Space-resolved spectral diagnosis of line-shaped laser-produced magnesium plasmas

Space-resolved spectra of line-shaped laser-produced magnesium plasmas in the normal direction of the target have been obtained using a pinhole crystal spectrograph. These spectra are treated by a spectrum analyzing code for obtaining the true spectra and fine structures of overlapped lines. The spatial distributions of electron temperature and density along the normal direction of the target surface have been obtained with different spectral diagnostic techniques. Especially, the electron density plateaus beyond the critical surface in line-shaped magnesium plasmas have been obtained with a fitting technique applied to the Stark-broadened Ly-alpha wings of hydrogenic ions. The difference of plasma parameters between those obtained by different diagnostic techniques is discussed. Other phenomena, such as plasma satellites, population inversion, etc., which are observed in magnesium plasmas, are also presented.

Time and space-resolved measurement of a gas-puff laser-plasma x-ray source

The dynamic interaction processes between a nano-second laser pulse and a gas-puff target, such as those of plasma formation, laser heating, and x-ray emission, have been investigated quantitatively. Time and space-resolved x-ray and optical measurement techniques were used in order to investigate time-resolved laser absorption and subsequent x-ray generation. Efficient absorption of the incident laser energy into the gas-puff target of 17%, 12%, 38%, and 91% for neon, argon, krypton, and xenon, respectively, was shown experimentally. It was found that the laser absorption starts and, simultaneously, soft x-ray emission occurs. The soft x-ray lasts much longer than the laser pulse due to the recombination. Temporal evolution of the soft x-ray emission region was analyzed by comparing the experimental results to the results of the model calculation, in which the laser light propagation through a gas-puff plasma was taken into account. (C) 2003 American Institute of Physics.

Theoretical and experimental study on femtosecond laser induced damage in CaF2 crystals

We report on the damage threshold in CaF2 crystals induced by femtosecond laser at wavelengths of 800 nm and 400 nm, respectively. The dependences of ablation depths and ablation volumes on laser fluences are also presented. We investigate theoretically the coupling constants between phonon and conduction band electrons (CBE), and calculate the rates of CBE absorbing laser energy. A theoretical model including CBE production, laser energy deposition, and CBE diffusion is applied to study the damage mechanisms. Our results indicate that energy diffusion greatly influences damage threshold and ablation depth.

Effect of cerium oxide on the precipitation of silver nanoparticles in femtosecond laser irradiated silicate glass

We investigated the effect of cerium oxide on the precipitation of Ag nanoparticles in silicate glass via a femtosecond laser irradiation and successive annealing. Absorption spectra show that Ce3+ ions may absorb part of the laser energy via multiphoton absorption and release free electrons, resulting in an increase of the concentration of Ag atoms and a decrease of the concentration of hole-trapped color centers, which influence precipitation of the Ag nanoparticles. In addition, we found that the formed Ag-0 may reduce Ce4+ ions to Ce3+ ions during the annealing process, which inhibits the growth of the Ag nanoparticles.

Distribution of the microcrystallites generated in borate glass irradiated by femtosecond laser pulses

This paper deals with the distribution of generated microcrystallites in borate glass irradiated by 120 fs laser pulses at a central wavelength of 800 nm. Raman spectroscopy is used to investigate the distribution of the high and low temperature phases of barium metaborate crystals generated in the borate glass. In combination with a microexplosion model, bond-breaking induced by laser irradiation is served as the origin of the formation of BBO crystals. Depending on the laser fluence and cooling conditions, the distribution mechanisms have been discussed. (c) 2006 Elsevier B.V. All rights reserved.

Application of sample-and-hold circuits in the laser frequency-shifting

A new method of frequency-shifting for a diode laser is realized. Using a sample-and-hold circuit, the error signal can be held by the circuit during frequency shifting. It can avoid the restraint of locking or even lock-losing caused by the servo circuit when we input a step-up voltage into piezoelectric transition (PZT) to achieve laser frequency-shifting.

Coherent spectroscopy of the Rb-87 Fg=1 -> F-e=0 closed transition

This paper has observed linewidth narrowing of dark states in rubidium cell by using the Hanle configuration. The reduction of the coherent resonance width under the transition of Rb-87 F-g = 1 -> F-e = 0 is observed and the qualitative explanation about its mechanism is presented. Multiple subnatural width dips are obtained with a linearly polarized laser beam for the transition of Rb-87 F-g = 0, 1, 2. The feature of negative and positive slope, namely dispersionlike feature, is observed in the transmitted light.

Microstructure of the crystals generated in borate glass irradiated by femtosecond laser pulses

In this study, we examined the microstructure of crystals generated in borate glass by femtosecond laser irradiation (FSLI). The distribution of the high-temperature and low-temperature phases of barium metaborate crystals produced in the borate glass is analyzed using Raman spectroscopy. We then propose the possible mechanism for the generation of crystals in glass by FSLI.

