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We developed appropriate conditions to use a laser with 60 femtosecond pulses, a frequency of 1 KHz and a wavelength of 266 nm to efficiently crosslink proteins to DNA in human nuclei for the purpose of using immunoprecipitation to study the binding of specific proteins to specific sequences of DNA under native conditions. Irradiation of nuclei for 30 min with 1-3 GW/cm(2)pulses crosslinked 10-12% of total protein to DNA. The efficiency of crosslinking was dose and protein specific. Histones H1 and H3 were crosslinked by 15 min of irradiation with 20-25% efficiency, at least 10 times more strongly than the other histones, consistent with experiments using conventional UV light. Irradiation for 15 min did not damage proteins, as assayed by SDS-PAGE of Ku-70 and histones. Although the same level of irradiation did not cause double-strand breaks, it did make the DNA partially insensitive to Eco RI restriction enzyme, probably through formation of thymidine dimers. Immuno-analysis of crosslinked nucleoprotein showed that Ku crosslinking to nuclear DNA is detectable only in the presence of breaks in the DNA, and that nucleosomes are bound to a significant fraction of the telomeric repeat (TTAGGG) (n).

By use of the recently developed technique of guided-wave frequency conversion, the generation of sub-10-fs light pulses in the UV has been demonstrated for what is believed to be the first time. Cross-phase modulation of the light in a hollow waveguide produced a bandwidth of 16 nm, with a center frequency of 270 nm, at 1 kHz. A simple grating pair was used to compress the pulses to a duration of 8 fs, as measured by self-diffraction frequency-resolved optical gating. In the experiment the compressed energy was greater than 1 muJ , with a peak power of >100 MW ; the technique can be scaled to higher energy. Further improvements should make it possible to generate pulses as short as approximately 3 fs with this technique.

Phase-matched harmonic conversion of visible laser light into soft x-rays was demonstrated. The recently developed technique of guided-wave frequency conversion was used to upshift light from 800 nanometers to the range from 17 to 32 nanometers. This process increased the coherent x-ray output by factors of 10(2) to 10(3) compared to the non-phase-matched case. This source uses a small-scale (sub-millijoule) high repetition-rate laser and will enable a wide variety of new experimental investigations in linear and nonlinear x-ray science.

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 of >4muJ 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 0.2TW and 4W, 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 125K. 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.

We demonstrate an apparatus for measuring time-dependent x-ray diffraction. X-ray pulses from a synchrotron are diffracted by a pair of Si(111) crystals and detected with an x-ray streak camera that has single-shot resolution of better than 1 ps. The streak camera is driven by a photoconductive switch, which is triggered by 100-fs laser pulses at a repetition rate of 1 kHz. The laser and the streak camera are synchronized with the synchrotron pulses. In the averaging mode, trigger jitter results in 2-ps temporal resolution. We measured the duration of 5-keV pulses from the Advanced Light Source synchrotron to be 70ps.

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 present results of a three-dimensional model of a Kerr-lens mode-locked titanium-doped sapphire laser in the sub-10-fs regime of operation. We find that space-time focusing of the pulse within the laser crystal imposes a limitation on the pulse duration as a function of the crystal length. Thus good compensation of the net group-velocity dispersion of the cavity is not sufficient for successful sub-10-fs operation of the laser. The dependence of the pulse duration on the intracavity dispersion is also presented, which demonstrates that pulses as short as 6 fs can be generated directly from the laser.

We describe a simple method for generating sub-20-fs ultraviolet light pulses with useful average powers, using a kilohertz Ti:sapphire laser system. By focusing a 22-fs, 1-mJ laser pulse in air, we obtain ultraviolet pulses with an energy of 1 microJ and at a wavelength of 266 nm and with an average power of 1 mW. The pulse duration of the ultraviolet pulses was measured to be 16 fs with frequency-resolved optical gating.

We present what is to our knowledge the first full spatial plus temporal model of a self-mode-locked titaniumdoped sapphire laser. The self-consistent evolution of the pulse toward steady state imposes strong space-time focusing in the crystal, where both the space and time foci are located. This combined focusing significantly improves the discrimination properties of the nonlinear resonator for shorter pulses and reduces the transient stage of pulse formation. Our theoretical results are in very good agreement with experiment.

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 report the generation of 26-fs-duration pulses, with an energy of 60 mJ, from a simple multipass Ti:sapphire amplifier system. The peak power of our amplified pulses is 2 TW, and the repetition rate is 10 Hz. Our amplifier design consists of two highly doped multipass amplifiers and is simple and compact. We use an all-reflective, low-groove-density grating stretcher and compressor, combined with a relatively short material path length in the amplifier. This design allows us to minimize higher-order dispersion. The result is a laser system that generates multiterrawatt transform-limited pulses, with good beam quality and low amplified-spontaneous-emission levels, at a duration near the theoretical limit imposed by gain narrowing in Ti:sapphire.

