O. G. Romanov

Efficient 3D numerical approach for temperature prediction in laser irradiated biological tissues

Temperature prediction in biological tissues irradiated by an optical source is frequently required in some medical applications, like Thermotherapy, Hyperthermia or tissue ablation. In this work we propose a new numerical approach to solve the bio-heat equation. It is based on the two steps 3D modified Du Fort-Frankel algorithm, which allows a better convergence, more accuracy and a faster computation than previous numerical methods developed by other authors. The model also includes adaptive spatial mesh and time step refinement. These improved results for opto-thermal temperature distribution could be used for choosing appropriate laser treatment parameters in medical praxis.

Formation of self-trapping waveguides in bulk PMMA media doped with Phenanthrenequinone

Experimental and theoretical investigations of light self-trapping waveguides in a bulk polymeric medium based on polymethylmethacrylate (PMMA) with photosensitive phenanthrenequinone (PQ)-molecules are examined. Self-channeling was generated for the first time in this nonlinear bulk PQ-PMMA media with a thickness up to several millimeters and 0.1 mol. % PQ-concentration. The experimental formation of volume waveguide structures with a length of 2 - 3 cm at different laser wavelengths (405 nm, 488 nm, and 514.5 nm) was demonstrated. The calculations based on a model for the laser beam propagation in the bulk PQ-PMMA medium with competitive nonlinearities are in a good agreement with the experiments.

Frequency up-conversion of coherent images by intracavity nondegenerate four-wave mixing

This work presents theoretical and experimental studies of the processes of light field transformations upon frequency-nondegenerate fourwave mixing (NFWM) in a nonlinear Fabry-Perot interferometer (FPI). The principal aims are the development of a theory for intracavity four-wave mixing in complex molecular media in conditions of scattering from dynamic gratings and resonator feedback; determination of a mechanism of light field transformations in dynamic holograms, and also by nonlinear interferometers; working out and introduction of novel nonlinear-optical methods to control the characteristics of light beams. High diffraction efficiency (up to 13.5%) with simultaneous infrared-to-visible frequency conversion of coherent images has been experimentally obtained by intracavity NFWM.

Nonlinear interaction and reflection of incoherent light beams

The interaction dynamics between two incoherent light beams in a medium with thermal nonlin-earity is studied both experimentally and theoretically. It is shown that in the mode of a nonlinear reflection the transformation of a spatial structure of a weak signal beam can be theoretically described in the framework of a spatially distributed model for a nonlinear change in the refractive index of the medium.

Necrosis prediction of photodynamic therapy applied to skin disorders

The great selectivity and the lack of side effects of Photodynamic Therapy make it more advantageous than radiotherapy or chemotherapy. The application of PDT to skin diseases is particularly appropriate, due to the accessibility of this tissue. Common disorders like nonmelanoma skin cancer, that includes basocelullar or squamous cell carcinomas, can be treated with PDT. Conventional procedures, like surgery or radiotherapy, are not so efficient and do not, in general, obtain the same favourable results. PDT in dermatology medical praxis uses fixed protocols depending on the photosensitizer and the optical source used. These protocols are usually provided by the photosensitizer laboratory, and every lesion is treated with the same parameters. In this work we present a photo-chemical model of PDT applied to skin disorders treated with topical photosensitizers. Optical propagation inside the tissue is calculated by means of a 3D diffusion equation, solved via a finite difference numerical method. The photosensitizer degradation or photobleaching is taken into account, as the drug looses efficiency with the irradiation time. With these data the necrosis area is estimated, so this model could be used as a predictive tool to adjust the optical power and exposition time for the particular disease under treatment.

Multiwave mixing of singular light beams in resonant media

A method for transforming the topological structure of a singular light beam under multiwave mixing in the media with resonant nonlinearity is justified theoretically. The effect of self-and cross-modulation of the light waves under condition of nonlinear variation of the refractive index on stability of the optical vortices appearing in the multiwave mixing is studied.

Frequency transformation of optical vortices upon nondegenerate multiwave interaction in dye solutions

A method for inverting the topological charge of singular light beams with their simultaneous frequency transformation under multiwave interaction in media with resonant and thermal nonlinearities is proposed. Frequency-nondegenerate four-wave and six-wave interactions of optical vortices are experimentally implemented using an ethanol solution of a polymethine dye 3274U as a nonlinear medium.

Analysis of a semiconductor semimagnetic microcavity for optical amplification and wavelength conversion applications

The importance of nonlinear optical devices is increasing due to their hopeful characteristics such as small size, high speed or even low power consumption. These devices integrated in all-optical systems achieve the best results because of the elimination of optoelectronic or electro optic conversions that imply great reductions in these advantages. Therefore the main effort should be directed to make as many functions as possible by optical means. Among these functions, wavelength conversion or amplification seem to be likely to implement with a nonlinear device. In this work a structure called Semiconductor Semimagnetic Microcavity (SSM), for optical amplification and wavelength conversion, is introduced and studied. This study requires a suitable method for nonlinear series devices. It must take into account each wave and its relationship with the others. An Extended Yeh matrix is appropriate for the characterization of this structure. The method reveals that if the microcavity is exposed to an input signal and a pumping input signal and also matches the conditions of degenerate four-wave mixing, another wave at a different wavelength appears. What is more, the original input signal becomes greatly amplified depending on the pumping input signal. The process of obtaining these results with the Extended Yeh matrix applied to a SSM is shown. Optical wavelength conversion and optical amplification in a microcavity is demonstrated by means of this matricial method.

Transformation of optical vortices by polarization dynamic holograms

Results of theoretical and experimental studies of regularities in the transformation of the topological and polarization structures of optical vortices by polarization dynamic holograms formed by pulse Gaussian and singular light beams in dye solutions are presented.

Self-trapping waveguiding structures in nonlinear photorefractive media based on Plexiglas with phenanthrenequinone molecules

The paper presents theoretical and experimental investigations of light beam self-trapping in a photorefractive medium based on Plexiglas (polymethylmethacrylate, PMMA) with photosensitive phenanthrenequinone (PQ)- molecules. It is shown that the self-trapping of a laser beam is generated due to the self-interaction of the propagating light wave under the conditions of a well balanced concurrence of the effects of light diffraction and nonlinear focusing. A new method for controlling the waveguide cross-section by changing the ratio of two competing mechanisms of the nonlinear refractive-index variation (namely the formation of the photoproducts and the heating of the medium while varying the power of the light beam) is proposed. The recording of self-trapping structures implemented in PQ-PMMA layers has been realized with two laser sources (405 nm and 514.5 nm) with an average power of several mW. It is shown that the photoattachment of the PQ-molecules to the polymeric chains and the formation of the photoproduct play the decisive role for the light-induced increase of the refractive index. Besides, the formation of the waveguide is strongly influenced by heating of the medium, which results in an additional thermal defocusing of the light beam. It has been established that the parameters of the waveguide (cross-section and length) are strongly dependent on the wavelength and the power of the laser radiation, as well as on the concentration of the PQ-molecules. Waveguiding structures with a diameter of 100 μm were recorded in samples with a high PQ-concentration (up to 4 mol.%) for the wavelength of 514.5 nm. Reducing the dye-concentration by one order requires shorter (blue) wavelengths (405 nm). The dependence of the waveguide parameters and the optimal laser wavelength on the concentration of PQ-molecules is confirmed by the numerical calculation including a 3D-model of the light self-trapping.