Absorption and fluorescence in Rb sub-micron cell on D₂ line have been studied when the thickness L varies in the range of 0.5 λ - 3.5 λ, and the laser intensity is 0.1 - 1 W/cm² (λ = 780 nm, λ_L ~ 5 MHz). When L = (2n +1) λ/2 (n-integer) and I ~ 0.1 W/cm², revival of peaks of Dicke-type coherent narrowing effect with some modification of the spectra (as compared with low intensity regime) are still observable. The experimental results obtained at I ~ 1 W/cm² for L = λ/2 show that there are no "dips" of a reduced absorption in the spectra, though there is a strong spectral broadening of the absorption (and fluorescence), as well as a reduction of the absorption (in contrast, I ~ 1 mW/cm² is sufficient to create the dips when L = λ). Probably, this is caused by a strong Dicke narrowing regime when L = λ/2. Several additional differences of fluorescence and absorption spectra behaviour at high intensities are revealed.

Nonlinear Doppler-free spectroscopy of atom gases in counterpropagating light waves is investigated. We consider a general case of elliptic polarizations and the energy level degeneracy. New results are obtained in comparison with cases of linear and circular wave polarizations. It is shown that nonlinear resonance versus the light wave detuning has asymmetry and shift for elliptic polarizations.

Advances in modeling lithium along with improved measurements have enabled precise tests of QED and allowed for the determination of nuclear radii. These are reviewed and prospects to improve the determination of the fine structure constant are discussed.

Theoretical model for the frequency-modulation spectroscopy of dark resonances is discussed in detail on example of a three-level quantum system in A-configuration driven by resonant laser field(s) with and without frequency modulation using two simulation techniques-the density matrix and quantum trajectories analysis.

Potoconductive properties and relaxation of free charges in high pure natural and CVD diamond were investigated and compared in the spectral range 1 .5-30 eV using a variety of pulsed laser sources including harmonics of radiation of 50 fs Ti:Sa laser. The resulting spectra were compared to experimental data in surface photoemission and speculated in terms of free carriers multiplication, effect of the surface layer, saturation of absorption, trapping of carriers and the following charge transfer between deep electron centers.

Cs* He_n exciplexes are produced by laser excitation of cesium atoms embedded in solid helium arid can be identified by their characteristic fluorescence spectra. Based on calculations of binding energies and emission spectra the fluorescence spectra observed in our experiments are assigned to the decay of specific Cs*He_n exciplexes: an apple-shaped Cs(AΠ_3/2)He₂ and Cs(AΠ_1/2)He₂ and a dumbbell-shaped Cs(AΠ_1/2)He₇ exciplex. Our observations contradict the commonly accepted belief that Cs*He_n dumbbell-shaped exciplexes with n > 2 do not exist. The contradiction arises probably from the higher He density in the pressurized crystal compared to gaseous or liquid helium environments.

A rather simple method is discussed to study at once optical absorption spectra, excited-states nonradiative relaxation and propagation of the nonequilibrium phonons emitted during this process in doped dielectrics. The technique utilizes the time-resolved superconductive bolometer detection of these phonons at temperature T=2 K and optical excitation with a pulsed tunable dye laser. Observed, at fixed delay time after the laser pulse action, excitation spectra of the phonons well coincide with already known absorption spectra of both single doped ions and pairs of the nearest neighbors. In some crystals both the satellite lines and uncontrollable impurities lines (Nd³⁺ in system LaF₃:Pr₃₊) are well resolved in the phonon excitation spectra. This talks about high sensitivity of the used method. Time evolution of the phonon excitation spectra contains in itself the information on energy transfer mechanisms and has allowed to reveal the new types of doped ion cites (Cr³⁺ in ruby). The study of a time dependence of bolometer signal versus the phonon propagation distance at fixed laser wave length has allowed to determine nonradiative relaxation time, a phonon propagation type and parameters of the phonon scattering rates.

