We report the generation and measurement of 804 nm pulses with durations as short as 20 fs and with peak powers as high as 500 kW from a regeneratively initiated, self-mode-locked Ti:sapphire laser. Pulse duration is shown to decrease, and spectral content to increase, as intracavity power is increased. Control of intracavity focusing and a high-modulation-depth, acousto-optic modulator allow the intracavity power to be maximized. Cavity cubic phase error is minimized by correct design and placement of a GDD compensating prism pair. Methods for accurate determination of the pulse duration without assumption of pulse shape are discussed. Interferometric autocorrelation is accomplished with an interferometer which intrinsically balances dispersion and loss in each arm. Techniques for eliminating pulse distortions during amplification are also presented.

The third-order dispersion of a modelocked Ti:Al₂O₃ laser can be reduced by using quartz prisms for dispersion control. Two such lasers have been constructed, one which uses a novel sequence of four quartz prisms and a second which uses a pair of quartz prisms. The laser which incorporates four quartz prisms allows transform-limited 13-fs pulses to be generated with a gain crystal 20 mm long. The second laser has four times less third-order dispersion but does not produce pulses shorter than 15 fs. Evidence is presented to show that third-order dispersion is not limiting pulse duration.

In this paper, we present results on the generation of 10.9 fs duration pulses directly from a self mode-locked Ti:Sapphire laser. Ultrashort-pulse operation of the laser was obtained by optimizing the intracavity dispersion compensation. A short, highly-doped Ti:Sapphire crystal and fused silica prism pair were used to achieve this. We also present preliminary results on frequency doubling of the 800 nm fundamental beam, both inside and outside the laser cavity. Detailed calculations have been made on a number of nonlinear crystals to determine the optimum crystal choice. Our calculations indicate that intracavity doubling should lead to the generation of sub-20 duration ultraviolet pulses.

We have developed a novel series of femtosecond amplifiers based on a combination of materials including Ti:Al₂O₃, Cr:LiSrAlF₆, alexandrite, and Nd:glass with the intent of increasing the average power capability of high intensity laser sources.

We describe a complete solid state 1.5 terawatt, 150 femtosecond laser system operating at 10 Hz repetition, based on titanium-doped sapphire amplifiers and use of the technique of chirped-pulse amplification (CPA). The design and performance of the system is described. Special emphasis on the tunability of the system from 760 nm to 860 nm is also discussed.

We have developed a system for continuously variable independent tuning of the higher order frequency dependent phase of ultrashort laser pulses. This technique relies on geometric aberrations that arise from adjustments to the relative alignment of the elements of an air spaced doublet lens in systems such as a diffraction grating stretcher in which the spectral components of the optical pulses are spatially dispersed. Modeling results are compared to experimental measurements for a non optimized pulse stretcher/compressor combination showing the higher order phase aberrations that limit the performance of a chirped pulse amplification system. Numerical results are presented indicating these higher order phase terms can be compensated by a properly adjusted air spaced doublet design within the pulse stretcher.

Feedback stabilization of the pulse repetition rate is demonstrated for an external cavity mode- locked semiconductor diode laser. The design procedure for repetition rate feedback stabilization is described. We propose feedback stabilization as a method of pulse stabilization for high repetition rate monolithic devices. These monolithic devices are expected to play an important role in applications such as optical clock generation and millimeter-wave clock distribution, but previously no timing stabilization technique had been developed which could be used up to the maximum repetition rates.

The generation of ultrafast optical pulses from semiconductor diode lasers is extremely attractive owing to the compact and efficient properties of these devices. Applications of these devices range from photonic switching, electro-optic sampling optical computing, optical clocking, applied nonlinear optics and other areas of ultrafast laser technology. There have been many recent advances in ultrafast pulse generation from diode lasers in the past few years, with many researchers concentrating on device fabrication, device physics, theoretical modeling and systems applications. With the advent of high power semiconductor lasers devices, experimental results have shown the potential for generating relatively high peak power optical pulses from a semiconductor diode laser system. In this paper, the techniques and underlying physics involved in generating high power ultrashort optical pulses from semiconductor optical amplifiers will be covered. The main concepts that are to be stressed experimentally are: (1) the elimination of the residual facet reflectivity which causes multiple pulse outputs by using angled striped semiconductor traveling wave optical amplifiers, (2) generation of short pulses by using intracavity MQW saturable absorbers, (3) exploitation of the frequency chirp impressed on the pulse by performing pulse 'compression' techniques, and (4) creation of high output powers by amplification techniques.

