In order to produce ultrashort far-infrared (FIR) laser pulses we have investigated the superradiance and Raman emission of various molecular transitions optically pumped with the pulses of 10 micrometers -hybrid- or high-pressure single-mode-CO₂ lasers truncated within 10 ps. Furthermore we have truncated directly the FIR laser pulses with the first FIR plasma shutter. In our experiments we have observed new phenomena which include e.g. new interrelations between superradiance, swept-gain superradiance and Raman emission, anticorrelated fluctuations of pump radiation versus FIR emission, first optical free induction decay of 10 micrometers -CO₂-laser pulses with FIR emitting molecular gases. Finally, we have proved the first spontaneous infrared solitons in stimulated far-infrared Raman scattering on NH₃. We have observed solitons in the depletion region of the infrared pump radiation with corresponding dark solitons in the Stokes pulses and detected the laser phase change connected with the soliton generation.

The intensity of radiation stimulated by laser pulses of arbitrary shape is derived up to the fourth order terms. It is shown that there is a linear fourth order mechanism of emission that depends on linear susceptibilities of molecules. This linear mechanism reveals collective, superradiant features. The intensity of laser-driven spontaneous emission (LDSE) is proportional to N² for samples smaller than (lambda) ³, where (lambda) is a wavelength of emitted radiation, and N is the number of molecules. For extended volumes >> (lambda) ³, the intensity of LDSE is proportional to N^4/3. Equations of motion describing collective radiation damping are derived.

A novel regime of collective spontaneous emission is an extended sample of excited molecules with quasi-equidistant energy spectrum is investigated. The limiting parameters of the ultrashort pulse and the optimal density and length of the sample are evaluated.

The self-consistent solution to the problem of collective spontaneous emission of an active medium in any-Q resonator, taking into account the nonlinear coupling of counter-propagating wave, is found. The optical parameters of resonator are evaluated.

The simulation of superfluorescence of a thin layer of two-level atoms was performed on the basis of semiclassical approach without using approximation of slowly varying in space amplitudes of electric field and medium polarization (paraxial approximation). The evolution of atomic characteristics and the structure of the radiation directivity pattern inside the total solid angle have been investigated. The comparison of the results obtained with the results based on the paraxial approximation has been done.

Phenomenon of nonlinear interference as a result of no less than two monochromatic light wave with far difference of optical frequencies is discussed by means of the function of correlation for the electrical field amplitude. The extension of the nonlinear interference theory on nonmonochromatic light waves for multiphoton resonances is represented.

Interference effects in quantum transitions, giving rise to amplification without inversion, optical transparency and to enhancements in nonlinear optical frequency conversions are considered. Review of the relevant early theoretical and experimental results is given. The role of relaxation processes, spontaneous cascade of polarizations, local field effects, Doppler- broadening, as well as specific features of the interference in the spectral continuum are discussed.

Quantum beats in transient absorption of PIC-J-aggregates, modelled by a modified 3-level- system are calculated. There is considered the influence of the temperature dependent phase relaxation on the temporal evolution of quantum beat signals.

Novel experimental results of anomalous luminescence and optical bleaching of large molecules concentrated in solution, influenced by intense laser pumping, are presented. The theory based on resonance 4-photon interaction with two zero vacuum oscillation is developed.

The free induction decay (FID) after saturation by a laser radiation pulse of finite duration is studied for systems with spectral diffusion. The exact solutions of the FID signal shape have been obtained in the framework of telegraph noise model as well as model of noncorrelated spectral exchange. These solutions take into account the finite duration of saturating field and are valid at arbitrary value of the spectral exchange rate and the amplitude of the coherent field. A selfconsistent explanation of the field dependence of the FID rate found by Szabo and Muramoto is obtained under the slow noncorrelated spectral diffusion.

Novel coherent transient effects of population locking in driven three-level media are demonstrated theoretically. With the help of exact solutions of the evolution equation with the time-dependent Hamiltonian we `design' laser and rf fields to lock population under the conditions of nonstationary two- and tri-photon resonances.

The problem of stationary-gained modes formation under conditions of transient double resonance inspired by 2(pi) -pulse of pump and low-intensity signal radiation is investigated. One demonstrates the possibility of different shapes stationary-gained signal pulse formation and investigates the conditions of their arising and stability. The effect of inhomogeneous broadening of spectral transition lines on the mode formation is studied too.

The method of analysis of supermodes composition in laser with short-term periodic modulation of losses--nonstationary supermodes--has been suggested and investigated. This method is based on the property of invariance supermode to Fourier-transformation. In experiment this is displayed by a regular modulation of integral spectrum of generation such laser in case, when individual supermode is generated. Realization of method makes possible to obtain detailed data about dynamics of generation laser and also to solve problems of control the parameters of characteristic for a given laser high-energy ultrashort pulses.

