Function Of Incident Laser Intensity Research Articles

High-throughput UV-photofragmentation studies of thymine and guanine.

High-throughput photofragmentation studies of thymine and guanine were performed at 257 and 343 nm and for a wide range of ionisation laser intensities. Combining a continuous laser-based thermal desorption source with femtosecond multiphoton ionisation using a 50 kHz repetition rate laser allowed us to produce detailed 2D maps of fragmentation as a function of incident laser intensity. The fragmentation was distinctly soft, the parent ions being at least an order of magnitude more abundant than fragment ions. For thymine there was a single dominant fragmentation channel, which involves consecutive HNCO and CO losses. In contrast, for guanine there were several competing ones, the most probable channel corresponding to CH2N2 loss through opening of the pyrimidine ring. The dependence of parent ion abundance on the ionisation laser intensity showed that at 257 nm the ionisation of thymine is a 1 + 1 resonance enhanced process through its open-shell singlet state.

Molecular emission dynamics from a femtosecond filament induced plasma plume

In this study, we investigated the filament-induced plasma properties and the associated molecular emission features from three different non-metallic samples. Graphite, polymethyl methacrylate, and Teflon samples have been ablated using a tightly focused filament, and their emission spectra were analyzed using a time-integrated optical emission spectroscopy technique. The temporal responses and evolution dynamics of molecular species such as CN and C2 from these samples in ambient conditions are compared. The tightly focused filament was generated by focusing the Ti:Sapphire femtosecond pulses having a pulse duration of 29 fs using a short focal length external focusing system. The time-integrated intensified charge-coupled device images of air filament show that the filament survives up to a few nanosecond time duration after the onset of air plasma. The influence of physical and chemical properties of the samples during filament ablation has also been studied by characterizing the optical emission spectra. We find that the molecular signal intensity strongly depends on the sample properties and the position of the sample in the filament. The increase in molecular emission intensity from a graphite sample as a function of incident laser intensity suggests that the tightly focused filament surpasses the intensity clamping value.

Solute Effects on Interfacial Thermal Conductance

ABSTRACTThe thermal conductance of a gold/water interface has been found to change as a function of the surrounding’s adhesion energy. We measure the thermal conductance of a lithographically prepared gold nanowire with a thin film nanoscale thermal sensor composed of AlGaN:Er3+. The temperature of the nanowire is measured as a function of incident laser intensity. The slope of this plot is inversely proportional to the thermal conductance of the nanoparticle/surrounding’s interface. We show that the conductance of the nanoparticle/water interface increases with the molality of the solution. This was tested with multiple solutes including NaCl, and D-Glucose. The interfacial conductance of pure water is reported to be 44 MW/m2K and the conductance saturates to 100 MW/m2K at a molality of 0.21 m.

Optimizing electron-positron pair production on kilojoule-class high-intensity lasers for the purpose of pair-plasma creation

Expressions for the yield of electron-positron pairs, their energy spectra, and production rates have been obtained in the interaction of multi-kJ pulses of high-intensity laser light interacting with solid targets. The Bethe-Heitler conversion of hard x-ray bremsstrahlung [D. A. Gryaznykh, Y. Z. Kandiev, and V. A. Lykov, JETP Lett. 67, 257 (1998); K. Nakashima and H. Takabe, Phys. Plasmas 9, 1505 (2002)] is shown to dominate over direct production (trident process) [E. P. Liang, S. C. Wilks, and M. Tabak, Phys. Rev. Lett. 81, 4887 (1998)]. The yields and production rates have been optimized as a function of incident laser intensity by the choice of target material and dimensions, indicating that up to 5 x 10 (11) pairs can be produced on the OMEGA EP laser system [L. J. Waxer, Opt. Photonics News 16, 30 (2005)]. The corresponding production rates are high enough to make possible the creation of a pair plasma.

Site-Selective, Intrinsically Bistable Luminescence ofYb3+Ion Pairs inCsCdBr3

We report the first observation of intrinsic optical bistability in ${\mathrm{CsCdBr}}_{3}:1%$ ${\mathrm{Yb}}^{3+}$ and show, using site-selective spectroscopy, that only the asymmetric, strongly coupled ${\mathrm{Yb}}^{3+}$ ion-pair minority site gives rise to hysteresis of near-infrared and cooperative up-conversion luminescence as a function of incident laser intensity below 15 K.

The elementary static magnetic field of the photon

It is argued that the photon generates on a fundamental level a static magnetic field, a flux density operator B̂ Π = B 0 Ĵ/ h ̷ where Ĵ is the photon's angular momentum operator and B 0 the scalar magnetic flux density amplitude of a beam of N photons. A key experimental test of the theory is proposed, whereby a circularly polarised laser beam is reflected at right angles from a beam of electrons, and the frequency shift Δ f (hertz) = ( m̂£ B̂ Π)/ h measured in the reflected laser beam as a function of incident laser intensity I 0. Here m̂ is the magnetic dipole moment operator of the electron.

Resolution limitations in focused ion beam damage-induced masking for photoelectrochemical etching of GaAs

The use of ion beam damage to locally suppress photoelectrochemical etching of GaAs has been previously described. This study examines the limitations on the resolution of patterns formed by this technique, using a focused ion beam. Directly written patterns of damage were formed in Si-doped n+ GaAs (1×1018 cm−3) by implantation with a 120 keV focused beam of Ga ions. Photoelectrochemical etching of the substrate was suppressed, not only in the implanted regions themselves, but also within a region extending a few micrometers beyond the implanted region. This suppression distance limits the smallest pattern feature or resolution achievable with this technique. The resolution was examined as a function of incident laser intensity, implant dose, the dimensions of the damaged areas, and distance between the damaged areas. A simple one-dimensional model suggests that the diffusion of the photogenerated carriers, relative to their reaction rate at the surface, determines the resolution achievable.

