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We demonstrate femtosecond time-resolved dynamic Gabor holography using highly coherent extreme ultraviolet light generated by high harmonic upconversion of a femtosecond laser. By reflecting this light from an impulsively heated surface, we implement a simple and robust single-reflection geometry for phase-sensitive holographic detection at extreme UV wavelengths. Using this setup, we study the ultrafast deformation and subsequent acoustic oscillations within a thin metal film. These measurements exhibit subpicometer spatial sensitivity in the vertical dimension.

Researchers are developing tools and techniques that use coherent light at ultrashort wavelengths - in the extreme ultraviolet and soft X-ray regions of the spectrum.

Efficient reflection grisms for pulse-compression and material-dispersion compensation have been designed and demonstrated in a 40 fs, 300 microJ, 5 kHz downchirped pulse amplification system for the first time to our knowledge. A grism design for 800 nm femtosecond laser pulse dispersion compensation applications is realized by using standard, commercial diffraction gratings.

We present a novel ultrafast multipass laser amplifier design optimized for sub-millijoule output energy and capable of being operated at repetition rates exceeding 40 kHz. This ti:sapphire based system makes use of a grism based stretcher, a cryogenically cooled ti:sapphire crystal and an astigmatically compensated multipass amplifier design that allows for pumping with significantly lower pump pulse energies than has been demonstrated to date. We also make use of the downchirped pulse amplification scheme to minimize loss in the pulse compression process. Preliminary experiments demonstrate an output pulse energy of 290 muJ at 10 kHz and 270 muJ at 15 kHz with a pulse duration of 36 fs.

The authors demonstrate the use of coherent extreme ultraviolet (EUV) light in laser induced transient grating spectroscopy. In this work, the authors study the dispersion of pulsed laser induced surface acoustic waves in a thin uniform nickel film by monitoring the diffraction of 30nm EUV light from the surface. Transient deformation of the sample surface leads to absolute diffracted intensities approaching 10−3 for 1A displacement, an ∼700-fold increase in sensitivity compared with optical probing. EUV probing provides a convenient geometry for observing short wavelength acoustic propagation.

We demonstrate for the first time that up to 0.45 microJ pulses can be obtained from a cavity-dumped Ti: sapphire oscillator stably operating in the positive dispersion regime. The output pulse can be compressed to 60 fs and used to generate a white light continuum through self-filamentation in a thin sapphire plate.

We demonstrate a general procedure for varying the repetition rate of a modelocked Ti:sapphire laser using an asymmetric focusing geometry. Using this procedure, we have made an extended length cavity with a repetition rate of 45 MHz, and a reduced length cavity with a repetition rate of 275 MHz, each of which generates sub-20 fs pulses. This procedure allows the repetition rate of the laser to be more precisely tailored for a variety of applications without compromise in performance.

In this work, we experimentally investigate the effect of a grating based pulse stretcher/compressor on the carrier-envelope phase stability of femtosecond pulses. Grating based stretcher-compressor (SC) setups have been avoided in past demonstrations of chirped pulse amplification (CPA) of carrier envelope phase (CEP) stabilized femtosecond pulses, because they were expected to introduce significantly stronger CEP fluctuations than material-based SC systems. Using a microstructure fiber-based detection setup, we measure CEP fluctuations of PhiCE,SC = 340 milliradians rms for a frequency range from 63 mHz to 102 kHz for pulses propagating through the SC setup. When bypassing the beam path through the SC, we find CEP fluctuations of PhiCE,bypass = 250 milliradians rms. These values contain significant contributions from amplitude-to-phase conversion in our microstructure fiber-based detection setup for PhiCE. Hence, we do not unambiguously measure any added CEP noise intrinsic to the SC setup. To distinguish between intrinsic SC effects and amplitude-to-phase conversion, we introduce controlled beam pointing fluctuations alpha and again compare the phase noise introduced when passing through / bypassing the SC. Our measurements do not reveal any intrinsic effects of the SC system, but allow us to place an upper limit on the sensitivity of our SC system of PhiCEintrinsic,SC / alpha13000 rad/rad. Our results demonstrate experimentally that there is not a strong coupling mechanism between CEP and beam pointing through a stretcher/compressor , as well as measuring significantly smaller CEP fluctuations than experimental results reported previously.

