Modulation behavior of semiconductor injection lasers

GaAs double heterostructure semiconductor injection lasers which now exhibit more than 25000 h cw room temperature lifetime are of great interest for future use as directly modulated transmitters for high bit-rate fiber optical communications. The effects limiting this application are modulation distortions, spectral width and additional spectral broadening in the case of modulation and spontaneous fluctuations of the output power. The dynamic and spectral behavior of injection lasers, the methods of high bit-rate modulation and the improvement of the high bit-rate modulation capability by coupling two lasers are discussed.     

Spectral broadening enhancement in silicon waveguides through pulse shaping

Spectral broadening in silicon waveguides is usually inhibited at telecom wavelengths due to some adverse effects related to semiconductor dynamics, namely, two-photon and free-carrier absorption (FCA). In this Letter, our numerical simulations show that it is possible to achieve a significant enhancement in spectral broadening when we properly preshape the input pulse to reduce the impact of FCA on spectral broadening. Our analysis suggests that the use of input pulses with the correct skewness and power level is crucial for this achievement.     

Fermi-surface truncation from thermal nematic fluctuations

We analyze how thermal fluctuations near a finite temperature nematic phase transition affect the spectral function A(k,ω) for single-electron excitations in a two-dimensional metal. Perturbation theory yields a splitting of the quasiparticle peak with a d-wave form factor, reminiscent of a pseudogap. We present a resummation of contributions to all orders in the Gaussian fluctuation regime. Instead of a splitting, the resulting spectral function exhibits a pronounced broadening of the quasiparticle peak, which varies strongly around the Fermi surface and vanishes upon approaching the Brillouin-zone diagonal. The Fermi surface obtained from a Brillouin-zone plot of A(k,0) seems truncated to Fermi arcs.     

Study of Be δ-doped GaAs/AlAs multiple quantum wells by the surface photovoltage spectroscopy

We report a surface photovoltage and differential surface photovoltage (DSPV) study of Be δ-doped GaAs/AlAs multiple quantum wells (QWs) with widths ranging from 3 to 20 nm and sheet doping densities from 2 × 10¹⁰ to 2.5 × 10¹² cm⁻² per well aiming to characterize their electronic properties and structural quality. From a line shape analysis of room temperature DSPV spectra the interband excitonic transition energies and broadening parameters for a large number of QW-related subbands have been established. A study of well-width and quantum number dependencies of the excitonic linewidths allowed us to evaluate the various broadening contributions to the spectral line shapes in QWs of different design. It was found that an average half monolayer well-width fluctuations are the dominant broadening mechanism of the excitonic line for QWs thinner than 10 nm. In QWs thicker than 10 nm, the spectral line broadening originates mainly from thermal broadening as well as Stark broadening due to random electric fields of ionized impurities and exciton scattering by free holes.     

Description of Electromagnetic Radiation Propagating in a Four-Dimensional Space-Time with a Fluctuating Metric Tensor

The effect of spectral broadening of electromagnetic radiation propagating in a four-dimensional space-time with fluctuations in the space curvature caused by relict gravitational radiation is predicted. It is demonstrated that distortion of spectral line profiles of electromagnetic radiation caused by the fluctuating metric is at the level of resolution of the available spectral instrumentation.     

Monitoring statistical magnetic fluctuations on the nanometer scale

Statistical fluctuations of the magnetization are measured on the nanometer scale. As the experimental monitor we use the characteristic photoluminescence signal of a single electron-hole pair confined in one magnetic semiconductor quantum dot, which sensitively depends on the alignment of the magnetic ion spins. Quantitative access to statistical magnetic fluctuations is obtained by analyzing the linewidth broadening of the single dot emission. Our all-optical technique allows us to address a magnetic moment of only approximately equal 100 micro(B) and to resolve statistical changes on the order of a few micro(B).     

Optoelectronic Devices: Design,Modeling, and Simulation,by Xun Li

A historical survey of the development of solid-state detectors is given, and it is shown why semiconductor detectors are superior to the earlier crystal counters. The physical processes which occur during the detection of nuclear radiation in a solid-state device are considered in detail, and the merits of the reverse-biased semiconductor junction in silicon or germanium are set out. Factors which determine the energy resolution of such a detector are analysed, and also the effects of radiation damage. The preparation of such detectors is not treated in detail, but the physical principles on which the important types of detector depend are described. The final section surveys the field of applications of solid-state detectors in nuclear physics, radiochemical analysis, space research, medicine and biology.     

