A. F. Bunkin

Thermal melting and ablation of silicon by femtosecond laser radiation

The space-time dynamics of thermal melting, subsurface cavitation, spallative ablation, and fragmentation ablation of the silicon surface excited by single IR femtosecond laser pulses is studied by timeresolved optical reflection microscopy. This dynamics is revealed by monitoring picosecond and (sub)nanosecond oscillations of probe pulse reflection, which is modulated by picosecond acoustic reverberations in the dynamically growing surface melt subjected to ablation and having another acoustic impedance, and by optical interference between the probe pulse replicas reflected by the spalled layer surface and the layer retained on the target surface. The acoustic reverberation periods change during the growth and ablation of the surface melt film, which makes it possible to quantitatively estimate the contributions of these processes to the thermal dynamics of the material surface. The results on the thermal dynamics of laser excitation are supported by dynamic measurements of the ablation parameters using noncontact ultrasonic diagnostics, scanning electron microscopy, atomic force microscopy, and optical interference microscopy of the modified regions appearing on the silicon surface after ablation.

Laser beam profile influence on LIBS analytical capabilities: single vs. multimode beam†

Single vs. multimode laser beams have been compared for laser ablation on steel samples. Laser plasma properties and analytical capabilities (precision, limit of detection) were used as key parameters for comparison. Peak fluence at focal spot has been observed to be higher for Gaussian beam despite ∼14-fold lower pulse energy. A comparison of Gaussian and multimode beams with equal energy was carried out in order to estimate influence of beam profile only. Single mode lasing (Gaussian beam) results in better reproducibility of analytical signals compared to multimode lasing while laser energy reproducibility was the same for both cases. Precision improvements were attributed to more stable laser ablation due to better reproducibility of beam profile fluence at laser spot. Plasma temperature and electron density were higher for Gaussian laser beams. Calibration curves were obtained for four elements under study (Cr, Mn, Si, Cu). Two sampling (drilling and scanning procedures) and two optical detection schemes (side-view and optical fiber) were used to compare Gaussian and multimode beam profile influence on analytical capabilities of LIBS. We have found that multimode beam sampling was strongly influenced by surface effects (impurities, defects etc.). For all sampling and detection schemes, better precision was obtained if Gaussian beam was used for sampling. In case of single-spot sampling better limits of detection were achieved for multimode beam. If laser sources have the same wavelength and equal energy then quality of laser beam becomes a crucial parameter which determined laser ablation and analytical capabilities of LIBS.

Temperature evolution of the relative concentration of the H2O ortho/para spin isomers in water studied by four-photon laser spectroscopy

The four-photon laser spectroscopy of molecular motions [1] of distilled water in the terahertz and subterahertz spectral ranges is employed to observe resonant lines related to the rotational transitions of ortho and para nuclear spin isomers of the H2O molecule. It is demonstrated that the intensity ratio of the lines of the H2O ortho/para spin isomers in several water samples decreases by a factor of 2.0–2.5 in comparison with the gas-phase ratio. A violation of the equilibrium ortho/para ratio upon the condensation of vapor is interpreted as a manifestation of the spin selectivity in the formation of the H-bonded complexes of the H2O para isomers. The nonequilibrium ortho/para ratio characterizes water at room temperature as an unstable liquid with respect to the spin temperature.

Single-shot and single-spot measurement of laser ablation threshold for carbon nanotubes

A simple and convenient procedure for single-shot, single-spot ablation threshold measurement has been developed. It is based on the employment of cylindrical lens to obtain elliptical Gaussian laser spot. The ablated spot chords which are parallel to the minor axis were measured across the spot major axis which is proportional to the fluence cross-section thus providing wide range dependence of damaged spot size versus fluence in one spot measurement. For both conventional and new-developed procedures the ablation threshold for typical Nd:YAG laser parameters (1064 nm, 10 ns) has been measured as 50 mJ/cm2 which is one order of magnitude lower than that for a bulk graphite.

