Table of contents

The formation of two-dimensional nonspreading atomic wave packets produced in the interaction of a beam of two-level atoms with two standing light waves polarised in the same plane is considered. The mechanism providing a dispersionless particle dynamics is the balance of two processes: a rapid decay of the atomic wave function away from the field nodes due to spontaneous transitions to nonresonance states and the quantum broadening of the wave packets formed in the close vicinity of field nodes. Coordinate-dependent amplitudes and phases of the two-dimensional wave packets were found for the j_g=0 ↔ j_e=1 transition.

The possibilities of creation and manipulation of subradiant states in an extended atomic system by coherent 2π pulses are analysed. It is shown that excitation of the atomic system to collective subradiant states eliminates the superradiant broadening of the resonance line in quantum optical memory devices. The scheme of a nonlinear sign-shift two-qubit gate is proposed, which can be used in optical quantum computers.

The effective Hamiltonian of a system describing electrons of two quantum dots in a two-frequency electromagnetic field upon Raman resonance of the field with a pair of the electronic levels of different dots is obtained by the method of equivalent transformation. The states of the continuous spectrum (conduction bands) are correctly taken into account. The role of the terms describing the dynamic Stark effect in a two-body system is analysed. A new mechanism of particle transfer between quantum wells in a non-resonance electromagnetic field is proposed, which is free from disadvantages of previous models.

The algebraic solution of a 'complex' problem of synthesis of phase-coded (PC) sequences with the zero level of side lobes of the cyclic autocorrelation function (ACF) is proposed. It is shown that the solution of the synthesis problem is connected with the existence of difference sets for a given code dimension. The problem of estimating the number of possible code combinations for a given code dimension is solved. It is pointed out that the problem of synthesis of PC sequences is related to the fundamental problems of discrete mathematics and, first of all, to a number of combinatorial problems, which can be solved, as the number factorisation problem, by algebraic methods by using the theory of Galois fields and groups.

Coherent backscattering of light by optically dense atomic ensembles is considered. The spectrum of scattered radiation is studied. The dynamics of the total intensity and enhancement factor is considered in the case of scattering of pulsed radiation. The spectral and temporal characteristics of coherent backscattering are analysed depending on the observation conditions. As an example, calculations are performed for an ensemble of ⁸⁵Rb atoms in a magnetooptical trap. It is shown that analysis of the correlation and dynamic properties of scattered radiation makes it possible to separate contributions from different orders of scattering and thereby to study the process of radiation trapping in dense media in more detail.

Superconducting single-photon ultrathin NbN film detectors are studied. The development of manufacturing technology of detectors and the reduction of their operating temperature down to 2 K resulted in a considerable increase in their quantum efficiency, which reached in the visible region (at 0.56 μm) 30%—40%, i.e., achieved the limit determined by the absorption coefficient of the film. The quantum efficiency exponentially decreases with increasing wavelength, being equal to ∼20% at 1.55 μm and ∼0.02% at 5 μm. For the dark count rate of ∼10^-4s^-1, the experimental equivalent noise power was 1.5×10^-20 W Hz^-1/2; it can be decreased in the future down to the record low value of 5×10^-21 W Hz^-1/2. The time resolution of the detector is 30 ps.

The propagation of few-cycle electromagnetic pulses through an anisotropic medium is considered. The effect of a constant dipole moment of the resonance transition on the formation of self-induced transparency pulses and their spectral composition is studied.

High-power lasing in a single spatial mode of a vertical-cavity surface-emitting semiconductor laser (VCSEL) can be achieved by controlling the mode composition by means of an external cavity. Among the important factors which affect the mode composition is the interference of the fields inside the VCSEL and the field which returns after reflection from the external mirror. An approximate theoretical analysis of the optical modes is performed for a VCSEL with an external mirror. The mode frequencies and the degree of longitudinal mode discrimination are determined in a plane wave approximation. A condition for the existence of a single longitudinal mode is obtained. A two-dimensional equation for the effective refractive index is derived in the explicit form, which describes the transverse field distributions. Explicit expressions are obtained for the fields of transverse modes in a VCSEL with a plane external mirror and a parabolic profile of the gain.

The effect of harmonic modulation of the coupling coefficients of counterpropagating waves on the type of generation of a solid-state Nd:YAG laser is studied. It is shown that, as the modulation depth is increased, transient generation regimes appear in the laser, which are accompanied by considerable frequency nonreciprocity. The boundaries of the regions of existence of different lasing regimes are found.

The dispersion and waveguide characteristics of two-layer microstructure fibres with different layer filling factors are studied theoretically and experimentally. It is shown that by changing the filling factor of the second layer, it is possible to achieve lower effective cross sections for a mode and to control dispersion characteristics in a broader range. Frequency conversion during four-wave mixing with a frequency shift of 5460 cm^-1 (1098–687 nm) and efficiency of 0.1% at 200 mW of cw pump power was obtained for the first time in two-layer fibres fabricated for experiments.

