Structures with variable dimensionality of electronic states for unipolar lasers

We review our study aimed at the creation of a unipolar laser with the efficient suppression of nonradiative relaxation between the laser subbands owing to strongly asymmetric-in-height barriers surrounding an active element. The asymmetry of the barriers causes the existence of a lower laser subband only within a narrow region of in-plane momenta. In such a way, one-phonon relaxation between the laser subbands can be significantly suppressed.     

Nonlinear dynamics of a directly modulated semiconductor laser with cavity detuning

A detailed numerical study of dynamical behavior of a semiconductor laser under current modulation and cavity detuning has been performed on the base of four different models of the active medium which take into account direct transitions between ground subbands, transitions with no k-selection rule between ground subbands and contribution of excited subbands in each of the above-mentioned cases. We have shown that different nonlinear regimes (period doubling, chaos, generalized bistability) can be obtained either with cavity detuning from the gain band maximum or near the laser threshold.It has been established that the shape and principal peculiarities of amplitude detuning characteristics are determined by the relation between the current modulation frequency and maximum resonance frequency of the laser.PACS numbers: 05.45.Pq, 42.55.Px, 42.65.Sf     

Spin relaxation of the photoexcited electron system in P-type InSb

By means of far-infrared (84 μm) laser cyclotron resonance, it is found that the photoexcited electron system in p-type InSb can enter a spin-hot state. The spin temperature cools down from ∽60 K with a time constant of several microseconds at the lattice temperature 4.2 K. Crude calculation for spin-flipping between 0^- and 0⁺ Landau subbands by electron- impurity scattering yields a reasonable time constant quite comparable with experimental observation.     

Radiative and nonradiative recombination times in semiconducting films

The radiation emitted spontaneously by a semiconductor which has been excited for a very short time decays exponentially with a time constant that depends on the recombination rate of electrons and holes. This recombination rate is the combination of radiative and nonradiative transition rates between conduction and valence bands of the semiconductor. The radiative recombination rate depends on the density of states of the electromagnetic field, which can be made to be dependent on the geometry. In this paper, we report on the dependence of the fluorescence lifetime upon the thickness of active thin films. For systems in which the radiative recombination rate is dominant over the nonradiative ones, the total recombination time can be changed by suitable modifications of the thickness of the film. In this situation, the nonradiative rate can be evaluated. We present experimental results for the case of cadmium sulphide (CdS) thin films.     

Dependence of hole effective mass on nitrogen concentration in W-type strained InAs(N)/GaSb/InAs(N) quantum well lasers

We have investigated the effects of nitrogen N concentration on the properties of hole subbands and effective mass in dilute-nitride type-II InAsN/GaSb laser diodes on InAs substrate with “W” design. Using a 5-bands k·p model, we obtained interesting numerical results for the heavy-hole (hh) and the light-hole (lh) subbands. The hole effective masses were found to be very sensitive to the nitrogen concentration and to the differences in the Luttinger parameters between the well and the barrier. In addition, the hole effective masses are found to be strongly affected by band-anticrossing (BAC) model.     

Gain and threshold properties of InGaAsN/GaAsN material system for 1.3-μm semiconductor lasers

The gain properties and valence subbands of InGaAsN/GaAsN quantum-well structures are numerically investigated with a self-consistent LASTIP simulation program. The simulation results show that the InGaAsN/GaAsN has lower transparency carrier density than the conventional InGaAsP/InP material system for 1.3-μm semiconductor lasers. The material gain and radiative current density of InGaAsN/GaAsN with different compressive strains in quantum well and tensile strains in barrier are also studied. The material gain and radiative current density as functions of strain in quantum well and barrier are determined. The simulation results suggest that the laser performance and Auger recombination rate of the 1.3-μm InGaAsN semiconductor laser may be markedly improved when the traditional GaAs barriers are replaced with the AlGaAs graded barriers.     

Model calculations of diffusion limited trapping dynamics in quantum well laser structures

We present a phenomenological theoretical model to treat the trapping of carriers into quantum wells of semiconductor laser structures. We consider explicitely the transport within the barrier layers by solving the continuity equation with the appropriate boundary conditions taking into account surface recombination, radiative and nonradiative recombination in the barrier layers and trapping of carriers into the quantum wells. The experimental findings for the trapping dynamics in GaAs/AlGaAs quantum well structures can be consistently interpreted by the model calculations.     