Noise spectrum of a multicircuit oscillator

We study the inherent power fluctuations in multicircuit self-oscillating systems, which can be represented by an oscillating circuit with a nonlinear element connected in series, and by a multiresonance linear system. We obtain in a general form the expressions for the spectral density of power fluctuations. We show that the shape of the spectral line of a multicircuit self-oscillator is, in general, asymmetric, and the pedestal has a number of maxima. The structure of the pedestal is determined by the roots of the characteristic stability equation. An estimate of the width of the spectral line is given. As examples, we consider the line shape of two- and three-circuit frequency stabilization systems. We take into account the noise contributions of separate circuits at various temperatures.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 27, No. 1, pp. 71–78, January, 1984.In conclusion, the authors express their gratitude to I. I. Minakova for the useful discussions of the results of the paper.     

Photon correlation probing of spectral diffusion in impurity glasses

A method of photon correlation probing of spectral diffusion at ultralow temperatures is suggested for polymer glasses. The method makes it possible to determine the ensemble-averaged autocorrelation function of fluctuations in the spectra of individual chromophores from the fluctuations of the fluorescence intensity of a sample, as well as to select subensembles of impurity molecules by the amplitudes of spectral jumps of their absorption lines due to the transitions of molecules between equilibrium states closely spaced in energy. The method has a number of advantages in comparison with the conventional methods of investigation of spectral diffusion: single-molecule spectroscopy, hole-burning spectroscopy, and the measurement of hole broadening with time.     

Hot electron and hot phonon contributions to radiative emission spectra in CdS at high excitation intensities

The dependence of excitation frequency of photoluminescence spectral shape of CdS at high excitation intensities allowed discrimination between hot phonon and hot electron contributions to the broadening of the emission band on one side and the broadening due to many body effects on the other. It was found that the shift towards lower energies of the emission peak is mainly due to induced high carrier density and that the broadening is related to the presence of high densities of optical phonons and hot carriers.     

Kinetic theory of radiation in non-equilibrium relativistic plasmas

Many-particle QED is applied to kinetic theory of radiative processes in many-component plasmas with relativistic electrons and non-relativistic heavy particles. Within the framework of non-equilibrium Green’s function technique, transport and mass-shell equations for fluctuations of the electromagnetic field are obtained. We show that the transverse field correlation functions can be decomposed into sharply peaked (non-Lorentzian) parts that describe resonant (propagating) photons and off-shell parts corresponding to virtual photons in plasmas. Analogous decompositions are found for the longitudinal field correlation functions and the correlation functions of relativistic electrons. As a novel result a kinetic equation for the resonant photons with a finite spectral width is derived. The off-shell parts of the particle and field correlation functions are shown to be essential to calculate the local radiating power in relativistic plasmas and recover the results of vacuum QED. The influence of plasma effects and collisional broadening of the relativistic quasiparticle spectral function on radiative processes is discussed.     

Quantum Dot Versus Quantum Well Semiconductor Optical Amplifiers for Subpicosecond Pulse Amplification

The performance of quantum well and quantum dot semiconductor optical amplifiers was theoretically investigated. The effects on subpicosecond pulse propagation due to gain and refractive index dispersion, calculated using a microscopic polarization equation and a reduced wave equation in the linear regime including the background refractive index dispersion, were used in the comparison. In particular, the spectral shift and phase modulation imposed on the pulse were compared. It is shown that quantum dot amplifiers suffer comparable spectral shifts to the quantum well amplifier, strong linear frequency chirp and large pulse broadening. In quantum dot amplifiers with small inhomogeneous broadening, similar pulse break-up is shown as that calculated for the quantum well amplifier. In quantum dot amplifiers with large inhomogeneous broadening, the background refractive index dispersion makes the linear frequency chirp the dominant feature. In the light of our calculations, the advantages and disadvantages of quantum dot and quantum well amplifiers are discussed.     

