Modeling of clusters in a strong 248-nm laser field by a three-dimensional relativistic molecular dynamic model.

A relativistic time-dependent three-dimensional particle simulation model has been developed to study the interaction of intense ultrashort KrF (248 nm) laser pulses with small Xe clusters. The trajectories of the electrons and ions are treated classically according to the relativistic equation of motion. The model has been applied to a different regime of ultrahigh intensities extending to 10(21) W/ cm(2). In particular, the behavior of the interaction with the clusters from intensities of approximately 10(15) W/cm(2) to intensities sufficient for a transition to the so-called "collective oscillation model" has been explored. At peak intensities below 10(20) W/cm(2), all electrons are removed from the cluster and form a plasma. It is found that the "collective oscillation model" commences at intensities in excess of 10(20) W/cm(2), the range that can be reached in stable relativistic channels. At these high intensities, the magnetic field has a profound effect on the shape and trajectory of the electron cloud. Specifically, the electrons are accelerated to relativistic velocities with energies exceeding 1 MeV in the direction of laser propagation and the magnetic field distorts the shape of the electron cloud to give the form of a pancake.

Optimization of power compression and stability of relativistic and ponderomotive self-channeling of 248 nm laser pulses in underdense plasmas.

The controlled formation in an underdense plasma of stable multi-PW relativistic micrometer-scale channels, which conduct a confined power at 248 nm exceeding 10(4) critical powers and establish a peak channel intensity of approximately 10(23) W/ cm(2) , can be achieved with the use of an appropriate gradient in the electron density in the initial launching phase of the confined propagation. This mode of channel formation optimizes both the power compression and the stability by smoothing the transition from the incident spatial profile to that associated with the lowest channel eigenmode, the dynamically robust structure that governs the confined propagation. A chief outcome is the ability to stably conduct coherent energy at fluences greater than 10(9) J/ cm(2) .

Stable relativistic/charge-displacement channels in ultrahigh power density (approximately 10(21 W/cm3) plasmas.

Robust stability is a chief characteristic of relativistic/charge-displacement self-channeling. Theoretical analysis of the dynamics of this stability (i) reveals a leading role for the eigenmodes in the development of stable channels, (ii) suggests a technique using a simple longitudinal gradient in the electron density to extend the zone of stability into the high electron density/high power density regime, (iii) indicates that a situation approaching unconditional stability can be achieved, (iv) demonstrates the efficacy of the stable dynamics in trapping severely perturbed beams in single uniform channels, and (v) predicts that approximately 10(4) critical powers can be trapped in a single stable channel. The scaling of the maximum power density with the propagating wavelength lambda is shown to be proportional to lambda-4 for a given propagating power and a fixed ratio of the electron plasma density to the critical plasma density. An estimate of the maximum power density that can be achieved in these channels with a power of approximately 2 TW at a UV (248 nm) wavelength gives a value of approximately 10(21) W/cm3 with a corresponding atomic specific magnitude of approximately 60 W/atom. The characteristic intensity propagating in the channel under these conditions exceeds 10(21) W/cm2.

Measurement of 160-fs, 248-nm pulses by two-photon fluorescence in fused-silica crystals.

Measurements of 160-fs, 248-nm ultrashort pulses are obtained through a two-photon fluorescence measurement based on the two-photon-induced color-center fluorescence in fused-silica crystals. The method proved to be reliable and advantageous in comparison with two-photon fluorescence techniques employing other materials, both solid state and gaseous.

Short-pulse amplification at 745 nm in Ti:sapphire with a continuously tunable regenerative amplifier.

We describe the design of a continuously tunable Ti:sapphire regenerative amplifier that is capable of sustaining amplification over a wavelength range from 730 nm to more than 800 nm. The amplifier cavity is tuned with a prism pair in combination with a spherical mirror. It sustains an oscillation bandwidth of 5 nm and produces a chirped output pulse energy of 1.2 mJ compressible to a duration of 140 fs at 745 nm. We present a calculation of the theoretical bandwidth.

Biomedical three-dimensional holographic microimaging at visible, ultraviolet and X-ray wavelengths.

A new imaging technology is under advanced development that has several key advantages over conventional forms of microimaging performed with standard light microscopes, confocal light microscopes, and electron microscopes. The image created by this microscope possesses several unique features: It is intrinsically three-dimensional; it can be formed with very high contrast, a characteristic of the phase-sensitive nature of the recording; the information contained in the image is obtained in a single exposure of the specimen, a feature that eliminates the accumulation of damage to living systems that can occur with techniques utilizing multiple exposures; the method of image construction is fundamentally free from aberration (distortion), thereby obviating the need to employ complex procedures for correction; the exact focal plane of any optical section is digitally determined through computation and is not based on any mechanical adjustments; and the principles of operation, including the computational processes and modalities of image presentation, are uniform over the full range of spectral coverage spanning from the visible (approximately 500 nm) to the X-ray (approximately 0.3 nm) regions. The application of this new technology is expected to open new cost-effective avenues to the understanding, prevention and treatment of broad areas of human disease.

Method of concentration of power in materials for x-ray amplification.

Recent experimental and theoretical results indicate that a new technique for the controlled concentration of power in materials may be feasible. The power levels that are potentially achievable are sufficient for the generation of amplification of x-ray wavelengths in the kilovolt range. The method of power concentration involves the combination of (1) a new ultrahigh brightness subpicosecond laser technology, (2) multiphoton coupling to atoms and molecules, and (3) a new channeled mode of electromagnetic propagation. The energy scaling of this approach is the most important consideration, and it is shown that the control of the propagation is the key factor that enables high levels of amplification in the kilovolt regime to be achieved with a total excitation energy of ~1 J.

