Thermodynamical considerations on the quantum efficiency of anti-stokes co-operative luminescence

It is shown that the second law of thermodynamics imposes an additional restriction on the quantum efficiency of the anti-Stokes co-operative luminescence besides the energy conservation law. This restriction is very severe for the excitation by ordinary light, while it can be neglected at very high intensities of the exciting radiation (e.g. laser excitation).     

衰减材料对高增益相对论速调管自激振荡的抑制

在高增益相对论速调管放大器中,自激振荡严重影响器件的正常工作并导致脉冲缩短。通过粒子模拟研究了高次模式自激振荡的激励机制,并提出在器件内加入微波衰减材料来抑制该类自激振荡。详细研究了衰减材料的电导率和几何参数与高次模式衰减常数的关系,并开展了微波衰减体的冷测实验。冷测表明微波衰减体对高次模式的衰减达11.25 dB,其性能满足器件实验要求,可以用于高增益相对论速调管的热腔实验。     

Self-modulation of the radiation from fiber lasers with microcavity mirrors

The domain of excitation of self-oscillations is experimentally studied in a system consisting of an erbium fiber laser and a microcavity. The dependences of the self-oscillation frequency on the parameters of the system are found. The features of the self-oscillations are analyzed for the case where the laser radiation simultaneously interacts with several microcavities.     

Heat diffusivity and heat conductivity of Ni near the Curie point

The heat diffusivity of Ni near the Curie point was measured by an a.c. oscillating method. The period of self-oscillation is related to the heat diffusivity. The principle and some technical details of the experimental method is given. The critical behavior of heat diffusivity near the Curie temperature is analysed. Using known specific heat data the heat conductivity for Ni can be calculated. A conclusion is made that the Wiedemann-Franz law is not valid near the ferromagnetic phase transition.     

The hard excitation regime for a relativistic backward-wave oscillator

We study theoretically the processes which take place in a relativistic backward-wave oscillator (BWO) in the parameter region corresponding to hard excitation of oscillations. The onset of both stationary oscillations near the boundary of the hard excitation regime and hard nonstationary oscillations realizing the self-modulation regimes of BWO operation are analyzed along with the dynamics of a BWO-based amplifier. In particular, it is shown that hard excitation of self-oscillation can restrict significantly the output characteristics of a relativistic BWO operated in the regenerative-amplification regime. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 8, pp. 675–681, August 2008.     

高增益相对论速调管放大器自激振荡的分析与抑制

通过二维和三维粒子模拟研究了高增益相对论速调管放大器中高次模式的激励机理,两者结果的一致性排除了高次非对称模式激励的影响。由此建立了高次模式起振的理论模型,给出了高次模式的起振电流。并通过在器件内加入微波衰减材料来抑制高次模式自激振荡,模拟获得了1.95GW的微波输出,增益62.8dB。     

热驱动谐振管回热器的时变数学模型

从自激振荡的稳定性入手,分别讨论了热声网络的周期时均和时变数学模型.用参数谐振理论讨论了热声自激振荡系统中回热器的参数储能实质.时变数学模型的建立对于深入理解热驱动谐振管回热器的热与声的能量转换实质具有重要的指导意义.     

Experimental investigation of acoustic self-oscillation influence on decay process for underexpanded supersonic jet in submerged space

Intensification of mixing between the gaseous working body ejected through a jet nozzle with ambient medium is an important scientific and technical problem. Effective mixing can increase the total efficiency of power and propulsion apparatuses. The promising approach, although poorly studied, is generation of acoustic self-oscillation inside the jet nozzle: this impact might enhance the decay of a supersonic jet and improve the mixing parameters. The paper presents peculiar properties of acoustic self-excitation in jet nozzle. The paper presents results of experimental study performed for a model injector with a set of plates placed into the flow channel, enabling the excitation of acoustic self-oscillations. The study reveals the regularity of under-expanded supersonic jet decay in submerged space for different flow modes. Experimental data support the efficiency of using the jet nozzle with acoustic self-oscillation in application to the systems of gas fuel supply. Experimental results can be used for designing new power apparatuses for aviation and space industry and for process plants.     

