Doppler-free fluorescence induced by strong quasi-resonant laser field

Alteration of the frequency-correlation properties and Doppler widths of the double-quantum and double-step processes with the increase of the field strength is considered. Possibilities to observe laser-induced Doppler-free fluorescence at the electronic-vibrational molecular transitions are shown. Various transitions can be involved into the Doppler-free regime when the pump field strength is varied.     

Four-wave mixing under conditions of Doppler-free resonance induced by strong radiation

The possibilities of enhancing the nonlinear optical response of a gaseous medium and the radiation conversion efficiency in four-wave mixing processes through the use of atomic coherence effects and the induced elimination of the Doppler broadening of resonances are investigated. Numerical illustrations of the effects are given for experiments in progress. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 2, 98–103 (25 January 1999)     

Method for observing Doppler-free hot-band transitions

A novel spectroscopic method for highly sensitive detection of hot-band transitions based on infrared-infrared double resonance in a molecular beam was successfully demonstrated. The intensity of the infrared transitions from a vibrationally excited state is enhanced by two orders of magnitude by resonant laser pumping. The hot-band transitions are identified in connection with the assigned transitions in the fundamental band. Doppler-free spectral lines are observed. The experiment was carried out on the 2v ₃←v ₃ transitions of CH₃F with CO₂ laser pumping and probing.     

Phase transitions induced by a strong magnetic field in electron-doped manganites

The magnetic and magnetoelastic properties of single crystals of electron-doped rare-earth manganites La_1?x Sr _x MnO₃ are studied. Phase transitions from the A-type antiferromagnetic phase to the C-type anti-ferromagnetic phase in a strong magnetic field are revealed in La_1?x Sr _x MnO₃ manganites with a strontium content x = 0.65. A similar phase transition is observed in manganites with a strontium content x = 0.8, at which the La_0.2Sr_0.8MnO₃ manganite is assumed to transform from the C-type antiferromagnetic phase to the G-type antiferromagnetic phase.     

Modified Maxwell-Bloch equations for systems under strong optical excitation

The response to a strong monochromatic electromagnetic field is investigated for a system with two energy bands. Modified Maxwell-Bloch equations are derived using a Master-equation approach in the rotating frame. Our results imply the vanishing of the stationary polarization in the rotating frame and are equivalent to the replacement of the equilibrium population in the ordinary linear response with the actual population determined by the strong excitation.     

Population inversion on vibrational transitions of molecules under nonresonant optical excitation

The possibility of obtaining a population inversion on vibrational transitions of molecules through nonlinear effects when the pump radiation is absorbed in the wings of spectral lines is investigated theoretically. We show that a population inversion can be produced in molecules on vibrational transitions when intense pump radiation is absorbed in the blue wing of the R branch of the vibrational-rotational molecular spectrum. This effect is related to inequality of the probabilities of the absorption and stimulated emission of radiation and is attributable to collisional transitions between rotational levels. We have ascertained that the larger the rotational constant of the molecule and the higher the pump radiation intensity, the higher the effective frequency of the collisions that give rise to a population inversion. Using the carbon monoxide (CO) molecule as an example, we show that when intense (∼1010 W cm⁻²) pump radiation is absorbed in the blue wing of the R branch, a noticeable population inversion can be produced and the gain at the center of the R and P branches of the molecular spectrum can reach 0.011 and 0.250 cm⁻¹ at temperatures T = 300 and 100 K, respectively.     

Calculation of the probability of optical transitions in strong electric fields

An algorithm is proposed for calculating the spectrum of the cross section for photoionization of carriers on deep centers in electric fields on the basis of the form function of an optical transition.An experiment and calculations were performed for the complex V_Ga-S_As in GaAs. The proposed model is compared with theoretical works based on the single-coordinate approximation. It is concluded that the single-coordinate model is applicable for describing the field-dependence of the cross section for photoionization of an electron on a V_Ga-S_As center. Data on the influence of an external electric field on the change in the moments of the form function of the absorption band of the complex V_Ga-S_As in GaAs are obtained. It is concluded that an electric field influences the adiabatic potentials of the center investigated.     