Mega-electron-volt (MeV) electron emission from the interaction of an ultrafast (τ∼29 fs), intense (>1018 W/cm2) laser pulse with underdense plasmas has been studied. A beam of MeV electrons with a divergence angle as small as 1° is observed in the forward direction, which is correlated with relativistic filamentation of the laser pulse in plasmas. A novel net-energy-gain mechanism is proposed for electron acceleration resulting from the relativistic filamentation and beam breakup. These results suggest an approach for generating a beam of femtosecond, MeV electrons at a kilohertz repetition rate with a compact ultrafast intense laser system.

An electrostatically deformable, gold-coated, silicon nitride membrane mirror was used as a phase modulator to compress pulses from 92 to 15 fs. Both an iterative genetic algorithm and single-step dispersion compensation based on frequency-resolved optical gating calibration of the mirror were used to compress pulses to within 10% of the transform limit. Frequency-resolved optical gating was used to characterize the pulses and to test the range of the deformable-mirror-based compressor.

We describe theoretically the performance of a new design for quasi-phase matched harmonic generation in a gas medium. A hollow core fiber in which thick glass plates are periodically introduced allows good phase control of the fundamental light. The generated x-rays are transmitted throught small holes in the plates. An increase of the harmonic yield of up to three orders of magnitude is predicted as a result of the structure. This esign can be considered as a phase-locked travelling-wave x-ray laser.

We study mode locking of a titanium-doped sapphire laser with a slow saturable absorber in the cavity. We show that the presence of the absorber stabilizes the pulsed mode of laser operation over a broader range of cavity mirror position and for a wider aperture. However, the basic principle of operation is unchanged from that of a conventional Kerr-lens mode-locked laser, in that, in all cases, we observed strong space–time focusing of the pulses into the laser crystal. Thus the absorber cannot lead to shortening of the pulse below the limit imposed by the Kerr-lens mode locking.

We present spatially and spectrally resolved measurements of light emerging from the focus of an intense 25-fs laser pulse in gaseous media. The pulses are focused inside a chamber backfilled with either argon or air to nominal vacuum intensities of up to 5×1015 W/cm2, far in excess of that necessary for ionization. We have recorded spectral-spatial maps for various pressures and pulse energies and have compared the results of argon and air. This survey is done in the regime where ∼0.1% of the laser light is converted into the third harmonic, which is of practical importance since it is the largest conversion efficiency measured to date while maintaining such a short pulse duration. These measurements are, to our knowledge, the first of their type and are performed with a laser beam of very high spatial quality. The dynamics of an intense laser focusing into a gas cell is complicated, and these measurements provide a way to compare numerical simulations with an array of experimental information. The experiments also determine the conditions for optimal third-harmonic spatial beam quality in this regime.

We present what is believed to be the first experimental demonstration of guided-wave phase-matched frequency mixing and harmonic conversion in gases. Broad-bandwidth ultrafast pulses, tunable around 270 nm, were generated from an ultrafast Ti:sapphire amplifier system using 2? + 2? - ? parametric wave mixing in a capillary waveguide. We achieved nonresonant phase matching by coupling both the fundamental and the second-harmonic light into the lowest-order mode. The output 3? pulses have an energy of4muJ at a 1-kHz repetition rate. Simple extensions of this method can generate higher-energy 10-20-fs pulses tunable throughout the vacuum ultraviolet.

We have developed a 1-kHz repetition-rate Ti:sapphire laser system that can simultaneously generate high peak and average powers of 0.2 TW and 4 W, respectively. The laser system generates 4-mJ energy pulses with a 20-fs pulse width. We eliminated thermal lensing in the system by cooling the Ti:sapphire crystal to 125 K. The output 20-fs pulses were fully characterized by use of the new technique of transient-grating frequency-resolved optical gating. We demonstrate experimentally that the pulse duration at the output is limited only by fifth-order dispersion.