The important characteristics of actively mode-locked semiconductor lasers are identified and recent advances in numerical techniques for the optimization of these characteristics are discussed. Numerical results from a comprehensive numerical model, the Transmission-Line Laser Model, illustrate the applicability of numerical models to the design and understanding of mode-locked lasers.

We demonstrate femtosecond pulses from 400 - 1600 nm, both at 76 MHz at high average power and at 1 kHz repetition rate at high pulse energy, using optical parametric techniques. The signal wavelength of the KTiOPO₄ (KTP)-based optical parametric oscillator is tunable in the 1.20 - 1.34 micrometers and 1.45 - 1.70 micrometers regions with two mirror sets. This oscillator is synchronously pumped and has a repetition rate of 76 MHz and a signal output power as high as 200 mW. In addition we have used amplified Ti:sapphire femtosecond pulses at 1 kHz repetition rate to generate a white light continuum which extends at least from 400 nm to 1.6 micrometers . Frequency mixing techniques are used to up-convert the continuum to the ultraviolet (UV) region or to amplify the continuum in the infrared (IR) region with LiB₃O₅(LBO) crystals.

Two methods for experimentation with ultrashort mid-infrared light pulses are presented. The techniques utilize high repetition rate sources, and are therefore suitable for use with lock-in modulation detection.

Femtosecond pulse have an appreciable spectral width and it is not possible to satisfy the phase matching condition for all the possible frequency components in the spectrum. This allows pulse propagation with a group velocity that differs from the phase velocity. The difference in the group velocity between the fundamental and harmonic radiation complicates the second harmonic generation. In this paper, the characteristics of SHG from femtosecond optical pulse are discussed. Analytical solutions for the harmonic pulse shape and conversion efficiency in non-depletion condition are presented. In time domain coupled equations in general condition are solved by numerical method. Phase mismatching effect on the harmonic pulse shape and conversion efficiency is analyzed.

Methods to determine the amplitude and phase of ultrashort signals are reviewed. Most of these methods involve a nonlinear process. A new linear method is described that provides complete characterization of recurrent or single shot fs signals.

Induced-grating autocorrelation (IGA) is a simple technique that measures the electric field autocorrelation function. We examine the use of IGA in characterizing the properties of self- phase-modulated pulses in optical fibers. Excellent agreement is found between theory and experiment.

The amplitude and phase of ultrashort optical pulses generated by modelocked lasers yield information about the physical mechanisms that shape the pulse inside the laser. The form of the electric field of pulses with a duration of several tens of femtoseconds can only be retrieved through indirect diagnostic techniques, however. A number of protocols for determining the pulse field envelope have been developed in the past several years and, in this paper, we discuss the application of two pulse measurement techniques, including a novel linear interferometric method, to the measurement of the pulses from a colliding pulse modelocked (CPM) dye laser and a self-modelocked Ti:Sapphire laser.

We report and demonstrate a new technique to measure the full intensity and phase of a single femtosecond laser pulse. The technique, which we call 'frequency resolved optical gating' (FROG), is inexpensive, easy to implement, and provides an output that graphically displays the instantaneous frequency vs. time of the pulse. Using almost any instantaneous nonlinear- optical interaction of two replicas of the ultrashort pulse to be measured, FROG involves measuring the spectrum of the signal pulse as a function of delay between the two replicas. The resulting trace of intensity vs. frequency and delay yields an intuitive display of the pulse, similar to the pulse spectrogram. We show that the problem of inverting the FROG trace to obtain the pulse intensity and phase is a two-dimensional phase-retrieval problem. As a result, the FROG trace yields, in principle, an essentially unique pulse intensity and phase. In this work, we show that this is the case in practice, also. In addition, we present an iterative- Fourier-transform algorithm for inverting the FROG trace.

A single-shot phase sensitive autocorrelator for the characterization of the pulse width and linear chirp coefficient of high intensity, femtosecond ultraviolet lasers is presented. The two photon excited exciton fluorescence of barium fluoride is used as a nonlinear detector. Pulse widths of 210 - 450 fs and the chirp of a subpicosecond KrF excimer laser are measured and shown to agree with an analysis of the laser spectrum.

A novel method to measure a short light pulse using the photorefractive effect is proposed. Since photorefractive crystals are volume holograms, if we assign one axis to time, we can use them as materials which record the change of spatially two dimensional information. We recorded 3.5 ps pulses of a mode-locked Nd:YAG laser in a Fe:LiNbO₃ crystal and read out the information by cw He-Ne laser by measuring the amplitude and phase of the diffracted wave.