The results of the theoretical study and computer simulations on the ultrashort pulse propagation in a medium of the two-level atoms are discussed. We find the solution of the nontruncated Maxwell-Bloch equations corresponding to the selfsimilar pulse propagation. It is shown that these pulses are solitons. We discuss the dependency of the solitary pulse parameters on the incident pulse amplitude, frequency and temporal width.

The propagation of a gap 2(pi) -pulse of self-induced transparency within the forbidden frequency gap band of a resonant Bragg structure with an inhomogeneously broadened line is described theoretically as an exact solution of coupled-mode Maxwell-Bloch equations. It is shown that the inelastic collision of two zoomeron-like pulse leads to creation of a stable breather pulse.

When Rabi frequency is greatly less than frequency detuning from rigorous one- and two- photon resonance the evolution equations describing an ultra-short pulse propagation in a fiber containing resonant impurities were obtained. The influence of impurities on optical soliton dynamics investigated under these conditions.

Principles for optical digital data processing based on photon echo phenomenon in resonant media are considered. The various schemes of optical processors realization with the pixel structure and of holographic type based on using the digital multiplication by the analog convolution algorithm is suggested. Methods of optical data flow control of digital informational on the basis of photon echo phenomenon are discussed. Interconnections' switching schemes in a sequential in time and parallel in space codes are suggested.

A strong decreasing of the intensity of the inverse 2-pulse, 3-pulse and long lived 3-pulse photon echo was observed when a current pulse passed through the strip evaporated onto the surface of the sample (current pulse was applied before or after the laser pulses). The strip is heated by the current pulse and there is the voltage difference at the ends of the strip. The voltage difference creates instantly the nonuniform electric field in the region of optical excitation so the echo intensity is decreased by the short-lived nonuniform line width rise. The line width rise is differentiated for the distinct type ion sites in the LaF₃ unit cell. The strip current-heating gives rise to a phonon pulse. When phonon pulse reaches the optically excited volume it causes the shift of energy levels of Pr³⁺ ions, effecting a phase accumulation and the echo intensity change as a consequence.

The possible applications of polarization and temporal peculiarities of long-lived stimulated photon echo in LaF₃:Pr³⁺ were analyzed. The influence of the excited pulses polarization on time-profile of LSPE was considered. It's discussed the possibility of using time-polarization processing of information in systems of optical memory and optical echo- processors.

We are predicted a new optical phenomenon a photon echo with a double frequency in the system of two-level atoms, when the excitation of the system is realized by two laser pulses with a primary frequency. On the basis of this phenomenon theory we consider conditions of its observation.

The time evolution of non-equilibrium polariton condensate has been studied in the framework of microscopic approach. It has been shown that due to polariton-polariton scattering a significant condensate depletion takes place in a comparatively short time interval. The condensate decay occurs in the form of multiple echo signals. Distribution function dynamics of non-condensate polaritons has been investigated. It has been shown that there are stochastic ripples on the smooth curve of the distribution function.

Femtosecond photon echo decay due to ultrafast optical dephasing in condensed molecular systems can be related to an impurity optical band shape. A relation is found and discussed.

The occurrence of optical echo phenomena in two-level media is analyzed beyond the mean field approximation. The polarization of the medium and the intensity of the echo signal depend in a complex way on both the areas of the pulses and on the area of the `material' field and the relation between (on the one hand) the photon densities of each pulse and (on the other) the density of two-level atoms (excitons) of the medium.

We review the recent achievements both in the theory and experiment on lasing without inversion and discuss the prospects for the further development of this domain.

A diagrammatic representation of the steady-state solution for the density matrix of a multilevel atom under the action of coherent classical fields is presented. It provides an intuitive interpretation of the terms corresponding to quantum interference and dynamical Stark splitting in the equations describing the effects of atomic coherence, such as lasing without inversion, electromagnetically induced transparency and ultra-large index of refraction.

We propose a new class of intersubband lasers and amplifiers that achieve net gain without population inversion. The laser scheme is based on a unipolar semiconductor double quantum- well structure where gain occurs at a transition between conduction band subbands. In order to achieve net gain without inversion, we utilize Fano-type interferences. The semiconductor laser scheme that we are considering is analogous to the atomic lambda system that has been extensively analyzed in the context of electromagnetically induced transparency and lasing without inversion. A coherent coupling field however, is not required in the present scheme. The electronic coherence necessary for Fano-type interferences is established by resonant tunneling. For nonlinear optics applications, the asymmetry of the structure allows for (chi) ⁽²) processes and therefore higher conversion efficiency or parametric gain.

The simple model is proposed for amplification of a probe optical radiation without population inversion and enhancement of refractive index at a point of vanishing absorption when a strong driving field is present at the adjacent two-photon transition. The conditions for observation of the effects have been determined.

We derive the generation condition of amplification without inversion in three-level atoms driven by the microwave coherent pumping. Analyzing this condition we show the restriction and the possibility of observing the amplification without inversion under initial thermodynamics equilibrium between atoms and thermal radiation field.

