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A larger harmonic cutoff with a higher harmonic yield is a necessary requirement for producing intense ultrashort extreme ultraviolet (XUV) and X-ray sources. In this paper, we present a potential method to improve the harmonic cutoff and harmonic yield from the superposition of initial states in the He atom driven by the low-intensity homogeneous and inhomo-geneous combined field. The results can be separated into two parts, that is, (i) the extension of harmonic cutoff via the optimization of three-color homogeneous and inhomogeneous field (covering the carrier envelope phase, delay time, intensity ratio, and inhomogeneous effect); and (ii) the enhancement of harmonic yield via the superposition of ground state and excited state as the initial state. Consequently, a water window harmonic plateau with an intensity enhancement of six orders of magnitude can be obtained. Moreover, the harmonic plateau is contributed by a single harmonic emission peak. Finally, through the Fourier transformation of some selected harmonics on the spectral continuum, a number of sub-50 as water window attosecond pulses with the intensity enhancement of six orders of magnitude can be obtained. Key words: high-order harmonic generation, water window single-attosecond pulse, superposition of multicolor laser beam, homogeneous and inhomogeneous laser field, superposition of initial state as the initial state. Une coupure d'harmoniques a un ordre plus eleve, accompagnee d'un plus grand rendement harmonique, constitue la condition necessaire pour la production d'impulsions X et XUV intenses et tres courtes. Nous presentons ici une methode potentiellement interessante pour ameliorer la coupure et le rendement harmonique en usant de superposition dans l'etat initial dans un atome de He excite par des champs combines homogene et inhomogene de faible intensite. Les resultats peuvent etre divises en deux parties : (i) l'extension de la coupure harmonique via l'optimisation d'un champ 3-couleurs homogene et inhomogene (couvrant la phase porteuse-enveloppe, le temps de delai, le rapport d'intensite et l'effet d'inhomogeneite); et (ii) l'augmentation du rendement d'harmoniques via la superposition du fondamental et d'un etat excite comme etat initial. Ainsi, on peut obtenir un plateau harmonique du type fenetre X de l'eau, avec une augmentation de six ordres de grandeur de l'intensite. De plus, le plateau harmonique provient d'une seule raie d'emission harmonique. Finalement, via la transformation de Fourier de quelques harmoniques selectionnees du spectre continu, on peut obtenir un nombre d'impulsions attoseconde de la fenetre X de l'eau sous les 50 as, avec une augmentation d'intensite de six ordres de grandeur. [Traduit par la Redaction] Mots-cles : generation d'harmoniques d'ordre eleve, generation d'impulsions attosecondes simples, physique des champs forts, champ laser homogene et inhomogene, superposition de faisceaux lasers multi-couleurs., 1. Introduction Since the first demonstration in 2001 [1], the single-attosecond pulses (SAPs), as an attractive tool to observe and control the ultrafast electronic dynamics in atoms, molecules, and solids, [...]

The time it takes a bound electron to respond to the electromagnetic force of light sets a fundamental speed limit on the dynamic control of matter and electromagnetic signal processing. Time-integrated measurements of the nonlinear refractive index (1) of matter indicate that the nonlinear response of bound electrons to optical fields is not instantaneous; however, a complete spectral characterization of the nonlinear susceptibility tensors (2)--which is essential to deduce the temporal response of a medium to arbitrary driving forces using spectral measurements--has not yet been achieved. With the establishment of attosecond chronoscopy (3-5), the impulsive response of positive-energy electrons to electromagnetic fields has been explored through ionization of atoms (6) and solids (7) by an extreme-ultraviolet attosecond pulse (8) or by strong near-infrared fields (9-11). However, none of the attosecond studies carried out so far have provided direct access to the nonlinear response of bound electrons. Here we demonstrate that intense optical attosecond pulses synthesized in the visible and nearby spectral ranges allow subfemtosecond control and metrology of bound-electron dynamics. Vacuum ultraviolet spectra emanating from krypton atoms, exposed to intense waveform-controlled optical attosecond pulses, reveal a finite nonlinear response time of bound electrons of up to 115 attoseconds, which is sensitive to and controllable by the superoctave optical field. Our study could enable new spectroscopies of bound electrons in atomic, molecular or lattice potentials of solids (12), as well as light-based electronics operating on sub-femtosecond timescales and at petahertz rates (13-15)., Intramolecular atomic motion dynamically distorts the interatomic electron cloud and thereby gives rise to a transient electronic polarizability (16) and a finite nonlinear response of matter to light fields. Timeresolved [...]

