We report a new finite-difference time-domain (FDTD) computational model of the lasing dynamics of a four-level two-electron atomic system. Transitions between the energy levels are governed by coupled rate equations and the Pauli Exclusion Principle. This approach is an advance relative to earlier FDTD models that did not include the pumping dynamics, or the Pauli Exclusion Principle. Further, the method proposed in this paper is more versatile than the conventional modal expansion of the electromagnetic field for complex inhomogeneous laser geometries constructed in photonic crystals or light-localizing random media. For such complex geometries, the lasing modes are either difficult or impossible to calculate. The present work aims at the self-consistent treatment of the dynamics of the 4-level atomic system and the instantaneous ambient optical electromagnetic field. This permits in principle a much more robust treatment of the overall lasing dynamics of four-level gain systems integrated into virtually arbitrary electromagnetic field confinement geometries.

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Finite-difference time-domain model of lasing action in a four-level two-electron atomic system

We report studies defining the diameter-dependent location of electrically active dopants in silicon (Si) and germanium (Ge) nanowires (NWs) prepared by nanocluster catalyzed vapor-liquid-solid (VLS) growth without measurable competing homogeneous decomposition and surface overcoating. The location of active dopants was assessed from electrical transport measurements before and after removal of controlled thicknesses of material from NW surfaces by low-temperature chemical oxidation and etching. These measurements show a well-defined transition from bulk-like to surface doping as the diameter is decreased

http://cml.harvard.edu/assets/PNAS_106-15255_Xie.pdf

Diameter-dependent dopant location in silicon and germanium nanowires

We report new methods of identifying the effects of aging on the light–current (L–I) and current–voltage (I–V) characteristics of AlInGaP light-emitting diodes (LEDs). We believe that these methods are also applicable to other III–V compound semiconductors. We observe a broadening of the nonlinear range of the L–I characteristic accompanied by a shift to higher currents in the I–V characteristic. These features can be attributed to an increase of nonradiative recombination processes in the active layer. A second process, however, can lead to an increase of the LED output power. We conclude from an analysis of the current dependence that this process is due to a different mechanism.

Identification of aging mechanisms in the optical and electrical characteristics of light-emitting diodes

Adv. Mater. 2010, 22, 2311–2315 2010 WILEY-VCH Verlag G T IO N Hydrogels can be composed of natural or synthetic polymers. They form a three-dimensional (3D) scaffold that can absorb a large quantity of water (>99% by volume). They can mimic the extracellular matrix, having good biocompatible and biodegradable qualities, which enables them to support the growth of cultured cells. Among the various polymers for forming hydrogels, short peptides are an important group. It has been shown that several peptides can undergo gelation and form the 3D structure of a hydrogel, such as peptides that influence the selective differentiation of neural cells, short alternating charged amino acid peptides, and peptide clusters from the nuclear pore complex. Moreover, the addition of the protective group N-fluorenylmethoxycarbonyl (Fmoc) to the short peptides may improve the hydrogel formation process. Naturally self-assembled nanostructures of protein fibrils are associated with neurodegenerative conditions such as Alzheimer’s disease, where the fibril structure is made of amyloid-b (Ab) peptides. It has been found that the minimal core recognition motif of Ab peptides is a diphenylalanine element. The chemically synthesized dipeptide NH2Phe-Phe-COOH (FF) can self-assemble into well-ordered peptide nanotubes (PNTs). By using the FFmotif connected to an Fmoc moiety (Fmoc-FF), we have shown the formation of a peptidebased hydrogel made from a PNT network. In the work presented here, we have examined the optical properties, optical absorption and photoluminescence (PL), of hydrogels that were self-assembled from Fmoc-FF building blocks. From the optical properties we were able to follow the formation of a quantum confined structure within the hydrogel nanotubes. The most common example of quantum confinement (QC) is a 2D-QC—also called a quantum well (QW)—system of GaAs. This system contains various structures, the most noticeable of which is GaAs/AlGaAs, along with GaAs/InGaAs and InGaN/GaN. These structures have a double heterostructure consisting of a thin layer of GaAs ca. 10 nm thick, whose bandgap is smaller than that of the surrounding AlGaAs bulk. Another QC system, which was first revealed by Canham in 1990, is found in mesoporous silicon layers. Canham showed that by increasing the pore size, hence decreasing the size of the bulk silicon skeleton between the pores, a QC effect can occur. In order to observe the QC effects, the bulk silicon dimensions need to be less than the dimensions of the free exciton Bohr radius of bulk silicon of ca. 50 Å. Recently QC has been demonstrated in inorganic nanotubes, such as nanowires of ZnO or InGaN/ GaN multiple QWs. QW structures can produce remarkable changes in the optical and electrical properties of semiconductor structures; hence QW structures are most commonly characterized by spectroscopy measurements. We have very recently demonstrated the QC phenomenon in several peptide nanostructures. We showed a QW structure within PNTs, which were formed by vapor deposition of the FF building blocks. Furthermore, we showed a quantum dot (QD) structure within self-assembled peptide spheres, made from an analogue of the FF building block. By understanding the phenomenon, we were able to demonstrate a unique blue luminescence from the bioinspired PNTs. The hydrogel’s self-assembled PNT network is composed of Fmoc-FF building blocks (Fig. 1A) with a final diameter of 7–15 nm (Figs. 1B,C). When the environment of the solution is not suitable for their self-assembly, the Fmoc-FF molecules form aggregates in the solution. The optical absorption of the hydrogel and the aggregates is presented in Figure 2. In Figure 2A in the curves of the high-concentration samples, characterized by PNT network formation, pronounced step-like behavior of the absorption spectra is observed. The spectrum indicates a major step in the UV region with a peak at its ‘‘red’’ edge with wavelength

Quantum confinement in self-assembled bioinspired peptide hydrogels.

p-Type-doped ZnO films were grown on nominally undoped n-ZnO, which is deposited on p-Si:B by electrochemical deposition. The electrical properties of individually doped ZnO:Na/Mg/Sb/Ti films were determined by means of the Hall system at the room temperature. Our results showed that doped ZnO films had high mobility ranging from 11 to 638 cm2/V s, carrier concentration ranging from 6.00 × 1016 to 3.37 ×1019 cm−3, and low resistivity ranging from 1.62 × 10−2 to 21.9 × 10−2 Ω cm. Electroluminescence measurements of obtained homojunctions exhibited three main broad luminescence bands peaking at 2.8, 2.5, and 1.98 eV, respectively. In addition to these broadbands, Fabry–Perot oscillations were also observed.

ZnO homojunction white light-emitting diodes

Semiconductors - crystal structure properties of semiconductors - electronic doping and carrier transport optical propetries of semiconductors excitonic effects and modulation semiconductor junction theory optoelectronic detectors noise and the photoreceiver the light emitting diode laser diode - static properties semiconductor lasers - dynamic properties modulation and switching devices optical communication systems list of symbols important properties of semiconductors important quantum mechanics concepts bipolar transistor action optical waves in waveguides and crystals.

Jasprit Singh

Papers

Semiconductor Optoelectronics: Physics and Technology