Tuning linear and nonlinear optical properties of wurtzite GaN by c-axial stress

We study the linear and nonlinear optical properties of wurtzite GaN under c-axial stress field, using density functional theory calculations. The fully structural optimization at each c-axial strain was performed. The calculated dielectric functions show that tensile c-axial strain effectively improves the linear optical efficiency, especially for the band-edge transitions, and significantly increase the mobility of electrons in the conduction band. Second-order nonlinear optical susceptibilities show that the tensile c-axial strain will enhance the zero- and low-frequency nonlinear responses of GaN. The enhancement of the nonlinear optical property is explained by the reduction of the polarization of wurtzite GaN under tensile c-axial strains. Based on these findings, we propose a method for improving the electrical and optical properties of the crystal through imposing appropriate stress on the high symmetry crystalline directions

Abruptness Improvement of the Interfaces of AlGaN/GaN Superlattices by Cancelling Asymmetric Diffusion

973 program [2012CB619301, 2011CB925600]; "863" program [2011AA03A111]; FRFCU [2012121011, 2011121042]; National Natural Science Foundation [61204101, 61227009, 90921002]; Science and Technology Program of Fujian and Xiamen of ChinaInterfacial abruptness plays a critical role in affecting the quantum confinement effect in heterostructures. Here, we accurately determine the inter-diffusion depth across the AlGaN/GaN interfaces and propose a simple blocking scheme to effectively improve the superlattice abruptness. It is found that the Al diffusion depth at the upper and lower interfaces of the AlGaN barrier appears considerably asymmetric. Such difference leads to the gradient interfacial region and the asymmetric quantum well shape. A pair of ultra-thin blocking layers is introduced to the GaN/AlGaN interface to block the Al downward diffusion. After the blocking treatment, the interfacial abruptness is improved and the light emission intensity from the superlattice can be effectively enhanced. (C) 2013 The Japan Society of Applied Physic

Bioheterojunction Effect on Fluorescence Origin and Efficiency Improvement of Firefly Chromophores

We propose the heterojunction effect in the analysis of the fluorescence mechanism of the firefly chromophore. Following this analysis, and with respect to the HOMO-LUMO gap alignment between the chromophore's functional fragments, three main heterojunction types (I, II, and I*) are identified. Time-dependent density-functional theory optical absorption calculations for the firefly chromophore show that the strongest excitation appears in the deprotonated anion state of the keto form. This can be explained by its high HOMO-LUMO overlap due to strong bio-heterojunction confinement. It is also found that the nitrogen atom in the thiazolyl rings, due to its larger electronegativity, plays a key role in the emission process, its importance growing when HOMO and LUMO overlap at its location. This principle is applied to enhance the chromophore's fluorescence efficiency and to guide the functionalization of molecular optoelectronic devices.Comment: 7 pages, 6 figure

Coupled donor states in InP by Sn doping

Coupled donor states induced by Sn doping into InP and their behaviors were studied by employing photoluminescence (PL) measurements and first-principles calculations. InP: Sn crystals were grown in the [100] orientation by a modified liquid-encapsulated vertical Bridgman technique. The PL data show a donor-acceptor recombination line and its phonon replica at 898 nm and 926 nm, respectively, and another recombination line around 800 nm, which uniquely appears at the top in high Sn concentration. The first-principles calculations for the InP:Sn system support these results by observing two separate donor levels below (D(Sn1)) and above (D(Sn2)) the bottom of the conduction band, which are responsible for the recombination lines. They deviate from the bottom of the conduction band with the increase of Sn concentration. A peak at 879 nm from the band-to-band transition could only be visible at room temperature. The PL mapping of this line shows the increase of Sn concentrations with the crystal growth, which helps to reduce the dislocation density and increase the lifetime of excess carriers

Full Color Modulation of Firefly Luciferase through Engineering with Unified Stark Effect

International audienceThe firefly luciferase has been a unique marking tool used in various bioimaging techniques. Extensive color modulation is strongly required to meet special marking demands; however, intentional and accurate wavelength tuning has yet to be achieved. Here, we demonstrate that the color shift of the firefly chromophore (OxyLH2-1) by internal and external fields can be described as a unified Stark shift. Electrostatic microenvironmental effects on fluorescent spectroscopy are modeled in vacuo through effective electric fields by using time-dependent density functional theory. A complete visible fluorescence spectrum of firefly chromophore is depicted, which enables one to control the emission in a specific color. As an application, the widely observed pH-correlated color shift is proved to be associated with the local Stark field generated by the trace water–ions (vicinal hydronium and hydroxide ions) at active sites close to the OxyLH2-1