The excitation and ionization processes of the hydrogen atom by intense femtosecond chirped laser pulses are investigated by solving numerically the three-dimensional time-dependent Schrodinger equation. We study, in three distinct cases, the interaction of the atomic system with chirped pulses of different central carrier frequencies that correspond to some harmonics of a Ti-sapphire laser. We analyze, in each case, the mechanism of the population transfer from the ground to the excited electronic states and show how it differs depending on the sign of the chirp parameter. We also show that the ionization yield is sensitive to the chirp parameter when resonant or near-resonant transitions are induced by the chirped pulses. The latter are efficient to control the excitation process by inducing selective population transfer when compared with unchirped pulses. PACS Nos.: 32.80.Rm, 42.50.Hz. Nous etudions les processus d'ionisation et d'excitation de l'atome d'hydrogene avec des impulsions lasers intenses a derive de frequence, a l'aide d'une methode numerique basee sur la resolution de l'equation de Schrodinger dependante du temps a trois dimensions. Trois etudes distinctes sont menees, dans lesquelles les frequences centrales lasers utilisees correspondent a des harmoniques du laser Ti-saphir. Nous analysons dans chaque cas l'influence de la derive de frequence laser sur le mecanisme de transfert de population entre l'etat fondamental et les etats electroniques excites. Nous etudions plus particulierement comment les processus d'ionisation et d'excitation sont influences par le signe du parametre de derive de frequence lorsque des transitions resonantes sont produites par le champ laser. Nous montrons que les impulsions lasers a derive de frequence sont des outils efficaces pour optimiser le transfert de population et ainsi controler les processus d'ionisation et d'excitation., 1. Introduction The unceasing progress in ultrashort and intense laser pulse generation techniques has opened the way to extensively investigate the strong field effects on matter [1-8]. Theoretical and experimental [...]

The active steering of the pathways taken by chemical reactions and the optimization of energy conversion processes (1-3) provide striking examples of the coherent control of quantum interference through the use of shaped laser pulses. Experimentally, coherence is usually established by synchronizing a subset of molecules in an ensemble (4-7) with ultra-short laser pulses [8]. But in complex systems where even chemically identical molecules exist with different conformations and in diverse environments, the synchronized subset will have an intrinsic inhomogeneity that limits the degree of coherent control that can be achieved. A natural--and, indeed, the ultimate--solution to overcoming intrinsic inhomogeneities is the investigation of the behaviour of one molecule at a time. The single-molecule approach (9,10) has provided useful insights into phenomena as diverse as biomolecular interactions (11-13), cellular processes (14) and the dynamics of supercooled liquids (15) and conjugated polymers (16). Coherent state preparation of single molecules has so far been restricted to cryogenic conditions (17), whereas at room temperature only incoherent vibrational relaxation pathways have been probed (18). Here we report the observation and manipulation of vibrational wave-packet interference in individual molecules at ambient conditions. We show that adapting the time and phase distribution of the optical excitation field to the dynamics of each molecule results in a high degree of control, and expect that the approach can be extended to achieve single-molecule coherent control in other complex inhomogeneous systems., For few-atom molecules, coherent control schemes can be designed on the basis of theory (19). For more complex systems, such as large (bio-)molecules at ambient conditions, ab initio quantum mechanical [...]

Time-delay-compensated grating monochromators for the spectral selection of extreme-ultraviolet femtosecond pulses in a broad spectral region are designed by adopting both classical diffraction and the off-plane mount. The optical design requires six optical elements: four concave mirrors and two plane gratings. I compare here the two designs in terms of the tolerances in the alignment for time-delay compensation in the femtosecond time scale. The off-plane mount is demonstrated to be far superior to the classical diffraction mount. OCIS codes: 050.1950, 050.1960, 320.0320, 300.6540.

