High resolution emission spectra of laser-excited molecular iodine as the excitation frequency is tuned through and away from resonance

We have studied the ΔJ = 2 rotationally shifted emission lines in the region of the strong absorption of molecular iodine which occurs within the 5145 Å argon ion laser line. We used an etalon tuned, single frequency argon ion laser with a linewidth of 20 MHz to excite the iodine emission and recorded the spectra of the rotational lines with Fabry-Perot spectrometers having resolutions up to 70 MHz (0.0023 cm^-1). To overcome Doppler linewidth limitations we took spectra of the emission at small angles to the exciting beam and found the lines to have widths less than our instrumental resolution and frequencies which accurately tuned with the incident laser frequency. We recorded the emission lines for laser frequencies in the absorption line center and out into the absorption wing. Our spectra show that the intensity of the emission lines follows the absorption line profile while the frequency of the emission lines is determined by the laser frequency; the intensity is maximum at the absorption line center, falling by 10⁴ as the laser frequency is moved off the line center, while the line position maintains a constant frequency shift from the laser frequency.     

Volume effect of laser produced plasma on X-ray emissions

An investigation of x-ray emission from Cu plasma produced by 1.054 μm Nd:glass laser pulses of 5 ns duration, at 2 × 10¹² − 2 × 10¹³ W cm⁻² is reported. The x-ray emission has been studied as a function of target position with respect to the laser beam focus position. It has been observed that x-ray emissions from ns duration plasma show a volume effect similar to subnanosecond plasmas. Due to this effect the x-ray yield increases when target is moved away relative to the best focal plane of the laser beam. This result supports the theoretical model of Tallents and has also been testified independently using suitably modified theoretical model for our experimental conditions. While above result is in good agreement with similar experimental results obtained for sub-nanosecond laser produced plasmas, it differs from result claiming filamentation rather than pure geometrical effect leading to x-ray enhancement for ns plasmas.     

Spectral analysis of K-shell X-ray emission of magnesium plasma produced by ultrashort high-intensity laser pulse irradiation

Spectral analysis of K-shell X-ray emission of magnesium plasma, produced by laser pulses of 45 fs duration, focussed up to an intensity of ～10¹⁸ W cm^?2, is carried out. The plasma conditions prevalent during the emission of X-ray spectrum were identified by comparing the experimental spectra with the synthetic spectra generated using the spectroscopic code PrismSPECT. It is observed that He-like resonance line emission occurs from the plasma region having sub-critical density, whereas K-α emission arises from the bulk solid heated to a temperature of 10 eV by the impact of hot electrons. K-α line from Be-like ions was used to estimate the hot electron temperature. A power law fit to the electron temperature showed a scaling of I ^0.47 with laser intensity.     

Nonlinear mixing between the emission of a cw laser and a femtosecond laser

We mix the emission of a femtosecond Ti:sapphire laser with the emission of a continuous wave infrared laser in a beta-barium borate crystal. Green light with a center wavelength of 527 nm and a spectral width of 2.5 nm resulting from sum frequency generation is detected. An intensity study verifies that a nonlinear χ⁽²⁾ process is at the origin of the green light generation. The experimentally obtained conversion efficiency of 7 × 10⁻¹⁰ is in good agreement to simple theoretical considerations.     

Optimization of soft x-ray line emission from laser-produced carbon plasma with laser intensity

Absolute measurement for He-α resonance (1s^{2 1}S₀?1s2p¹ P ₁, at 40.2 Å) line emission from a laser-produced carbon plasma has been studied as a function of laser intensity. The optimum laser intensity is found to be ≈1.3×10¹² W/cm² for the maximum emission of 3.2 × 10¹³ photons sr^?1 pulse^?1. Since this line lies in the water window spectral region, it has potential application in x-ray microscopic imaging of biological sample in wet condition. Theoretical calculation using corona model for the emission of this line is also carried out with appropriate ionization and radiative recombination rate coefficients     

Laser-produced plasma helium-like titanium Kα x-ray source and its application to Rayleighben-Taylor instability study

