Modeling single-frequency laser-plasma acceleration usingparticle-in-cell simulations: the physics of beam breakup

We investigate electron acceleration from space-charge waves driven by single-frequency lasers using a fully explicit particle-in-cell (PIC) model. The two dimensional (2-D) simulations model ~100 fs pulses at densities near n=4×10¹⁹ cm^-3 for 1-μm lasers. The pulses are found to break up due to a combination of longitudinal and transverse bunching of the laser intensity via Raman forward scattering type instabilities. The ponderomotive force of these intensity modulations generates large amplitude plasma waves. Large numbers of self-trapped electrons and multiple Raman forward scattering satellites are observed. The relevance of these simulations to experiments is discussed     

Noncritically phase-matched frequency conversion in Gd(x)Y(1-x)Ca(4)O(BO(3))(3) crystal

A nonlinear optical crystal YCa(4)O(BO(3))(3) (YCOB) is phase matchable for third-harmonic generation (THG) of a Nd:YAG laser by type I mixing between 1.064 and 0.532mum . By partial substitution of Gd for Y in YCOB, a solid solution Gd(x)Y(1-x)Ca(4)O(BO(3))(3) gradually changes the phase-matching angles of THG to (theta, ?)=(90( degrees ), 90 ( degrees )) . We present the properties of noncritically phase-matched THG in Gd(x) Y(1-x)Ca(4)O(BO(3))(3) (x=0.28) .     

Continuous-wave diode-pumped laser action of Nd³⁺-doped photo-thermo-refractive glass

Laser action of the photo-thermo-refractive (PTR) glass, which is the photosensitive material for holographic recording of volume Bragg gratings (VBGs), was demonstrated for the first time by introducing Nd3?. Nd:PTR glass has a bandwidth of 27.8 nm and 16.0 nm for luminescence and absorption, respectively. An uncoated 2 mm thick Nd:PTR element generated cw laser output of 124 mW, with a slope efficiency of 25%, by laser diode pumping. This Nd:PTR glass also performed wide bandwidth laser action at 1053.9-1063.3 nm, where the decrease of the pump-absorption efficiency was held off below 30%, even under a 3.5 nm shift of pump wavelength from its absorption center.     

Spatial distribution of AT- and GC-rich DNA within interphase cell nuclei of Triatoma infestans Klug

Heterochromatin bodies in single- and multichromocentered interphase cell nuclei of Triatoma infestans, a vector of Chagas disease, have been suggested to contain AT-rich DNA, based on their positive response to Q-banding and Hoechst 33248 treatment. No information exists on whether GC-rich DNA is also present in these nuclei and whether it plays a role on chromatin condensation. Considering that methodologies more precise than those previously used to determine DNA base composition in situ are currently available, and that the spatial distribution of chromatin areas differing in composition in interphase cell nuclei of different species is a matter of interest, the localization of AT- and GC-rich DNA in T. infestans nuclei is revisited here. The methodologies used included DAPI/AMD and CMA(3)/Distamycin differential staining, Feulgen-DNA image analysis following Msp I and Hpa II enzymatic digestion, 5-methylcytidine immunodetection, AgNOR response, confocal microscopy, and the 5-aza-2'-deoxycytidine (5-AZA) demethylation assay. The results identified the presence of AT-rich/GC-poor DNA in chromocenters and evenly distributed AT and GC sequences in euchromatin. A GC-rich DNA zone encircling the chromocenters was also found but it could not be associated with NOR regions. To corroborate the DNA AT-richness in T. infestans nuclei, bioinformatic analyses were also performed. Methylated cytosine was evident at some points of the chromocenters' edge in single- and multichromocentered nuclei and at the euchromatin of multichromocentered nuclei and could be transiently affected by the 5-AZA treatment. The present results suggest that in the particular case of chromocenters of the hemipteran T. infestans, cytosine methylation is not a relevant factor involved in chromatin condensation.     

Paraconductivity for a d-wave superconductor in short-wavelength fluctuation regime

A theoretical study of the fluctuation conductivity above T_c (paraconductivity) is reported for a d-wave superconductor with resonant scattering impurities. A d-wave system is modeled by tight-binding electrons in the two-dimensional squared lattice, and the impurity scattering is treated in the T-matrix approximation in a unitary limit. In calculating the Aslamazov–Larkin (AL) and the Maki–Thompson (MT) terms, we also consider effects of a short-wavelength cutoff in the fluctuation spectrum. The d-wave character in the AL and MT terms manifests itself to renormalization effects on the fluctuation amplitude and reduced temperature, whereas an anomalous-MT term is absent. The present calculations can describe fairly well experiments on the paraconductivity in zinc-doped cuprate superconductors provided that effects of a total-energy cutoff are taken into account.     

Characterization of preformed plasmas with an interferometer for ultra-short high-intensity laser-plasma interactions

The evolution of an Al preformed plasma produced by a prepulse was observed before and after the arrival of the main pulse by an interferometer using a femtosecond probe pulse. A central density depression due to the ponderomotive force of the main laser pulse in the preformed plasma with a 100 m scale length was clearly visible after the main pulse irradiation at an intensity of 5×10¹⁶ W/cm². The temporal profiles of the prepulse, characterized by a cross-correlation in conjunction with a precise density profile measurement by an interferometer, contribute to the better understanding of femtosecond laser-matter interactions. PACS 52.38.-r; 52.50.Jm; 52.70.-m     

Modification of the physical properties of chemical vapor-deposited nanostructure diamond by argon-hydrogen plasma surface treatment

Nanostructure diamond (NSD) film with a hardness as high as 70 GPa and an average surface roughness of 10 nm has been synthesized by the two-step negative substrate bias method combined with post-growth Ar-H₂ plasma irradiation. The Ar-H₂ plasma irradiation has been confirmed to improve the uniformity of grain size and shape and increase the hardness of the NSD film.