We study theoretically and experimentally the gating performance of photocathode-gated image tube. A cross-correlation method is proposed to analyze the rising and falling speed and width of the image tube gain. Femtosecond pulses generated by a fiber laser are used as the light source of ultrahigh temporal resolution and trapezoid electrical signals are applied to a photocathode electrode as gating pulses. By adjusting the time delay between the laser pulse and the electrical gating pulse, various acceleration procedures for the photoelectrons generated at the photocathode can be observed. The photoelectrons arriving at the multichannel plate (MCP) with different kinetic energies receive different gain according to Eberhardt's MCP gain model. The gain profile is obtained by measuring the output light power of the fluorescent screen at the output port of the tube. The theoretical analysis and experimental result show that the shape of the output gain curve of the image tube is deformed and the width is broadened in comparison with the symmetric electrical gating pulse. OCIS codes: 100.4550, 110.2970, 120.1880, 320.7100.

The effect of instantaneous cubic nonlinearity on second-harmonic generation of femtosecond laser pulses is investigated. The possibility of improving generation efficiency and shortening pulse duration is analyzed. Energy conversion efficiency of 60% was obtained in an experiment in a 1 mm thickness KDP crystal at the peak intensity of 0.6 TW/[cm.sup.2] and the duration of 60 fs. OCIS codes: 190.0190, 190.2620, 190.5940.

A nanosecond 1.3 [micro]m Nd:YAG laser-pumped external cavity eye-safe Raman laser based on [BaWO.sub.4] crystal is efficiently demonstrated. The average power of 1.5 [micro]m was 0.60W operating at a pulse repetition rate of 1.7 kHz and an incident pump power of 4.14W. Conversion efficiency of 14.5% and a high slope efficiency of 54.5% have been achieved. Pulse shortening was obviously observed. The pulse duration of the first Stokes was about 40ns, much shorter than the fundamental pulse with a duration of ~220 ns. OCIS codes: 140.3550, 140.3580, 190.5650, 230.5750.

The wavelength of the free electron laser (FEL) in Osaka University can be continuously varied in the range of 5.0-20.0 [micro]m. The FEL has a double-pulse structure, consisting of a train of macropulses of pulse duration 12 [micro]s. Each macropulse contains a train of 330 micropulses of pulse duration 5 ps. The tunability and picosecond pulses afford new medical and biological applications. However, a macropulse of long pulse duration leads to undesirable secondary effects. Precise control of the macropulse duration is essential for the high-precision applications of the FEL. An FEL pulse control system using acousto-optic modulators has been developed to investigate mechanical (shock-wave) effects of the FEL on living tissues. With this system, we have controlled photoinduced shock waves and determine the mechanism of interaction during FEL-induced tissue ablation. OCIS codes: 140.2600, 170.1020, 170.1065.

By tightly focusing 27 fs laser pulses from a Ti:sapphire oscillator with 1.3 nJ pulse energy at 93 MHz repetition rate, we are able to fabricate optical waveguides inside hydrogel polymers containing ~36% water by weight. A tapered lensed fiber is used to couple laser light at a wavelength of 632.8 nm into these waveguides within a water environment. Strong waveguiding is observed due to large refractive index changes. A large waveguide propagation loss is found, and we show that this is caused by surface roughhess which can be reduced by optimizing the waveguides. OCIS codes: 230.7370, 160.5470, 320.7110.

A practical ytterbium-doped mode-locked fiber source producing 89 fs pulses without an external bulk compensator was developed by investigating the pulse propagation dynamics in a mode-locked fiber laser with small average dispersion. Negatively chirped pulses are taken from the cavity and compressed in a standard output fiber resulting in high-quality pulses. OCIS codes: 140.4050, 140.3510, 320.5520.

The design of aperiodic reflecting multilayer (ML) structures for attosecond physics in the extreme ultraviolet spectral region is presented. An optimization procedure based on 'evolutive strategy' has been developed in order to get coating structures reflecting high photon fluxes in ultrashort duration pulses. The MLs are designed for a specific (75-105 eV) spectral interval with suitable reflectance and phase characteristics, in particular high total spectral reflectivity coupled with very wide bandwidth, spectral phase compensation, and amplitude reshaping. Furthermore, to take into account manufacturing tolerances, solutions stable with respect to random layer thickness variations are selected. To test the reliability of the proposed design procedure, examples of Mo/Si ML structures designed to reflect ultrashort pulses with different amplitude profiles and phase behavior are considered. The performances of the various structures are analyzed. OCIS codes: 320.1590, 320.5520, 320.5540, 310.1620.

