Showing papers on "Femtosecond published in 1991"

The first step in vision: femtosecond isomerization of rhodopsin.

Abstract: The kinetics of the primary event in vision have been resolved with the use of femtosecond optical measurement techniques. The 11-cis retinal prosthetic group of rhodopsin is excited with a 35-femtosecond pump pulse at 500 nanometers, and the transient changes in absorption are measured between 450 and 580 nanometers with a 10-femtosecond probe pulse. Within 200 femtoseconds, an increased absorption is observed between 540 and 580 nanometers, indicating the formation of photoproduct on this time scale. These measurements demonstrate that the first step in vision, the 11-cis----11-trans torsional isomerization of the rhodopsin chromophore, is essentially complete in only 200 femtoseconds.

Optics of Femtosecond Laser Pulses

Abstract: Contents: Short Optical Pulses in Linear Dispersive Media. Self-action of Optical Pulses Self-modulation, Self-compression, Solitons, and Instabilities. Parametric Interactions and Coherent Scattering of Femtosecond Pulses. Fast Phase Control. Compression and Shaping of Optical Pulses. Optical Solitons. Picosecond and Femtosecond Pulses in Optical Information Systems.

Fluorescence-detected wave packet interferometry: Time resolved molecular spectroscopy with sequences of femtosecond phase-locked pulses

Abstract: We introduce a novel spectroscopic technique which utilizes a two‐pulse sequence of femtosecond duration phase‐locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in‐phase, in‐quadrature, and out‐of‐phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule.These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct de...

Direct Observation of Vibrational Coherence in Bacterial Reaction Centers Using Femtosecond Absorption Spectroscopy

Abstract: It is shown that vibrational coherence modulates the femtosecond kinetics of stimulated emission and absorption of reaction centers of purple bacteria. In the DLL mutant of Rhodobacter capsulatus, which lacks the bacteriopheophytin electron acceptor, oscillations with periods of approximately 500 fs and possibly also of approximately 2 ps were observed, which are associated with formation of the excited state. The kinetics, which reflect primary processes in Rhodobacter sphaeroides R-26, were modulated by oscillations with a period of approximately 700 fs at 796 nm and approximately 2 ps at 930 nm. In the latter case, at 930 nm, where the stimulated emission of the excited state, P*, is probed, oscillations could only be resolved when a sufficiently narrow (10 nm) and concomitantly long pump pulse was used. This may indicate that the potential energy surface of the excited state is anharmonic or that low-frequency oscillations are masked when higher frequency modes are also coherently excited, or both. The possibility is discussed that the primary charge separation may be a coherent and adiabatic process coupled to low-frequency vibrational modes.

Femtosecond time-resolved molecular multiphoton ionization: The Na2 system.

Abstract: We report here the first experimental study of femtosecond time-resolved molecular multiphoton ionization. Femtosecond pump-probe techniques are combined with time-of-flight spectroscopy to measure transient ionization spectra of ${\mathrm{Na}}_{2}$ in a molecular-beam experiment. The wave-packet motions in different molecular potentials show that incoherent contributions from direct photoionization of a singly excited state and from excitation and autoionization of a bound doubly excited molecular state determine the observed transient ionization signal.

Femtosecond pulses from a continuously self-starting passively mode-locked Ti:sapphire laser

Abstract: We show that an external coupled cavity containing a nonlinear quantum-well reflector can continuously self-start a dispersion-compensated Ti:sapphire laser, which produces stable time-transform-limited pulses as short as 70 fs in a TEM00 mode. In this mode of operation, the quantum wells do not control the mode-locking process, as in previous research on the resonant passive mode-locked laser. By separating the mode-locking and starting processes, we show that the presence of higher-order spatial modes is not required to start or sustain mode locking.

Ultrafast electronic disordering during femtosecond laser melting of GaAs.

Abstract: We have observed an ultrarapid electronic phase transformation to a centrosymmetric electronic state during laser excitation of GaAs with intense femtosecond pulses. Reflection second-harmonic intensity from the upper 90 atomic layers vanishes within 100 fs; reflectivity rises within 0.5 ps to a steady value characteristic of a metallic molten phase, long before phonon emission can heat the lattice to the melting temperature.

Measurement of femtosecond ionization dynamics of atmospheric density gases by spectral blueshifting.

Abstract: The new plasma diagnostic technique of spectral blueshifting of femtosecond pulses is used for the first time to analyze quantitatively the ionization of noble gases under the influence of intense, femtosecond illumination. The two processes of strong-field tunneling ionization and electron impact ionization are found to play competing roles on these time scales

Femtosecond non-Markovian optical dynamics in solution.

