Table of contents

Using the radiative transfer theory and Monte Carlo simulations, we analyse the effect of scattering in a medium and of the size of the detector pinhole on the formation of the fluorescent signal in standard two-photon fluorescence microscopy (TPFM) systems. The theoretical analysis is based on a small-angle diffusion approximation of the radiative transfer equation, adapted to calculate the propagation of focused infrared radiation in media similar to the biological tissues in their optical properties. The accuracy of the model is evaluated by comparing the calculated excitation intensity in a highly scattering medium with the results of Monte Carlo simulations. To simulate a tightly focused Gaussian beam by the Monte Carlo method, the so called 'ray-optics' approach that correctly takes into account the finite size and shape of the beam waist is applied. It is shown that in the combined confocal and two-photon scanning microscopy systems not equipped with an external 'nondescanned' detector, the scattering significantly affects both the nonlinear excitation efficiency in the medium and the fluorescence collection efficiency of the system. In such systems, the rate of the useful TPFM signal in-depth decay is 1.5 — 2 times higher than in systems equipped with a 'nondescanned' detector.

Diffuse optical tomography (DOT) makes it possible to reconstruct, in two or three dimensions, the internal structure of the biological tissues based on the distribution of the absorption coefficient and the reduced scattering coefficient, using optical measurements at multiple source — detector positions on the tissue surface. The measurement of the light intensity transmitted through the tissue can be also used to compute the haemoglobin and oxyhaemoglobin concentrations, measuring the selective absorption of the main blood chromophores by near infrared spectroscopy (NIRS). The spectral selectivity of the system and the evaluation of the blood volume and blood oxygenation (BV and OXY distributions), together with the reconstruction of the inner structure of the tissue, can improve the accuracy of early cancer diagnosis, based on the tissue angiogenesis characterisation.

A non-invasive device for measuring blood oxygen variations in human brain is designed, implemented, and tested for MRI compatibility. The device is based on principles of near-IR reflectometry; power LEDs serve as sources of probing radiation delivered to patient skin surface through optical fibres. Numerical Monte Carlo simulations of probing radiation propagation in a multilayer brain model are performed to evaluate signal levels at different source — detector separations at three operation wavelengths and an additional wavelength of 915 nm. It is shown that the device can be applied for brain activity studies using power LEDs operating at 830 and 915 nm, while employment of wavelength of 660 nm requires an increased probing power. Employment of the wavelength of 592 nm in the current configuration is unreasonable.

We consider the problem of the visibility of the diffraction pattern that is observed in scattering laser radiation on the erythrocyte suspension in ectacytometer. The theoretical estimates show that 10% variation in the particle size reduces the diffraction pattern visibility by 1% only.

Impact of cw laser radiation on autofluorescence features of human skin is studied. Two methods of autofluorescence detection are applied: the spectral method with the use of a fibreoptic probe and spectrometer for determining the autofluorescence recovery kinetics at a fixed skin area of ∼12 mm², and the multispectral visualisation method with the use of a multispectral imaging camera for visualising long-term autofluorescence changes in a skin area of ∼4 cm². The autofluorescence recovery kinetics after preliminary laser irradiation is determined. Skin autofluorescence images with visible long-term changes — 'signatures' of low power laser treatment are acquired.

Methods for solving analytically and numerically the problem of multiscale modelling of the laser hyperthermia processes in a medium with nanoparticles are developed with regard to composite spherical nanoparticles (nanoshells). The features of the laser radiation field localisation on nanoscale inhomogeneities are investigated. Issues related to the control of the tissue hyperthermia processes by choosing the parameters of spatiotemporal localisation of the laser beam and of the absorbing nanoparticles are discussed.

Adsorption of such blood plasma proteins as albumin and g-globulin on diamond nanoparticles of size around 5 nm and around 100 nm is observed and studied using laser-optical methods. The adsorption of blood plasma proteins at physiological pH 7.4 is found weaker than that of enzyme protein lysozyme. The observed variations in the Fourier Transform Infrared (FTIR) spectra of proteins may be due to structural transformations of the adsorbed protein. Using the lysozyme as a test protein we show that the protein adsorption leading to observable changes in the FTIR spectrum (the band of Amide I) also induces a significant decrease in the protein functional activity. It is also found that the influence of ∼5-nm diamond nanoparticles on the protein structure and functions is more significant than that of ∼100-nm nanodiamonds.

Nanoparticles of gadolinium borate polycrystals and borate glasses, activated by Nd3+ ions, are obtained from macroscopic samples of the corresponding compositions by mechanical grinding and ultrasonic dispersion in water. A spectroscopic study of these nanoparticles in the near-IR region is performed to determine their potential as luminescence biosensors and radiopharmaceutical preparations for cancer diagnostics by radiosensitive methods. A twofold increase in the lifetime of the metastable ⁴F_3/2 state of Nd³⁺ ions at the transition from submicron polycrystalline particles to nanoparticles is experimentally found. A study of the nanoparticle distribution over organs and tissues of laboratory animals, performed with a 810-nm laser for exciting luminescence and a multichannel fibre spectrometer for measuring fluorescence in the range of 0.8 — 1.2 mm, showed this technique to be sufficiently sensitive to reliably determine the nanoparticle concentration in biological tissues and the dynamics of its change.

