On the stability of a one-frequency laser based on an inhomogeneously broadened active substance

An investigation is performed of the stability of one-frequency lasing in the one-dimensional model of a traveling-wave laser based on an inhomogeneously broadened active substance relative to a broad class of perturbations which depend on the coordinate and on time. Analytic expressions are derived for the shape of the contour of the amplification line for the spectral components of the perturbations of the field for an arbitrary ratio between the width of the luminescence line and the natural width. It is shown that with an increase of this ratio the critical excess of the pumping above the threshold value at which instability develops decreases.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 15, No. 1, pp. 19–26, January, 1972.The authors thank V. I. Talanov for his constant interest in and attention to the work, V. I. Bespalov, Ya. I. Khanin and E. I. Yakubovich for their helpful discussion, and likewise V. I. Petrukhina for performing the programming and calculation on the electronic computer.     

Single particle inclusive spectra of charged particles from antiproton-proton interactions at 5.7 GeV/c

A systematic study of single particle inclusive spectra of charged particles from ¯pp interactions at 5.7 GeV/c is presented and discussed in the framework of the Mueller-Regge approach to inclusive reactions. A comparison of our results with data at other energies points to specific properties of antiproton-proton interactions.I have greatly benefited from discussions with Dr. J. Cvach and Dr. V. imák. To Dr. V. imák I am also indebted for the encouragement during the preparation of this paper.     

Rotations of eigenvibration direction in anisotropic crystals

The electric field of incident light induces dipoles in anisotropic media, vibrating in two perpendicular directions of the principal axes. Because of the tensor property of the dielectric constant, an induced dipole is subject to a torque, tending to rotate it about the axis parallel to the propagation direction. The directions of eigenvibration of the ordinary (o-ray) and extraordinary (e-ray) waves are no longer perpendicular in this sense. We propose here the relationships to describe the rotation of the induced dipole in the perpendicular electric fields. The rotation angles are found to increase with increasing dielectric constants and electric field strength of the incident light, exhibiting large values near the resonance frequencies in the infrared range at the azimuth angle /4 of the polarized incident light. Piezoelectric and ferroelectric crystals have a large value of the dielectric constant in the infrared frequency range. Rotations of the vibration direction of the o-ray and the e-ray waves are shown in the infrared transmission spectra recorded by incidence of the polarized light and transmission through piezoelectric and ferroelectric crystals (-quartz, LiNbO₃, and LiTaO₃). Interference of the o-ray and the e-ray waves transmitted through the crystals confirms the rotations of the vibration directions, a self-modulation effect of light in piezoelectric and ferroelectric crystals induced by the electric field of the propagating light.     

Determining optical constants by frustrated total internal reflection spectrophotometry

The possibility of using the method of frustrated total internal reflection (the two-media method) to determine the optical constants n and k in the value ranges 1. 3相似文献   

Nonlinear interferometry and phase measurements for surface second-harmonic generation in a dispersive geometry

Direct measurement of the phase of the surface nonlinear susceptibility is based on the interference of nonlinear optical signals. Up to now, this has not been possible for Second-Harmonic Generation (SHG) in geometries such as Total Internal Reflection (TIR) due to the refractive dispersion of harmonic and fundamental light created in TIR. We demonstrate two schemes which enable us to overcome this dispersion, leading to interference between two second-harmonic signals generated consecutively by the same laser. The advantages and limitations of the two approaches are discussed. We use this technique to check the theoretical predictions for the nonlinear Fresnel factors for SHG in the TIR geometry.Paper presented at the 129th HE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Optical phase conjugation and waveguide coupling by cascading transverse second-harmonic and difference-frequency generation in a vertical cavity

We propose a novel scheme for implementing optical phase conjugation (OPC) by cascading transverse second-harmonic and difference-frequency generation in a vertical cavity. We propose to achieve phase conjugation in the pump and input-output two-waveguide structure. For a symmetric waveguide structure, the structure can be used as an optical phase conjugate mirror. For a realistic structure based on III–V semiconductor materials, a pump power per unit waveguide of 0.74 W m^-1 is required to reach a conversion efficiency of 9.4%. Combined with another phase-conjugate mirror, the reflectivity of the proposed phase-conjugate mirror can be efficiently modulated by control of the pump power. For an asymmetric waveguide structure, a gain is predicted. In addition, by cascading second-harmonic generation (SHG) and parametric oscillation, we propose to use the same structure as a waveguide coupler. There is a threshold to achieve coupling.     