色散关系和纵向耦合腔(C3)对半导体激光器的模谱行为的影响

本文从理论上探讨了色散关系的作用,引进α_e和F因数,修正了半导体激光器的光谱线宽公式,发现通过波导结构的设计不但可减小α_e因数,还可增大F因数来进一步抑制光谱线宽,用所提出的等效反射率法分析了纵向耦合腔(C³)的选模和压缩线宽的机制,指出实现单纵模窄线宽的条件。 关键词：     

QED background to heavy lepton production ine + e − annihilation

The paper presents two measures, which are infrared stable and which easily relate to known properties of soft hadronic jets. The measures allow for an approximate addition of contributions from individual phases in multistep processes; the contributions from typical initial parton configurations, from evoluting jets and from final state (confinement) fluctuations are evaluated with suitable approximations in a general quantum chromodynamical framework.     

Picosecond time-resolved photoluminescence at detection wavelengths greater than 1500 nm

We report what is to our knowledge the first application of high-efficiency InGaAs/InP photon-counting diode detectors in time-resolved photoluminescence measurements at wavelength greater than 1500 nm. When they were cooled to 77 K and used in conjunction with the time-correlated single-photon counting technique, the detectors were capable of an instrumental response of 230 ps and a noise equivalent power of 2x10(-17)W Hz(-1/2) . Preliminary measurement of a semiconductor heterostructure indicates sensitivity at photogenerated carrier densities as low as 10(14)cm (-3) . This development facilitates the detailed characterization of dominant recombination mechanisms in semiconductor optoelectronic materials and devices designed to operate in the third telecommunications spectral window.     

Fine structure of emission spectrum of a single-frequency injection laser

A theoretical analysis of single-frequency lasing is carried out in the framework of the quasimonochromatic field method with variable field amplitude and phase. The spectral optical-line profile, the spectral distribution of the laser output-power fluctuations, and the spectral distribution of the active-region density fluctuations (of the p-n voltage fluctuations) are obtained. In contrast to the known work in this field, the analysis in the present paper is carried out consistently starting with the wave equation and with account taken of the wavelike character of the radiation propagation in the active region. This has made it possible to express the final distributions in terms of quantities that are most readily measurable. Allowance for thermodynamic fluctuations has revealed an additional lasing-line broadening independent of the laser power. The analysis is not confined to lasing in the diode's own cavity, but takes into account also the possibility of using an external cavity, and corresponding varying the optical line spectral distributions, the output-power fluctuations, and p-n junction voltage fluctuations.Optoelectronics Laboratory, Lebedev Physics Institute Academy of Sciences of the USSR. Translated from Preprint No. 256 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1988.     

Density of states of narrow superconducting channels in the regime of quantum fluctuations of the order parameter

The current–voltage characteristics of superconductor–insulator–semiconductor (S1–I–S2) tunnel junctions, where superconducting electrode S2 is a thin nanowire, are studied experimentally. The observed blurring of the gap singularities is interpreted as a manifestation of the order parameter quantum fluctuations. We propose a model taking into account the broadening of the density of states due to the interaction of electrons with the Mooij–Schön plasmon mode emerging in a quasi-one-dimensional superconducting channel in the regime of quantum fluctuations of the order parameter. The model gives results that are in a reasonable qualitative agreement with the experimental data.     

Detector electronics for experiments at the large hadron collider

The state of the art of a tracking detector and calorimeter electronics that are being developed for experiments at the Large Hadron Collider (LHC) is discussed. Construction of the detectors is briefly described. The problems of fabrication of integrated circuits based on a radiation-resistant technology are considered, as well as the solution to the problem of microconnections between sensitive elements and readout amplifiers in two-coordinate semiconductor detectors. The parameters and block diagrams of both analog and digital integrated circuits are given; these circuits are used for amplifying and shaping the signals measured by tracking detectors of elementary particles and calorimeters. The contributions of Russian experimenters and physicists of the Joint Institute for Nuclear Research to the development of detector electronics for experiments at the LHC is described.     

Power Broadening of Two-Photon Coherent Resonances on Rydberg Atomic Transitions in a Magneto-Optical Trap

We study power broadening of coherent two-photon resonances on Rydberg transitions in lithium-7 atoms in a magneto-optical trap (MOT). We apply the spectroscopic technique based on the reduction of resonance fluorescence in the MOT. One of the main contributions to the spectral broadening is associated with heating of the trapped atoms by laser cooling beams. We suggest a new nondestructive method to measure the atomic temperature in a working MOT using two-photon spectroscopy with variable directions of probe beams.