Several experiments are performed on the ShenGuang-II laser facility to investigate an x-ray source and test radiography concepts. X-ray lines emitted from laser-produced plasmas are the most practical means of generating these high intensity sources. By using a time-integrated space-resolved keV spectroscope and pinhole camera, potential helium-like titanium Kα x-ray backlighting (radiography) line source is studied as a function of laser wavelength, ratio of pre-pulse intensity to main pulse intensity, and laser intensity (from 7.25 to ～11.3× 10¹⁵ W/cm²⁾. One-dimensional radiography using a grid consisting of 5 μm Au wires on 16 μm period and the pinhole-assisted point projection is tested. The measurements show that the size of the helium-like titanium Kα source from a simple foil target is larger than 100 μm, and relative x-ray line emission conversion efficiency ξ_x from the incident laser light energy to helium-like titanium K-shell spectrum increases significantly with pre-pulse intensity increasing, increases rapidly with laser wavelength decreasing, and increases moderately with main laser intensity increasing. It is also found that a gold gird foils can reach an imaging resolution better than 5-μm featured with high contrast. It is further demonstrated that the pinhole-assisted point projection at such a level will be a novel two-dimensional imaging diagnostic technique for inertial confinement fusion experiments.     

High-Power Diode-End-Pumped Slab Composite Tm:YLF Compact Laser

We design a continuous-wave Tm:YLF laser with a composite slab crystal end-pumped by two fiber-coupled laser diodes at room temperature. We achieve a maximum continuous wave output power of 105 W for the bonded slab Tm:YLF laser; the corresponding slope efficiency is 47.7% and the optical-to-optical conversion efficiency is 42.0% with respect to the incident pump power. The laser operated at 1,907.5 nm with a beam quality factor of M² ～3.2 at the highest output power.     

Observations of MeV ions emitted from a laser produced plasma

Fast-ion emission from tungsten plasmas has been studied with neodymium laser intensities from 5x10¹² to 2x10¹⁵ W/cm². Ion energies into the MeV region have been observed. The variation of ion energy (E_I) with laser intensity (I) was found to be of the form E_I∞I^0.65.     

Yb effect on the spectroscopic properties of Er-Yb codoped SnO2 thin films

We have investigated the optical properties of sol-gel thin films of tin dioxide (SnO₂) codoped with Er³⁺-Yb³⁺ as a function of Yb³⁺ concentration. The Judd-Ofelt model has been applied to absorption intensities of Er³⁺ (4f¹¹) transitions to establish the so-called Judd-Ofelt intensity parameters: Ω₂, Ω₄, Ω₆. Various spectroscopic parameters were obtained to evaluate their dependence and the potential of the samples as a laser material in the eye-safe laser wavelength (1.53 μm) as a function of Yb³⁺ concentration. An amelioration of the quality factor Ω₄/Ω₆ was found with Yb content. Both the IR photoluminescence (PL) intensity and the up-conversion emission, from Er³⁺ ion in SnO₂, were found to increase with Yb concentration. We show that the Yb³⁺ ion acts as sensitizer for Er³⁺ ion and contributes largely to the improvement of the spectroscopic properties of SnO₂:Er. The mechanism of up-conversion emission is discussed and a model is proposed. The results showed that sol-gel SnO₂ is promising gain media for developing the solid-state 1.5 μm optical amplifiers and tunable up-conversion lasers.     

Photoluminescence properties of Eu in lithium titanate

Lithium titanate ceramics doped with Eu were synthesized and their photoluminescence (PL) and emission spectral characteristics were investigated. PL spectra of the sample showed peaks corresponding to the ⁵D₀→⁷F_J (J=0, 1, 2, 3 and 4) transitions under 230 nm excitation. The fluorescence lifetimes of Eu³⁺ ions were found out to be 645 μs. Strong red emission coming from the hypersensitive ⁵D₀→⁷F₂ transition of Eu³⁺ ion suggested the presence of the dopant ion in highly asymmetric environment. Further analysis of the emission spectrum revealed that the symmetry of the metal ion is very low i.e. C₂. The emission intensity of the sample was compared with a commercial phosphor to get an idea about the commercial utility of the phosphor. Various emission properties for the system namely Judd-Ofelt intensity parameters, spontaneous emission probabilities, branching ratios, radiative lifetimes and quantum efficiency were evaluated for the dopant ion by adopting standard procedure.     