Investigation of atomic transitions of cesium in strong magnetic fields by an optical half-wavelength cell

It has been demonstrated that the use of the λ/2 method allows one to effectively investigate individual atomic levels of the D ₂ line of Cs (with the most complicated spectrum among all alkali metals) in strong magnetic fields up to 7 kG. The method is based on strong narrowing of the absorption spectrum (which provides sub-Doppler resolution) of a cesium-filled thin cell with the thickness L equal to a half-wavelength (L = λ/2) of the laser radiation (λ = 852 nm) resonant with the D ₂ line. In particular, the λ/2 method has allowed us to resolve 16 atomic transitions (in two groups of eight atomic transitions each) and to determine their frequency positions, fixed (within each group) frequency slopes, the probability characteristics of the transitions, and other important characteristics of the hyperfine structure of Cs in the Paschen-Back regime. Possible applications are mentioned. Two theoretical models have been implemented. The values of the magnetic field have been indicated at which the models describe the experiment well.     

Magnetization behavior and metamagnetic transitions in electron-doped manganites induced by a strong magnetic field

The behavior of magnetization M of the R_xA_1?xMnO₃ manganites (R=La, Pr, Nd, Sm, etc., A=Ca, Sr, Ba) in the electron doping region (x<0.4) is studied as a function of external magnetic field H. The M(H) relations for homogeneous magnetic structures are obtained by performing band calculations in the double-exchange model. Three different types of magnetization behavior corresponding to three electron concentration ranges (x<0.14, 0.14<x<0.27, x>0.27) are revealed. The M(H) relations are interpreted in terms of the phase diagram for the homogeneous ground state of the manganites calculated for H=0, and the results agree qualitatively with experimental data on the magnetization of Sm_xCa_1?xMnO₃.     

Doppler-free two-photon millimeter wave transitions in OCS and CHF3

Doppler-free two-photon rotational transitions J = 13<--<--11 and J = 12<--<--10 of OCS and J = 8<--<--6 and J = 7<--<--5 of CHF (3) were detected in the frequency range 134-156 GHz, using a novel, highly sensitive intracavity-jet technique. The sub-Doppler narrowing of the observed peaks (down to 40 kHz full width at half maximum as compared to 300 kHz of the Doppler width) demonstrates the potential of this new technique for high precision millimeter wave spectroscopy. The possibilities of the further reduction of the two-photon absorption line widths are considered.     

On the theory of nonradiative transitions in optical excitation of gaseous media

A new approach and a theoretical model for describing experimentally observed nonradiative effects are considered. Nonradiative transitions in an ensemble of polyatomic molecules are interpreted as conversion of absorbed energy of optical radiation to energy of thermal translational motion of gas molecules. The model includes explicit account for intermolecular interactions. The form of the corresponding model Hamiltonian in the approximation of pairwise interactions of molecules and methods for its calculation are proposed. It is shown that all required matrix elements of the theory are easy to calculate, and the method is suitable for both qualitative and quantitative analysis of nonradiative effects. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 289–295, May–June, 2000.     

Nonstationary photovoltage induced in tin disulfide crystals under strong surface excitation

The specific features of nonstationary photovoltage excitation in molecular crystals of tin disulfide are investigated. A substantial change in the dependence of the photocurrent amplitude on the spatial frequency of the interference pattern is revealed. The observed behavior is explained in terms of the model of a photoconductor with different-type charge carriers of the same sign.     

Doppler-free spectroscopy of neon atoms dressed by optical photons in nearly-degenerate four-wave mixing

We have investigated nearly degenerate four-wave mixing in neon on the 607 nm line. We have considered the case where one pump beam saturates the medium while the two other incident beams have a much weaker intensity. The frequency of the most intense pump beam is equal to the frequency of the probe beam. When the frequency of the second pump beam is scanned, narrow resonances are observed. These resonances correspond to transitions between the energy levels of the atom dressed by the photons of the intense pump beam.     

Population inversion induced by collisions in a two level system under nonresonance optical excitation

When the gas particles, which can be considered as a two level quantum system, are affected by frequent collisions with buffer gas particles (thermal reservoir), the spectral densities of the Einstein coefficients for absorption and induced emission are unequal to each other far from the absorption (emission) line. The relationship between the spectral densities at the specified condition reveals new physical phenomena. The mostly convincing example is the fact that population inversion is established in a two level system under nonresonance optical excitation. This effect is experimentally observed as a lasing on sodium resonance transitions.