Fourier-transform holographic microscope.

We describe a holographic microscope with a spatial resolution approaching the diffraction limit. The instrument uses a tiny drop of glycerol as a lens to create the spherically diverging reference illumination necessary for Fourier-transform holography. Measurement of the point-spread function, which is obtained by imaging a knife edge in dark-field illumination, indicates a transverse resolution of 1.4 microm with wavelength lambda = 514.5 nm. Longitudinal resolution is obtained from the holograms by the numerical equivalent of optical sectioning. We describe the method of reconstruction and demonstrate the microscope's capability with selected biological specimens. The instrument offers two unique capabilities: (1) it can collect three-dimensional information in a single pulse of light, avoiding specimen damage and bleaching; and (2) it can record three-dimensional motion pictures from a series of light pulses. The conceptual design is applicable to a broad range of wavelengths and we discuss extension to the x-ray regime.

Plasma production from ultraviolet-transmitting targets using subpicosecond ultraviolet radiation.

Plasma produced from ultraviolet-transmitting solid targets undergoing intense (>10(16) W/cm(2)) subpicosecond (~600 fs) ultraviolet (248 nm) irradiation have been studied under conditions for which no interfering prepulse plasma is generated. Time and spatially integrated measurements of the x-ray emission for H-like and He-like Mg and Si were found to be consistent with LASNEX calculations that model the laser-target interaction.

Protection of fiber function by para-axial fluid flow in interstitial laser therapy of malignant tumors.

In the past, interstitial laser therapy frequently has failed because of the damage to the bare fiber tip due to intense heat generated at the point of contact. Using a rat mammary tumor model, we describe a method of placing a 600 micron fiber inside a gauge 19 needle cannula after its insertion into the tumor. With this device continuous wave Nd:YAG laser is delivered to the target tumor while 0.9% saline flows para-axially into the tumor. Significant coagulation necrosis was induced with 500 joules at 5 watts, 100 seconds and 1 cc per minute of saline while the needle-fiber is pulled out of the tumor by 10 mm. The mean transmission loss after 500 joules was 2% in ten experiments. The tumor edema due to 1.5 ml of saline was transient. We conclude that successful hyperthermic coagulation necrosis by Nd:YAG laser can be achieved with minimal transmission loss by employing the above technique.

Ultrahigh-intensity KrF* laser system.

The operational characteristics of an ultrahigh-intensity subpicosecond large-aperture KrF* laser system are described. Measurements show the achievement of a focal spot diameter of less than 1.7 microm. Combined with measurements of the pulse width and pulse energy, this yields an average intensity of ~2 x 10(19) W/cm(2), a value corresponding to a peak electric field of ~24 (e/a(0)(2)). Light sources of this nature will find application in a broad range of studies of the nonlinear properties of matter in the strong-field regime.

Measurement of 248-nm, subpicosecond pulse durations by two-photon fluorescence of xenon excimers.

A technique for measuring the duration of single ultrashort pulses at KrF(*) wavelengths has been developed that employs fluorescence from xenon excimers excited by two-photon absorption of atom pairs. Pulses of ~350-fsec duration have been measured at 248 nm.

Shortening of KrF(*) laser pulses using stimulated Brillouin scattering.

Pulses as short as 43 psec were generated by Brillouin backscattering at 248 nm in cyclohexane. The measurements show that the process is accompanied by considerable spectral broadening of the reflected radiation.

Subpicosecond KrF* excimer-laser source.

A subpicosecond KrF* laser system capable of producing 20 +/- 2-mJ pulses has been developed. The means of producing ultrashort seed pulses for the KrF* amplifier system and characteristics of the full system are described. It is shown that efficient subpicosecond energy extraction is possible.

Multiphoton ionization of atoms.

Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a(2)(0)), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.

Optically excited XeF* excimer laser in liquid argon.

Stimulated emission on the B ? X band of the XeF* molecule in the liquid phase at 404 nm was observed following transverse optical pumping at 351 nm. The energy of this emission was measured to be ~ 70 microJ, and the pulse had a FWHM of ~5 nsec. The stimulated-emission spectrum showed considerable narrowing compared with the spontaneous- emission spectrum. The temporal behavior of the 404-nm pulse was investigated, and gain saturation of the lasing medium was observed.

Third-harmonic generation using an ultrahigh-spectral-brightness ArF source.

Tunable coherent radiation in the vicinity of 64 nm has been produced by third-harmonic generation of the output of an ultrahigh-spectral-brightness ArF*source in various simple gaseous media. The following performance parameters of the ArF* system are used: pulse energy, ~300 mJ; pulse duration, ~7 nsec; spectral width, <260 MHz; beam divergence, ~5microradx15microrad; and repetition rate up to 10 Hz. Third-harmonic generation with peak powers up to 30 W has been observed in an apparatus that eliminates photoabsorption of the third harmonic in the differential pumping stages.

Generation of high-spectral-brightness tunable XUV radiation at 83 nm.

High-spectral-brightness coherent XUV radiation has been produced by third-harmonic generation of a transformlimited- bandwidth KrF* laser in gaseous xenon. The observed XUV output, which was continuously tunable from 82.8 to 83.3 nm, had a peak power of 40 mW, a bandwidth <0.01 cm(-1), and absolute frequency control to within 0.04 cm(-1). The utility of this XUV source for high-resolution spectroscopic applications is demonstrated by absorption studies in molecular hydrogen.