E. V. Barmina

Effect of visible light on biological objects: Physiological and pathophysiological aspects

Visible light is a necessary condition for the existence of life on the Earth. The food and oxygen consumed by humans, directly or mediately, are products of photosynthesis: conversion of light energy into the energy of chemical bonding in organic matter. Visible light not only provides photosynthesis in plants; it is also involved in the regulation of many reactions (from molecular to behavioral) in the animal world. This review is devoted to the questions concerning the role of light in the life of biological objects and the physical mechanisms of interaction of visible light with matter. The effect of light on the molecular and physiological reactions of a human and its microflora is analyzed. The existing methods and the application potential of light effects in diagnostics and therapy are considered.

Optical properties of nanocomposites based on polymers and metal nanoparticles

The optical properties of nanocomposites of metal nanoparticles and polymers of two types have been studied. Gold and silver nanoparticles were obtained by laser ablation of corresponding metal targets in acetone and chloroform. The thus formed colloidal solutions were used to prepare nanocomposites of these nanoparticles in polymer matrices of polymethylmethacrylate (PMMA) and fluorine-containing polymer LF-32. The polymer matrix is found to promote aggregation of the metal nanoparticles under study into elongated chains. In turn, metal nanoparticles affect the polymer matrix. In the case of PMMA, suppression of the vibrational peaks of polymer in the low-frequency region of its Raman spectrum occurs. In the case of LF-32, gold and silver nanoparticles amplify the Raman signal of the polymer matrix. In addition, the Raman spectra of nanocomposites indicate aggregation of disordered carbon around the nanoparticles obtained by laser ablation in organic solvents. The possibilities of studying the ultrafast (about 1 ps) optical response of the nanocomposites obtained in order to use it in photonics elements are discussed.

Nanostructuring of single-crystal silicon carbide by femtosecond laser irradiation in a liquid

The formation of nanostructures on the surface of single-crystal silicon carbide under ablation by femtosecond laser pulses in liquid ethanol has been experimentally investigated. A 800-nm Ti:sapphire laser with a pulse duration of 210 fs was used as a radiation source. Single-scan irradiation of SiC surface leads to the formation of periodic grooves with a period of about 200 nm. Double exposure with a sample rotation by 90° between the scans gives rise to a regular array of nanostructures with average lateral size of 10 to 15 nm. It is determined that the wettability of nanostructured SiC surface is improved in comparison with the initial surface. It is shown that nanostructuring of SiC surface leads to an increase in the red light transmission by a factor of more than 60.

Self-organization of hydrogen gas bubbles rising from the surface of the laser-irradiated aluminum target under its etching in a dilute alkaline solution

Self-organization of gas bubbles that are formed in etching of an aluminum target in a dilute alkaline solution is described. The rising gas bubbles move in the direction of their higher concentration. This positive feedback results in the fact that the rising bubbles form various stationary structures whose symmetry determined by the configuration of the etching area. Bubbles are aligned along the bisectors of the etching area contour. The self-organization is simulated on the basis of the numerical solution of the Bernoulli equation. The results of the simulation are in good qualitative agreement with the experimental observations.

Laser-Assisted Formation of Elongated Au Nanoparticles and Subsequent Dynamics of Their Morphology under Pulsed Irradiation in Water

The processes of laser-assisted formation of elongated Au nanoparticles and their subsequent agglomeration and fragmentation have been experimentally investigated. Elongated gold nanoparticles were formed by laser ablation of a solid target in water. IR radiation of ytterbium-doped fiber laser with a pulse width of 200 ns and a pulse energy of 0.5 to 1 mJ was used to this end. The extinction spectra and transmission electron microscopy images indicate the formation of elongated gold nanoparticles. The interaction of laser radiation with aqueous colloids of elongated nanoparticles in dependence of the pulse energy and exposure time has been analyzed. Possible processes of laser-assisted formation of elongated Au nanoparticles and their subsequent transition from agglomeration to fragmentation of gold nanoparticles, induced by laser irradiation are discussed.

Solar Cells Based on Laser-Modified Silicon

The fabrication of solar cells based on laser-modified silicon (LMS) has been experimentally investigated. Laser ablation of single-crystal Si in air results in the formation of a quasi-periodic array of microcones. After removing the oxide layer, this array is coated by a transparent conducting tin oxide layer. A comparison of the spectral dependence of the mirror reflection of laser-modified Si with that of the initial flat Si surface shows a 10 to 15% drop in reflectivity. The light-to-electricity conversion efficiency is shown to exceed that of the initial flat Si surface by more than 20%.

Fabrication of Materials with Low Optical Reflectance Based on Laser-Microstructured Metal Surfaces

It has experimentally been shown that the specular reflectance of some metals can be reduced by one to two orders of magnitude by subjecting them to multipulse laser ablation in air. An increase in the coefficient of grayness has been implemented for copper, nickel, aluminum, and stainless steel. Multipulse ablation of the corresponding targets leads to the formation of a quasi-periodic microstructure with an amplitude relief ranging from 30 to 50 μm. The specular reflectance of duralumin has been reduced using anodic oxidation of the microstructures formed by laser ablation and filling the newly formed pores with carbon nanoparticles. The thus obtained surfaces are close to ideal black body in their optical characteristics.

Hydrogen generation by laser irradiation of colloids of iron and beryllium in water

The hydrogen generation under irradiation of aqueous colloids of iron and beryllium nanoparticles by 10-ns pulses of the first harmonic of a Nd : YAG laser with an energy density of about 80 J cm−2 in the solution is experimentally investigated. The partial hydrogen content in the atmosphere above the colloidal solutions of nanoparticles first increases with increasing irradiation time and then reaches a stationary level (400 − 500 Torr). The hydrogen generation rate, being nonzero in the case of irradiation of pure water, passes through a maximum with an increase in the nanoparticle concentration. The results obtained are discussed in terms of the dissociation of water molecules under a direct electron impact from the optical breakdown plasma formed in the liquid.

Laser Alloying of Co Nanorods and Al Nanoparticles in a Liquid

Experimental results are presented on laser alloying of Co nanorods with Al spherical nanoparticles by laser exposure of their mixture in liquid ethanol. It is demonstrated that the initial morphology of the nanoparticles is modified upon laser exposure. The Co nanorods are transformed into Co nanospheres covered by a layer of Al. The evolution of the size distribution is studied as a function of the time of laser exposure. The average size of the nanoparticles has a maximum level with a corresponding optimal irradiation time. The results are discussed from the viewpoint of the interaction of molten nanoparticles inside a cavitation bubble filled with a liquid vapor.