Using second-harmonic frequency-resolved optical gating and a tricyanovinylaniline polymer thin film, we have measured the amplitude and the phase of mode-locked Ti:sapphire laser pulses as short as 13 fs. These thin films are ideally suited for ultrashort-pulse diagnostics because they eliminate the angle tuning associated with birefringent phase-matched crystals, minimize pulse distortions introduced by group-velocity dispersion, and exhibit excellent photochemical stability.

We have developed a Ti:sapphire amplifier system capable of producing pulses of 1 mJ, with 20-22-fs pulse duration, at a 1-kHz repetition rate. The amplifier has a unique design consisting of a three-mirror multipass ring configuration with a highly doped Ti:sapphire crystal as the gain medium. Pulses of 15-fs duration from a Ti:sapphire oscillator are temporally stretched and injected into the amplifier, which is an eight-pass system with a total gain of 10(6). The amplif ier is more than 10% efficient, and the shot-to-shot energy fluctuation of the output is less than 2%. The output beam focuses to 1.8 times the diffraction limit.

We measure the intensity and phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser using the recently developed technique of frequency-resolved optical gating. These results represent to our knowledge the shortest complete optical waveform characterization measurements performed to date. We also verify recent theoretical calculations that predict that the main limitation on the pulse duration from these lasers is the presence of uncompensated higher-order dispersion.

We present what is to our knowledge the first three-dimensional model of a femtosecond pulse propagating in a Ti:sapphire laser crystal, which includes dispersion, self-focusing, and finite response time of the medium and does not assume the slowly varying envelope approximation. The combined action of material dispersion and phase modulation leads to a dramatic space-time focusing on the pulse. Dispersion prevents catastrophic selffocusing and self-steepening of the pulse, even though the peak power of the pulse is much greater than the critical power for self-focusing filamentation. Extrapolation of these results to shorter-pulse durations shows that this space-time focusing mechanism can operate even for pulses with durations close to the response time of the Kerr nonlinearity.

We demonstrate that by operating near the zero second- and third-order dispersion point in a self-mode-locked Ti:sapphire laser we can generate sub-10-fs pulses. Our numerical simulations show that the pulse duration is limited by fourth-order dispersion and that shorter pulses will be possible if this can be reduced. Also, by inserting a pellicle in various positions in a Ti:sapphire cavity, we have measured the intracavity pulse duration and chirp of the circulating pulse in the laser. Our results demonstrate that the pulse is shortest near the middle of the laser crystal, in one direction of propagation. In the other direction of propagation, the pulse is positively chirped and several times longer.

By optimizing the intracavity dispersion compensation in a self-mode-locked Ti:sapphire laser, we have generated pulses of 10.95-fs duration. Dispersion within the laser cavity is reduced by use of a short 4.5-mm highly doped Ti:sapphire crystal and fused-silica prisms. The output from the laser has an average power of as much as 500 mW, with a wavelength centered at 780 nm and a bandwidth of 62 nm. Our results demonstrate that the exceptionally broad bandwidth of Ti:sapphire can be utilized to generate pulses that, to our knowledge, are shorter than has been possible with any other type of laser material to date.

Ultrashort-duration, high-energy laser pulses have been generated using a titanium-doped-sapphire oscillator and amplifier system. The pulses have an energy of 0.45 J with a duration of 95 fs and an energy of 0.23 J with a duration of 60 fs.

Tunable radiation can be produced by Doppler upshifting light from a laser-generated moving ionization front. We analyze this technique as a method of generating ultrashort tunable light pulses in the UV to extreme UV (XUV) regions of the spectrum and demonstrate the possibility of generating subfemtosecond short-wavelength light pulses. Using realistic and technologically achievable experimental conditions and including dispersion of a pulse reflected from a finite-rise-time multiphoton-ionization-produced ionization front, we show that a single-cycle far-infrared (FIR) pulse can be compressed in time and frequency into a pulse substantially shorter than any light pulse demonstrated to date. We show that arbitrarily large upshifts are achievable using this technique, making it a potentially competitive way of producing tunable XUV radiation. The multiphoton ionization mechanism by which the ionization front is created is an important factor in determining the reflectivity of the ionization front.