We describe a Fourier-transform spectrometer appropriate for use in an undergraduate optics laboratory. The modular design, which uses off-the-shelf equipment, is suitable for determining the spectra of ultrashort pulsed and continuous light sources. The spectrometer does not require equal step sizes for the motion of the mirror. An algorithm interpolates the phase of both a reference and an unknown light beam at equal intervals before performing the Fourier transform. The interferometer scan and the phase and spectral analysis are performed in a few seconds, making the apparatus a useful tool for teaching concepts of temporal coherence and Fourier spectroscopy. [DOI: 10.1119/1.2218358]

The Einstein, Podolski, and Rosen (EPR) experiment in the case of the breakup of polyatomic molecule into two mutually entangled fragments is analyzed. The experiments have shown that each separated fragment can retain coherences induced in its parent molecule by a broad band laser pulse, regardless of whether a measurement is performed on its entangled partner.

A simple system that simultaneously supports active mode-locking and self-seeding schemes for wavelength-tunable optical short-pulse generation is proposed. The system consists of a gain-switched Fabry--Perot laser diode, an erbium-doped fiber amplifier, a tunable optical filter, and two circulators. The mode-locked optical pulses exhibit good stability, a high side-mode suppression ratio of more than 31 dB over a wide wavelength tuning range of 42 nm, and a pulse width of around 35 ps at a repetition frequency of -2.8 GHz.

Imaging of femtosecond laser filaments is accomplished by utilizing a recently developed diagnostic which is not damaged by the filaments and which greatly reduces the nonlinearities in the detection system. This calibrated detection system allows quantities such as the filament energy and fluence distribution to be determined with greater detail accuracy, and confidence than was previously possible. The diagnostic is placed on a rail so that filament images can be obtained at a large number of positions along the propagation path. lt is found that filaments formed from 450-fs pulses carry higher fluence and propagate farther than filaments formed from 50-fs pulses. Index Terms--Air propagation, filamentation, laser-plasma interaction, ultrashort pulses.

A study of the excited-state dynamics of (SO2)(sub m) clusters following excitation by ultrafast laser pulses in the range of 4.5 eV (coupled (super 1)A2, (super 1)B1 states) and 9 eV (F band) is presented. Results for the coupled (super 1)A2 and (super 1)B1 states are in good agreement with published computational work on the properties of these coupled states.

Highly efficient stimulated Raman scattering of near-infrared, visible, and UV picosecond laser plses in BaWO[subscript 4] crystals is investigated and discussed. Single-pass, double-pass, and Raman cavity configurations are used.

A study of femtosecond 248nm laser ablation uses a high-power femtosecond Ti3+:Al2O3/KrF laser system and a sample of liquid toluene.

A study has been conducted to examine laser emission to more than 20 lines in the UV-visible region. It has been found that when plasma is excited with a long current pulse, a double-pulsed output is produced. A plasma model has been developed to investigate the temporal characteristics of the spontaneous emission of a low density pulsed discharge plasma. Findings have revealed that in most of the laser lines, both the electron impact and recombination mechanisms are responsible for population inversion.

The quantum control of the predissociation of NaI was studied theoretically. Control pulses that accelerate the predissociation was designed using a local control method. The control method, combined with the backward time propagation method, was applied to the shaping of the target packet with the shortest lifetime. Results demonstrate that the approximated control scheme efficiently enhances the predissociation reaction on a time scale of several picoseconds.

The damage fluences of gratings for diffraction of ultraviolet radiation, which are used in high-order harmonic generation experiments, have been measured with respect to the fundamental laser beam radiation. We have tested gold and platinum coatings of 40- and 50-nm thickness, respectively, deposited onto fused-silica substrates, after irradiation of high-energy, spatially filtered, 20-fs laser pulses at 780 nm. The damage appears at a fluence of [approximately]0.3 J [cm.sup.-2] for gold and at a fluence of [approximately]0.4 J [cm.sup.-2] for platinum. Scanning electron microscopy of the irradiated regions revealed different damaging mechanisms for the two coatings.

Research was conducted to examine the optical creation and modulation of magnetic flux quanta in a YBa2Cu3O(70delta) thin film loop by observing terahertz emission excited with femtosecond laser pulses. The mechanism of the optical creation of the magnetic flux in the loop was also investigated. Results indicate a possible application of the superconductive loop as an optical flux-trap memory for the detection and storage of the femtosecond optical signals.