Upconversion luminescence from E-2 state of Cr3+ in Al2O3 crystal by infrared femtosecond laser irradiation

Visible upconversion luminescence was observed in Cr3+: Al2O3 crystal under focused femtosecond laser irradiation. The luminescence spectra show that the upconversion luminescence originates from the E-2-(4)A(2) transition of Cr3+. The dependence of the fluorescence intensity of Cr3+ on the pump power reveals that a two-photon absorption process dominates in the conversion of infrared radiation to the visible emission. It is suggested that the simultaneous absorption of two infrared photons produces the population of upper excited states, which leads to the characteristic visible emission from E-2 state of Cr3+.

Sites structure and spectroscopic properties of Yb-doped and Yb, Na-codoped CaF2 laser crystals

For the first time, the effect of Na+ on crystal structure, valence state of Yb ions, spectroscopic properties of YbF3-doped CaF2 system was systematically studied. Na+ can greatly suppress the deoxidization of Yb3+ to Yb2+. Absorption and emission spectra showed codoping Na+ with different Na:Yb ratios can modulate the spectroscopy and photoluminescence properties of Yb3+ ions in CaF2 lattice in a large scope. The emission lifetime and quantum efficiency of Yb3+ in CaF2 were greatly enhanced by the codopant of Na+. The potential laser performances of the new Yb, Na-codoped CaF2 crystals were predicted. (c) 2005 Elsevier B.V. All rights reserved.

Thermal stress-induced birefringence in borate glass irradiated by femtosecond laser pulses

Thermal stress-induced birefringence in borate glass which has been irradiated by 800-nm femtosecond laser pulses is observed under cross-polarized light. Due to the high temperature and pressure formed in the focal volume, the material at the edge of the micro-modified region is compressed between the expanding region and the unheated one, then stress emerges. Raman spectroscopy is used to investigate the stress distribution in the micro-modified region and indicates the redistributions of density and refractive index by Raman peak shift. We suggest that this technique can develop waveguide polarizers and Fresnel zone plates in integrated optics.

Study of crystal formation in titanate glass irradiated by 800 nm femtosecond laser pulse

The structure of the titanate glass is destroyed during irradiation by the femtosecond laser pulses, and (TiO6)(8-) and (TiO4)(4-) anion units are exsolved from the network of the titanate glass. These anion units are rearranged to form some crystals such as anatase and Ba2TiO4 crystals. By Raman spectroscopy, it is found that these crystals have a strong dependence on the intensity of the femtosecond laser pulses. The relation between the generation of these crystals and space distribution of the femtosecond laser power intensity is qualitatively explained. (c) 2005 Elsevier B.V. All rights reserved.

Effects of annealing on laser-induced damage threshold of TiO2/SiO2 high reflectors

The mechanism of improving 1064 nm, 12 ns laser-induced damage threshold (LIDT) of TiO2/SiO2 high reflectors (HR) prepared by electronic beam evaporation from 5.1 to 13.1 J/cm(2) by thermal annealing is discussed. Through optical properties, structure and chemical composition analysis, it is found that the reduced atomic non-stoichiometric defects are the main reason of absorption decrease and LIDT rise after annealing. A remarkable increase of LIDT is found at 300 degrees C annealing. The refractive index and film inhomogeneity rise, physical thickness decrease, and film stress changes from compress stress to tensile stress due to the structure change during annealing. (c) 2007 Elsevier B.V. All rights reserved.

Investigation of damage threshold to TiO2 coatings at different laser wavelength and pulse duration

Laser-induced damages to TiO2 single layers and TiO2/SiO2 high reflectors at laser wavelength of 1064 nm, 800 run, 532 urn, and pulse width of 12 ns, 220 ps, 50 fs, 8 ns are investigated. All films are prepared by electron beam evaporation. The relations among microstructure, chemical composition, optical properties and laser-induced damage threshold (LIDT), have been researched. The dependence of damage mechanism on laser wavelength and pulse width is discussed. It is found that from 1064 nm to 532 nm, LIDT is mainly absorption related, which is determined by film's extinction coefficient and stoichiometric defects. The rapid decrease of LIDT at 800 nm is due to the pulse width factor. TiO2 coatings are mainly thermally by damaged at long pulse (tau >= 220 ps). The damage shows ablation feature at 50 fs. (C) 2007 Elsevier B.V. All rights reserved.

High temperature annealing effect on structure, optical property and laser-induced damage threshold of Ta2O5 films

Ta2O5 films were deposited by conventional electron beam evaporation method and then annealed in air at different temperature from 873 to 1273 K. It was found that the film structure changed from amorphous phase to hexagonal phase when annealed at 1073 K, then transformed to orthorhombic phase after annealed at 1273 K. The transmittance was improved after annealed at 873 K, and it decreased as the annealing temperature increased further. The total integrated scattering (TIS) tests and AFM results showed that both scattering and root mean square (RMS) roughness of films increased with the annealing temperature increasing. X-ray photoelectron spectroscopy (XPS) analysis showed that the film obtained better stoichiometry and the O/Ta ratio increased to 2.50 after annealing. It was found that the laser-induced damage threshold (LIDT) increased to the maximum when annealed at 873 K, while it decreased when the annealing temperature increased further. Detailed damaged models dominated by different parameters during annealing were discussed. (C) 2008 Elsevier B. V. All rights reserved.