Computational electromagnetics for nanowire solar cells

This review article provides an overview of various novel nanowire array solar cells and highlights the aspects of electromagnetic simulations that are a valuable tool for understanding the optical processes leading to their distinct properties. As the computational methods commonly used for the task are well established, we focus on the question how numerical modeling can be used to assess the performance of a design and reveal the working principle of the devices. We conclude that scientific literature identifies numerical simulations as paramount for design and interpretation of experimental dat

Part 1: Design, Synthesis, and Evaluation of Novel Gram-positive Antibiotics Part 2: Synthesis of Dihydrobenzofurans Via a New Transition Metal Catalyzed Reaction Part 3: Design, Synthesis, and Evaluation of Bz/gabaa Α6 Positive Allosteric Modulators

Part 1. Lead compound SK-03-92 represents a new scaffold for antibiotic drug discovery. Development of a new process for the synthesis of analogs has led to the development of a number of new ligands with even more potent activity against gram-positive bacteria, including drug-resistant strains of S. aureus. Compounds 36 and 38 represent some of the most potent analogs developed thus far, and preliminary results indicate that they are also not cytotoxic. Research into a Heck-mediated transition metal catalyzed pathway towards electron-rich stilbenoid analogs has greatly expanded the scope of future SAR studies. This development has led to 14 new analogs with minimum inhibitory concentrations (MICs) in pharmaceutically acceptable ranges and it is presumed that further SAR expansion will lead to even more potent compounds. Mechanism of action studies have shown that these compounds prove difficult to induce mutations in bacteria that lead to drug-resistance. This has made determination of the mechanism/mode of action difficult, and to date it is still not known, but is promising in that a lack of developed resistance may show that these compounds act on pathways that are novel and unlikely to form resistance. An enzyme catalyzed pathway involving tyrosinase is postulated as a plausible mechanism for these stilbenoid compounds. This process would involve the formation of quinones, which might be toxic to the bacteria, causing the observed bactericidal nature of these potent analogs. Further, this may explain some of the observed activity for a number of analogs synthesized in this study. The need for new antibiotics is clear, and these novel compounds represent a new scaffold for antibiotic drug discovery. Part 2. Dihydrobenzofurans are an important class of compounds, a number of which are natural products and/or biologically active. A new transition metal catalyzed pathway was developed to synthesize novel dihydrobenzofurans. This new process was modified from the Heck reaction developed in Part 1. Initially, the dihydrobenzofurans synthesized by the Heck mediated process were in very low yields. Optimization of the conditions for this reaction were successful in improving the conversion to nearly quantitative levels. A preliminary examination of the scope of the reactions indicated that a number of electron-rich aryl bromides were well tolerated and high yields for nearly all attempted aryl bromides were reported. The scope of vinyl arenes includes both aryl and heteroaryl vinylic compounds, many of which were conveniently synthesized from inexpensive starting materials. This reaction sequence is similar to work reported by Larock, however differs in a number of significant ways. Part 3. The α6 subunits of GABAA receptors exhibit a quite restricted regional distribution in the brain. They are predominantly expressed in the granule cells of the cerebellum, and in the cochlea nuclei. Our recent study revealed that the α6 GABAAR in the cerebellum plays an important role in controlling the sensorimotor gating function, a deficit of this function is manifested in several neuropsychiatric disorders, such as schizophrenia, tic disorders, attention deficit hyperactivity disorder, obsessive compulsive disorder. We have designed a series of pyrazoloquinolinone ligands that are functionally selective for α6β2,3γ2 GABAA receptors and are positive allosteric modulators at this subtype. Preliminary data show analogs such as Compound 6 and Compound 11 are effective in an animal model with sensorimotor gating deficit, reflecting the impairment of prepulse inhibition of the acoustic startle response (PPI) induced by methamphetamine. Recently, the α6 GABAAR was shown to be expressed in both neurons and satellite glia of the trigeminal ganglia. The α6 subunit positive neuronal cell bodies in the trigeminal ganglia project axons to the temporomandibular joint and likely to the trigeminal nucleus caudalis and upper cervical region (Vc–C1), and might modulate orofacial pain and inflammatory temporomandibular joint nociception and might modulate orofacial pain and inflammatory temporomandibular joint nociception. Rats with 30% knock down of the α6 subunit of GABAA receptors in trigeminal ganglia were hypersensitive to TMJ inflammation, measured by a prolong meal time. The prevalence of TMJ disorders in the United States is estimated at 4.6% and these disorders are the leading cause of chronic orofacial pain. Importantly, trigeminal ganglia also send projections to the trigeminal nucleus caudalis (TNC) and upper cervical region (Vc–C1), the trigeminal cervical complex. Activation of the TNC plays an important role in the neuropathogenesis of migraine. In an animal model of migraine, we have found a selective α6-GABAA receptor PAM, Compound 6, effectively decreased the number of activated neurons in the TNC induced by intracisteral (i.c.) injection of capsaicin. This suggests the potential of selective α6-GABAA receptor PAMs for the treatment of migraine

Reliable k ⋅ p band structure calculation for nanostructures using finite elements

