S. V. Garnov

Color-center generation in silicate glasses exposed to infrared femtosecond pulses

The optical properties of silicate glasses under high-power, 850-nm femtosecond laser irradiation have been studied. Photoinduced processes occurred at irradiances well below the threshold for laser-induce damage. Laser spectral line broadening leading to supercontinuum generation in the visible and UV spectral regions was observed in all the glasses studied. Color-center generation and intrinsic luminescence were found in boro-silicate and alkali silicate glasses. It is believed that these processes result from linear and two-photon absorption of the short-wavelength component of the supercontinuum, causing ionization of the glass matrix. No color-center absorption in the visible region was observed in fused silica at irradiances up to the laser-damage threshold.

Ablation of CVD diamond with nanosecond laser pulses of UV-IR range

Etch rates of CVD diamond upon irradiation by nanosecond (5‐9 ns) pulses at three diVerent wavelengths 1078, 539 and 270 nm at laser fluences in the range 1‐1000 J/cm2 were measured. A Nd:YAP laser system operated at first, second and fourth harmonics was used in the ablation experiments. Both shallow (<15 microns) and through holes were etched in a 95-mm thick free-standing diamond film grown by microwave plasma CVD. The ablation rate was found to be wavelength-independent, this result being ascribed to surface blackening caused by amorphization/graphitization as confirmed by Raman analysis. The maximum etch rate approached 600 nm/pulse. The etch rate depended on the crater depth, which was ascribed to the eVect of laser‐plasma interaction inside the deep channel. The possibility of cutting trenches of high aspect ratio has been demonstrated. In a separate experiment, a batch of thin diamond films diVering in thermal conductivity (k=2‐5 W/cmK ) was ablated with a KrF excimer laser (l=248 nm). No dependence of ablation rate on film quality was observed, which could be explained assuming grain boundaries to be the main source of thermal resistance.

Laser ablation of metals and ceramics in picosecond–nanosecond pulsewidth in the presence of different ambient atmospheres

Abstract Ablation tests of AlN, Si3N4, SiC, Al2O3 ceramics, steel and aluminum have been carried out in vacuum, air and argon atmospheres using UV (270 nm), visible (539 nm) and IR (1078 nm) picosecond (100÷150 ps) and nanosecond (6÷9 ns) laser pulses. Ablation rate dependencies have been measured in the range of laser energy densities varied from (2÷5)×101 J/cm2 to (5÷10)×103 J/cm2. Peculiarities of laser ablation processes at different wavelengths, pulsewidths and ambient gases are discussed. In particular, the efficiencies of laser ablation in picosecond and nanosecond regions are compared. The scanning electron microscope (SEM) pictures of high quality microstructures, deep and narrow cuts and holes produced in ceramics with typical size of tens microns and aspect ratio as high as 20, are demonstrated.

New lasers based on c-cut vanadat crystals

Spectroscopic and lasing properties of c-cut Nd-doped Nd:Gd0.7Y0.3VO4, Nd:YVO4, and Nd:GdVO4 crystals were investigated. Spectral tuning from 1062 to 1067 nm was demonstrated. CW, Q-switching and mode-locking regimes for two-color laser operations were realized. A novel THz source based on Q-switch two-color diode-pumped solid state c-cut Nd:GdVO4 laser with Filter Lio as selective element and the GaSe nonlinear optical crystals as convertor was demonstrated. Terahertz radiation with wavelength 436 mm (0.56 THz) was detected. One picosecond laser pulses in mode-locking diode pumped c-cut vanadat lasers with a Kerr-lens and PbS-doped glasses as saturable absorbers are observed.

Time-resolved microwave technique for ultrafast charge-carrier recombination time measurements in diamonds and GaAs

Recombination times of laser-excited charge carriers in natural diamond crystals, polycrystalline chemical vapor deposited (CVD) diamond films, and GaAs wafers were measured with 1 ns time resolution by a microwave-radiation technique. A waveguide scheme was applied to record time-dependent reflection and transmission of 140 GHz cw radiation. The measured recombination carrier lifetimes in the bulk of natural and CVD diamond samples were found to be of 1–3 ns. In GaAs, a distinguishing difference between the bulk (15 ns) and surface (3.5 ns) recombination times was observed. To validate the applicability of the developed technique, a computer simulation of the microwave-radiation interaction with excited plane–parallel specimens has been performed applying the Fabry–Perot resonator theory and the classical Drude model.

Spatio-temporal dynamics of electron density in femtosecond laser microplasma of gases

The formation and evolution of femtosecond laser plasma produced in microvolumes of gases at different pressures upon their multiply ionization by high intensity pulses of fundamental and second harmonics of a Ti:sapphire laser is studied. The interferometric technique for precise ultrafast optical diagnostics of such plasma was applied. The numerical technique of interferogram processing and reconstruction of instant spatial distribution of refractive index and free electron density in laser-induced plasma applied for this proposes is described. The spatiotemporal distribution of the refractive index and free electron density were studied with a spatial resolution of ∼1 μ m and a temporal resolution of ∼70 fs.

Two-stage laser-induced synthesis of linear carbon chains

A method is proposed for the synthesis of linear carbon chains under irradiation of colloidal systems consisting of schungite nanoparticles by an ytterbium fibre laser with a pulse duration of 100 ns and an energy up to 1 mJ. Colloidal systems have been previously prepared by laser ablation of a schungite target in distilled water using a Nd3+ : YAG laser with a pulse duration of 1 ms and an energy up to 10 J.

Superluminal source of directional pulsed wideband electromagnetic radiation

A photoemission source of directional pulsed wideband electromagnetic radiation in the microwave region is developed, and the time profile of the generated pulse is investigated. The source is a vacuum photodiode of a parabolic shape in which a Cherenkov radiation pulse is formed by an electron current wave excited by an incident laser pulse and propagating along the surface of the anode mesh with a phase velocity higher than the speed of light.

Diode pumped Nd:YAG laser with active Q-switching and mode locking for hole drilling

A diode-pumped Nd:YAG laser operating with active-passive Q-switch mode locking, has been developed. The acousto-optic repetition train was one kilohertz with generated pulse train widths 65 ns, single pulse widths 200 ps and an average power of 6.5 W. Improvement of efficiency of small diameter deep holes laser drilling in different materials was studied.

Effect of low-threshold air breakdown on material ablation by short laser pulses

Ablative formation of channels in steel by picosecond and nanosecond pulses of Nd lasers was studied. It was found that significant screening of the incident energy (up to 80–90%) in this pulse duration range is caused by breakdown of air contaminated with ablated microparticles. The breakdown threshold, size of particles, and time of their settling down were estimated. It was shown that this kind of plasma screening results in a decrease in the ablation rate and significant channel widening. Practical approaches to eliminate the low-threshold breakdown induced by microparticles were proposed and implemented. These approaches are based on experimental results of the study of the dependences of laser ablation on the pressure and repetition rate. It was shown that a moderate decrease in the pressure below 300–400 mbar makes it possible to avoid screening. In high-repetition-rate ablation, it was found that values above several kilohertz correspond to quasi-vacuum conditions in the ablation spot.