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Terahertz quantum cascade-lasers (QCLs) are able to produce higher optical output power at higher temperatures when operated in pulsed mode. Predicting a laser's behavior under pulsed operation in order to achieve performance requirements is, however, a nontrivial exercise: the complex and nonlinear interplay between current, electric field, and thermal transients gives rise to complex responses in both optical output power and emission frequency. In applications where it is important to predict and control these behaviors, establishing the link between current drive, emission frequency, and optical output power is necessary. In this paper, we demonstrate, via a realistic laser-specific model, that by appropriate manipulation of the drive pulse we can not only obtain a higher optical output at increased operating temperature but also both extend and linearize a QCL's frequency sweep. We suggest that consideration of laser behavior through realistic and comprehensive modeling is not only useful but is also required in any pulsed application in which emission frequency change is likely to affect performance.

The design, modeling, fabrication, and characterization of single-photon avalanche diode detectors with an epitaxial Ge absorption region grown directly on Si are presented. At 100 K, a single-photon detection efficiency of 4% at 1310 nm wavelength was measured with a dark count rate of ~ 6 megacounts/s, resulting in the lowest reported noise-equivalent power for a Ge-on-Si single-photon avalanche diode detector (1×10-14 WHz-1/2). The first report of 1550 nm wavelength detection efficiency measurements with such a device is presented. A jitter of 300 ps was measured, and preliminary tests on after-pulsing showed only a small increase (a factor of 2) in the normalized dark count rate when the gating frequency was increased from 1 kHz to 1 MHz. These initial results suggest that optimized devices integrated on Si substrates could potentially provide performance comparable to or better than that of many commercially available discrete technologies.

We make a systematic study to find the dependence of the threshold current density of a mid-infrared quantum-cascade laser on the waveguide ridge width. The optical and thermal parameters of the waveguide are calculated with a finite-element method, and gain is found from a self-consistent rate equation model with energy-balance. The results show the existence of an optimal ridge width that minimizes the threshold current density of the laser. The influence of the interface roughness parameters on laser performance is also discussed.

In recent years, SiGeSn alloys have been considered as promising materials for optoelectronic applications because they offer the possibility of a direct bandgap, and are compatible with Si-based technology; therefore, they have prospective applications such as in interband lasers and detectors, solar cells, etc. Here, we consider another possible application of nanostructures based on these materials: to extend the suite of Si-based optoelectronic devices for interband electroabsorption (EA) modulators. Using the 8-band k.p method, we have designed asymmetric double quantum wells that are optimized, by varying the well and barrier widths and material composition, to show large optical transmission sensitivity to the applied bias. Generally, these structures are useful for EA modulators in the midinfrared spectral range.

Judd-Ofelt analysis is performed on measurements of bulk samples of Tm3+- and Ho3+-doped tellurite glass from which the host-dependent Judd-Ofelt intensity parameters are extracted. These have then been used to calculate the radiative rates and branching ratios in this particular material system. A rate-equation approach is then used to model an experimentally realized ~2.1 μm Tm3+/Ho3+ codoped tellurite fiber laser and extract values of the energy transfer and upconversion rate parameters in TeO2-ZnO-Na2O (TZN) glass. Excellent agreement is found between simulated and experimental data, which indicates the validity of the approach.

A rate equation approach is used to model an experimentally realized ~ 2 mum Tm3+-doped tellurite fiber laser utilizing an in-band pumping scheme (3H6 rarr 3F4) . Excellent agreement between the theoretically predicted and experimentally measured slope efficiency is obtained. It is shown that in order to achieve agreement between the predicted and measured thresholds it is important to include cross-relaxation mechanisms with the 3H4 level, even with the in-band pumping scheme. By fitting the results of the model with level lifetime measurements and the threshold pump powers we have extracted the cross-relaxation parameters of Tm-doped tellurite glass.