We report studies of new short-wavelength lasers in xenon at 108.9 nm and krypton at 90.7 nm. The lasers are pumped by photoionization of inner-shell electrons followed by Auger decay into excited states. Experiments include measurement of gain and the lifetimes and quenching of the lasing states, the use of a mirror for multipass gain, and studies of the output pulse duration. Numerical simulations which predict gain and the time evolution of the laser output are compared with experiment.

When an intense, sub-picosecond laser pulse is focused onto the surface of a solid, a high-temperature, high-density, X-ray emitting plasma is produced. In this paper we describe experimental and theoretical studies of such a plasma, which show that it is a useful source of short X-ray pulses with a duration of less than 1.5 ps. Spectroscopic measurements indicate the presence of a short-lived, high-density plasma, with an emission spectrum in reasonable agreement with theoretical predictions.

The interpretation of upcoming cosmic soft X-ray spectroscopic observations may require a vast improvement in the state of our knowledge of basic atomic physics and radiation transfer processes, particularly for the important class of accretion-powered systems such as cataclysmic variables, X-ray binaries, and active galactic nuclei. For these sources, short wavelength spectral features are formed in the transfer of X-radiation from a powerful central source outward through a cooler, surrounding medium. The line emitting gas is photoionized and the level populations are determined largely by recombination cascades and photoexcitation as opposed to collisional effects. These processes can be rather complex and many uncertainties remain. We are developing a new approach to this problem using time-resolved studies of laser produced plasmas. Experiments which are currently underway include the measurements of: (a) inner and outer shell photoabsorption cross-sections of intermediate ions of abundant elements, and (b) fluorescent excitation processes due to wavelength coincidences of resonant transitions from different ions. In connection with this program, we have developed a novel reflection grating spectrometer coupled to a streak camera which provides high resolution spectra at soft X-ray and EUV wavelengths at the sub-nanosecond level. We describe our laboratory facility and some of our preliminary experimental results.

Pulses at soft X-ray wavelengths with a duration of less than 1.5 ps were produced by focusing high-intensity, ultrafast visible laser pulses onto a solid. Reflectivity measurements indicate that the laser pulses are absorbed by a plasma formed just inside the surface of the solid before expansion occurs. The observed emission spectrum is characteristic of a high-temperature, short-lived, high-density plasma. Applications of the ultrafast X-ray pulses, including an X-ray laser scheme, are discussed. >

We report the observation of gain in a new type of short-wavelength laser system at 108.9 nm. Soft x rays from a laser-produced plasma photoionize an inner-shell electron from xenon, and a population inversion is created by subsequent Auger decay to particular excited states. A model for this system is described, spectroscopic data are presented, and gain limitations due to parasitic amplified spontaneous emission are discussed. A maximum gain of 0.8 cm/sup -1/ over a length of 9 cm is reported.

We describe temporal and spectroscopic measurements of high-density plasmas produced by focusing intense, 160-fsec laser pulses on solids. Soft x-ray emission with a duration of 2 +- 2 psec is observed up to photon energies of a kilovolt. We observe reduced emission from long-lived spectral lines, indicating the presence of a short-lived, high-density plasma. Reflectivity measurements indicate that absorption of the laser pulse occurs at the surface of the solid before it expands.

We present data characterizing an x-ray streak camera which uses a microchannel plate as a photocathode. The microchannel plate provides good sensitivity to soft x-ray photons and adjustable gain in addition to one dimensional spatial imaging. The streak camera was used in conjunction with a high sensitivity CCD camera and a computer data acquisition system. The resulting detection system has single photon sensitivity, a time resolution of better than 200 psec, and a dynamic range exceeding 103.