We analyze perspectives for creation of a novel, stable ⁸⁷Rb-based frequency standard. Although the stability ~10^-14 is challenging, achieving this goal looks ultimately possible. We considers three different all-optical repumping schemes aimed to improve the contrast of the dark resonance in thermal ⁸⁷Rb vapor: excitation by a σ⁺ -polarized bichromatic laser radiation, and σ⁺ - σ^- bithromatic co-propagating fields, and a novel excitation scheme with transversal laser pumping. Qualitative considerations are confirmed by numerical calculations.

A new method for frequency stabilizing of an external cavity diode laser without the use of direct injection current modulation is presented. A Pound-Drever FM sideband technique with AOM operating in a single pass Raman-Nath diffraction mode as an external phase modulator and atomic resonance as the frequency reference are used to produce an error signal for frequency locking of the laser diode to the resonances within caesium D₂ spectral line.

In the present work we consider theoretically the influence of an arbitrary oriented additional magnetic field on the main properties of the Coherent Population Trapping (CPT) resonances in fluorescence and/or in absorption. The exact numerical solutions are obtained for the tensor components Ρ^k₀ (ρ = f, φ) which describe the population and the longitudinal alignment of the resonant (f) levels with quantum numbers F_f 0 and F_f =1, exited with a single frequency laser field from the ground (φ) state with F_φ=l and F_φ=2. Numerical simulations was used to obtain the width and the amplitude dependences of the resonances under different parameters and parasitic fields. The results are in qualitative agreement with resent experimental investigations.

The results of the experimental and theoretical study on associative ionization of laser excited Na Rydberg atoms in collisions with ground-state atoms and on thermal ionization by blackbody radiation in single and crossed effusive atomic beams are reported and discussed.

The results of theoretical investigations of the off-axis Z-scan technique for the measurement of nonlinear refraction in materials are presented. The normalized transmittances are calculated for different aperture radii and positions. The dependence of both the normalized transmittance amplitude (T_pv) and the distance between maximum and minimum (Z_pv) on the aperture radius is analyzed.

Polarization dependence of amplitude for resonant magnetic-field-induced coherent mixing of two collinear laser waves may be used for separating M1 and E2 contributions to n₁ ²P_1/2(3/2) - n₂ ²P_3/2(1/2) dipole-forbidden transition in atoms. The interference between E2 and Ml terms of the amplitude may cause linear polarization and circular dichroism effects. The dependencies of the two effects on the width and splitting of resonance levels, on resonance detuning, on relative directions of steady and alternating field vectors are determined. Numerical data are presented for frequency summation of two laser waves on Cs, Rb and K atoms.

A static electric field may induce essential changes to the cross section for the magnetic-field-induced coherent frecuency mixing of two laser waves, modifying the cross-section dependence on the magnetic field and polarization vectors of incident waves, which depends on the ratio of M1/E2 contributions to n₁ ²P_{1/2 (3/2)} - n₂ ²P_3/2(1/2) radiation transition amplitude. Relations between the M1/E2 ratio and quantitative characteristics of polarization asymmetry in cross section are determined. Calculated numerical data are presented for coherent mixing of two laser waves on Cs, Rb and K atoms.

The values of tensor components of nonlinear susceptibilities responsible for self-action effects have been measured in quadratic nonlinear-optical crystals at the Nd:YAG laser wavelength. The angular dependences of the nonlinear susceptibilities have been determined on the base of measured relations between tensor components. The variation of the high-order nonlinear susceptibilities at various spectral ranges has been discussed. The measurements have been conducted on the basis of the analysis of polarization dependences of focused radiation transmittance in the Z-scan scheme.

The paper discusses and compares the concepts, performance potential, and most recent experimental results of both classical and novel active two-terminal for low-noise RF power generation at submillimeter-wave frequencies up to 1 THz. These devices use transit-time, transferred-electron, and quantum-mechanical effects (or a combination of them) to create a negative differential resistance at the frequency of interest. Examples of state-of-the-art results are output power levels of more than 3.5 mW at 300 GHz and more than 1.6 MW at 327 GHz from an InP Gunn device as well as more than 140 μW at 355 GHz from a GaAs tunnel-injection transit-time diode.