Self-oscillations in a hybrid bistable optical device

The Van der Pol, Krylov-Bogoliubov method for self-oscillation calculations in nonlinear systems is applied to a piezoelectrically driven Fabry-Perot interferometer which is operated as a hybrid bistable optical device. It is shown that in cases where the time delay in the feedback signal cannot be ignored, or when a reactive element is inserted in the appropriate place in the electrical circuit, self-oscillation will result. Experiments in which a piezoelectric element serves as the basic component of a hybrid bistable device demonstrate that self-oscillation occurs with a frequency which is very near the piezoelement self-frequency.     

Dissociative excitation of the cesium atom in e-CsCl collisions

The dissociative excitation of the cesium atom in collisions of slow electrons with cesium chloride molecules is studied by the method of extended crossed beams with detection of the optical signal from the crossing region. Forty-seven cross sections for the excitation of spectral lines of CsI in the spectral region 347–852 nm are measured at an electron energy of 100 eV, and 10 optical excitation functions (OEFs) are recorded. It is found that the dependence of the excitation cross sections on the principal quantum number of the upper level in the spectral series of Cs I corresponds to a power law. The characteristic values of the constants in the power law are determined for each of the investigated series. Zh. Tekh. Fiz. 69, 29–34 (January 1999)     

Elementary process rate constants in excitation of zinc sulfide luminescence by hydrogen atoms

Elementary process rate constants are determined for excitation of zinc sulfide-based luminophores by a mixture of atomic and molecular hydrogen, together with conditions for development of self-oscillation of the luminescence intensity. A model is constructed for a hydrogen atom absorption center on the zinc sulfide surface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 15–21, April, 1990.The author thanks A. A. Grosberg for his assistance in the study.     

Optoelectronic oscillator using a LiNbO3 phase modulator for self-oscillating frequency comb generation

Optical frequency comb generation by using a novel optoelectronic oscillator (OEO) is proposed and demonstrated with the emphasis placed on self-oscillating operation. In the OEO, a wideband LiNbO3 phase modulator is driven with a large-amplitude radio-frequency (RF) feedback signal to generate a deeply phase-modulated light wave; accordingly, an optical frequency comb with a bandwidth greater than the RF signal is generated by self-oscillation. Although it generates multifrequency components, the OEO exhibits characteristics of a single-mode oscillator. Its operation is stable and self-starting. An optical frequency comb with a 120 GHz bandwidth and 9.95 GHz frequency spacing was successfully generated by self-oscillation at a single frequency.     

Self-propagating crystallization waves in the TiCu amorphous alloy

Self-propagating crystallization waves are detected and experimentally demonstrated in the Ti₅₀Cu₅₀ amorphous alloy obtained by the melt spinning (ultrafast quenching) method. High-speed thermographic recording has shown that crystallization waves can appear spontaneously at the heating of an amorphous strip to 300–350°С or at the local initiation by a hot tungsten coil of a small segment of the strip preliminarily heated to 230–250°С. In the former case, the crystallization wave propagates at a velocity of ~7 cm/s; in the latter case, the crystallization wave propagates in a self-oscillation mode at an average velocity of ~1.2 cm/s. The temperature gradient across the wavefront is about 150°С. The samples crystallized in the self-oscillation mode have a characteristic banded structure with a smaller grain in depression regions. The crystallization product in all samples is the TiCu tetragonal intermetallic phase.     

Transform method in resonance Raman scattering with quadratic Franck–Condon and Herzberg–Teller interactions

A new method of considering quadratic vibronic interactions and deviations from the Condon approximation in resonance Raman scattering (RRS) is proposed. The method is used for the generalization of the transform law between absorption and Raman excitation profiles, derived earlier for a basic model. In the case of arbitrary mixing of modes with similar frequencies, a simple generalization of the transform law for first-order RRS is obtained. For an arbitrary change of frequencies without mode mixing the transform law for the nth-order RRS is given. The equations are valid for an arbitrary number of modes and also take temperature effects into account.     

Effects of linear absorption on the steady-state behavior of a photorefractive phase-conjugate mirror

A photorefractive phase-conjugate mirror with linear absorption is examined. An objective of this study is to identify possible effects of absorption on the self-oscillation conditions of the mirror and the properties of the excited conjugate wave. It has been found that self-oscillation is still possible in the presence of absorption. However, the coupling coefficient required for self-oscillation must increase in magnitude with increasing absorption coefficient to counteract the depressing effects of absorption. A consequence of self-oscillation in the presence of absorption is enhancement of the mirror reflectivity. A parametric study of the absorption-enhanced mirror is presented.The authors wish to acknowledge the contributions of Dai Sieh during the initial phase of this study. The work that he completed as part of his Master of Science thesis was an important element in our study of PCM reflectivity enhancement. The authors also acknowledge the assistance of Sahaid Rauf in carrying out some of the numerical solutions of Eq. 10, the governing equation of the self-oscillatory modes.     