High statistics study of the chiral condensate in quenched lattice QCD

We present the results of a high statistics study of the chiral condensate in quenched lattice QCD on an 8⁴ lattice at β = 5.4, 5.5, 5.6, 5.7, 5.8 and 6.0. We see clear evidence for deviation from asymptotic scaling in the range of β considered. Our results are in agreement with the behaviour anticipated from recent Monte Carlo renormalisation group studies of the β-function. We find indications of a common scaling behaviour for the condensate, the string tension and the deconfinement temperature.     

Hot electrons produced from long scale-length laser-produced droplet plasmas

Microplasmas produced from 15 μm methanol droplets irradiated by 100 fs laser pulses in the intensity range 10¹⁴–10¹⁶ W cm^?2 are investigated via measurements of the hot electron temperature and x-ray yields under different conditions of intensity, polarization state, and plasma scale-length. The scale length of the drop-let plasma is increased with an intentional prepulse that is 10 ns ahead of the main pulse. Hot electron temperatures up to 48 keV have been measured at intensities of 2.5 × 10¹⁵W cm^?2 and the scaling of temperature as a function of intensity is determined for a long scale-length droplet plasma. The polarization and ellipticity dependence of the hard x-ray yield from the microdroplet plasmas are used to probe the shape of the droplet after irradiation by a prepulse.     

High-power master-oscillator power-amplifier with optical vortex output

A high-power master-oscillator power-amplifier with optical vortex output is reported. The master oscillator for an optical vortex seed beam is a simple two-mirror Nd:YAG laser using a fiber-based pump beam conditioning scheme. The seed is amplified in a double-clad multimode fiber amplifier end-pumped by a high-power diode laser at 975 nm yielding 10.7 W of continuous-wave output at 1064 nm in the first-order Laguerre–Gaussian beam with M ² ≈ 2.11 for an absorbed pump power of 17.5 W, corresponding to a slope efficiency of ~59 %. The ring-shaped intensity profile and the wave front handedness of the seed beam were well preserved in the fiber amplifier. The prospects of power scaling via this approach are discussed.     

Bright multi-keV harmonic generation from relativistically oscillating plasma surfaces

The first evidence of x-ray harmonic radiation extending to 3.3 A, 3.8 keV (order n>3200) from petawatt class laser-solid interactions is presented, exhibiting relativistic limit efficiency scaling (eta approximately n{-2.5}-n{-3}) at multi-keV energies. This scaling holds up to a maximum order, n{RO} approximately 8{1/2}gamma;{3}, where gamma is the relativistic Lorentz factor, above which the first evidence of an intensity dependent efficiency rollover is observed. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy hnu>1 keV is found to be into a cone angle approximately 4 degrees , significantly less than that of the incident laser cone (20 degrees ).     

Spectroscopic investigations and luminescence spectra of Nd and Dy doped different phosphate glasses

Spectral properties of Nd³⁺ and Dy³⁺ ions in different phosphate glasses were studied and several spectroscopic parameters were reported. Covalency of rare-earth-oxygen bond was studied in these phosphate glass matrices with the variation of modifier in host glass matrix. Using Judd-Ofelt intensity parameters (Ω₂, Ω₄ and Ω₆), radiative transition probabilities (A) and radiative lifetimes (τ_R) of certain excited states of Nd³⁺ and Dy³⁺ ions are estimated in these glass matrices. From the magnitudes of branching ratios (β_R) and integrated absorption cross-sections (Σ), certain transitions of both the ions are identified for laser excitation. From the emission spectra, peak stimulated emission cross-sections (σ_P) are evaluated for the emission transitions observed in all these phosphate glass matrices for both Nd³⁺ and Dy³⁺ ions.     

Visible and NIR luminescence of nanocrystalline β-Ga2O3:Er prepared by solution combustion synthesis

In this paper we report on facile solution combustion synthesis of erbium doped β-Ga₂O₃ with urea as fuel. The product was characterized using powder X-ray diffraction and transmission electron microscopy (TEM). X-ray diffraction and TEM showed that the material is nanostructured. Luminescence properties of β-Ga₂O₃:Er are studied with excitation in near infrared (Nd:YAG laser at 1064 nm) and visible (argon laser at 514.5 nm). A strong NIR emission of Er³⁺ in the window of minimal optical loss in silica based optical fibers, due to the ⁴I_13/2→⁴I_15/2 transition at 1.55 μm has been observed. Codoping with Yb³⁺ significantly increases the intensity of that important emission.     