A theoretical model of a waveguide free-electron laser is developed which includes the mismatches of group and cavity synchronism conditions as well as the waveguide dispersion. Under the assumption of a high quality resonator, a parabolic equation for the evolution of the profile of electromagnetic pulse is derived. The condition of self-excitation and the parameters of stationary generation are found. It is shown that the waveguide dispersion makes it possible the operation of free-electron laser not only under positive but also under negative mismatch between the period of electron bunch injection and the period of electromagnetic pulse round trip. The transient and nonlinear stages of the free-electron laser operation are analyzed, and the optimal mismatches of group and cavity synchronism conditions are found, ensuring, in particular, utmost efficient generation of the terahertz pulses.

A concept of intracavity nonlinear wave mixing using modes generated in semiconductor injection lasers as an intracavity optical pump for the mixing is developed. This approach utilizes very high nonlinear susceptibilities of III-v semiconductors that cannot be used as a passive externally pumped crystal because of the strong absorption of the optical pump and the absence of a convenient phase-matching scheme. In particular, it opens prospects for mastering the terahertz frequency range on the basis of a standard heterolaser technology. We review recent theoretical and experimental investigations in the field and discuss applications.

Butt-joint diode lasers for the efficient nonlinear optical mixing in semiconductors are suggested and successfully implemented. First experimental demonstration of the sum-frequency and second-harmonic continuous-wave room-temperature generation in the InGaAs/GaAs/InGaP butt-joint diode lasers in the edge-emitting geometry is reported. Specific features of the butt-joint device, where two butt-joined diode lasers are optically coupled but injection pumped separately, are investigated and the prospects of the difference-frequency (far-infrared) generation are outlined.

The method for calculation of of in THz range is developed. Dependencies of χ⁽³⁾ in semiconductors AIIIBV are calculated for different frequencies, temperatures and concentrations of charge carriers.

Propagation of terahertz waves in segmented waveguides (1 -D photonic crystal) is experimentally demonstrated. Data are well fitted using a modified FDTD method. Dispersion curves of the first guided modes are determined.

We report on optical rectification at bulk and thin film noble metal (Ag and Au) surfaces investigated by means of terahertz time-domain spectroscopy. The experimental data are well modelled using a nonlinear surface susceptibility tensor. The main contribution comes from the charges moving along the surface normal. For gold films, the terahertz signal decreases dramatically for thicknesses below 150 nm. We tentatively explain this phenomenon by screening of the nonlinear polarization by the field induced by charges that have moved across the layer.

We describe the investigations on terahertz radiation emission from surfaces of several semiconductors illuminated by femtosecond Ti:sapphire laser pulses. This radiation is caused by numerous physical mechanisms, therefore in-deep analysis of its causes can give valuable information about the properties of the material. It is demonstrated that, e.g., in the case of InAs important contribution to the terahertz emission comes from the nonlinear-optical rectification effect induced by the electric field in the surface inversion layer, whereas in CdHgTe alloys this emission is solely caused by the photo-Dember effect. Femtosecond laser pulse absorption in Ge is accompanied by intense electron redistribution between different conduction band valleys. Efficient terahertz emission was, for the first time, observed also from femtosecond laser illuminated thin layers of CuInSe₂.

We report on the studies of kinetics of broad-band generation via optical rectification into terahertz domain using a sample of LiNbO₃ partially filling a hollow metallic waveguide to increase the coherence length, enabling the observation of transition between phase-mismatched and phase-matched mechanisms of terahertz frequency generation.

We present a review on terahertz diagnostics in dentistry. Time domain and spectral domain imaging studies are compared. Imaging built on mapping time-domain dependencies delivers the information on mechanisms of light transport in tissue: ballistic, quasi-ballistic and diffusive photon transfer. Imaging utilizing transform in frequency-domain provides a wealth of data on specific characterization of object: spectral and chemical content, tissue structure and composition and various kind of tissue abnormalities.