Tunneling spectroscopy of electron space charge layers on (111)-Si

MgSiO₂Si tunnel junctions were prepared on nondegenerate substrates in a planar structure. By suitable technology-steps a low-resistance connection between the diffused drain contact and an electron enhancement channel, which was generated by the work function difference of the electrodes and charged defects in the barrier region, can be realized; therefore low-temperature measurements of the current-voltage characteristic and its derivatives were possible. In the $\text{d}{}^2\text{I}\text{dU}{}^2$ vs. U-traces various structures have been observed, which were caused by band edge effects of numerous surface subbands, by phonon-assisted tunneling and by a zero-bias anomaly. At nearly constant surface charge density the energy separation of the subbands and its dependence on depletion charge density, varied by the application of a substrate bias, has been measured. Because of the direct reading of the subband energies relative to the Fermi-level the surface density could be estimated.     

Multiple wavelength electroluminescence and laser generation in p-i-n resonant tunneling heterostructures

We report a design and electroluminescence (EL) investigation of a p-i-n resonant tunneling device based on an Al_0.4Ga_0.6As/GaAs graded-index waveguide heterostructure. The intrinsic region of the structure consists of a quantum well (QW) surrounded by multiple barrier energy filters providing simultaneous resonant occupation of electron and heavy-hole second excited subbands in the QW. Several peaks are observed in the EL spectra, confirming occupation of the excited subbands. The EL efficiency displays a resonant behavior accompanied by an S-shaped negative differential resistance region in the voltage–current characteristic. Current bistability is demonstrated, leading to bistability in the EL and laser generation spectra.     

Modeling of the Kinetics of Intramolecular Processes and Transient Spectra with Allowance for Nonradiative Transitions

It has been shown that the kinetics of intramolecular processes and time-resolved spectra with allowance for the quantum beats of the resonant states of isomers or isolated subsystems of levels of one isomeric form can be described with the use of a molecular model interpreting the effect of beats as a nonradiative transition. We have obtained an expression for the nonradiative transition probability, which is directly proportional to the beat frequency and depends oscillatorily on time, thus modeling the effect of beats. The parameter of the molecular system model is the beat frequency directly related to the parameter characterizing the intramolecular interisomeric interactions (the corresponding nondiagonal element of the energy matrix) rather than the value of the nonradiative transition probability. The character of the change in the level populations and, accordingly, in the band intensities in the spectra in the proposed model is in good agreement with the experiment, including the fine structure of the time dependences — oscillations of the line intensities. In analyzing the temporal experiment with a high resolution, it is necessary to take into account the instrument function leading to quantitative and qualitative changes in the time dependences. The traditional model of nonradiative transitions with a constant probability value has a very limited range of applicability — very high beat frequencies compared to the probability of optical transitions.     

Observation of the quasidiffusive propagation of phonons in Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>:Ho<Superscript>3+</Superscript> using time-resolved photo-phonon spectroscopy

The energy flux of phonons produced due to the nonradiative laser-induced transitions of Ho³⁺ impurity ions in forsterite from the ⁵F₅ states has been measured using a superconductor bolometer at a temperature of 2 K. The dependence of the flux on the laser wavelength, the time elapsed after the action of a laser pulse, and the phonon propagation path length is analyzed. It is found that the excitation of Ho³⁺ to some states leads to the diffusive propagation of emitted phonons in the spontaneous frequency decay mode (quasidiffusive mode of propagation): the time of arrival of a phonon pulse is almost a linear function of the path length, but it is several times longer than the longest ballistic time of flight (for transverse phonons). The diffusion coefficient and the nonradiative relaxation time are determined from the best fit to the experiment.     

Determination of energy difference and width of minibands in GaAs/AlGaAs superlattices by using Fourier transform photoreflectance and photoluminescence

In this work, Fourier transform photoreflectance (in a form of fast differential reflectance spectroscopy) has been used to study the interband optical transitions in molecular beam epitaxially grown GaAs/AlGaAs superlattices. The dependence of the measured features on the growth parameters (QW and barrier widths) has been studied. The minibands widths and energy differences between them have been obtained and matched to these coming from effective mass calculations. In addition, it has been shown that Fourier transform photoluminescence measurement might be used in the far infrared region (up to ∼15 μm) to the direct detection of the energies of intraband transitions between the electron minibands (subbands) in the superlattice and QW system.     