Research was conducted to examine singled-picosecond electrical pulse excitation and detection in YBa2Cu3O7-x (YBCO) transmission lines containing microbridges and grain-boundary Josephson junctions. The structures were patterned in YBCO films developed by laser ablation on MgO bicrystals. Results provide evidence of the potential of YBCO for ultrafast optical and electrical transient detection and processing.

Irradiation with the second harmonic of a Ti:sapphire laser induces multiphoton reduction of Eu(super 3+) to Eu(super 2+) in methanol. This was observed in an experiment in which light pulses were delivered from a chirped-pulse amplified Ti:sapphire laser system. Conversion efficiencies were raised to 0.3 for short pulse excitation compared to on the order of 10(super -5) for a nanosecond dye laser pulse. Results suggest a three-photon process that pumps a deep charge-transfer state from which Eu(super 3+) efficiently dissociates to Eu(super 2+). The Eu(super 2+) concentration increased shot by shot and subsequently approached a plateau.

We describe an oscillator-amplifier laser system for the generation of high-power femtosecond pulses near 1.25 [[micro]meter] based on chromium-doped forsterite. Chirped-pulse amplification at a 1-kHz repetition rate raises the pulse energy to >350 [Mu]J. The nearly transform-limited 200-[Mu]J, 135-fs-long recompressed pulses have a peak power of [approximately equal to]1.5 GW.

The intensity and the phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser operating in the vicinity of zero group-delay dispersion (GDD) have been completely characterized by the technique of frequency-resolved optical gating (FROG). For small values of negative GDD, the appearance of a dispersive wave in the pulse spectrum is manifested in the measured FROG trace, and pulse retrieval directly shows its association with a broad leading-edge pedestal. For positive GDD, we confirm previous experimental observations of picosecond pulses with large positive chirp and report a new operating regime in which the output pulses are of picosecond duration but are intensity modulated at 20 THz. The physical origin of this modulation is discussed by analogy with similar effects observed during pulse propagation in optical fibers, and the experimental results are compared with a model of intracavity four-wave mixing about the cavity zero GDD wavelength.

We demonstrate a new configuration of self-pumped phase conjugation in BaTi[O.sub.3] with 450-nm femtosecond pulses. The pump beam enters the -c face of the crystal at an acute angle with respect to the +c axis. This geometry is different from all known geometries of self-pumped phase conjugations in BaTi[O.sub.3]. Phase-conjugate reflectivity of 1.7% and a rise time of [approximately]24 s are obtained at an average pump power of 10 mW. The ability of the phase conjugator to serve phase distortions correctly is demonstrated. We propose that the femtosecond phase conjugator operate with the hologram mechanism sharing as the double phase conjugator. The self-pumped phase-conjugate pulses are analyzed by femtosecond electric-field cross correlation.

A technique for compressing and amplifying a weak picosecond optical pulse using a copropagating intense pump pulse in semiconductor optical amplifiers is presented. Numerical simulations showed that the technique can simultaneously compress and amplify probe pulses by simply adjusting the time delay between two pulses. Results also showed a compressed pulse shape with strongly damped wings, with the shortest achievable pulsewidth displaying dependency on the frequency detuning and pump pulse properties.

A picosecond laser was used to study the harmonic generation from solid-vacuum interfaces. The harmonic intensity was found to be strongest for p-polarized driving lasers and about half for s-polarized ones. The ratio of between the fourth and second harmonic intensities was found to be relatively higher than that predicted by simulation. Results also showed that the transverse mode and divergence of the harmonics were very similar to those of the pump laser for preplasma free conditions.

A method for direct shape measurement with short laser light pulses and digital holography with a CCD array is proposed. An in-line holographic setup is used in which the reference beam is reflected from a blazed reflection grating, i.e., a Littrow setup. By this method a relatively large optical delay is created between the reference and the object beams even with a small object-reference angle, which is necessary because of the limited resolution of the CCD. The delay varies continuously across one axis of the CCD array. In this way different object sections are reconstructed from different parts of the CCD, which in turn correspond to a certain path length from the object. By putting the sections together, one can evaluate the three-dimensional shape. Theoretical as well as experimental results are presented.