The possibility of improving the spatial resolution of diffuse optical tomograms reconstructed by the photon average trajectories (PAT) method is substantiated. The PAT method is based on a concept of an average statistical trajectory for transfer of light energy, the photon average trajectory (PAT). The inverse problem of diffuse optical tomography is reduced to a solution of an integral equation with integration along a conditional PAT. As a result, the standard filtered back projection algorithm can be used for fast reconstruction of diffuse optical tomograms. The shortcoming of the PAT method is that it reconstructs the images blurred due to averaging over spatial distributions of photons, which form the signal measured by the receiver. To improve the resolution, I apply a spatially variant blur model based on an interpolation of the spatially invariant point spread functions simulated for the different small sub-regions of the image domain. Two iterative algorithms for solving a system of linear algebraic equations: the conjugate gradient algorithm for least squares problem and the modified residual norm steepest descent algorithm are used for deblurring. It is shown that the restoration procedure enhances the tomogram profiles and allows an obvious gain in spatial resolution to be obtained.

Optoacoustic (OA) imaging is based on the generation of thermoelastic stress waves by heating an object in an optically heterogeneous medium with a short laser pulse. The stress waves contain information on the distribution of structures with enhanced optical absorption that can be used for early cancer diagnostics. This technique has already been applied in-vivo for breast cancer diagnostics and yielded higher contrast of obtained images than that of X-ray or ultrasonic images. The resolution was comparable with that yielded by ultrasonic imaging. Therefore, OA imaging is a very promising technique and it is being rapidly developed. Research in the area is now mostly targeted to the development of OA wave detection systems and image reconstruction algorithms. In this work a new design of receiving array transducer, that allows to enhance image resolution is proposed. The array consists of 64 focused piezo-elements made of PVDF slabs imposed on a spherical surface. Resolution yielded by the array in different directions is determined. Several tissue irradiation geometries and laser wavelengths are considered for optimization of the OA image contrast. Obtained results are used for maximum imaging depth studies. All the investigations include both numerical modelling and experiment.

Supercontinuum light source generated from a photonic crystal fiber has been applied to coherent anti-Stokes Raman scattering (CARS) microscopy. Owing to an ultrabroadband spectral profile of the supercontinuum, multiplex CARS detection has been performed in the range from 825 to 3650 cm^-1. We have successfully obtained CARS spectra and images of a multi-lamellar vesicle and a living yeast cell.

A kinetic model of the photodynamic fullerene-oxygen action on biological tissues is developed. The efficiency of generation of singlet oxygen is studied in relation to the intensity of exciting radiation and the concentrations of fullerene and oxygen. The spectral efficiencies of singlet oxygen generation upon irradiation of a biological tissue by a lamp and different lasers are studied and compared with each other. It is shown that the singlet oxygen generation effectiveness increases with exciting wavelength decreasing. The spatial distributions of singlet oxygen in allantoic fluid is studied. The oxidation of lipids of external membranes is assumed to be the basic mechanism of cell damage. The characteristic time of this oxidation is calculated. The sensitivity of Semliki Forest Virus to photodynamic action is estimated. It's value can be used in calculations of the dose curves for this virus culture in other illumination conditions and for other fullerene and oxygene concentrations. The effectiveness of singlet oxygen generation using pulsed excitation is compared with the one using CW.