Abstract: Femtosecond photon-echo experiments on sodium resorufin in dimethylsulfoxide at room temperature show that optical dephasing in solution is of non-Markovian character. A single Gauss-Markov stochastic modulation process is used to interpret both the femtosecond light-scattering results and the steady-state absorption spectrum. The optical dynamics of this system appears to fall in the intermediate modulation regime. No evidence was found for contributions of slower dynamical processes to the absorption line breadth.

Imaging objects hidden in highly scattering media using femtosecond second-harmonic-generation cross-correlation time gating.

Abstract: A combination of 100-fs cross-correlation time gating and lock-in amplifier detection is shown to be a versatile technique to image objects hidden in highly scattering media. The image is formed from the ballistic component of the ultrafast laser pulse while the diffuse component is eliminated by time gating using cross-correlation second-harmonic generation. The low-noise lock-in amplifier detection method and the high-repetition-rate milliwatt pulsed laser enable us to image through a random medium as thick as 28 scattering mean free paths with submillimeter resolution.

Femtosecond soliton propagation in fibers with slowly decreasing dispersion

Abstract: We obtain the adiabatic compression of femtosecond solitons in fibers with slowly decreasing dispersion (FSDD’s) and discover the effect of soliton pulse-width stabilization in FSDD’s. A theoretical model for the description of femtosecond-pulse propagation in FSDD’s is presented. The model takes into account higher-order nonlinear and dispersive effects in fibers. We present numerical simulations of a generalized nonlinear Schrodinger equation and the adiabatic soliton approximation theory as well. Good quantitative agreement between the theory and the experiments is seen. The effect of soliton pulse-width stabilization in FSDD’s is described as a result of the combined action of the third-order dispersion and Raman self-scattering effects. We also show the possibility of obtaining high-quality pulses with less than a 15-fs pulse width in the 1.6-μm spectral region by soliton compression in FSDD’s.

Femtosecond multiple-pulse impulsive stimulated Raman scattering spectroscopy

Abstract: Optical control of elementary molecular motion through impulsive stimulated Raman scattering is enhanced by means of timed sequences of femtosecond pulses that are produced by pulse-shaping techniques. In particular, terahertz-rate trains of femtosecond pulses are used for repetitive impulsive excitation of individual phonon modes in an α-perylene molecular crystal. When the pulse repetition rate is matched to the desired phonon frequency, mode-selective vibrational amplification is achieved. A comparison of data acquired with the transient-grating and the pump–probe experimental geometries reveals the timing dynamics of the induced phonon oscillations with respect to the driving femtosecond pulse sequence.

Femtosecond time-resolved interferometry for the determination of complex nonlinear susceptibility

Abstract: We propose and demonstrate sensitive interferometry that permits the separation of the real and imaginary parts of the nonlinear susceptibility with a femtosecond time resolution by a single measurement. A special reference interferometer compensates for any fluctuations of the fringe and provides high sensitivity to detect a fringe shift as small as 0.025 rad (lambda/250) by averaging only 100 shots with a low-repetition-rate laser. This method can be applied to materials with optical anisotropy and/or absorption with high sensitivity. We apply the method to two materials, CS(2) and CdS(x)Se(1-x) microcrystallite-doped glass.

Non-Markovian dephasing of molecules in solution measured with three-pulse femtosecond photon echoes

Abstract: The electronic dephasing of large molecules in solution is investigated with three-pulse photon echoes generated by 6-fs optical pulses. The observed dephasing behavior, which is analyzed with a model based on a stochastic coupling of the molecules with their heat bath, is clearly shown to be non-Markovian. Moreover, the correlation function of the fluctuations asymptotically approaches an exponential with characteristic times varying from 20 to 80 fs, depending on the solvent environment.

Femtosecond transition-state dynamics

Abstract: This article presents the progress made in probing femtosecond transition–state dynamics of elementary reactions. Experiments demonstrating the dynamics in systems characterized by a transition region and by a saddle-point transition state are reported, and comparison with theory is made.

Femtosecond temporal encoding in barium titanate.

Abstract: We describe two-beam coupling and temporal encoding experiments in barium titanate. Volume gratings are created in the photorefractive material by 50-fs optical pulses. Information in the writing pulses may be encoded as spatially distributed volume gratings in the crystal. Femtosecond temporal waveform reconstruction is demonstrated.

