Effect of the near-lying level on the kinetics of the field of superradiance of a thin layer

The effect of the local field on the competition of transitions in the superradiance of an incompletely excited thin layer of three-level atoms (Λ-diagram of working transitions) is studied. Analysis is made for the conditions when one of the sublevels of the lower doublet is initially unpopulated. For an initially unpopulated upper sublevel of the lower doublet, a new superradiance regime is found.     

Conditions for the onset of superradiance regime in a relativistic electron bunch

Problems associated with the formation of coherent oscillations of an ensemble of classical oscillators and their superradiance instability are considered. The dispersion properties of an electron bunch and the conditions for the generation of nonequilibrium radiation are determined in the quasi-steady anharmonic oscillator approximation.     

Inversionless superradiance of a thin layer of three-level atoms: Coherent Raman transition between sublevels of the lower doublet

The effect of a local field on the transition competition in the process of superradiance of a thin layer of three-level atoms (Λ-configuration of working transitions) in the presence of an initial coherence of the lower doublet is investigated. It is shown that under the action of a local field, a coherent Raman transition between sublevels of the lower doublet can take place.     

Superradiance and superradiance without inversion of a thin layer of three-level atoms: The consideration of the sample extent

The effect of the sample extent in the process of superradiance and superradiance without population inversion of a thin (L ≤ λ) layer of three-level atoms (Λ-configuration of working transitions) is investigated. The limits of applicability of the spatially homogeneous approximation are determined. The consideration of the extent is shown to introduce new effects into the kinetics of populations and the field.     

Model of stochastization of the oscillatory regime of superradiance

Based on the analysis of the experimental data on the observation of the fluorescence and superradiance of praseodymium ions in a matrix of lanthanum trifluoride, a model of superradiance of three-level radiators with two close upper levels is developed in the mean-field approximation and studied. The uppermost level is coherently pumped by an ultrashort pulse of electromagnetic field, after which the excitation is transferred to the close energy level, from which the superradiance transition occurs to the lower level. In limiting cases, the considered model is reduced to the known models of superradiance and describes the ordinary regimes of monopulse and multipulse (oscillatory) superradiance. However, in a certain region of parameters, the model under discussion describes such a multipulse superradiance signal in which electromagnetic field spikes composing it follow in time with random intervals and amplitudes, so that the regime of regular chaotic dynamics is demonstrated in a single superradiance signal. In a certain time interval, the proposed model can be described by the Lorenz equations with the parameters corresponding to the chaotic dynamics of spikes composing the superradiance signal. The presented results of the numerical simulation of the model equations qualitatively correspond to the picture of stochastic pulsations observed experimentally.     

Effect of the initiating-pulse area on induced superradiance

The formation of reflected and transmitted waves in induced superradiance is studied. It is shown that the relationship between reflected and transmitted waves periodically varies with increasing initiating-pulse area. The effect is attributed to the periodic cooperative exchange between forward and backward waves in a sample and its termination at the moment corresponding to the pulse delay time.     

Reconciling the eigenmode analysis with the Maxwell-Bloch equations approach to superradiance in the linear regime

The superradiance from a slab of inverted two-level atoms is theoretically analyzed in the linear regime from both the perspective of the expansion in eigenfunctions of the integral equation with the Lienard-Wiechert potential as kernel, and that of linearizing the Maxwell-Bloch equations. We show the equivalence of both approaches. We show that the so-called Reduced Maxwell-Bloch equations do not yield even approximately the correct solution when applied in the obvious way, but that they can be made to give the correct solution by adding a fictitious input field.     

Effect of multiple scattering on the emission of ultrarelativistic electrons in a thin layer of matter

We develop a quantitative theory of the effect of multiple scattering on the bremsstrahlung of ultrarelativistic electrons in a thin layer of matter. The effect is an analog of the Landau-Pomeranchuk-Migdal (LPM) effect of suppression of the radiation emitted by high-energy particles in an infinite amorphous medium, but certain differences do exist. On the basis of our approach we analyze the data recently obtained at SLAC (E-146) in experiments set up to verify the LPM effect. We show that in addition to the LPM effect, this experiment exhibited the suppression of bremsstrahlung in a thin layer of matter, theoretically predicted in our earlier papers. Zh. éksp. Teor. Fiz. 113, 58–70 (January 1998)     

Thermodynamics and kinetics of shear-induced melting of a thin layer of lubricant confined between solids

Using Landau’s phenomenological theory of phase transitions, the shear-induced melting of a thin layer of substance confined between two crystalline surfaces is considered. The kinetics of melting and solidification is considered for static and alternating loads. The possibility of two consecutive “melting” (solid-to-liquid) transitions is discussed. As a result of the first transition, modulation of the microscopic density becomes zero only in the direction of shear (partial melting) and as a result of the second, it also disappears in the normal direction (complete melting).     

