E. V. Chelnokov

Two-photon pumped random laser in nanocrystalline ZnO

Thin film of ZnO nanoparticles with dimension of about 5–10nm were fabricated by the pulsed laser ablation method. By using a femtosecond laser beam at 700nm to pump micrometer-thick films the authors observed two-photon-induced lasing at 385nm. Experimentally obtained dependence of the threshold on the excitation spot radius r0 is closer to (1∕r02) than to (1∕r0), thus suggesting efficiency of the feedback by scattering-random lasing. The experimental data on nonlinear transmission of the film at the wavelength of pumping are presented.

Nonreciprocal XeCl laser-induced aggregation of β-crystallins in water solution

The aggregation of a β-crystallin water solution exposed to XeCl laser radiation demonstrates the dependence of scattering-exposure curve (scattering versus exposure) on laser intensity. The main features of this dependence can be understood by the relaxation of a partly denaturated state of a protein within some finite relaxation time. These photoactivated states originate from the absorption of UV photons. Two partly denaturated (photoactivated) monomers, as well as other aggregates, can aggregate, giving rise to sharply increasing probe light scattering after some lag time of irradiation.

Kinetics of low-scattering biotissue photodenaturation induced by the UV harmonics of a Nd:YAP laser and by Nd:YAG laser at a wavelength of 1440 nm

We study the effect of laser radiation on initially low scattering media such as egg white and crystalline substance. These media become highly scattering under laser irradiation. We perform and discuss three kinds of experiments, which elucidate the time dynamics of scattering of probe radiation. In the experiment of the first kind we investigate the pure photothermal denaturation of tissues under the effect of a Nd:YAG laser at a wavelength of 1440 nm (absorption coefficient in water is 26 cm-1). The theoretical model is derived which enables us to estimate the temperature rise involved. It also allow us to estimate the corresponding kinetics parameters of photodenaturation and characteristics of probe light scattering. In the experiment of the second kind we perform multiple pulse irradiation of tissues by UV harmonics (the fourth and the fifth) of a Nd:YAP laser with the aim to determine the characteristics of pure photochemical modification of materials. In these experiments the fluences were small enough to prevent heating of the materials. The results of the above experiments both of the first and the second kind allow us to estimate the relative contributions of photochemical modification of a tissue and photothermal protein denaturation within the experiments of the third kind, in which we use the mentioned above UV solid state laser harmonics at fluences high enough to produce heating of the materials.

Investigation of photo-aggregation of proteins irradiated by XeCl laser light

The effect of XeCl laser radiation on carbonic anhydrase solution is studied. It is investigated that kinetics of protein aggregation is strongly influenced by both laser fluence and repetition rate. The theoretical model is constructed which allows one to explain qualitatively the features of experimental findings.

Photoaggregation of water-soluble protein (carboanhydrase) induced by the ultraviolet radiation of Xe-Cl laser.

Molecular mechanisms of the origin and development of certain forms of cataract, including photocataractogenesis, are based on the degradation and aggregation of the main water-soluble proteins of the eye lens ( α -, β -, and γ -crystallines) [1–4]. The light-induced degradation of these proteins and the whole lens substantially simulates the processes associated with the development of senile cataract, one of the most widespread forms of cataract [5]. Detailed studies of the mechanisms of the UV-induced degradation and aggregation of crystallines in solution should be regarded as an appropriate model of the processes of lenticular opacity, primarily, the UV-induced lenticular opacity.

Aggregation of crystallins induced by pulsed laser UV light (308 nm)

Here we compile and analyze the data on photoaggregation of a model protein carboanhydrase and the main eye lens proteins α-, β-, γ-crystallins under the action of pulsed UV irradiation from a Xe-Cl laser (308 nm) with broad variation of pulse energy density and repetition rate. The aggregation efficacy proves to be a nonlinear function of these parameters and protein concentration. A theoretical model is proposed that qualitatively explains the experimental data. It is shown that N-arm-truncated βA3-crystallin is more prone to UV-induced aggregation than the full-sized protein; such defects caused by mutation or aging may aggravate the development of lenticular opacity. Analyzed is the effect of some low-molecular compounds on the aggregation of β-crystallin and its mixture with α-crystallin. A combination of short peptides prepared on this basis markedly impedes crystallin aggregation and retards the development of UV-induced cataract in rats.

Photoaggregation of crystallins (main proteins of eye lens) under the effect of XeCl laser radiation

UV light is one of primary factors associated with cataract formation in the eye lens. α-, β-, γ-Crystallins maintain lens transparency, and damage to these proteins plays a major role in cataract formation. The effect of XeCl laser radiation (308 nm) on βL-crystallin solution is studied. The strong dependence of protein aggregation kinetics on both laser fluence (w) and repetition rate (F) is investigated. The kinetics features are similar to those of carbonic anhydrase photoaggregation studied previously.

Two-photon pumped zinc oxide random laser

Zinc oxide is a promising material for creation of novel ultraviolet light sources. In this work we study random laser action in a thin ZnO nanocluster film under two-photon pumping. The results are compared with the case of single-photon pumping. A theoretical model is developed, which shows the effect of boundary conditions on lasing in the film. Measurement results of nonlinear transmission are presented and compared with classical two-photon absorption model.

Study of molecular mechanisms of UV-induced aggregation of crystallins and possibility of maintaining eye lens transparency

The effect of D-pantethine and L-carnosine on the rate of UV-induced (XeC1 laser λ = 308 nm) aggregation of a mixture of βL-crystallin and α-crystallin is studied. We also demonstrate that the suggested by us combination of short-chain peptides shows better protective properties with respect to UV-induced aggregation than known anti-cataract agents.