Femtosecond studies of excited-state proton and deuterium transfer in benzothiazole compounds

Abstract: the femtosecond time scale. The reaction steps occurring immediately after photoexcitation are temporally resolved in pump and probe measurements, where the onset of emission of the reaction product is monitored. For HBT in nonpolar solvents, the transfer of the proton occurs with a time constant of 160

Third‐order nonlinearity and two‐photon‐induced molecular dynamics: Femtosecond time‐resolved transient absorption, Kerr gate, and degenerate four‐wave mixing studies in poly (p‐phenylene vinylene)/sol‐gel silica film

Abstract: Femtosecond response and relaxation of the third‐order optical nonlinearity in a newly developed poly (p‐phenylene vinylene)/sol‐gel silica composite are investigated by time‐resolved forward wave degenerate four‐wave mixing, Kerr gate, and transient absorption techniques using 60 fs pulses at 620 nm. Using a theoretical description of two‐ and four‐wave mixing in optically nonlinear media, it is shown that the results obtained from simultaneous use of these techniques yield valuable information on the real and imaginary components of the third‐order susceptibility. In the composite material investigated here, the imaginary component is derived from the presence of a two‐photon resonance at the wavelength of 620 nm used for the present study. This two‐photon resonance is observed as transient absorption of the probe beam induced by the presence of a strong pump beam. It also provides fifth‐order nonlinear response both in transient absorption and in degenerate four‐wave mixing. The fifth‐order contributions are derived from the two‐photon generated excited species which can absorb at the measurement wavelength and therefore modify both the absorption coefficient and the refractive index of the medium.

Pulse repetition-rates in a passive, self-starting, femtosecond soliton fibre laser

Abstract: Several unusual pulsing modes are observed in a self-starting, passively mode-locked erbium fibre soliton laser capable of generating pulses with durations as short as 320 fsec.

Terahertz radiation induced by subband-gap femtosecond optical excitation of GaAs

Abstract: We have observed terahertz radiation induced by subband-gap femtosecond optical excitation from GaAs. We belive this is due to screening of the surface depletion field by virtual photocarriers (the inverse Franz-Keldysh effect).

Two-photon absorption in diamond and its application to ultraviolet femtosecond pulse-width measurement

Abstract: Intensity-dependent transmission measurements of 310-nm femtosecond pulses show that diamond has a twophoton absorption coefficient of 0.75 +/- 0.15 cm/GW, in approximate agreement with universal scaling formulas for two-photon absorption in diamond-structure materials. We then demonstrate that two-photon absorption is strong enough to permit simple measurements of ultraviolet femtosecond pulse widths in single-crystal diamond plates that are thin enough (250 microm) to be both inexpensive and dispersion free. Autocorrelation measurements of 10-50-nJ, 0.18-1.4-ps pulses are presented. The method requires no phase matching and can be applied to pulses in the wavelength range of 220-550 nm.

Multikilohertz Ti:Al 2 O 3 amplifier for high-power femtosecond pulses

Abstract: We amplified femtosecond pulses at repetition rates of as much as 7.2 kHz in a Ti:Al2O3 regenerative amplifier pumped by a frequency-doubled Q-switched Nd:YLF laser. Energies of as much as 1 mJ at 1 kHz and 85 μJ at 7.2 kHz in a 150-fs pulse have been obtained. This source was used to produce 1 W of femtosecond white light through spectral continuum generation.

Pulse repetition rates in passive, selfstarting, femtosecond soliton fibre laser

Abstract: Several unusual pulsing modes are observed in a selfstarting, passively mode-locked erbium fibre soliton laser capable of generating pulses with durations as short as 320 fs.

Ultrafast optical Kerr dynamics studied with incoherent light

Abstract: Femtosecond optical Kerr dynamics in various transparent liquids were measured using incoherent light with a 60 fs autocorrelation width. From the measurement of the optical Kerr effect (OKE) of binary mixtures of CS2 and various liquids, the contribution of the intermolecular interaction‐induced polarizability change to the OKE was found to be affected remarkably by the femtosecond molecular dynamics of CS2. Especially, data from diluted solutions of CS2 in nonviscous solvents composed of molecules with a low molecular weight are consistent with the binary collision model in free space. An oscillatory feature, which was attributed to an intermolecular vibrational mode, was found in the OKE dynamics of neat benzene and several benzene derivatives. A theoretical expression for the delay‐time dependence of the signal intensity was also derived with no restriction on the statistical properties of the incoherent light. It is expressed in terms of the autocorrelation function of the intensity fluctuation of th...