Circuit-field coupled finite element analysis method for an electromagnetic acoustic transducer under pulsed voltage excitation

This paper presents an analytical method for electromagnetic acoustic transducers (EMATs) under voltage excitation and considers the non-uniform distribution of the biased magnetic field. A complete model of EMATs including the non-uniform biased magnetic field, a pulsed eddy current field and the acoustic field is built up. The pulsed voltage excitation is transformed to the frequency domain by fast Fourier transformation (FFT). In terms of the time harmonic field equations of the EMAT system, the impedances of the coils under different frequencies are calculated according to the circuit-field coupling method and Poynting’s theorem. Then the currents under different frequencies are calculated according to Ohm’s law and the pulsed current excitation is obtained by inverse fast Fourier transformation (IFFT). Lastly, the sequentially coupled finite element method (FEM) is used to calculate the Lorentz force in the EMATs under the current excitation. An actual EMAT with a two-layer two-bundle printed circuit board (PCB) coil, a rectangular permanent magnet and an aluminium specimen is analysed. The coil impedances and the pulsed current are calculated and compared with the experimental results. Their agreement verified the validity of the proposed method. Further- more, the influences of lift-off distances and the non-uniform static magnetic field on the Lorentz force under pulsed voltage excitation are studied.     

Effect of dipole–dipole interaction on self-control magnetization oscillation in double-domain nanomagnets

A double-domain model with long-range dipole–dipole interaction is proposed to investigate the self-oscillation of magnetization in nano-magnetic systems driven by self-controlled spin-polarized current. The dynamic behavior of magnetization oscillation is calculated by a modified Landau–Lifshitz–Gilbert equation in order to evaluate the effects of the long-range dipole–dipole interaction. While the self-oscillation of magnetization can be maintained substantially, several self-oscillation regions are experienced as the dipole–dipole interaction increases gradually.     

APPLICATION OF THE SPECTRAL FINITE ELEMENT METHOD TO TURBULENT BOUNDARY LAYER INDUCED VIBRATION OF PLATES

The spectral finite element method and equally the dynamic stiffness method use exponential functions as basis functions. Thus it is possible to find exact solutions to the homogeneous equations of motion for simple rod, beam, plate and shell structures. Normally, this restricts the analysis to elements where the excitation is at the element ends. This study removes the restriction for distributed excitation, that in particular has an exponential spatial dependence, by the inclusion of the particular solution in the set of basis functions. These elementary solutions, in turn, build up the solution for an arbitrary homogeneous random excitation. A numerical implementation for the vibration of a plate, excited by a turbulent boundary layer flow, is presented. The results compare favourably with results from conventional modal analysis.     

Optical control of the coherent dynamics of a bismuth lattice at liquid-helium temperature at low and high excitation levels

The fully symmetric A_1g phonons of bismuth have been investigated at liquid-helium temperature by the coherent control method for various crystal excitation levels. It has been found that large-amplitude coherent phonons exhibit the “rigidity” of the phase, which is absent at a small amplitude. The impossibility of changing the phase of phonon oscillations appears at the excitation levels at which their amplitude relaxation law changes from exponential to power. The modification of the phase properties and relaxation law of the excitations of the crystal lattice can be understood in terms of the concept of the condensation of phonons, which occurs with an increase in the crystal excitation level.     

Doppler optomechanics of a photonic crystal

A laser beam directed at a mirror attached onto a flexible mount adds friction to its mechanical motion by the Doppler effect. For a normal mirror the efficiency of this radiative Doppler friction is very weak and practically masked by laser shot noise. We find that it can become very efficient using a photonic crystal mirror near its photonic band gaps. As an example, a Bragg mirror used at the long wavelength edge of its band stop can be efficiently optically cooled using the Doppler friction. The opposite effect opens new routes for optical pumping of mechanical systems: a laser pointing at a Bragg mirror and tuned at its short wavelength edge induces amplification of the vibrational excitation of the mirror leading eventually to its self-oscillation. These new effects rely on the strong dependency of a photonic crystal reflectivity on the wavelength.