The formation of domains in quantum cascade structures is one of the mechanisms strongly affecting the operation of quantum cascade lasers, quantum-well infrared detectors, and other devices. In this paper, we consider the problem of domain formation in p-doped Si/SiGe quantum cascades, using a carrier scattering transport framework. In effect, the hole flow along the cascade is described via scattering between quantized states belonging to neighboring periods, caused by phonons, alloy disorder, and carrier-carrier interactions. The generation of either periodic or of nonperiodic domains is studied in uniformly doped cascades, as well as the influence of modulation doping of cascades on the domain formation.

We propose an optically pumped laser based on intersublevel transitions in InAs-GaAs pyramidal self-assembled quantum dots. A theoretical rate equations model of the laser is given in order to predict the dependence of the gain on pumping flux and temperature. The energy levels and wave functions were calculated using the 8-band k/spl middot/p method where the symmetry of the pyramid was exploited to reduce the computational complexity. Carrier dynamics in the laser were modeled by taking both electron-longitudinal optical phonon and electron-longitudinal acoustic phonon interactions into account. The proposed laser emits at 14.6 /spl mu/m with a gain of g/spl ap/ 570 cm/sup -1/ at the pumping flux /spl Phi/=10/sup 24/ cm/sup -2/ s/sup -1/ and a temperature of T=77 K. By varying the size of the investigated dots, laser emission in the spectral range 13-21 /spl mu/m is predicted. In comparison to optically pumped lasers based on quantum wells, an advantage of the proposed type of laser is a lower pumping flux, due to the longer carrier lifetime in quantum dots, and also that both surface and edge emission are possible. The appropriate waveguide and cavity designs are presented, and by comparing the calculated values of the gain with the estimated losses, lasing is predicted even at room temperature for all the quantum dots investigated.

The design of ZnCdSe-ZnMnSe-based quantum wells is considered, in order to obtain a large shift of the peak absorption wavelength in the far infrared range, due to a giant Zeeman splitting with magnetic field, while maintaining a reasonably large value of peak absorption. A triple quantum-well structure with a suitable choice of parameters has been found to satisfy such requirements. A maximal tuning range between 14.6 and 34.7 meV is obtained, when the magnetic field varies from zero to 5 T, so the wavelength of the absorbed radiation decreases from 85.2 to 35.7 /spl mu/m with absorption up to 1.25% at low temperatures. These structures might form the basis for magnetic field tunable photodetectors and quantum cascade lasers in the terahertz range.

A method is proposed for the design and optimization of structural parameters of GaN-AlGaN Bragg-confined structures with respect to peak intersubband absorption from the ground to the first excited state,1 /spl rarr/ 2 electronic transition, in the near infrared spectral range. An above-the-barrier bound state was used to extend the range of transition energies above the values available in conventional quantum wells. Intrinsic polarization fields and nonparabolicity effects were taken into account. The selection of optimal parameters, maximizing the absorption at wavelengths of 1.55 and 1.3 /spl mu/m, was performed by using a simulated annealing algorithm, and optimal structures with infinite superlattices as confinement regions were thus designed. These optimal parameters were then used to set realistic, finite structures with a small number of layers, the performance of which was re-evaluated by solving the Schrodinger-Poisson equation self-consistently for a few different levels and profiles of doping.

The influence of permanent dipole moments of quantized states on intersubband second harmonic generation based on quantum-interference induced transparency in semiconductor quantum wells is explored using the harmonic balance method. The permanent moments are found to be quite important: they affect the transparency condition, especially at larger pump intensities. Hence, both the conversion efficiency and the optimal interaction path length change significantly when accounting for the permanent moments, and the conversion efficiency is reduced.

A method is described for the optimized design of quantum-well (QW) structures, in respect to maximizing the stimulated gain in optically pumped intersubband lasers. It relies on applying supersymmetric quantum mechanics (SUSYQM) to an initial Hamiltonian, in order to both map one bound state below the spectral range of the initial Hamiltonian, and to generate a parameter-controlled family of isospectral Hamiltonians with the desired energy spectrum. By varying the control parameter, one changes the potential shape and, thus, the values of dipole matrix elements and electron-phonon scattering matrix elements. The use of this procedure is demonstrated by designing smoothly graded and stepwise-constant Al/sub x/Ga/sub 1-x/As ternary alloy QWs, with the self-consistent potential taken into account. Finally, the possibility of employing layer interdiffusion to get optimal smooth potentials is discussed.