Four-wave mixing (FWM) spectroscopy is used to detect the rotational resonances of H₂O and H₂O₂ molecules in DNA and denatured DNA aqueous solutions in the range ±10 cm^-1 with a spectral resolution of 3 GHz. It is found that the resonance contribution of the rotational transitions of these molecules increases significantly in solutions rather than in distilled water. This fact is interpreted as a manifestation of specific properties of a hydration layer at DNA—water and denatured DNA—water interfaces. Analysis of the FWM spectra shows that the concentration of H₂O₂ molecules in the hydration layer of the DNA solution increases by a factor of 3 after denaturation. The FWM spectra of aqueous solutions of α-chymotrypsin protein are obtained in the range ±7cm^-1 at the protein concentrations between 0 and 20 mg cm^-3. It is found that the hypersound velocity in the protein aqueous solution, measured by the shift of Brillouin components in the scattering spectrum, obeys a cubic dependence on the protein concentration and reaches a value of about 3000 m s^-1 at 20 mg cm^-3.

An optical fibre with a core based on thulium-doped germanate glass (45SiO₂ — 55GeO₂) and a quartz glass cladding is developed for the first time. Lasing on Tm³⁺ ions (λ =1.862 μm) with an output power up to 70 mW at a differential efficiency of 37% is obtained in a laser based on this fibre.

It is established for the first time that the intensities of deformation and valence Raman scattering (RS) bands in water vary oppositely in the processes of degassing and crystallising. Water degassing shifts the centre of a valence OH band to higher frequencies by approximately 2 cm^-1, which points to a reduced contribution of structural complexes to the RS spectrum.

The output characteristics of a laser based on polycrystalline Cr²⁺:ZnSe obtained by diffusion annealing are studied upon pumping by a cw-diode-pumped Q-switched Tm:YLF laser operating at a wavelength of 1.908 μm with a pulse repetition rate of 2 kHz. Efficient pulse-periodic lasing is obtained in the region of 2.32 μm with a total efficiency of ∼17%, a pulse duration of ∼45 ns, and an average power of ∼1 W.

The lasing properties of polymer matrices based on commercial polyurethane compound activated by sulforhodamine 101 dye are studied. Lasing with an efficiency of 26 % and pulse energy of 76 mJ is obtained using microsecond transverse pumping at a wavelength of 587 nm. The service life (time of operation to a decrease in the output energy by 50 % upon excitation by 0.3 J cm^-2 pulses) amounts to 2500 pulses. A particular attention is given to the bichromatic lasing spectra of the samples tested. Based on the experimental data a model explaining the two-band emission spectrum is proposed and discussed.

Laser on a self-contained transition of a helium atom is studied under excitation of the helium mixture with molecular gases by single long-duration (up to 700ns) or double nanosecond pulses. In He — H₂O and He — NH₃ mixtures, no limitations were found on the pulse repetition rate and the laser pulse duration obtained was equal to that of the pump pulse.

A model is developed and numerical calculations are performed for the regenerative amplification of seed picosecond 10-μm radiation pulses in a high-pressure optically pumped CO₂ laser. It is established that by varying the cavity parameters, the conditions of efficient regenerative amplification may be optimised for the case of a relatively short-duration (50 ns) solid-state Cr:Er:YSGG laser and 'long-duration' (250 ns) electrodischarge chemical HF laser. It is shown that the schemes of a CO₂ amplifier with optical pumping presented allow obtaining the pulses with the duration of 3ps, output energy of 1–5 mJ, and peak power of ∼1 GW at the pump energy of 150–200 mJ.

A laser system is designed for operation with a photocathode electron gun for a linear accelerator with the following parameters of radiation at a wavelength of 262 nm (the fourth harmonic of a Nd:YLF laser). The pulse trains (macropulses) with a repetition rate of 5 Hz and a duration of 900 μs consist of 8-ps micropulses with an energy of 1.4 μJ and a repetition rate of 2.708 MHz. This repetition rate is variable within ±32 kHz and is stabilised by an external signal with an accuracy of 10 Hz. Due to the use of a feedback-controlled acousto-optic modulator, the root-mean-square deviation of the micropulse energy in the first and second harmonics is 2.5% and 3.6%, respectively. Using the decaying branch of the dependence of the second-to-fourth harmonic conversion efficiency on the second harmonic intensity, we decreased the root-mean-square deviation of the energy of the fourth-harmonic micropulses to 2.3% at the first-to-fourth harmonic conversion efficiency of 27%.

The theory of three-wave SRS is developed, which takes into account nonlinear dispersion of a medium for arbitrary phases of the pump waves at the input to the medium. The effect of interference suppression of SRS is predicted for values of the total phase of the three-wave pump (2n+1)π (n=0, ±1, ±2...), the effect being caused by the destructive interference of polarisations of the nonresonant dipole-allowed transitions. The relation between the contributions of the linear and nonlinear dispersions to the SRS is found. It is shown that at a sufficiently large wave detuning, the anti-Stokes wave amplitude experiences spatial oscillations.

A photonic bandgap fibre with an ytterbium-doped silica glass core is fabricated and investigated. The possibility of implementing single-mode operation of such fibres in a wide spectral range at a large (above 20 μm) mode field diameter makes them promising for fibre lasers and amplifiers. To ensure a high quality of the beam emerging from the fibre, particular attention is paid to increasing the optical homogeneity of the ytterbium-doped core glass.

The prospects of the long lateral shear interferometry for controlling the quality of diffractive elements are shown. In this case, use is made of two diffractive elements, one of which is under investigation, and the other is the model one. A universal device is proposed to control the quality of the diffractive elements of both transmission and reflection types. We present the results of experimental realisation of the technique for the quality control of production of two-dimensional amplitude masks and reflection diffraction gratings.

It is established for the first time by Raman scattering of second-harmonic pulses of an Nd:YAG laser that there are molecular complexes with frequencies of ∼3600, ∼3450, and ∼3200 cm^-1 (which are characteristic of dimers, tetramers, and ice-like hexamers observed in H₂O molecular beams) in water at temperatures of 13 — 99°C. The frequency of the ice hexameric component is found to shift only slightly with an increase in the water temperature, whereas its contribution decreases but remains significant, with a simultaneous increase in the contribution of small complexes.