Stokes pulses of high energy and high-average power were obtained by stimulated Brillouin scattering (SBS) compression of long Nd:YAG laser pulses. The SBS medium used in the single-cell compressor was liquid ultrafiltered Fluorinert FC-75 fluorocarbon. An output pulse duration of 0.9 ns and a peak-power enhancement by 1 order of magnitude were observed for 10-ns, 0.57-J input pulses at a 10-Hz repetition rate. The compressor internal SBS efficiency reached a value of [[Eta].sub.SBS] = 94% and the overall device efficiency a value of [[Eta].sub.dev] = 87%; both values are the highest reported so far to the best of our knowledge. The simple single-cell SBS geometry provided excellent energy and pointing stability of the Stokes pulse. Its temporal shape turned out to be somewhat less stable. The SBS process also partially improved the laser beam quality. The Stokes pulses proved to be capable of generating radiation in the extreme-ultraviolet and soft-x-ray regions over a period of two months without any significant output deterioration.

As much as 1.6-W average output power was emitted in a simple setup from a diffusion-bonded Nd:YAG rod with 70-100-ns Q-switched pulses at 946 nm and repetition frequencies between 15 and 45 kHz at 22-W incident diode-pump power. A [Cr.sup.4+]:YAG crystal with a bleachable loss of approximately 2.5% and a length of 0.5 mm was used as a saturable absorber. The extraction efficiency was 47% in comparison with the continuous-wave laser output power of 3.37 W in the free-running regime.

A theory for doubling the pulse repetition frequency of two orthogonal high-order transverse modes of a vertical-cavity surface-emitting laser through alternating current modulation is presented. An analysis of the proposal reveals that, by modulating a single high-order transverse mode, the pulse repetition frequency can be doubled without increasing the density levels of the injected current. This spatial switching results in high-frequency beam steering in the azimuthal direction of the laser.

Research was conducted to develop a Nd:YLF laser system with a special amplifier chain which is optimized for generation of trains of picosecond pulses. It produces 0.8-ms-long pulse trains having 800 individual micropulses of 30-ps duration and 1.0 microns spacing in time. A food stability and flatness of the pulse train was obtained if the start of the flashlamp pulse, its maximum current and a correcting slop were tuned appropriately.

A theoretical analysis of a thermal defocusing optimal limiter's performance in a transient environment is presented. The situation occurs when the duration of the laser pulse is comparable to the time of the acoustic transit. The analysis, which used space- and time-dependent refractive index changes to calculate the nonlinear pulse change experienced by the pulse as it travels through the medium, supports the idea that the performance of the optical limiter is dependent on the laser pulse's duration.

Research was conducted to examine the tunable 50-fs pulse generation in the 250-310-nm ultraviolet range. The optical parametric amplifier visible pulses and corresponding second harmonic ultraviolet outputs pumped by a new high-repetition-rate ultrafast regenerative amplifier that is designed to produce transform limited sub-50-fs pulses at 100-300-kHz repetition rates is described. The system could be further optimized for the production of even shorter pulses at these ultraviolet wavelengths.

We present a set of experiments that introduce students to ultrafast science. We discuss the relationship between the description of an ultrashort laser pulse in the frequency domain and the time domain. Using experimental results we demonstrate that this relationship is constrained by the lower limit of the Heisenberg uncertainty principle. Students carrying out the experiments will become familiar with ultrafast techniques, such as autocorrelation and laser cavity design, as well as various other concepts such as dispersion, Fourier transformation, interference, and nonlinear optics.

Picosecond pulses emitted from a gain-switched laser diode have been amplified in a Ti:sapphire regenerative amplifier indirectly pumped by a 4-W laser diode. This all-solid-state system produced microjoule pulses tunable from 803 to 840 nm at repetition rates up to 25 kHz with durations of 70-100 ps. By frequency doubling and tripling the output, we generated blue and UV pulses tunable from 401 to 420 nm and from 268 to 280 nm, respectively. Average powers larger than 4 mW were reached in these two wavelength regions.

Research was conducted to reexamine the theory of transmission control of dark solitons through nonlinear gain. A direct perturbation approach for the dark one-soliton is outlined and an adiabatic solution for the problem of nonlinear gain is provided. Results are in agreement with previous results, in addition to a shift of the soliton center which is also significant to the problem of signal transmission.

