Sergei M. Klimentov

Hole formation process in laser deep drilling with short and ultrashort pulses

The drilling process in different materials (diamond, steel, ceramics and PMMA) was studied for a large range of pulse lengths from about 100 fs to 10 ns using different approaches. In transparent materials the penetration process was visualized with high-speed video analysis and microscopy. The drilling rate as well as the relation between processing energy density and ablation threshold were determined in situ. The penetration of the laser beam inside the channel and the influence of laser-ignited plasma were investigated by transmission measurements. Mechanisms of energy coupling and heat losses were examined by applying simple analytical calculations. Proposals for the basic understanding of the drilling process are presented.

Plasma effects during ablation and drilling using pulsed solid-state lasers

Plasma and vapor plumes generated by ultrashort laser pulses have been studied by various optical methods for both single pulse ablation as well as high-repetition rate drilling. Time-resolved shadow and resonance absorption photographs enable to determine the plume and vapor expansion behavior and, by means of an analytical shock wave model, allow to estimate an energy balance that can be refined by plasma transmission measurements. The results furthermore suggest that several types of laser-induced plasmas can be distinguished according to their origin: the material vapor plasma originating at the ablated surface even at moderate intensities, a breakdown plasma at increased power densities occurring in cold vapor or dust particles left from previous ablations during repetitively-pulsed processing and, finally, the optical breakdown in the pure atmosphere at high intensities. The latter also gives rise to nonlinear scattering phenomena resulting in a strong redistribution of the energy density in the beam profile.

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.

Propagation of short-pulsed laser radiation and stages of ablative deep-channel formation

Experiments on deep drilling of steel by 300 ps, 1 ps and 125 fs laser pulses are reported. The ablation rate dependence on the channel depth was studied and energy losses in through channels for different radiation parameters were measured. The low-threshold cluster-assisted air breakdown was revealed to play an important role in ablation by 300 ps pulses. The ablated particles remaining inside the channel between laser shots provide substantial reduction of the air breakdown threshold. Laser-induced spark produces noticeable shielding effect and, presumably, is main reason of observed deep channel widening. Pronounced strengthening of light shielding by laser-induced spark was observed under steel target ablation comparing with pure air without target for ultrashort (125 fs, 1 ps) laser pulses. The dramatic reduction of the drilling rate in deep channels was observed for all examined pulsewidths. In the case of 300 ps pulses, the drilling rate falls down sharply by two order of magnitude at a certain critical channel depth increasing with the incident laser fluence. It was found that the integral plasma transmittance (breakdown plus ablation) remains unchanged when the drilling rate decreases.

Effect of nonlinear scattering of radiation in air on material ablation by femtosecond laser pulses

This work investigates the role of ambient atmosphere in material ablation by ultra-short intense laser pulses. It is shown, that ablative action of femtosecond pulses reveals limitations imposed by nonlinear optical response of gases resulting in significant modification or the incident laser beam. This phenomenon called conical emission (CE) manifests itself as strong scattering or emission of radiation in the forward direction developed at focusing of intense pulses of Ti:Sa laser (π=110÷1500 fs) in air. Transformation of the nearly Gaussian spatial profile into a wide angle cone is followed by spectral conversion of the fundamental laser frequency into a broad spectrum with relatively shorter wavelengths extending up to the visible range. Thresholds, converted energy, spectra and profiles of scattered radiation were measured at variable laser pulse duration and the ambient pressure. It was found, that more than 70% of the incident pulse energy can be scattered at conventional focusing of the beam by a long focal length lens. Effect of CE on material ablation in air was investigated, and the data obtained allowed to explain paradox morphology of steel channels drilled by high power femtosecond pulses.

