T. Tsang

Photoemission studies on metals using picosecond ultraviolet laser pulses

We report the quantum efficiency of various metals irradiated by 266‐nm, 4.66‐eV laser pulses of 10‐ps duration. The highest quantum efficiency obtained is 7.25×10−4 with samarium photocathodes. Current densities exceeding 66 and 21 kA/cm2 have been obtained from an area of 0.05 and 7 mm2, respectively. The maximum currents and current densities obtainable in these experiments are limited by the space charge. For surface fields exceeding 5×107 V/m on gold, the efficiency increases linearly with the field for the values investigated. Based on the quantum efficiency and optical damage threshold measurements, current densities exceeding 100 kA/cm2 seem feasible without damaging the photocathode.

FREQUENCY-RESOLVED OPTICAL-GATING MEASUREMENTS OF ULTRASHORT PULSES USING SURFACE THIRD-HARMONIC GENERATION

We demonstrate what is to our knowledge the first frequency-resolved optical gating (FROG) technique to measure ultrashort pulses from an unamplified Ti:sapphire laser oscillator without direction-of-time ambiguity. This technique utilizes surface third-harmonic generation as the nonlinear-optical effect and, surprisingly, is the most sensitive third-order FROG geometry yet.

Target studies with BNL E951 at the AGS

We report initial results of exposing low-Z solid and high-Z liquid targets to 150-ns, 4/spl times/10/sup 12/ proton pulses with spot sizes on the order of 1 to 2 mm. The energy deposition density approached 100 J/g. Diagnostics included fiberoptic strain sensors on the solid target and high-speed photography of the liquid targets. This work is part of the R&D program of the Neutrino Factory and Muon Collider Collaboration.

SURFACE-PLASMON-ENHANCED THIRD-HARMONIC GENERATION IN THIN SILVER FILMS

Optical third-harmonic generation (THG) is dipole allowed. I report a 10(3) signal enhancement of THG that is due to the excitation of a surface plasmon in thin silver films by an attenuated-total-internal-ref lection geometry. Because the THG signal depends on the cube of the incident intensity and the second-harmonic generation depends on the square of the intensity, the THG overtakes the second-harmonic generation at laser intensities beyond 6 x 10(11) W/cm(2).

In vitro efficiency and mechanistic role of indocyanine green as photodynamic therapy agent for human melanoma

BACKGROUND Photodynamic therapy (PDT) is a promising treatment for superficial cancer. However, poor therapeutic results have been reported for melanoma, due to the high melanin content. Indocyanine green (ICG) has near infrared absorption (700-800 nm) and melanins do not absorb strongly in this area. This study explores the efficiency of ICG as a PDT agent for human melanoma, and its mechanistic role in the cell death pathway. METHODS Human skin melanoma cells (Sk-Mel-28) were incubated with ICG and exposed to a low power Ti:Sapphire laser. Synchrotron-assisted Fourier transform infrared microspectroscopy and hierarchical cluster analysis were used to assess the cell damage and changes in lipid, protein, and nucleic acids. The cell death pathway was determined by analysis of cell viability and apoptosis and necrosis markers. RESULTS In the cell death pathway, (1)O(2) generation evoked rapid multiple consequences that trigger apoptosis after laser exposure for only 15 min including the release of cytochrome c, the activation of total caspases, caspase-3, and caspase-9, the inhibition of NF-kappaB P65, and the enhancement of DNA fragmentation, and histone acetylation. CONCLUSION ICG/PDT can efficiently and rapidly induce apoptosis in human melanoma cells and it can be considered as a new therapeutic approach for topical treatment of melanoma.

Intense electron emission due to picosecond laser‐produced plasmas in high gradient electric fields

Picosecond laser pulses at a wavelength of 266 nm have been focused onto a solid metal cathode in coincidence with high gradient electric fields to produce high brightness electron beams. At power densities exceeding 109 W/cm2, a solid density plasma is formed and intense bursts of electrons are emitted from the target accompanied by macroscopic surface damage. An inferred ∼1 μC of integrated charge with an average current of ∼20 A is emitted from a radio‐frequency cavity driven at electric field gradients of ∼80 MV/m. In another experiment, where a dc extraction field of ∼6 MV/m is used, we observed an electron charge of ∼0.17 μC. Both results are compared with the Schottky effect and the Fowler–Nordheim field emission. We found that this laser‐induced intense electron emission shares many features with the explosive electron emission processes. No selective wavelength dependence is observed in the production of the intense electron emission in the dc extraction field. The integrated electrons give an ap...

Performance of the Brookhaven Photocathode RF Gun

The Brookhaven Accelerator Test Facility (ATF) uses a photocathode rf gun to provide a high-brightness electron beam intended for FEL and laser-acceleration experiments. The rf gun consists of 1{1/2} cells driven at 2856 MHz in {pi}-mode with a maximum cathode field of 100 MV/m. To achieve long lifetimes, the photocathode development concentrates on robust metals such as copper, yttrium and samarium. We illuminate these cathodes with a 10-ps, frequency-quadrupled Nd:YAG laser. We describe the initial operation of the gun, including measurements of transverse and longitudinal emittance, quantum efficiencies, and peak current. The results are compared to models.

Observation of high-order solitons from a mode-locked Ti:sapphire laser.

I report the observation of high-order solitonlike femtosecond pulses directly generated in an absorber jet-induced self-mode-locked Ti:sapphire laser. A soliton period of ~1 MHz was observed with a nearly linear dependence on the intracavity dispersion.

Efficiency enhancement using electron energy detuning in a laser seeded free electron laser amplifier

We report the experimental characterization of efficiency enhancement in a single-pass seeded free-electron laser (FEL) where the electron energy is detuned from resonance. Experiments show a doubling of the efficiency for beam energies above the resonant energy. Measurements of the FEL spectra versus energy detuning shows that the wavelength is governed by the seed laser. The variation in the gain length with beam energy was also observed. Good agreement is found between the experiment and numerical simulations using the MEDUSA simulation code.

PHOTOEMISSION FROM MG IRRADIATED BY SHORT PULSE ULTRAVIOLET AND VISIBLE LASERS

Quantum efficiency of diamond turned magnesium was measured with low intensity, 266 nm, 12 ps, p‐polarized radiation for various incident angles. The optimum quantum efficiency of 3×10−4 was obtained with p‐polarized light incident close to Brewster’s angle where the absorption is maximum. Investigations of the field dependence of the yield indicate that application of a high surface field is expected to lower the surface barrier by Schottky effect and increase the quantum efficiency further. Measurements of the electron yield with 632 nm radiation of 300 fs pulse duration indicate that at intensities exceeding 20 MW/cm2, the electric field due to the laser may become important and start contributing to the electron emission. Autocorrelation measurements at low intensities with this wavelength indicate that the electron emission is prompt.