Research was conducted to examine numerically the propagation of strong picosecond optical pulses in semiconductor optical amplifiers by considering carrier heating, two-photon absorption, ultrafast nonlinear refraction and spectral hole-burning. Results are in good agreement with the published experimental measurements in both the time and frequency domains for strong input pulses. Results reveal that the effects of two-photon absorption and ultrafast nonlinear refraction are very important in determining the output pulse properties for pulse energy larger than 1 pJ.

Research was conducted to examine the influence of spontaneous carrier and photon fluctuations on the dynamics of a nearly single-mode injection laser under conditions of short-pulse generation. The method of EFP equation, which allowed the derivation of a closed system of ordinary differential equations for the moments of carrier and photon density in the modes, was used for the analysis. Results indicate that the effect of spontaneous fluctuations of carrier and photon density led to the splitting of generation pulses under the strong modulation of the injection laser's bias current in the microwave band.

Research was conducted to develop a time-domain ABCD matrix formalism. The formalism was based on the propagation of a Gaussian pulse through a medium which is characterized by the pulsewidth and chirp. The ABCD matrix elements of AM and FM moculators, filter dispersion, lumped SPM and cavity dispersion were characterized and used to confirm that the matrices are very useful for deriving a steady-state solution of the Gaussian pulse output from various mode-locked lasers.

Molecular wave-packet simulations for a two-dimensional model based on ab initio potential-energy surfaces and dipole functions for HONO2 have discovered quasi-coherent molecular vibrations with energies well above the dissociation threshold in the ground electronic state of a two-dimensional model system. Specifically, selective infrared laser pulses in the femtosecond time domain may be used to prepare quasi-coherent molecular vibrations in the dissociative continuum of the ground electronic state. Femtosecond IR+UV pump-probe spectroscopy may be used to monitor such vibrations.

Femtosecond spectroscopy was used to investigate the biologically active far-red-absorbing form of the plant photoreceptor phytochrome Pfr. Experimental results suggest that immediately after excitation, bleaching of the Pfr ground-state absorption takes place both in red spectral region between 700 and 775 nm and in the blue spectral region between 400 and 425 nm. Such bleaching in the two spectral regions decays at a rapid rate with time constants of 700 +/- 100 femtoseconds and 4.5 +/- 0.5 ps.

Ulrafast multiphoton processes are analyzed using stationary and nonstationary multilevel models. Of specific focus are the electronic and nuclear coherence of a near-resonant intermediate state. Three intrapulse dynamical mechanisms, namely, sequential resonance, chirp-following and time-delay, are identified and characterized, allowing the understanding of the subtle chirp dependence in a near-resonant intrapulse pump-probe process and revealing it to be the combined consequence of these three mechanisms.

Wave packet dynamics are observed for the first time in frequency-resolved pump-probe experiments using pulses of 13 femtoseconds centered at 780 nm. An ultrafast red-shift of the electronic transition by about 6500 cm(super -1) is generated by wave packet dynamics launched by the pump pulse. An electronic two-level system strongly coupled to a librational motion of water molecules is used to explain this novel feature in the solation of electrons.

The time-dependent quantum mechanical wave packet strategy is used to examine the influence of a femtosecond laser pulse on an adsorbate molecule-metal surface system by considering a time-dependent coupling between the ground- and excited-state potential energy curves. Results reveal that the duration of the coupling and the image-charge attraction is a strong factor in the probability of desorption. Such probability also increases as vibrational excitation of the metal-adsorbate bond increases.

The collisional deactivation of the laser excited states A 2[[Sigma].sup.+] (v[prime] = 1, N[prime] = 4, 12) of OH in a flame is studied by measurement of spectrally resolved fluorescence decays in the picosecond time domain. Quenching and depolarization rates, as well as vibrational energy-transfer (VET) and rotational energy-transfer (RET) rates are determined. An empirical model describes the temporal evolution of the quenching and VET rates that emerge from the rotational-state relaxation. Fitting this model to the measured 1-0 and 0-0 fluorescence decays yields the quenching and VET rates of the initially excited rotational state along with those that correspond to a rotationally equilibrated vibronic-state population. VET from the higher rotational state (N[prime] = 12) shows a tendency for resonant transitions to energetic close-lying levels. RET is investigated by analysis of the temporal evolution of the 1-1 emission band. The observed RET is well described by the energy-corrected sudden-approximation theory in conjunction with a power-gap law.