Single-particle subband properties of quantum cables

We proposed a new kind of coupled coaxial cylindrical quantum wires structure - quantum cable, and calculated its single-electron energy subband spectrum for the varying structure parameters, in order to investigate its subband motion in the structure parameter space. It is shown that quantum cable has unique subband spectrum, which differs either from the (solid and hollow) cylindrical quantum wire or from the usual coupled double quantum wires (CDQWs) structure. Aside from the two-fold degeneracy induced by the cylindrical symmetry, crossings (accidental degeneracies) and anticrossings (repulsions) of quantum cable subbands with different azimuthal and radial quantum numbers are observed as one of the cable structure parameters varies. This introduces the dependence of the subband ladder on the structure parameters of the quantum cable structure. However, the subband with the lowest azimuthal and radial quantum numbers remains the lowest subband and never crosses with the other subbands irrespective of the value of structure parameters. As the coupling barrier is broadening (coupling becoming weak), some subbands bundling toward another subband is seen before the extreme isolating limit achieved. Moreover, the separation between neighboring subbands exhibits non-monotonous evolution as one changes the thickness of one of the cylindrical quantum wires, with a minimum existing in the separation between some two adjacent subbands. Interesting optical and transport phenomena arising from these unique subband properties of the quantum cable structure are also predicted. Received 22 March 2000 and Received in final form 6 June 2000     

微腔激光器速率方程分析

推出了至少有一维尺寸在发射波长量级的微腔激光器的光输出特性，首次得到了无辐射衰减速率不等于零时速率方程的角析解。结果表明，微腔激光器自发发射到腔模的较高的合效率使激发特性和传统的激光器有很大的不同。     

The inter molecular potential of NH+ 4-Ar II. Calculations and experimental measurements for the rotational structure of the v 3 band

The mid-infrared spectrum of the v ₃,(t ₂) transition of the NH⁺ ₄-Ar complex has been recorded at rotational resolution using photofragmentation spectroscopy. The spectrum is divided into perpendicular and parallel subbands corresponding to transitions between different hindered internal rotor states. The P and R branches of the strongest perpendicular subbands are rotationally resolved providing rotational and centrifugal distortion constants. The widths of individual rotational lines are limited by the laser bandwidth of 0.02 cm^?1, giving a lower limit of 250 ps for the lifetime of the excited states. Effective intermolecular separations for each internal rotor state are determined from its rotational constant, after correction for the contribution due to Coriolis coupling between the internal and total rotational angular momenta. The absolute energies, rotational and distortion constants for the first few intermolecular bending and stretching levels of the ground intramolecular vibrational state are determined in a numerical solution to the rotation-intermolecular vibration Hamiltonian, employing a three-dimensional ab initio intermolecular potential. The results are compared with the experimental constants in order to assess the accuracy of the calculated potential. The relative energy levels from this calculation are also compared with those from a two-dimensional representation of the potential energy surface (‘fixed-R’ model) in order to judge directly the influence of the radial dependence of the potential.     

A k.p model for hole capture into a quantum well

A k.p model for hole capture into a quantum well has been applied to the calculation of the rates for capture via phonon and alloy scattering into a 30Å In_0.7Ga_0.3As-InGaAsP quantum well, a system which is suitable for lasers operating at 1.55 μm. The well has three subbands, derived from the HH1, LH1 and HH2 zone centre states. Capture of holes into the HH2 subband is predicted to be dominated by polar optical phonon emission, whereas scattering into the other two bands proceeds mainly by non-polar optical phonon emission or alloy scattering. There is structure in the capture rate plotted as a function of the energy and in-plane wavevector of the incident hole, which is attributed to instances where incident holes in barrier states undergo strong transmission into the well region. Mixing between the quantum well subbands is important through the effect upon the density of final states in capture transitions, but is less significant with respect to its effect on the scattering matrix elements.     

Quantum Hall effect in a system with an electron reservoir

Precise measurements of the magnetic-field and gate-voltage dependences of the capacitance of a field-effect transistor with an electron system in a wide GaAs quantum well have been carried out. It has been found that the capacitance minima caused by the gaps in the Landau spectrum of the electron system become anomalously wide when two size-quantization subbands are occupied. The effect is explained by retention of the chemical potential in the gap between the Landau levels of one of the subbands owing to redistribution of electrons between the subbands under a change in the magnetic field. The calculation taking into account this redistribution has been performed in a model of the electron system formed by two two-dimensional electron layers. The calculation results describe both the wide capacitance features and the observed disappearance of certain quantum Hall effect states.     