Electron transport through quantum-dot (QD) cascades was investigated using the formalism of nonequilibrium Green's functions within the self-consistent Born approximation. Polar coupling to optical phonons, deformation potential coupling to acoustic phonons, as well as anharmonic decay of longitudinal optical phonons were included in the simulation. A QD cascade laser structure comprising two QDs per period was designed and its characteristics were simulated. Significant values of population inversion enabling lasing in the terahertz frequency range were predicted, with operating current densities being more than an order of magnitude smaller than in existing terahertz quantum-well-based quantum-cascade lasers.

Methods for systematic optimization of step-graded and continuously graded ternary alloy based quantum wells (QW's), in respect to second- or third-order intersubband nonlinear susceptibilities at resonance, are discussed. The use of these methods is examplified on the design of Al/sub x/Ga/sub 1-x/N and Al/sub x/Ga/sub 1-x/As-based QW's intended for resonant second harmonic or third harmonic generation with h/spl omega/=116 meV or h/spl omega/=240 meV pump photon energies, the objective being the largest susceptibility achievable with the chosen material. The obtained results exceed those previously reported in the literature.

Intersubband electron scattering transport in terahertz GaAs-AlGaAs quantum cascade lasers is analyzed, using a full 13-level self-consistent rate equation model. The approach includes all relevant scattering mechanisms between injector-collector and active region states in the cascade structures. Employing an energy balance equation which includes the influence of both electron longitudinal optical phonon and electron-electron scattering, the method also enables evaluation of the average electron temperature of the nonequilibrium carrier distributions in the device. The electron temperature is found to give a strong influence on the output characteristics, particularly at very low temperatures. The threshold currents and electric field-current density characteristics are in very good agreement with experiment, implying that the model has a strong predictive capability.

We describe a combined 6 × 6 k · p and one-band effective mass modelling tool to calculate absorption spectra in Ge-SiGe multiple quantum well (MQW) heterostructures. We find good agreement with experimentally measured absorption spectra of Ge-SiGe MQW structures described previously in the literature, proving its predictive capability, and the simulation tool is used for the analysis and design of electroabsorption modulators. We employ strain-engineering in Ge-SiGe MQW systems to design structures for modulation at 1310 nm and 1550 nm.

The permanent dipole-induced two-level Raman scattering in semiconductor quantum wells in a static electric field is analyzed. Calculations performed for GaAs/Al/sub x/Ga/sub 1-x/As multiple quantum wells (MQWs) indicate that reasonable, but not high, Stokes wave amplification could be obtained, and eventually used for IR generation. The gain is limited primarily by reststrahlen absorption and a rather large transition linewidth. >

A CMOS compatible interference-based Mach-Zehnder modulator with each arm comprising a metal-insulator-silicon-insulator-metal structure and a simpler, single-arm variant is considered for electro-optic and electroabsorption modulation, respectively, using finite-element electromagnetic simulations. In the calculation of electron density profiles in bias-induced accumulation layers, the size-quantization effects at oxide-silicon interfaces are considered. These are then used to find the complex refractive index profiles across the structure, in its biased and unbiased states, and eventually the modulator insertion loss and extinction ratio, as they depend on various structural parameters. Furthermore, an alternative modulator, comprising a metalized stub filled with SiGe/Ge multiple quantum wells or quantum dots in a silicon matrix, coupled to a dielectric waveguide structure is also investigated using 3-D finite-element simulations. The principle behind its modulation is the change of electromagnetic field profile after the stub, relative to the incoming mode, which happens when the absorption of the stub material varies. This affects the field coupling into a single-mode output waveguide, resulting in a controllable transmission.