Role of gas environment in the process of deep-hole drilling by ultrashort laser pulses

The detailed study of the role of air pressure in deep hole drilling by femtosecond and picosecond intense laser pulses (Ti:Al 2 O 3 and Nd:YAP lasers) was performed in the range 1÷1000 mBar. Steel sample plates were mostly tested, experimental data obtained for ceramic materials is also presented. The following ablation parameters were measured and analyzed: ablation rates and their dependence on the channel depth, ablated crater morphology, optical transmission in channels after through hole formation. Both percussion and helical drilling regimes were used. Special attention was paid to two strong gas assisted effects typical of sub-picosecond and sub-nanosecond material ablation, which are low threshold gas breakdown in deep channels and nonlinear interaction of ultra-short intense pulses with air resulting in conical emission. Unwanted aspects of both phenomena were shown to disappear in a moderate vacuum of ~100 mBar. A new approach to formation of such a vacuum in drilled channels was also proposed and experimentally modeled using ultra-high repetition rate nanosecond laser pulses.

Simple optical parametric oscillator-amplifier in unitary crystal, tunable over the visible range of spectrum and its application for two-photon spectroscopy

For application of optical parametric oscillator (OPO) to investigation of nonlinear interaction of laser radiation with matter the factor of importance is stability of light spatial distribution and spotsize position on a target through a tuning range. Collinear temperature tunable schemes show an advantage for these purposes, in particular, for small pump beam diameters. A simple and efficient visible range parametric converter can be realized using two-pass configuration, where parametric luminescence is excited on the first pass through a nonlinear crystal and amplified on the second pass after spatial filtering. Lack of resonator simplifies high power UV pumping and getting of relatively narrow emission spectrum. Using such an approach, we have made the oscillator-amplifier system temperature tunable in the range of 440 to 670 nm employing 4-cm-length ADP crystal pumped by 266 nm radiation from the single-mode YAG:Nd laser. The output energy of 3 mJ in about 1-ns pulsewidth has been achieved with total conversion efficiency of 10%. A spatial profile of the output beam kept its shape within the branch of the tuning curve. This allowed us to use the device as a proper tool for investigation of two-photon excitation in undoped CsI and KI single crystals. The OPO signal output was used to record photoconductivity spectra in these materials.

Pulse-width dependence of laser damage in optical materials: critical analysis of available data and recent results for nanopicosecond region

A critical analysis of available theoretical and experimental data for a pulse-width dependence of laser induced damage thresholds (LIDT) in various optical materials is given along with new results of our recent measurements obtained in nano-picosecond range at 1064 nm wavelength for alkali halide crystals (NaCL, KCl, KBr). Three YAG:Nd laser oscillator- amplifier systems operating in Q-switch and mode-locking modes provided highly stable light pulses at 2 ns, 15 ns, and 50 ps with Gaussian spatial profile of beams were used in the experiments. Special attention was paid to the comparability of the LIDT test conditions for various pulse widths, implying the carefully controlled similarity of beam intensity spatial distributions and temporal profiles. The experimental data are analyzed on the basis of theoretical predictions for different damage mechanisms. It is concluded that pulse-width scaling of LIDT is still a problem, and an adequate approach to its solution is formulated.

Effect of submillisecond radiation of the erbium laser on absorbing liquid

Pressure pulses occurring in water under the effect of submillisecond radiation of the Er3+:YAG laser on the free and covered liquid surface were measured. The behavior of pressure pulses is caused by the photoacoustic effect, explosive boiling of the superheated liquid surface layer, and cavitation processes developing after laser exposure.

Ultrashort-pulse generation in solid state dye lasers

Generation of ultrashort pulses (picosecond range duration) in pyrromethene-580 (PM-580) and xanthene 11B dyes doped modified PMMA polymer lasers is reported. Active polymer elements were pumped by second harmonic radiation of the 50 picosecond Nd:YAG laser. A single-pulse and train-pulse laser generation was observed at different conditions (variable optical resonator length and pump energy) with a pulsewidth ranging from 140 ps to 30 ps. The train-pulse generation is attributed to mode-locking of a laser resonator. The lifetime (tau) e, of an excited quantum state of a laser transition of the PM- 580 dye molecules has been estimated ((tau) e equals 200 divided by 250 ps) from the pulse-train duration.