Measurements on a proposed short wavelength laser system at 108.9 nm in doubly ionized Xenon are reported. Previous electron spectroscopy data indicate that a population inversion should be created between the Xe III states 5s0 5p6 1S0 and 5s0 5p6 1P1 due to selective Auger decay following photoionization of a 4d electron from the neutral atom. Using a high repetition rate laser produced plasma x-ray source and time correlated photon counting, we measured the lifetime of the 1S0 upper state to be 4.75 ±0.15 nsec with a collisional quenching rate of 2.8±0.2x107 (torr-sec)-1. We also determined the Auger branching ratio into the upper and lower states by measuring fluorescence intensities. Implementation of this laser scheme is discussed including limitations in photopumping short wavelength lasers due to amplified spontaneous emission.

Publisher Summary The chapter provides an overview of short-wavelength light pulses with a duration of less than a picosecond, which can be produced by focusing high-intensity, ultrafast laser pulses onto a solid. The intense, ultrafast laser pulses are absorbed by plasma formed inside the surface of the solid before ablation occurs. The resulting short-wavelength emission spectrum is characteristic of a high-temperature, short-lived, high-density plasma. In a study mentioned in the chapter, the importance of using low prepulse energy for the production of short pulse x-rays from laser-produced plasmas was demonstrated. Energy deposited before the arrival of the short pulse can cause ionized material to be ablated, leading to the formation of lower density plasma in front of the solid surface. Ultrashort laser pulse absorbs in that lower density region and can lead to relatively long x-ray emission times. The short-pulse laser is coupled into solid or near-solid density plasma with an associated high, metal-like reflectivity. At high electron densities, ionization depression reduces the number of bound levels to the first few excited levels and Stark broadening is severe; this can lead to rapidly varying line broadenings. Continuum emission is expected to dominate until the plasma cools and expands. Resonance line emission from long lived states has been observed to radiate for tens of picoseconds longer than the short pulse integrated emission observed using the streak camera.

We demonstrate that high harmonic generation from ions can reach higher energies than from neutral atoms. We extend the highest cutoff observed in argon using 800 nm light from 100 eV to 250 eV.

We demonstrate that using longer-wavelength driving light in high-order harmonic generation requires moderately ionized larger-density atoms to perfectly phase match the process, scaling conventional phase matching at low ionization to extremely high photon energies.

Summary form only given. We present measurements of the coherence of extreme ultraviolet (EUV) light generated using the process of high-harmonic upconversion (HHG) of light from a femtosecond laser and demonstrate coherent imaging applications for this unique source. When HHG is generated in a phase-matched hollow-fiber geometry, the EUV light generated exhibits the highest inherent spatial coherence of any source in this region of the spectrum to date.

We demonstrate enhancement of high harmonic generation in the technologically significant 95 eV photon energy region by combining phase matching, quasi phase matching, and pulse compression in a single gas-filled waveguide. The waveguide geometry allows us to use argon gas as the generating medium, which also leads to enhanced emission due to its higher nonlinearity. The induced shock front due to laser pulse compression in the ionizing gas medium also generates an X-ray continuum at low pressure, with a spectrum corresponding to an isolated, 50 attosecond, X-ray pulse.

We demonstrate bright high harmonic generation driven by UV lasers with ultra-high conversion efficiency approaching 10−3 and ultra-narrow single-harmonic bandwidth of ∼0.2%. The enhanced flux results from improved phase-matching combined with stronger single-atom yield.

We present the first observation of the laser-assisted photoelectric effect in surface photoemission. Illuminating a Pt(111) sample simultaneously with 1.6 eV and 42 eV pulses, we observe ldquodressingrdquo of the EUV photoelectron spectrum.

We extend ptychography CDI to allow for imaging of multiple areas on a sample simultaneously using multiple identical beams. This enables high throughput imaging of large samples without increased data collection or loss in resolution.

We demonstrate that high-harmonic generation can be phase-matched in the "water window" region of the spectrum using periodically-modulated hollow fibers. We also observe emission from ions.