Nonlinear Propagation and Filamentation on 100 Meter Air Path of Femtosecond Beam Partitioned by Wire Mesh.

High-intensity (∼1 TW/cm2 and higher) region formed in the propagation of ∼60 GW, 90 fs Ti:Sapphire laser pulse on a ∼100 m path in air spans for several tens of meters and includes a plasma filament and a postfilament light channel. The intensity in this extended region is high enough to generate an infrared supercontinuum wing and to initiate laser-induced discharge in the gap between the electrodes. In the experiment and simulations, we delay the high-intensity region along the propagation direction by inserting metal-wire meshes with square cells at the laser system output. We identify the presence of a high-intensity region from the clean-spatial-mode distributions, appearance of the infrared supercontinuum wing, and occurrence of the laser-induced discharge. In the case of free propagation (without any meshes), the onset of the high-intensity zone is at 40-52 m from the laser system output with ∼30 m extension. Insertion of the mesh with 3 mm cells delays the beginning of the high-intensity region to 49-68 m with the same ∼30 m extension. A decrease in the cell size to 1 mm leads to both delay and shrinking of the high-intensity zone to 71-73 m and 6 m, respectively. Three-dimensional simulations in space confirm the mesh-induced delay of the high-intensity zone as the cell size decreases.

Compact Radio Frequency Discharge-Pumped Slab CO Laser System with a Zinc Germanium Phosphide (ZnGeP2) Sum-Frequency Generator for Remote Sensing of the Atmosphere.

Based on our experimental study of spectral characteristics of an improved compact CO laser system, the prospects of using such a laser system for solving urgent problems of atmospheric sounding were demonstrated. An increase in the number of laser lines due to intracavity generation of the second harmonic and sum-frequencies in a nonlinear zinc germanium phosphide (ZnGeP2) crystal enriches capabilities of the laser. It was shown that such a CO laser system with frequency conversion in ZnGeP2 crystal can be used for detection and measuring concentration of at least 14 minor natural and pollution gases components of the atmosphere. The advantages and disadvantages of various laser operation modes, and the prospects for using its radiation for atmospheric remote sensing in various spectral ranges are discussed.

Terahertz emission pattern from a single-color filament plasma.

We study the angular distribution of different spectral components of the terahertz emission from a single-color laser filament plasma. The opening angle of a terahertz cone is experimentally demonstrated to be proportional to the inverse square root of both plasma channel length and terahertz frequency in the non-linear focusing mode, whereas in the case of linear focusing this dependence breaks down. We also experimentally show that any conclusions of terahertz radiation spectral composition require the angle range from which it is collected to be specified.

Similarity of angular distribution for THz radiation emitted by laser filament plasma channels of different lengths.

The influence of plasma channel length on an angular terahertz (THz) radiation distribution is experimentally studied for the channel formed under filamentation of an ultrashort laser pulse. It is shown that the angular distribution of the THz emission depends only on laser intensity in the filament and plasma density of the plasma channel and does not depend on the plasma channel length. A qualitative explanation of the THz emission screening by the filament plasma channel is proposed.

Surface-Enhanced IR-Absorption Microscopy of Staphylococcus aureus Bacteria on Bactericidal Nanostructured Si Surfaces.

Surface-enhanced IR absorption (SEIRA) microscopy was used to reveal main chemical and physical interactions between Staphylococcus aureus bacteria and different laser-nanostructured bactericidal Si surfaces via simultaneous chemical enhancement of the corresponding IR-absorption in the intact functional chemical groups. A cleaner, less passivated surface of Si nanoripples, laser-patterned in water, exhibits much stronger enhancement of SEIRA signals compared to the bare Si wafer, the surface coating of oxidized Si nanoparticles and oxidized/carbonized Si (nano) ripples, laser-patterned in air and water. Additional very strong bands emerge in the SEIRA spectra on the clean Si nanoripples, indicating the potential chemical modifications in the bacterial membrane and nucleic acids during the bactericidal effect.

Influence of air humidity on 248-nm ultraviolet laser pulse filamentation.

At first glance, the amount of water molecules naturally contained in humid air is negligibly small to affect filamentation of ultrashort laser pulses. However, here we show, both experimentally and numerically, that for ultraviolet laser pulses with 248 nm wavelength this is not true. We demonstrate that with increase of air humidity the plasma channels generated by the ultraviolet laser pulses in air become longer and wider, while the corresponding electron density in humid air can be up to one order of magnitude higher compared to dry air.

Nanosecond-Laser Generation of Nanoparticles in Liquids: From Ablation through Bubble Dynamics to Nanoparticle Yield.

A comprehensive picture of the nanosecond-laser generation of colloidal nanoparticles in liquids is nowadays the demand of their high-throughput industrial fabrication for diverse perspective biomedical, material science, and optoelectronic applications. In this study, using silicon as an example, we present a self-consistent experimental visualization and theoretical description of key transient stages during nanosecond-laser generation of colloidal nanoparticles in liquids: plasma-mediated injection of ablated mass into the liquid and driving the vapor bubble, finalized by the colloid appearance in the liquid. The explored fundamental transient stages envision the basic temporal and spatial scales, as well as laser parameter windows, for the demanded high-throughput nanosecond-laser generation of colloidal nanoparticles in liquids.

Direct laser writing of barriers with controllable permeability in porous glass.

Barriers were produced in porous glass through its local bulk density modification by direct femtosecond writing accompanied by СО2-laser surface thermal densification, to make functional microfluidic elements separated by such physical barriers with different controlled permeability. The separation of multi-component solutions into individual components with different molecule sizes (molecular separation) was performed in this first integrated microfluidic device fabricated in porous glass. Its application in the environmental gas-phase analysis was demonstrated.

Experimental study of fs-laser induced sub-100-nm periodic surface structures on titanium.

In this work the formation of laser-induced periodic surface structures (LIPSS) on a titanium surface upon irradiation by linearly polarized femtosecond (fs) laser pulses with a repetition rate of 1 kHz in air environment was studied experimentally. In particular, the dependence of high-spatial-frequency-LIPSS (HSFL) characteristics on various laser parameters: fluence, pulse number, wavelength (800 nm and 400 nm), pulse duration (10 fs - 550 fs), and polarization was studied in detail. In comparison with low-spatial-frequency-LIPSS (LSFL), the HSFL emerge at a much lower fluence with orientation perpendicular to the ridges of the LSFL. It was observed that these two types of LIPSS demonstrate different fluence, shot number and wavelength dependencies, which suggest their origin is different. Therefore, the HSFL formation mechanism cannot be described by the widely accepted interference model developed for describing LSFL formation.

Femtosecond laser treatment for the design of electro-insulating superhydrophobic coatings with enhanced wear resistance on glass.

Femtosecond laser treatment of a glass surface was used to fabricate a multimodal roughness having regular surface ripples with a period of a few micrometers decorated by aggregates of nearly spherical nanoparticles. UV-ozone treatment followed by chemisorption of the appropriate functional fluorosilanes onto the textured surface makes it possible to fabricate a superhydrophobic coating with a specific surface resistance on the order of petaohms on a glass surface. The main advantage of the fabricated coating under severe operating conditions with abrasion loads is the significant durability of its electro-insulating properties. The longevity of the high surface resistivity, even on long-term contact with a water vapor-saturated atmosphere, is directly related to the peculiarities of the surface texture and ripple structure.