Terahertz/optical mixing in symmetric semiconductor quantum wells embedded in optical microcavities

A quantum well (QW) in the simultaneous presence of a terahertz field polarized in the growth direction and an incident optical field near an excitonic resonance results in substantial frequency mixing between the terahertz and optical fields. In particular, a response at new frequencies given by the input optical frequency plus or minus multiples of the terahertz frequency occurs-the terahertz sidebands. In a symmetric QW, the dominant contribution to terahertz-sideband formation is the high-frequency modulation of the overlap integral of the relevant conduction- and valence-subband envelope functions that determine the strength of the interband dipole moment. terahertz-sideband generation is shown to be strongly enhanced in a high quality-factor optical microcavity. Numerical values of the reflected intensity into the first terahertz sideband normalized with respect to the reflected intensity at the fundamental as large as /spl sim/10% are estimated. This suggests that terahertz-sideband generation in semiconductor microcavities is a promising option worthy of exploration for wavelength conversion for wavelength-division multiplexing applications.     

Carla: A rule language for specifying communications architectures

Due to the unique requirements of a series of projects to specify communications architectures using graphical representations (Cara and MFD), we have developed the communications-oriented rule-based language Carla (Cara Rule Language), which provides an executable specification of the architecture being developed. Carla is designed to provide the ability to specify and simulate high-level, possibly incomplete, specifications of communications architectures, and to allow the developer to refine the specification through the addition of behavior-describing rules. Carla is also well-suited to creating black-box specifications of any system whose behavior depends on input/output history. We describe the features of the language, discuss various design issues, and provide examples of various communications protocols specified in Carla.     

Theory of gain in quantum-wire lasers grown in V-grooves

Theoretical calculations of gain, refractive index change, differential gain, and threshold current for GaAs-AlGaAs quantum-wire lasers grown in V-shaped grooves are presented. The theoretical model is based on the density-matrix formalism with intraband relaxation, and the subband structure is calculated within the effective bond-orbital model. For the quantum-wire geometry treated, agreement with the observed subband spacings is found. Because of the small overlap of the optical field with the active region for a single quantum wire, lasing threshold is reached only when several subbands are filled     

Using formalized temporal message-flow diagrams

Temporal message-flow diagrams (TMFDs), alternatively called sequence charts, interaction diagrams, event traces, or actor diagrams, are illustrations of a system's global message-passing activity over time, and a pictorial aid to understanding the system's behavior. They are widely used for requirements and documentation for network protocols and object-oriented applications. We present a general formalism for TMFDs, describe a suite of tools we have designed that employs this formalism, and present our experiences with these tools. The formalism and tools described serve to support and broaden the use of TMFDs in developing communicating systems.     

Numerical calculation of the terahertz field-induced changes in theoptical absorption in quantum wells

In the presence of a strong terahertz field, the optical (i.e., valence-to-conduction band) transitions in semiconductors occur between states which are altered by the terahertz field. This alteration has a direct impact on the optical absorption. We describe a numerical technique for calculating the optical absorption in this case by solving the Schrodinger equation for the electron-hole envelope function in real space. This technique correctly accounts for the Coulomb interaction between optically created electron-hole pairs and nonperturbative terahertz-field induced alteration of the states. We applied this technique to investigate the optical absorption of quantum wells of finite width in which terahertz/optical mixing can be significant and which cannot be treated analytically     

Optimized Asymmetric Double Quantum Well for High Electric-Field-Sensitivity Electroabsorption: Excitonic Mixing Effects

Asymmetric double quantum wells (ADQWs) are optimized to exhibit maximum optical modulation sensitivity by varying the barrier width, barrier position, and well width. Anticrossing of the two lowest excitons in ADQWs significantly enhances the modulation sensitivity. Consideration of exciton mixing is crucial to obtain accurate estimates of the effects. For a given linewidth of the excitonic peaks, we find optimum structural parameters that exhibit maximum modulation sensitivity. Values dalpha / dE ~ 6.51 x 10⁴ kV^-1of of at 4.2 K and ~1.25 x 10³ kV^-1of at 298 K are predicted in GaAs-based ADQWs. The result provides new design guidelines in fabricating high-sensitivity QW electroabsorption modulators.     

Electrooptic Properties of InGaAsP Asymmetric Double Quantum Wells: Enhanced Slope Efficiency in Waveguide Electroabsorption Modulators

We present detailed theoretical estimates of the optical properties of InGaAsP quantum well (QW) electroabsorption modulators (EAMs) operating at ~1550 nm wavelength. Absorption coefficients of QWs are obtained from the linear optical susceptibility. Exciton states are calculated in momentum space, which includes valence-band mixing, mixing of excitons originating in different subband pairs, and exciton spin-related optical selection rules. Various line-broadening mechanisms relevant to InGaAsP-QWs are also included. Extending the study further to asymmetric double QWs (ADQWs) suggests that the small-signal modulation efficiency can be enhanced significantly at substantially lower operating bias voltage. Simple optimization of ADQW band structure results in a maximum slope efficiency ~3.8 times larger than that of SQW EAMs at a reduced operating bias field of 34 kV/cm compared with ~70 kV/cm for comparable SQWs.     

Exciton dephasing and absorption line shape in semiconductor quantum dots

The homogeneous broadening of exciton absorption spectral lines in semiconductor quantum dots (QDs) in the strong confinement regime is studied theoretically. It is shown that the term linear in nuclear displacements in the difference of the phonon Hamiltonians of the ground and optically excited states does not lead to the zero-phonon line (ZPL) broadening. The ZPL width is contributed by the term quadratic in nuclear displacements associated with short-living optical phonons. This contribution is estimated for CdSe nanocrystals (NCs) and found to be much less than the linewidth observed in recent experiments. We conclude that the experimentally observed linewidth is due to the longitudinal lifetime associated with the exciton relaxation to the dark state. The shape of spectral wings originating from the exciton interaction with long-living acoustic phonons is studied at various temperatures for a CdSe NC embedded in a glass matrix.