AlGaAs doping superlattices grown by molecular beam epitaxy

AlGaAs doping superlattices (‘‘nipi structures’’) have been successfully grown in 30% AlGaAs by molecular beam epitaxy. Tunable photoluminescence (PL) as a function of incident laser intensity has been observed in samples with a wide range of intrinsic layer thicknesses over a temperature range from 2 to 120 K. Luminescence shifts as large as 230 meV were observed for a range of incident intensities of about 500. Low-temperature PL spectra showed a weaker dependence of the peak energy on incident intensity for thicker spacer layers. This decrease in tuning rate can be associated with the reduced probability for tunneling transitions with increasing spacer thickness.

On plasma surface coupling of 10.6 μm laser radiation with copper targets

Thermal coupling coefficients of CO 2 laser radiation to copper targets in air and in vacuum were measured as a function of incident laser intensity. It is shown that the addition of a small amount of NaF or NaCl on the surface of the copper target can change the coupling coefficient in air and in vacuum.

On plasma surface coupling of 1.06μm laser radiation with copper targets

The thermal coupling coefficient of neodymium glass laser radiation to copper targets was measured as a function of incident laser intensity. It was shown that the addition of a small amount of NaCl on the surface of the copper target can change the coupling coefficient considerably. A tentative explanation for this effect based on the change of plasma properties due to the presence of sodium is offered.

Theoretical Studies of the Laser Induced Plasma Profile Near the Critical Density Region for an Adiabatic Plasma Model

AbstractA theoretical study of the variation of density and flow velocity due to pondermotive force, near critical density region of a laser – produced plasma, obeying an adiabatic pressure law is reported. The ion‐continuity, momentum and wave equations are solved in a steady state. The density and flow velocity as a function of incident laser intensity are calculated and plotted for both underdense and overdense regions for adiabatic exponent γ = 4/3, 5/3, 2. The velocity decreases and density increases in the overdense region and velocity increase and density decreases in the underdense region with the increase of larger power at sonic point. It is found that the velocity is subsonic in the overdense region and supersonic in the underdense region. The nature of variation is similar to the isothermal model.

Detailed look at aspects of optical pumping in sodium

Calculations and measurements are presented of the increase in F\ifmmode\bar\else\textasciimacron\fi{}=1 ground-state population as a function of incident laser intensity in optically pumped sodium. The calculations involve numerical integration of a multilevel version of the optical Bloch equations. Agreement between experiment and theory is good when proper account is taken of the residual Doppler width in the atomic beam, which causes a larger increase in the F\ifmmode\bar\else\textasciimacron\fi{}=1 population. The F\ifmmode\bar\else\textasciimacron\fi{}=1 population increases by 3.5% at 35 mW/${\mathrm{cm}}^{2}$, the highest laser intensity investigated.

Numerical Method for Nonlinear Eigenvalue Problems

A new efficient numerical scheme is presented for analyzing nonlinear boundary value and eigenvalue problems. The method consists of combining the shooting method and Newton's iteration scheme. The method is applied to study the nonlinear interaction of strong laser light with plasma. By the numerical analysis of the structure resonance in laser plasma interaction, multi-valued refraction coefficient as a function of incident laser intensity, and associated anomalous transmission and absorption phenomena are obtained.

Multiphoton ultraviolet spectroscopy of some6plevels in krypton

The observation of two-photon excitation of selected $6p$ levels in krypton atoms using tunable ArF* laser radiation at 193 nm is reported. Using time-resolved detection of visible fluorescence lines in the vicinity of 430 nm arising from $6p\ensuremath{-}5s$ transitions, radiative lifetimes for the $6p{[\frac{1}{2}]}_{0}$ and $6p{[\frac{3}{2}]}_{2}$ states of 123 \ifmmode\pm\else\textpm\fi{} 5 and 115 \ifmmode\pm\else\textpm\fi{} 5 nsec, and collisional self-quenching rates of (4.1 \ifmmode\pm\else\textpm\fi{} 0.4) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{3}$/sec and (6.7 \ifmmode\pm\else\textpm\fi{} 0.7) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{3}$/sec, respectively, have been determined. By carefully measuring the visible fluorescence intensity as a function of incident laser intensity, the photoionization cross section of the $6p{[\frac{3}{2}]}_{2}$ state has been established as (3.2 \ifmmode\pm\else\textpm\fi{} 2.0) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$.. The two-photon transition rates for both states have been theoretically calculated, and good agreement is found with measurements of the relative excitation rates for the two transitions.

Hot-spot model of K-line emission from laser-heated plasmas

A nonhydrodynamic model is developed to describe x-ray emission from laser-heated plasmas on a subnanosecond time scale. The model is first correlated with the experiment and then is used to study the conversion efficiency of laser energy to K-line x rays in carbon, fluorine, and aluminum plasmas as a function of incident laser intensity. It is found that these conversion efficiencies peak at well-defined values of input energy per area for laser pulse widths smaller than the pumping times for K-line emission. In this case, the degree and not the rate of plasma heating is important in determining the peak conversion efficiency. It is also found that peak conversion to K radiation in excess of roughly 10 or 20% occurs only when the C, F, and Al plasmas are optically thick to the Ka lines.