Remote sensing of seawater and drifting ice in Svalbard fjords by compact Raman lidar

A compact Raman lidar system for remote sensing of sea and drifting ice was developed at the Wave Research Center at the Prokhorov General Physics Institute of the Russian Academy of Sciences. The developed system is based on a diode-pumped solid-state YVO(4):Nd laser combined with a compact spectrograph equipped with a gated detector. The system exhibits high sensitivity and can be used for mapping or depth profiling of different parameters within many oceanographic problems. Light weight (∼20 kg) and low power consumption (300 W) make it possible to install the device on any vehicle, including unmanned aircraft or submarine systems. The Raman lidar presented was used for study and analysis of the different influence of the open sea and glaciers on water properties in Svalbard fjords. Temperature, phytoplankton, and dissolved organic matter distributions in the seawater were studied in the Ice Fjord, Van Mijen Fjord, and Rinders Fjord. Drifting ice and seawater in the Rinders Fjord were characterized by the Raman spectroscopy and fluorescence. It was found that the Paula Glacier strongly influences the water temperature and chlorophyll distributions in the Van Mijen Fjord and Rinders Fjord. Possible applications of compact lidar systems for express monitoring of seawater in places with high concentrations of floating ice or near cold streams in the Arctic Ocean are discussed.

Water enrichment by H2O ortho-isomer: Four-photon and NMR spectroscopy

Ratio of H2O ortho-/para-spin-isomers in water of different treatment procedures (distilled or cavitation fountain) were studied by both Rayleigh wing four-photon spectroscopy and 1H nuclear magnetic resonance (NMR) spectroscopy. Low-frequency gas-like rotational resonances were observed in the 0.1–1.5 cm−1 (3–45 GHz) spectral range and NMR proton density was measured in both water samples. We established that the intensity of ortho-isomer line (616−523)0.74 cm−1 measured by four-photon spectroscopy increases by factor of ∼3.5 after cavitation treatment of distilled water. Moreover, the proton density measured by NMR spectroscopy in the same sample grows on ∼17%. We have suggested that the enrichment of the distilled water by ortho-H2O molecules was achieved due to cavitation bubbles collapse when the water passes through the supercritical state.

Spin isomeric selectivity of water molecules upon DNA hydration

Four-photon coherent scattering of laser radiation was used to study the influence of DNA on the content of quasi-free ortho and para isomers of water molecules in its aqueous solution. It was shown that the concentration of quasi-free molecules that form the rotational spectrum of spin isomers increases considerably in the hydration shell of the DNA molecule as compared with pure water. The increase in the concentration of spin isomers occurs disproportionally. In the presence of DNA, the intensity of the rotational spectrum of ortho isomers is on the average much greater than that of para isomers. It was also demonstrated that the character of hydration and the ortho/para ratio change noticeably upon DNA denaturation, which may be evidence of changes in preferable solvation of DNA during its denaturation. The data obtained allowed us to assume that the stability of different biologically important states of macromolecules can be changed by varying the relative concentration of water spin isomers in solution.

Observation of rotational resonances of ortho and para spin isomers of the H2O molecule in hexagonal ice using four-photon laser spectroscopy

Rotational resonances of ortho and para spin isomers of the H2O molecule are observed in hexagonal ice using four-photon spectroscopy of coherent light scattering. It is experimentally shown that the resonant contribution to the four-photon scattering signal from para H2O spin isomers in ice is about half as large as that in the liquid phase.

Ship wake detection by Raman lidar

We carried out a remote study of ship wakes by optical methods. Both Mie and Raman scattering signals and their evolution were simultaneously recorded by gated detector (intensified CCD). The Mie scattering signal was detectable within 1 min after water disturbance by a high-speed boat. According to an approximation of experimental data, Raman signal fluctuations can be detected for a much longer time under the same conditions. We have demonstrated that Raman spectroscopy is substantially more sensitive to water perturbation compared to conventional acoustic (sonar) technique and can be used for ship wake detection and monitoring.

Ice thickness measurements by Raman scattering

A compact Raman LIDAR system with a spectrograph was used for express ice thickness measurements. The difference between the Raman spectra of ice and liquid water is employed to locate the ice-water interface while elastic scattering was used for air--ice surface detection. This approach yields an error of only 2 mm for an 80 mm thick ice sample, indicating that it is a promising express noncontact thickness measurements technique in field experiments.