Double-lambda four-level configuration, coupled to two driving and two probe fields is considered. Explicit formulae, based on density-matrix approach, for the analysis of the conditions of amplification without population inversion both in V and (Lambda) configurations are given. Effects of inhomogeneous broadening of the transitions on coherent coupling are discussed. Crucial importance of these effects for the choice of the proper transition schematics is shown. Numerical analysis of the conditions for amplification without inversion and four-wave mixing in the Ne energy-level configuration, relevant to the experiments, which are under way, is given.

The theoretical and experimental investigation of the absorption correlation in the sum-mixing generation in Hg vapor was performed. The absorption correlation equals to 0.90 was observed for the first time. The corresponding increasing of the absorption length is as much as 10 times.

We study a three-level atomic system strongly driven by a monochromatic field resonant to one of atomic transitions. The second highest frequency transition is probed by a weak field. In such a system we predict a novel mechanism of probe field amplification without inversion via strong field induced spontaneous relaxation of the ground atomic state. Unlike the AWI in the (Lambda) -scheme this mechanism appears in case of sufficiently fast relaxation at the low frequency transition.

We present a theoretical study of the quantum non-demolition (QND) measurement of an optical intensity in an atomic three-level system, using a quantum linear response theory recently developed. The principle of this scheme is to use the two-photon crossed-phase- modulation effect, in order to couple the quantum intensity fluctuations of the `signal' beam to the quantum phase fluctuations of the `meter' beam. The predictions of the complete theory are compared with recent experimental results using a sodium atomic beam, which include the observation of both squeezing and QND effects.

Coherent population trapping can be used to achieve efficient optical phase conjugation in a double-(Lambda) system at pump intensities well below the saturation intensity of the optical transitions. Experimental evidence for this process in sodium vapor qualitatively verifies this prediction.

For j_g equals j yields j_e equals j - 1 and j_g equals j yields j_e equals j optical transitions (where j_g, j_e are the total angular momenta of ground (g) and excited (e) states) previously unknown exact solution of quantum self-consistent problem on resonant atom-electromagnetic field interaction is found. This solution describes arbitrary set of field modes having same uniform elliptical polarization and atoms being in noninteracting with field coherent population trapping (CPT)-state. Some peculiarities of this spontaneous CPT-effect are investigated. We define the geometrical conditions (the relative orientation of light and static fields) for the existence of spontaneous CPT-effect in atoms + quantized field system in the presence of external static magnetic and electric fields.

The general Fano-Friedrichs Hamiltonian is diagonalized via a proper extraction of all dressed decaying eigenquasistates with complex eigenquasienergies. Their non-adiabatic crossing is described using the true truncated non-Hermitian Hamiltonian with applications to DC + AC non-steady Stark effect.

The paper presents a theoretical investigation of propagation of two frequency laser radiation in resonant (Lambda) -medium driven by a radiofrequency field on the transition between the two bottom levels. It is established that there is dramatic dependence of the character of propagation of radiation along the medium on phase and amplitude of the radiofield and on initial phases and intensities of the optical fields. We show that both amplification of one wave at the expense of the energy of another wave and transparency of the medium produced by population trapping can take place.

We consider a case of interaction of a three-level atom with high-intensity electromagnetic field when the Rabi frequencies are greater than the atomic transition frequencies. We show that coherent population trapping can exist under the certain phase conditions in this case of the rotating wave approximation violation.

The nonlinear stationary solution of the Bloch kinetic equations with Lorentz correction to the local electric field for the thermal gas of two-level molecules is found. This solution shows that radiative coupling between high-frequency molecular dipoles can provide the second-order phase transition to the antiferroelectric state of coherent gaseous crystal.

Using the idea of achieving a high index of refraction on a transition via populating only one of the dressed states corresponding to this transition, we propose a (Lambda) -scheme that provides a ratio index/absorption approximately equals 300 on a strong field and give realistic numbers for the susceptibility (chi) .

The constructive and destructive nature of the interference for the spontaneous emission from a three-level atom have been discussed by using dressed states. There is an interference if the upper level of the spontaneous emission is driven by a coherent field, while there is no interference if the lower level of the spontaneous emission is driven by the field. What interference (constructive or destructive one) we will get at a fixed frequency depends on the initial condition of the atom.

Propagation of a weak probe optical beam through a three-level medium pumped with a microwave strong beam is numerically studied. P-scheme of transitions under the conditions providing the amplification of the probe field without inversion is taken to calculate the atomic polarization which enters the paraxial wave equations. Diffraction and self-action of beams due to nonlinear absorption and refraction are properly taken into account. These effects together with linear absorption and refraction form a complex distribution of gain across the beams. As a result the amplified beam acquires ring transverse patterns and slight asymmetry in the frequency characteristics.

Two experiments in which laser oscillation without population inversion was obtained for the first time are reported. Analytical solutions are presented to elucidate the principles of the experiments. Numerical simulations and experimental results in a Rb vapor cell and a Na atomic beam for the gain profile are compared.