Optical autocorrelator method for measurement of granularity of developed black-and-white photographic materials

Conclusions The introduced optical autocorrelator method is suitable for granularity measurements of developed black-and-white photographic materials in laboratory practice and is also useful for factories producing the photographic materials. This method was developed for the factory Fotochema n. p. Hradec Králové.The mentioned optical autocorrelator is in general also suitable for measurement of the relative autocorrelation functions of various transmittance distributions in plane which are similar with the grain distributions of developed photographic materials.Some parts of the introduced optical autocorrelator, designed by the author of this paper, were manufactured in the machine shop of Department of Physics of Faculty of Science Palacký University in Olomouc by L.Ambrz and S.Sedlá. I am obliged to them.     

The Split of the Dirac Hamiltonian into Precisely Predictable Energy Components

We are dealing with the Dirac Hamiltonian H = H₀ + V with no magnetic field and radially symmetric electrostatic potential V = V(r), preferably the Coulomb potential. While the observable H is precisely predictable, its components H₀ (relativistic mass) and V (potential energy) are not. However they both possess precisely predictable approximations H₀ and V which approximate accurately if the particle is not near its nucleus. On the other hand, near 0, H₀ and V are practically unpredictable, perhaps in agreement with the fact, that a neutrino also should be in the game. [We have not yet studied the corresponding observables for the ( 12-dimensional) problem of electro-weak interaction.] Mathematically we are focusing on the spectral theory of the unbounded self-adjoint operators H₀ and V . We can prove that V is unitarily equivalent to V(r) again, by a unitary map given as Wiener-Hopf-type singular integral operator in the standard separation of variables for radially symmetric Dirac Hamiltonians. [This is, as far as the continuous spectrum is concerned.] Very similar unitary equivalence holds for H ₀ and H ₀. We are tempted to regard this as a form of renormalization.     

Low-threshold optical power limiting of cw laser illumination based on nonlinear refraction in zinc tetraphenyl porphyrin

The increasing use of low power cw lasers in various applications calls for the design of optical limiters with low thresholds. To this end, the optical nonlinearity exhibited by zinc tetraphenyl porphyrin at low laser powers is utilized to design an optical limiter for low threshold operation. The basic parameter responsible for limiting action, the nonlinear refractive index of the medium, is measured using the Z-scan technique and found to have a value of at the helium–neon laser wavelength of 632.8 nm. The origin of nonlinearity is explained on the basis of the thermal lens model. It is shown that effective optical limiting at desired threshold values can be achieved by the optimal choice of aperture size and experimental geometry.     

Temperature stabilized optical waveguide modulator

A low drive-voltage optical modulator using a Ti-diffused UNbO₃ optical waveguide has been fabricated. Stabilization against ambient temperature change was realized by using a miniature halfwave plate. The halfwave voltage, 3 dB bandwidth, optical insertion loss and extinction ratio were 3·8 V (at 1·06m wavelength), 850 MHz, 10 dB and 13 dB, respectively. A reduction scheme for the optical absorption caused by metallic electrodes, and an analysis of the modulator high frequency response are also reported.     

Neutralization of antiprotons at nuclei

A method is given for calculating the cross section for neutralization of for medium and heavy nuclei in terms of the optical potential, by addition of the neutralization operator . The method is reasonable for energies . Explicit forms are obtained for the differential and total cross sections for an optical potential in the form of a complex rectangular well, V(r) = V₀., in which V₀ differs; from zero within the nucleus and . The value of V_o corresponding to the observed cross section for lead is derived. The dependence of the total cross section on V₀ for copper is given.I am indebted to P. E. Nemirovskii for direction and valuable advice.     

Optimized Peltier cooling via an array of quantum dots with stair-like ground-state energy configuration

With the advancement in fabrication and scaling technology, the rising temperature in nano devices has attracted special attention towards thermoelectric or Peltier cooling. In this paper, I propose optimum Peltier cooling by employing an array of connected quantum dots with stair-like ground-state eigen energy configuration. The difference in ground state eigen energy between two adjacent quantum dots in the stair-like configuration is chosen to be identical with the optical phonon energy for efficient absorption of lattice heat. I show that in the proposed configuration, for a given optical phonon energy, one can optimize the cooling power by tuning the number of stages in the array of quantum dots. A further analysis demonstrates that the maximum cooling power at a given potential bias under optimal conditions does not depend strongly on the optical phonon energy or the number of stages at which the maximum cooling power is achieved, provided that the optical phonon energy is less than kT. The proposed concept can also be applied to $2?D$ or bulk resonant tunnel and superlattice structures with stair-like resonant energy configuration.     