The k⋅p envelope function method is a popular tool for the study of electronic properties of III-V nanostructures. The equations are usually transferred to real-space and solved using standard numerical techniques. The powerful and flexible finite element method was seldom employed due to problems with spurious solutions. The method would be favorable for the calculation of electronic properties of large strained nanostructures as it allows a flexible representation of complex geometries. In this paper, we show our consistent implementation of the k⋅p envelope equations for nanostructures of any dimensionality. By including Burt-Foreman operator ordering and ensuring the ellipticity of the equations, we are able to calculate reliable and spurious solution free subband structures for the standard k⋅p 4×4, 6×6 and 8×8 models for zinc-blende and wurtzite crystals. We further show how to consistently include strain effects up to second order by means of the Pikus-Bir transformation. Finally, we analyze the performance of our implementation using benchmark example

tdkp/AQUA: Unified modeling of electroluminescence in nanostructures

This article summarizes the capabilities of the optoelectronic simulation framework tdkp/AQUA aimed at the description of electroluminescence in semiconductor nanostructures such as light-emitting diodes. tdkp is a stand-alone finite-element software able to accurately calculate strain, built-in fields due to spontaneous and piezoelectric polarization, bound quantum states using k · p theory, gain and luminescence spectra in zero- to three-dimensional structures. AQUA calculates transport through nanostructures using a model which accounts for the distinct behaviour of carriers confined to active regions and unconfined carriers. Furthermore, it computes electroluminescence spectra via a self-consistent coupling of the confined carriers to quantum-mechanical calculations using tdkp. Two examples are presented which highlight the versatility and generality of the developed framewor

Unified simulation of transport and luminescence inoptoelectronic nanostructures

Computer simulation of microscopic transport and light emission in semiconductor nanostructures is often restricted to an isolated system of a single quantum well, wire or dot. In this work we report on the development of a simulator for devices with various kinds of nanostructures which exhibit quantization in different dimensionalities. Our approach is based upon the partition of the carrier densities within each quantization region into bound and unbound populations. A bound carrier is treated fully coherent in the directions of confinement, whereas it is assumed to be totally incoherent with a motion driven by classical drift and diffusion in the remaining directions. Coupling of the populations takes place through electrostatics and carrier capture. We illustrate the applicability of our approach with a well-wire structur

Harmonic balance analysis for semiconductor lasers under large-signal modulation

The dynamic characteristics of an edge-emitting laser under large-signal modulation are analyzed in the frequency domain using a harmonic balance method on device level. The simulations reveal the nonlinearities of the carrier dynamics in the quantum well region which strongly influence the optical power in the higher harmonic

Single-mode performance analysis for vertical-cavity surface-emitting lasers

In this work, the simulation of the single-mode stability in vertical-cavity surface-emitting lasers (VCSELs) is presented using a microscopic electro-opto-thermal model. Experimental data for oxide-confined VCSELs emitting at 850 nm with different contact metal designs are also available. It is shown that detailed models for the optical losses in the cavity consisting of outcoupling and absorption are required in order to explain the experiments. The role of cavity losses and spatial hole burning in the nonlinear electro-opto-thermal simulation framework is discussed in a quantitative manne

Operator ordering, ellipticity and spurious solutions in k · p calculations of III-nitride nanostructures

We analyze the ellipticity of the standard k · p wurtzite model for the symmetrized and the Burt-Foreman operator ordering. We find that for certain situations the symmetrized Hamiltonian is unstable, leads to unplausible results and can cause spurious solutions. We show that the operator ordering in wurtzite must be completely asymmetric to be stable. The asymmetric operator ordering is elliptic and consequently no spurious solutions are obtained. Therefore we recommend the use of a complete asymmetric operator ordering for nitride device simulatio

Investigation of the Purcell effect in photonic crystal cavities with a 3D Finite Element Maxwell Solver

Photonic crystal cavities facilitate novel applications demanding the efficient emission of incoherent light. This unique property arises when combining a relatively high quality factor of the cavity modes with a tight spatial constriction of the modes. While spontaneous emission is desired in these applications the stimulated emission must be kept low. A measure for the spontaneous emission enhancement is the local density of optical states (LDOS). Due to the complicated three dimensional geometry of photonic crystal cavities the LDOS quantity has to be computed numerically. In this work, we present the computation of the LDOS by means of a 3D Finite Element (FE) Maxwell Solver. The solver applies a sophisticated symmetry handling to reduce the problem size and provides perfectly matched layers to simulate open boundaries. Different photonic crystal cavity designs have been investigated for their spontaneous emission enhancement by means of this FE solver. The simulation results have been compared to photoluminescence characterizations of fabricated cavities. The excellent agreement of simulations and characterizations results confirms the performance and the accuracy of the 3D FE Maxwell Solve

Investigation of the Purcell effect in photonic crystal cavities with a 3D Finite Element Maxwell Solver

Photonic crystal cavities facilitate novel applications demanding the efficient emission of incoherent light. This unique property arises when combining a relatively high quality factor of the cavity modes with a tight spatial constriction of the modes. While spontaneous emission is desired in these applications the stimulated emission must be kept low. A measure for the spontaneous emission enhancement is the local density of optical states (LDOS). Due to the complicated three dimensional geometry of photonic crystal cavities the LDOS quantity has to be computed numerically. In this work, we present the computation of the LDOS by means of a 3D Finite Element (FE) Maxwell Solver. The solver applies a sophisticated symmetry handling to reduce the problem size and provides perfectly matched layers to simulate open boundaries. Different photonic crystal cavity designs have been investigated for their spontaneous emission enhancement by means of this FE solver. The simulation results have been compared to photoluminescence characterizations of fabricated cavities. The excellent agreement of simulations and characterizations results confirms the performance and the accuracy of the 3D FE Maxwell Solve