Plaquette expansion study of compact U(1) lattice gauge theory in (2 + 1) dimensions

The plaquette expansion is applied to compact U(1) lattice gauge theory in (2 + 1) dimensions to high order: 1/N ⁸ for the ground state energy density and 1/N ⁷ for the anti-symmetric or photon mass gap, where N is defined as the number of plaquettes. Evidence of scaling in the photon mass gap is observed for low order (≤ 1/N ⁴) for inverse coupling values β = 0. 7 to 1. 25 with scaling behaviour given by the weak-coupling formula: M ²/β = exp(-5. 01β + 5. 82) in good agreement with other studies. Higher order results appear to diverge from the scaling slope past the transition point portenting the prospect that the strong coupling trial state in this region gives vise to an asymtotic series in 1/N for the photon mass gap.     

Laser interaction with low-density carbon foam

Experiments were performed with a 15 J/500 ps Nd:glass laser (λ = 1064 nm) focussed to an intensity >10¹⁴ W/cm². X-ray emissions from carbon foam and 5% Pt-doped carbon foam of density 150–300 mg/cc were compared with that of the solid carbon targets. The thickness of the carbon foam was 15 µm on a graphite substrate. X-ray emission was measured using semiconductor X-ray diodes covered with various filters having transmissions in different X-ray spectral ranges. It covered X-ray spectrum of 0.8–8.5 keV range. The X-ray emission in the soft X-ray region was observed to increase to about 1.8 times and 2.3 times in carbon foam and Pt-doped foam, respectively with respect to solid carbon. In hard X-rays, there was no measurable difference amongst the carbon foam, Pt-doped carbon foam and solid carbon. Scanning electron microscope (SEM) analysis demonstrates that foam targets smoothens the crater formed by the laser irradiation.     

White up-conversion emission in Ho/Tm/Yb tri-doped glass ceramics embedding BaF2 nanocrystals

Ho³⁺/Tm³⁺/Yb³⁺ tri-doped glass ceramics with white light emitting have been developed and demonstrated. Pumped by 980 nm laser diode (LD), intensive red, green and blue up-conversions (UC) were obtained. The green emission is assigned to Ho³⁺ ion and the blue emission is assigned to Tm³⁺ ion, whereas the red emission is the combination contribution of the Ho³⁺ and Tm³⁺ ions. The RGB intensities could be adjusted by tuning the rare-earth ion concentration and pump power intensity. Thus, multicolor of the luminescence, including perfect white light with CIE-X=0.329 and CIE-Y=0.342 in the 1931 CIE chromaticity diagram can be obtained in 0.15 Ho³⁺/0.2Tm³⁺/3Yb³⁺ tri-doped glass ceramics embedding BaF₂ nanocrystals pumped by a single infrared laser diode source of 980 nm at 500 mW. The up-conversion luminescence mechanism of Yb³⁺ sensitize Ho³⁺ and Tm³⁺ ions and the energy transfer from Ho³⁺ to Tm³⁺ in oxy-fluoride silicate glass ceramics were analyzed.     

Analysis of characteristic X-ray generation induced by laser plasma electrons accelerated by an electric field

The results of experiments devoted to the study of spectral, spatial, and time characteristics of a spectrally bright point x-ray source based on a vacuum diode with a laser-plasma cathode and a titanium needle anode with a photon energy approximately equal to 4.5 keV are presented. The experimental results revealed a considerable difference between the electron emission from laser plasma in a strong electric field and the explosive electron emission and demonstrated the effectiveness of laser plasma as an electron source. The optimization of the laser radiation power density, the accelerating voltage, and the interelectrode spacing made it possible to create a point x-ray source whose spectral brightness exceeds available sources in the class of small-size pulse x-ray instruments (tubes with explosive cathodes). It has been proved experimentally that the maximum contrast of the characteristic lines of the anode material is attained in the case of an optimal choice of accelerating voltage. The x-ray source has the following parameters: (1) spectral brightness of the K-lines of titanium of the order of 10²¹ photons/cm² s sr keV; (2) emitting region size of 250 mm; and (3) laser pulse duration less than 20 ns.