A practical method for generating femtosecond-time-scale VUV pulses in the spectral range 150 - 200 nm is discussed. The method is based upon four-wave difference-frequency mixing in Xe ((omega) ₃ equals 2(omega) ₁ - (omega) ₂). One can utilize a two-photon resonance in Xe to increase the VUV output energy by almost two orders of magnitude by choosing the beam at (omega) ₁ to be a KrF-excimer-amplified ultrashort pulse. This resonantly-enhanced mixing process is utilized to generate is congruent to 4-(mu) J, femtosecond- time-scale, pulses at is congruent to 156 nm, suitable for use as seed pulses to test the ultrafast amplification characteristics of F₂-discharges.

The saturation and power scaling of terahertz radiation produced by large-aperture photoconducting antennas under high electric fields and high optical fluences are described. From the saturation behavior, a large-aperture transmitter can be designed to produce the maximum pulse energy of terahertz radiation for a given photoconductor, optical pulse energy and electric field.

A theoretical model based on the nonlinear optical rectification at semiconductor surfaces and quantum wells is reviewed. The theory explains the experimental observations including the crystal orientation dependence of the terahertz signal on the crystal growth axis. Interesting properties, such as (1) the polarization of the radiated electromagnetic pulse is mostly in the plane of incidence (TM polarization), (2) the polarity of the radiation field changes in sign from an n-type doped sample to a p-type doped sample, (3) the radiated field decreases with increasing temperature, (4) the radiation field decreases with increasing doping concentration, and (5) the frequency response of the radiation field peaks near the terahertz range, can all be explained using the optical rectification theory. We also discuss how the same theory can explain the experimentally observed terahertz signals from coupled quantum-well structures. The theory shows that both optically rectified signals and quantum beats are generated.

Experimental demonstration of optical pulse slicing and pulse shaping by pumping a large core, liquid-filled waveguide with picosecond radiation are reported. Steepening of the optical pulses to the extend that the peak of an input pulse catches up and overpasses the rising edge is observed, which results in the pulse shape with a 'negative-like' rising edge. Significant pulsewidth reduction by over 20 fold is obtained. Based on the experimental results, a new mechanism of self trapping of picosecond radiation in a waveguide structure is proposed, which is shown to agree well with our experimental results.

Recently, we have shown that coherent lattice vibration can be induced and subsequently detected in certain semimetals and semiconductors (e.g. Bi, Sb, Te, Ti₂O₃) using short pulses of optical light. The time-resolved optical pump-probe data show in each case that only totally symmetric lattice modes (i.e. A₁ symmetry) are coherently excited, even though other symmetry modes of comparable Raman cross-section exist. Careful measurement of the coherent photon phase reveals that the excitation mechanism for coherent phonons is related to a pump-induced shift in the ion equilibrium configuration in these materials. Of particular interest in these experiments are the magnitudes of the observed reflectivity modulations. Implications of lattice modulations are discussed as well as the validity of employing the equilibrium calibration in the transient regime.

We report on the generation of terahertz pulses from the optical excitation of excitons in GaAs/Al_0.3Ga_0.7As quantum wells. In general there are two contributions to the THz signals for nonzero field. The first arises from the creation of excitons in states in which they are already polarized. This leads to a time-dependent polarization that grows with the integrated pulse energy and radiates a short electrical transient. The second one arises from the coherent excitation of both light hole and heavy hole excitons. The time evolution of the coherent superposition of these two exciton states give rise to charge oscillations that radiate a terahertz signal consisting of several oscillations of the electric field. By applying a field to the sample we can tune the oscillation frequency from 1.4 to 2.6 THz.

We present systematic femtosecond pump-probe studies of the initial scattering and cooling processes of hot electrons in intrinsic, n-type, and p-type GaAs for carrier densities from 8 X 10¹⁵ to 10¹⁹ cm^-3 excited at 2 eV. The role of electron- electron scattering in intra-(Gamma) -valley equilibrium, its dependence on the injected carrier density, and its influence on the amplitude and recovery rate of the initial absorption bleaching are established. For a low density (approximately 3 X 10¹⁶ cm^-3), the electron-electron scattering is observed to be most efficient when all the electrons are in Bloch states with the same (large) kinetic energy. The importance and time scales of electron-hole and electron-plasmon interactions are also revealed.