We analyze the performance of a Ti:sapphire self-mode-locked laser with near-zero second- and third-order dispersion. Our simulations show that, in the presence of fourth-order dispersion, solitary laser pulses can be supported within a wide parameter range, close to the experimental values of these parameters. We also conclude that it is possible to generate pulses much shorter than 10 fs if fourth-order dispersion is further reduced.

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.

We demonstrate the use of intracavity frequency doubling in a self-mode-locked Ti:sapphire laser to generate pulses as short as 14 fs. We use a 100-microm-thick lithium triborate (LiB(3)O(5)) crystal in a standard Ti:sapphire laser cavity for the frequency doubling. The average blue output power is 30 mW in each of two beams directly from the laser and 11 mW after compression. The repetition rate is 91 MHz, and the spectral bandwidth is a near-transform-limited 19.2 nm centered at 416 nm. To our knowledge, these results represent the shortest pulse duration yet demonstrated in the blue region of the spectrum.

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.

The technique of self-induced plasma shuttering can be used to suppress prepulse energy from an ultrashort pulse. If a femtosecond pulse is incident upon a transparent target, the leading edge passes through while the peak reflects owing to ionization breakdown at the surface. We describe a fluid jet, enclosed in a vacuum chamber, that allows this technique to be used at high repetition rates. The jet has excellent stability and a fast (~500 micros) recovery time. At normal incidence, we demonstrate a reflection efficiency of 70% with a prepulse-to-main-pulse energy suppression ratio of >10, while at Brewster incidence we measure a reflection efficiency of 38% with a suppression ratio of nearly 400.

We have generated sub-17-fs-duration pulses directly from a self-mode-locked Ti:sapphire laser. These pulses are near transform limited, with a wavelength centered at 817 nm, a pulse repetition rate of 80 MHz, and an average power of 500 mW. By minimizing the amount of material inside the laser cavity and choosing the correct glass for the intracavity prism pair, third-order dispersion in the laser can be significantly reduced compared with that in previous designs. Extracavity compensation for group-velocity dispersion in the output coupler and autocorrelator optics is necessary to measure this pulse width. To our knowledge this laser generates pulses substantially shorter than any other laser to date.

Recent advances in the production of ultrashort x-ray pulses by using femtosecond laser-produced plasmas coupled with the development of terawatt ultrashort-pulse lasers may make possible ultrashortpulse photoexcited x-ray lasers. I examine the creation of a population inversion on the K-alpha transition of neon at 1.5 nm by using the photoionization scheme first suggested by Duguay and Rentzepis in 1967. It is shown that this laser can be produced by using a pump laser of ~ 10 J in 50 fs, provided that a sufficiently bright laser-produced plasma x-ray source can be created. Recent experimental and theoretical results are discussed that verify the potential feasibility of this scheme.

We demonstrate the technique of self-induced plasma shuttering as a means of suppressing prepulse energy that accompanies high-energy, ultrashort laser pulses. This technique makes possible the generation of clean, high-intensity subpicosecond laser pulses even in the presence of high levels of amplified spontaneous emission from the laser system. Low prepulse energy is important in applications such as the generation of solid-density subpico-second plasmas.

A high-temperature plasma is created when an intense laser pulse is focused onto the surface of a solid. An ultrafast pulse of x-ray radiation is emitted from such a plasma when the laser pulse length is less than a picosecond. A high-speed streak camera detector was used to determine the duration of these x-ray pulses, and computer simulations of the plasmas agree with the experimental results. Scaling laws predict that brighter and more efficient x-ray sources will be obtained by the use of more intense laser pulses. These sources can be used for time-resolved x-ray scattering studies and for the development of x-ray lasers.

We demonstrate a novel technique for time-resolved spectroscopic studies of highly excited ions. The technique uses a laser-produced plasma as a short-pulse, soft-x-ray light source with a high repetition rate. A Nd:YAG laser with a pulse duration of 90 psec, a pulse energy of 70 microJ, and repetition rate of 10(4) pulses per second is focused onto a rotating metal target. Soft x rays from the resulting plasma photoionize a gas surrounding the target, and fluorescence from the gas is detected by using a spectrometer and a high-speed photodetector. Using the technique of time-correlated photon counting, we determined the radiative lifetime and collisional quenching rate of the Xe III 5s(0)5p(6)(1)S(0) state by observing its fluorescence at 108.9 nm. A time resolution of better than 400 psec was obtained. We also measured relative Auger decay yields of a core hole state in xenon using a higher-energy laser-produced plasma light source at a lower repetition rate.