Temperature and injection current dependent electroluminescence study of GaInP/AlGaInP quantum well laser diode grown using tertiarybutylarsine and tertiarybutylphosphine

GaInP/AlGaInP triple quantum well (TQW) lasers, grown by metalorganic chemical vapor deposition (MOCVD) using tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP), were fabricated with a pulsed anodic oxidation (PAO) process. The devices worked at room temperature (RT) with the lowest threshold current density (J_th) of 1.5 kA/cm² ever reported for GaInP/AlGaInP lasers grown using TBAs and TBP. Temperature dependent (35–250 K) electroluminescence (EL) study of the GaInP/AlGaInP laser diode showed almost the same luminescence quenching behavior at a high temperature region (120–250 K), independent of the injection current (100–150 mA). A model involving a nonradiative recombination mechanism was presented to interpret the EL quenching behavior over the experimental temperature range. The nonradiative recombination centers in the Al-containing barrier or cladding layer are believed to contribute to the loss of carriers via nonradiative recombination. PACS 78.60.Fi; 71.20.Nr; 78.67.De; 81.15.Gh; 42.55.Px     

Inductive-resonant theory of nonradiative transitions in lanthanide and transition metal ions (review)

The review is devoted to the theory of nonradiative transitions in tricharged ions of lanthanides and transition metals in the condensed phase, which was proposed in 1971. The theory is based on the phenomenon of nonradiative energy transfer from an electronically excited ion to surrounding molecular groups with excitation of resonant vibrational states and makes it possible to calculate the nonradiative transition rate constant (k _nr) by a formula that is similar to the Förster formula. The primary emphasis is placed on recent experimental works that directly confirm the proposed theory. It is shown that the theory satisfactorily quantitatively accounts for (i) the effect of deuteration of molecular groups surrounding ions on k _nr, (ii) the energy gap law, and (iii) the dependence of k _nr on the distance between the ion and deactivating groups. Furthermore, it is shown that (iv) the theory makes it possible to satisfactorily quantitatively calculate in the dipole-dipole approximation the constant k _nr of the electronic transition based on the knowledge of the radiative rate constant and the vibrational absorption spectra of molecular groups in the range of overlap with the luminescence spectrum of the ion; (v) the temperature dependence of k _nr; and (vi) the anomalously low k _nr in the case where the corresponding radiative transition is caused by the magnetic rather than the electric dipole. Literature data are presented that directly experimentally support the proposed theory of nonradiative transitions. In addition, works where this approach is used to calculate k _nr of transitions in laser media are described.     

Selected Problems of Relativistic Quantum Mechanics and Atomic Physics

The paper presents a brief review of the scientific work performed by the authors in the field of quantum mechanics and atomic, laser, and mathematical physics. The following problems are considered: the semiclassical theory of tunneling and multiphoton ionization of atoms and ions in a strong electromagnetic field; generalization of the Keldysh ionization theory to the relativistic case; calculation of the Coulomb corrections to the ionization rate of atoms for arbitrary values of the adiabaticity parameter γ: from γ ≪ 1 (the adiabatic region) to γ ≫ 1, when the laser field changes its direction and magnitude many times during the time of flight of the electron through the barrier; the Lorentz ionization of atoms moving in a constant magnetic field; the WKB approximation and the imaginary time method for describing electron tunneling through a time-varying barrier; the Stark effect in a strong field; the energy spectrum of a hydrogen atom in a strong and superstrong magnetic field; quantization with account of the barrier transparency; creation of electron-positron pairs from vacuum in a constant electric or intense pulsed (laser) field and the dependence of the number of pairs on the intensity and frequency of the laser field; the Feynman method of disentanglement of noncommuting operators and its applications: transitions between atomic states in an alternating magnetic field (the Majorana problem); a quantum oscillator with time-dependent frequency; and a singular oscillator. The mathematical problems of quantum mechanics are considered: the fall of a particle to the center; modification of the Bohr-Sommerfeld quantization condition for potentials with a barrier and the Kramers matching conditions; divergence of perturbation series and their summation; eigenvalues of the Casimir operators for irreducible representations of Lie groups, including the SU(2), SU(3), and SU(6) groups, which are widely used in physics.