It is shown that all the spectral—temporal and temperature anomalies of a nonlinear response of high-temperature superconductors (HTSCs) observed in the pump—probe spectroscopy at high excitation levels can be interpreted taking into account contributions of interband electronic transitions to the dielectric constant of an excited HTSC film with a 'frozen' energy gap in the electronic spectrum. A model is constructed which explains all these 'anomalies' by a drastic decrease in the rates of three-particle nonradiative recombination of excess carriers under strong degeneracy conditions. It is found that this situation takes place due to the specific distribution of the density of electronic states in a HTSC almost immediately after the 'opening' of the energy gap in the electronic spectrum.

It is shown that the spectral—temporal and temperature 'anomalies' of the kinetics of reflection and transmission coefficients of high-temperatures superconductors (HTSCs) at high 'impact' excitation levels have direct analogies also in the case when the instant state of samples is probed by using the self-diffraction of probe pulses made coincident in time and propagating at an angle to each other. The possibility of using the transient modification of the method of degenerate four-photon spectroscopy for the HTSC diagnostics substantially expands the scope of experiments that can confirm or refute the correctness of the model based on the consideration of the contribution of interband electronic transitions to the response of an excited HTSC film with the 'frozen' energy gap.

A new method is proposed to control the shape of spectral characteristics of Bragg gratings, which is based on the introduction of electrically controlled shifts of the average refractive index. The shape of the spectral characteristics of Bragg gratings with a complex step structure of the spatial distribution of the average refractive index is calculated. The operative electric control of their shape in a channel optical LiNbO₃ crystal waveguide is experimentally demonstrated.

Optical properties of a composite medium comprised of metallic nanospheres located at the sites of a three-dimensional lattice and embedded into a dielectric matrix are considered. It is shown that for a certain ratio of the dielectric constants, the refractive index of the composite medium may be equal to unity. A method taking into account the delay effects during interaction of nanospheres with radiation has been developed to describe the optical properties of the composite medium. It is shown that in the limit when the distances between nanospheres are negligibly small compared to the wavelength, the results coincide with the results of the Maxwell—Garnett theory.

Luminescence of crystals of natural spodumene and natural diamond of the type IIa is studied upon excitation by a laser at a wavelength of 222 nm and by a subnanosecond avalanche electron beam (SAEB) formed in air at the atmospheric pressure. The photoluminescence spectra of spodumene and diamond are shown to exhibit additional bands, which are absent upon SAEB excitation. It is demonstrated that SAEB excitation allows one to analyse various crystals under normal conditions without using any vacuum equipment.

The transient double resonance in the Λ scheme is numerically investigated in the case of a strong inhomogeneous spectral broadening of a medium and nonzero detunings of the frequencies of interacting pulses from the central frequencies of corresponding quantum transitions. It is shown that the signal pulse efficiently interacts with the pump pulse only when a certain condition imposed on the nonzero detunings is satisfied. If this condition is strongly violated at the input to the resonance medium, only a weak field of the rear edge of the input signal pulse mainly interacts with the pump field. In this case, two pulses with different frequencies are formed in the signal channel. This is explained by the specific interaction of radiation with Doppler-broadened quantum transitions.

The deactivation of the excited S₁ state of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H₂TMPyP4) in a complex with DNA is studied by the methods of kinetic absorption spectroscopy for different concentration ratios r of porphyrin and DNA base pairs. It is found that, when the DNA bound porphyrin is assumed to be in a monomer (non-aggregate) state (r=1:30), the S₁ state of porphyrin excited by high-power picosecond pulses is rapidly deactivated, which is manifested as an additional decay component with the decay time 200–250 ps. This effect is interpreted as singlet—singlet annihilation of excitation indicating the presence of sites on the DNA surface with a higher local concentration of porphyrin molecules. The decrease in the relative content of DNA in solution down to r=1:3 further accelerates the deactivation of the S₁ state of porphyrin and causes the hypsochromic shift of its absorption spectrum due to the formation of stack-like porphyrin aggregates along the DNA surface. It is assumed that the additional component with the decay time 44±7 ps observed in the decay curve of the S₁ state is caused by the annihilation of excitation in porphyrin aggregates.

Features of the formation of signals in wavefront sensors with the single-frequency light wave phase modulation and spatial separation of control channels are considered. Analysis is performed for sensors in which phase modulation is governed by a controlled element located in the pupil of the optical system of a sensor or in the focal plane of the objective of this system. Peculiarities of the signal formation for a tilted wavefront are considered separately for internal points of the exit pupil in the case of light wave phase modulation in the pupil. It is shown that a signal at the modulation frequency in these wavefront sensors for points located far from the pupil boundaries is determined by the wavefront curvature.

The results of experimental measurements of small surface oscillations using a femtosecond laser are presented. It is shown that an increase in the number of spectral components in the radiation leads to a decrease in the minimum detectable amplitude of surface oscillations.