The nonlinear optical response of photorefractive oxide materials was investigated using four- wave-mixing (FWM) techniques with laser pulses having durations in the pico- and sub- picosecond range. The specific materials studied are KNbO₃, KTaO₃, KTa_1-XNb_XO₃ (KTN), Sr_XBa_1-XNb₂O₆, and Bi₂TeO₅. In each case there are several different physical processes that contribute to the nonlinear signals and their origins are analyzed in the context of time intervals within and after the cross-correlation time of the two write beams pulses. The role of the nonlinear absorption in the four-wave mixing (FWM) processes is compared for the different materials. Variation in the build-up of the electro-optic photorefractive effect is discussed.

The polar low frequency vibrational and relaxational modes crucial to understanding the structure and phase transitions of ferroelectric perovskite crystals are examined using impulsive stimulated Raman scattering (ISRS). At wavevectors near the Brillouin zone center, these modes are strongly coupled to light, and the pure optic mode behavior is deduced from the wavevector dependence of the mixed phonon-polaritons.

Measurements of orientational relaxation over 6 decades in time have been made on the liquid crystal Methoxy Benzylidene butyl aniline (MBBA) using a Transient Grating Optical Kerr effect experiment (TG-OKE). The Slower dynamics have been shown to fit to Landau-de Gennes modified Debye Stoke Einstein Hydrodynamic equation. The faster dynamics show a power law behavior that is temperature independent for 43 degree(s) above the nematic-isotropic phase transition. The slower dynamics deviate from Landau-de Gennes behavior at the same temperature that the faster dynamics become temperature dependent. This is attributed to the domain size, the factor controlling the slow decay, becoming small enough that the local structure is disturbed. Two possible sets of processes are proposed for the power law dependence of the faster dynamics. A parallel process, the Forster direct transfer model, where there is a distribution of potential surfaces for the system to propagate along and the serial process (or Hierarchically constrained dynamics model) where some degrees of freedom are suppressed unless other degrees of freedom are in particular states. These results are compared to earlier work on pentylcyanobiphenyl(5CB). The same behavior is seen in both 5CB and MBBA.

The result of experimental studies of Photoluminescence (PL) in cis-transoid and trans-cisoid polyphenylacetylene were presented. The samples were prepared by a. newly-developed method using rare-earth metal catalyst. In contrast to polyacetylenea, the obtained PPA samples were soluble in common organic solvents and can be proceaaed into thin film. Comparing the results of diferent form of samples: powder, film and toluene solution samples, we concluded that, the observed fut PL decay is caused by the polaronic exciton radiative recombination, which will diminished when the inter-chain charge transfer took place, while the slow PL decay is related to the intra-chain bipolaron recombination.

The relaxation dynamics of photoexcited CdSe nanocrystallites (quantum dots) are dominated by the surface. Surface electronic properties of CdSe nanocrystallites have been probed using low temperature fluorescence line narrowing and time resolved luminescence. We find that the surface structure creates a random potential for the hole with a size dependent barrier for site to site hopping.

We demonstrate two complementary techniques: femtosecond ellipsometry and surface second harmonic generation, for characterization and diagnostics of semiconductor epilayers using unamplified femtosecond laser sources. Through femtosecond ellipsometry, we obtained the time-resolved change in the real and imaginary parts of the index of refraction in relaxed and strained Si_1-xGe_x alloys. Through surface second harmonic generation in conjunction with the Kerr Lens mode-locked (KLM) Ti:Sapphire laser, we obtained surface second harmonic signals in Si(100) and Diamond(111) with an unprecedented signal-to-noise ratio.

A general theoretical and experimental treatment of transient grating diffraction is presented for interfacial holographic gratings in thin film structures. The theoretical treatment allows for gratings that have nonuniform spatial amplitude throughout the sample. Interface selective transient gratings experiments are performed on oriented thin films (approximately 100 nm) of YBa₂Cu₃O_7-x, with MgO and SrTiO₃ substrates. Four different excitation and probe geometries are utilized such that each geometry results in a unique temporal decay. The grating has a significant amplitude on both sides of the film-substrate interface with a grating wave-vector parallel to the interface. The four experimental geometries comprise an over determined system that can be used to confirm the validity of the model assumptions. Numerical fits to the experimental data, using a straight forward diffusive model, are performed to obtain information on thermal diffusivity and to demonstrate the applicability of the technique to monitor anisotropic thermal relaxation processes in thin film- substrate structures. This analysis yields the anisotropic YBa₂Cu₃O_7-x thermal diffusivity constants and the thermal boundary resistance between the thin film and substrate.

