstract: Femtosecond laser pulses are used to probe nonequilibrium carrier distributions in high-Tc superconducting films. An increase in the relaxation time and peak fractional transmissivity change was observed when the films became superconducting. No such changes were observed for a nonsuperconducting film.
TL;DR: The results show that the method is most advantageous for incident pulses between 0.1- and 1.0-ps duration and microjoule and higher energies and that second harmonic pulse duration and conversion efficiency are not highly sensitive to optical misalignments of the order of 1 degrees.
Abstract: We theoretically analyze a method for matching group velocities of fundamental and second harmonic femtosecond pulses during phase matched frequncy doubling by predispersing the fundamental pulse with a prism. The method permits improved conversion efficiency by allowing crystal lengths of several millimeters without sacrificing second harmonic pulse duration. Second harmonic pulse energy and duration are analyzed for beta-BaB(2)O(4), and limiting experimental factors are discussed. The results show that the method is most advantageous for incident pulses between 0.1- and 1.0-ps duration and microjoule and higher energies and that second harmonic pulse duration and conversion efficiency are not highly sensitive to optical misalignments of the order of 1 degrees .
TL;DR: In this paper, the authors investigated the dynamics of resonant third-order optical nonlinearity of chemically prepared poly(3-dodecylthiophene) by the degenerate four wave mixing technique using 60 fs pulses at 620 nm.
Abstract: We have investigated the dynamics of resonant third‐order optical nonlinearity of chemically prepared poly(3‐dodecylthiophene) by the degenerate four wave mixing technique using 60 fs pulses at 620 nm. The measured effective value of χ(3) is 5.5×10−11 esu, sixfold smaller than that obtained with 400 fs pulses, emphasizing the pulse width dependence of effective χ(3) when the relaxation time of the photogenerated excitation responsible for the optical nonlinearity is comparable to the pulse width. Within the resolution of the optical pulse, the rise time of the nonlinear response is instantaneous and the dominant decay occurs within 200 fs, revealing that the short time, nonlinear response is derived from the initially photogenerated excitons. A detailed analysis of the total decay behavior is consistent with the polaron dynamics of the conformational deformation model proposed by Su, Schrieffer, and Heeger for a conjugated linear polymer with bond alternation.
TL;DR: In this paper, a femtosecond laser study of cis-stilbene isomerization is presented, which shows the absorption of the hot trans product decaying in 20 ps.
Abstract: Femtosecond laser studies of cis‐stilbene isomerization show the absorption of the hot trans product decaying in 20 ps. An initial internal temperature of 725±100 K is estimated which decays with a ca. 14 ps time constant. This temperature accounts for less than one‐half the photon energy implying significant energy loss by solvent friction.
TL;DR: A singly resonant optical parametric oscillator is demonstrated that uses a beta-barium borate crystal and is synchronously pumped by microsecond pulse trains of a Nd:glass laser to obtain parametric pulses as short as 65 +/- 7 fsec.
Abstract: A singly resonant optical parametric oscillator is demonstrated that uses a β-barium borate crystal and is synchronously pumped by microsecond pulse trains of a Nd:glass laser (0.8 psec, 527 nm). Broadly tunable pulses are generated in the wavelength range 0.7–1.8 μm with a duration of 160–260 fsec and an energy conversion of ∼3%. Steep pulse wings are observed, with a 1/e decay time of ≃90 fsec. Under special conditions, parametric pulses as short as 65 ± 7 fsec are obtained.
TL;DR: In this paper, it was shown that both population and phase control of the packet motion can be observed by a 2-D pulse sequence of variable delay times and phase angles, which can be extended to other type of control experiments.
TL;DR: In this paper, a simple kinetic model was proposed to describe the trap-rate constant and the maximum number of electrons which can be emitted per particle, as a function of pump fluence.