In recent years, there has been growing interest in the common use of laser-induced autofluorescence (LIAF) and reflectance spectroscopy (RS) to differentiate disease from normal surrounding tissue - so called optical biopsy method. The goals of this work were investigation of cutaneous benign and malignant lesions by the methods of LIAFS and RS. Spectra from healthy skin areas near to the lesion were detected to be used posterion to reveal changes between healthy and lesion skin spectra. Before spectral measurements lesions were classified clinically and dermatoscopically by specialist (P.T.). After procedures all lesions were excised and were investigated by standard histological methods. The set-up consists of a nitrogen laser (337 nm, 14 μJ, 10 Hz), halogen lamp (400-900 nm, 50 W), lenses, filters, optical fibers, and a microspectrometer (PC2000, 'Ocean Optics", USA). A computer controls these systems. The results, obtained were used to develop multispectral diagnostic algorithms based on the most prominent spectral features from the fluorescence and reflectance spectra of the lesions investigated. In comparison between normal skin and different cutaneous lesion types and between lesion types themselves sensitivities and specificities higher than 90 % were achieved in all cases. These results show a perspective possibility to develop an optical biopsy system for diagnosis and differentiation of the benign and malignant non-melanin pigmented cutaneous lesions with wide clinical applications. Non-invasive and high-sensitive in vivo detection by means of appropriate light sources and detectors should be possible, related to real-time determination of existing pathological conditions.

To identify the molecular photoacceptors and the mechanisms of primary photoreactions the relationships were discussed between the changes both in visible and infrared spectra and in the molecular structure of blood and blood components after intravenous laser irradiation.

We have investigated structural changes of human erythrocyte membranes under the action of interference light fields with different periods of spatial modulation. A local polarity in the surface regions of membranes was detected by means of the fluorescent probe method using stationary and time-resolved fluorescence spectroscopy. The electro-dipolar molecules 4'-diethylamino (FET) and 4'-N-(15-azacrown-5) (FCR) derivatives of 3-hydroxyflavone with coupled intramolecular charge and proton transfers were used as fluorescent probes. The FET molecules locate deeper in the membrane than FCR probes. It was established that the micropolarity of probes binding sites in membranes is changed after irradiation by the interference laser field. The largest effect was observed when erythrocytes were irradiated by the field with 7.2 μm period, which corresponds to maximal energy of interaction of an erythrocyte with a gradient light field.

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.

Glucose sensing is of great importance nowadays. In this paper, we apply the Monte Carlo technique to simulate the effect of glucose on pulse propagation within a 4% Intralipid skin phantom. This effect is based on refractive index matching of Intralipid vesicles relative to surrounding liquid (water) if glucose is added. Time-of-flight profiles of femtosecond pulses (906 nm) passed through a 2-mm thick plain glass cuvette are registered by two detectors 0.3 mm in diameter with numerical apertures of 0.19 and 0.29. It is found that glucose levels within the physiological range (100-500 mg/dl) and 1000 mg/dl can be detected due to the effect of glucose on peak location and intensity, and width of the registered pulses.

Light scattering and absorption in the visible and near-infrared ranges were studied in films of donor-acceptor blends of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with 2,4,7-trinitrofluorenone (TNF) or 1,5-dinitroantraquinone (DNAQ). Charge-transfer interaction in the electronic ground state of MEH-PPV/TNF and MEHPPV/DNAQ blends results in optical absorption in the bandgap of MEH-PPV. At the same time, because of donor-acceptor phase separation, light scattering can grow with increasing the acceptor content in the blends contributing to their optical transmission. To evaluate the amount of absorbed and scattered light separately, we have elaborated a simple method in which the optical transmission of the films is measured at several laser wavelengths using a wide aperture photodetector with or without a dump blocking the straight laser beam. Making some assumptions about the scattering indicatrix, the scattering and absorption coefficients of the blends can be evaluated. We have found that the amount of scattered light in MEH-PPV/TNF blends does not depend smoothly on the acceptor content. At the molar acceptor concentration MEH-PPV:TNF below 1:0.33, the light scattering is low and depends on the TNF content only weakly, whereas at higher ratios, the scattered light begins to grow dramatically. We assign this growth to phase separation in the blend owing to excess acceptor molecules that are not involved in charge-transfer interaction with the donor. In MEH-PPV/DNAQ blends, the phase separated DNAQ forms large species, which weakly contribute to the observed light scattering.