We demonstrate a new purely optical based real-time method for excitation and detection of acoustic and thermal disturbances in thin films. The technique is applied to the determination of viscoelastic properties of unsupported and silicon-supported polyimide thin (approximately 1 micron) films. By comparing data from supported films with that from unsupported films, we demonstrate the sensitivity of this technique to delaminations. We then present calculations that suggest how the same technique may be used to probe film-substrate adhesive quality.

The ultrafast dynamics of solid C₆₀ following optical excitation are discussed. Excitation into the lowest optical band using pulses 12 fs in duration centered at 620 nm results in coherent vibrational motion as well as nonexponential relaxation dynamics dominated by interactions between photoexcitations. Excitation at 500 nm into the next highest band reveals complex relaxation dynamics indicative of fast energy relaxation from the higher electronic state.

The time dependence of the carrier relaxation in undoped solid C₆₀ thin films has been studied by monitoring changes in the optical transmission upon photoexcitation at the HOMO- LUMO band edge. The relaxation closely follows a Kohlrausch-Williams-Watts decay with an exponent of (beta) equals 0.4. This functional form is typical of carrier relaxation associated with states of localized rather than extended character. Observation of this decay is highly dependent on film purity, which in turn is highly dependent on the methods used in the fullerene purification and separation processes. The effects of fullerene processing procedures on the observed carrier dynamics in solid C₆₀ is discussed.

We present a comprehensive report of time-resolved reflectivity and transmission measurements of the superconducting system Pr_xY_1-xBa₂Cu₃O_7-(delta ). For a fixed photon energy (1.98 eV), varying the Pr content permits optical probing of energy states both above and below the Fermi level where a superconducting gap is expected to occur. We find qualitatively different behavior in the sign, magnitude, and temporal responses as a function of Pr fraction. Our results indicate that a simple two-fluid model interpretation cannot account for the observed response and that the intrinsic band structure of Pr_xY_1-xBa₂Cu₃O_7-(delta ) plays a significant role in the dynamics of these systems.

We have performed a series of femtosecond reflectivity experiments on various YBa₂Cu₃O_7-x (YBCO) thin films at temperatures ranging from 12 K to 300 K. In particular, we have measured the dependence of the transient reflectivity signal on probing laser frequency, oxygen content, sample's thickness, ambient temperature, and pumping laser intensity. Results obtained at room temperature provide quantitative information on the position of the Fermi level for films with different oxygen contents. Systematic analysis of the measurements performed in the superconducting state indicate that the optical response associated with nonequilibrium properties of YBCO depends strongly on excitation intensity and sample thickness. The observed ultrafast refractive nonlinearity under the low laser excitation cannot be comprehensively interpreted as the relaxation dynamics of quasi-particles, and is speculated to be correlated to the existence of the nonequilibrium intermediate state.

By using the time-resolved photoluminescence technique, we have studied the temporal properties of the photoluminescence from several InGaAs/GaAs single quantum well samples at 77 K. The radiative recombination, the nonradiative recombination and the trapping processes have been investigated for these samples. From the excitation power dependence and temperature dependence measurement, the radiative and the nonradiative lifetime of the carriers and the excitons in these quantum well samples have been obtained which reveal the great influence of the excitation power, temperature and the parameters of the well on these recombination processes.

In this communication, the electron tunneling process in GaAs/AlGaAs asymmetric double quantum wells with different barrier thickness and doping pattern were studied by picosecond photoluminescence spectrum technique. Impurity-assisted sequence tunneling and coherent tunneling were excluded. An electron and hole resonant/nonresonant tunneling model were considered to explain the experimental data obtained.

Regeneratively-initiated, self-sustained, continuous-wave mode-locked operation of a chromium-doped forsterite laser operated at 3.5 degree(s)C is described. Without compensating for the positive group velocity dispersion of the cavity, regenerative, acousto-optic modulation produced pulses of between 41 and 6.5 psec (FWHM) at 1.23 micrometers with average output powers of between 280 and 380 mW, respectively. By employing intracavity negative group velocity dispersion compensation, nearly transform-limited femtosecond pulses of 48 fsec (FWHM) duration were generated with average TEM00 output powers of 380 mW at 1.23 micrometers . These represent the shortest and highest peak power pulses directly generated from this laser system to date.