Effective damping of proton transfer in the H-bond caused by asymmetry of the reservoir

When studying some relaxation phenomena, the thermal reservoir is usually taken as a system of harmonic oscillators. In the model used here this type of the reservoir is coupled to the proton in such a way, that it distinguishes whether the proton is in the left or in the right well of the potential. Due to this asymmetry it may be expected that the steady state of the proton will not be symmetric. Surprisingly, in the Born approximation the reservoir does not change the previous symmetry of the steady state of the proton but changes its character of relaxation.I would like to thank to Dr. P. Baacký and to Dr. V. ápek for many useful discussions.     

Another Return of “Return to Equilibrium”

The property of return to equilibrium is established for a class of quantum-mechanical models describing interactions of a (toy) atom with black-body radiation, or of a spin with a heat bath of scalar bosons, under the assumption that the interaction strength is sufficiently weak. For models describing the first class of systems, our upper bound on the interaction strength is independent of the temperature T, (with 0<TT₀<), while, for the spin-boson model, it tends to zero logarithmically, as T0. Our result holds for interaction form factors with physically realistic infrared behaviour. Three key ingredients of our analysis are: a suitable concrete form of the Araki-Woods representation of the radiation field, Mourres positive commutator method combined with a recent virial theorem, and a norm bound on the difference between the equilibrium states of the interacting and the non-interacting system (which, for the system of an atom coupled to black-body radiation, is valid for all temperatures T0, assuming only that the interaction strength is sufficiently weak).Acknowledgements We thank V. Bach and I.M. Sigal for countless discussions on related problems and spectral methods without which this work would never have been done. M.M. is grateful to V. Jaksi for illuminating discussions. We have enjoyed the hospitality of IHES during the initial and final stages of this work.     

Some Thought-Experiments Involving Macrosystems as Illustrations of Various Interpretations of Quantum Mechanics

I consider various experiments related to the so-called macroscopic quantum coherence experiment, which are probably at present in the class of thought experiment but are likely to become realistic in the next few decades. I explore the way in which outcomes consistent with the predictions of quantum mechanics would be interpreted by an adherent of, respectively, the Copenhagen, statistical, and Bohmian interpretations of the formalism.     

The K-SAT Problem in a Simple Limit

In this work we compute the thermodynamic properties of the 3-satisfiability problem in the infinite connectivity limit. In this limit the computation can be strongly simplified and the thermodynamic properties can be obtained with a high accuracy. We find evidence for a continuous replica symmetry breaking in the region of high number of clauses, >_c.     

Optical phonons of graphene and nanotubes

We review the optical phonon dispersions of graphene. In particular, we focus on the presence of two Kohn anomalies in the highest optical phonon branch at the and points of the Brillouin zone. We then show how graphene can be used as a model for the calculation of phonons in carbon nanotubes. Finally, we present the beyond Born-Oppenheimer corrections to their phonon dispersions. These are experimentally revealed in the Raman spectra of doped samples.     

The influence of an uneven surface on the diffraction pattern of a perfect crystal

A simple theoretical model is given for the diffraction of X-rays on a perfect crystal with uneven surface. It is assumed that the crystal surface can be approximated by a system of small surfaces, variously inclined from the diffraction planes, and that the diffraction can be calculated as the superposition of independent diffractions from the different small surfaces. The model was verified experimentally by measuring a perfect germanium crystal with an uneven surface on a triple-crystal diffractometer. The unevenness of the surface was studied by optical measurement. Very good agreement with theory was obtained.In conclusion the authors thank Dr. Z. Trousil for preparing the Ge single crystals and M. Prochodov and V. idová for carefully performing the numerical calculations and measurements.     

Radioactive probe atoms in semiconductors

Progress in semiconductor technology is accompanied by progress in knowledge and control of intrinsic and extrinsic defects in these materials, i.e. of vacancies or self-interstitials and dopant or impurity atoms, respectively. In spite of the large number of different experimental techniques, in particular electrical and optical ones, employed for the characterisation of these defects, there is a shortage of techniques that are able to identify the chemical nature, local structure, and dynamic properties of defects on an atomic scale. The missing sensitivity towards low defect concentrations in the range of 10¹⁶ cm^–3 excludes many techniques which are known to fulfil these requirements. There is a growing number of experimental techniques, however, having in common the application of radioactive isotopes, so-called probe atoms, that deliver the required information also at low defect concentrations. Among the nuclear techniques, which will be discussed in context of their applicability to semiconductors, are the perturbed angular correlation spectroscopy, Mössbauer spectroscopy, and-nuclear magnetic resonance. But also non-nuclear techniques, such as the classical electrical and optical methods, being highly sensitive to low defect concentrations, will be shown to gain the missing sensitivity to the nature of defects if radioactive atoms are employed. For elemental semiconductors as well as for III–V, II–VI, and IV-IV compounds, it will be illustrated to what extent radioactive probe atoms contribute to delivering the needed knowledge and control of defects.