Growth of sputter-deposited Ni-Ti thin films: Effect of a SiO2 buffer layer

In-situ X-ray diffraction (XRD) during the growth of Ni-Ti thin films was chosen in order to investigate their texture development using a deposition chamber installed at a synchrotron radiation beamline. Near-equiatomic films were co-sputtered from Ni-Ti and Ti targets. The texture evolution during deposition is clearly affected by the substrate type and the ion bombardment of the growing film. On naturally oxidized Si(100) substrates, the Ni-Ti B2 phase starts by stacking onto (h00) planes, and as the thickness increases evolves into a (110) fibre texture. For the deposition on thermally oxidized Si(100) substrates, this pronounced cross-over is only observed when a substrate bias voltage (-45 V) is applied. The oxide layer plays an important role on the development of the (100) orientation of the B2 phase during deposition on heated substrates (≈470 °C). If this layer is not thick enough (naturally oxidized Si substrate) or if a bias voltage is applied, a cross-over and further development of the (110) fibre texture is observed, which is considered as an orientation that minimizes surface energies. Electrical resistivity measurements showed different behaviour during phase transformation for the Ni-Ti film deposited on thermally oxidized Si without bias and those on thermally oxidized Si(100) with bias and on naturally oxidized Si(100) without bias. This is related to stresses resultant from the fact that the Ni-Ti films are attached to the substrates as well as with the existence of distinct textures. PACS 81.15.Cd; 61.10.Nz; 68.55.Jk     

Effect of a thin dielectric layer on terahertz transmission characteristics for metal hole arrays

We studied the role of surface-plasmon polaritons (SPPs) in a bandpass transmission property of two-dimensional metal hole arrays (2D-MHAs) by investigating the effect of thin dielectric layers on the 2D-MHA surfaces. We measured zero-order transmission spectra of the 2D-MHAs by changing the thickness of the dielectric layer and found that the bandpass transmission peak shifted to the lower-frequency side with increasing layer thickness, owing to the change of the resonant frequency of the SPP. This result shows that SPPs play a crucial role in the transmission property of 2D-MHAs in the terahertz region.     

Symmetry in the problem on wave-flow modes of a thin viscous-fluid layer

It is shown that a suitable transformation in the transverse-coordinate shift makes the set of equations describing the flow of viscous fluid films in the long-wavelength approximation invariant with respect to the reflection transformation. When constructing the flow models with the Galerkin spectral method, this symmetry in the equation for the longitudinal velocity stimulates its representation with the help of a set of functions symmetric with respect to the transverse coordinate.     

Enhanced superradiance effect in a system of interacting two-level atoms and crossover from coherent to many-atom multiphoton relaxation regime

We study effects of direct interatomic interaction on cooperative processes in atom-photon dynamics. Using a model of two-level atoms with Ising-type interaction as an example, it is demonstrated that interparticle interaction can promote cooperative radiative relaxation. For small number of atoms this results in inhibition of incoherent spontaneous decay leading to the regime of collective pulse relaxation. Above superradiance threshold increase in delay time and enhancement of superradiance is occurred. In the case of strong interaction (as compared to excitation energy of an atom) transition to the regime of multiphoton relaxation occurs, which we discuss using a simple model of two atoms in a high-Q single mode cavity. It is shown that such transition is accompanied by Rabi oscillations involving many-atom multiphoton states. Dephasing effect of dipole-dipole interaction and solitonic mechanism of relaxation are discussed as well.     

Microstructure of bidisperse ferrofluids in a thin layer

In this work we present a characterization of the bidisperse ferrofluid microstructures that appear in thin layers of ferrofluid. These layers have been studied by a combination of Langevin dynamics simulations and density functional theory. Our results allow us to compare the microstructures that exist in quasi-two-dimensional ferrofluid nanolayers with the microstructures found in three-dimensional bidisperse ferrofluids. Furthermore, our results allow us to explain the influence of the geometry of the sample on the topology and size-distribution of the observed aggregates of magnetic nanoparticles.     

Effect of emitter layer doping concentration on the performance of a silicon thin film heterojunction solar cell

A novel type of n/i/i/p heterojunction solar cell with a-Si:H(15nm)/a-Si:H(10nm)/ epitaxial c-Si(47μm)/epitaxial c-Si(3μm) structure is fabricated by using the layer transfer technique, and the emitter layer is deposited by hot wire chemical vapour deposition. The effect of the doping concentration of the emitter layer S d (Sd =PH3 /(PH3 +SiH4 +H2 )) on the performance of the solar cell is studied by means of current density-voltage and external quantum efficiency. The results show that the conversion efficiency of the solar cell first increases to a maximum value and then decreases with S d increasing from 0.1% to 0.4%. The best performance of the solar cell is obtained at S d = 0.2% with an open circuit voltage of 534 mV, a short circuit current density of 23.35mA/cm2 , a fill factor of 63.3%, and a conversion efficiency of 7.9%.     

Magneto-optics in the ultrafast regime: thermalization of spin populations in ferromagnetic films

We have characterized by pump-probe polarimetry the time-dependent dielectric tensor in a CoPt3 ferromagnetic film excited by 20 fs laser pulses. It is shown that, after the thermalization time of the electrons (approximately 50 fs), the dynamics of the real and the imaginary parts of the Voigt vector are identical. In addition, their relative variation is 10 times larger than that of the diagonal elements of the tensor, which allows one to infer that the spins dominate the magneto-optical response. During the thermalization process, the temporal behavior of the tensor elements opens new questions concerning the dynamics of the spins associated to a nonthermal electronic population in a ferromagnet.     

Decaying quasi-two-dimensional turbulence in a thin liquid layer

The results of laboratory measurements of decaying quasi-two-dimensional turbulence in a thin liquid layer are considered. It is shown that the enstrophy-to-energy ratio decreases according to a power law on a certain decay interval. The exponent in the power law is a function of the Reynolds number. The enstrophy decay is found to be anomalously slow as predicted in a number of numerical studies. It is shown that the anomalous decay in the quasi-two-dimensional flow under investigation is not due to intense vortex formation as in the numerical decaying turbulence, but due to the limited range of scales on which a flow can be regarded as two-dimensional.