Papers by Keyword: Optical Characteristics

Abstract: First-principles calculations are used within the framework of density functional theory to investigate the electronic, structural, magnetic and optical properties of Potassium Nitride (KN) in the bulk and monolayer states. This compound is dynamically stable according to phonon calculations. The results show that the energy gap decreases from the bulk to the monolayer. The equilibrium lattice constant increases when changing from bulk to monolayer, and the half-metallic (HM) character remains preserved in that case. According to the Slater–Pauling statute (Zt-4), the total magnetic moment equals 2 µ_B per unit cell. The electric field and biaxial strain affect the monolayer's electronic and magnetic characteristics were investigated. The magnitude of the spin-up channel concerning the energy gap changes under the biaxial strain. In particular, it decreases under tensile strain and increases under compression strain. Given that the values of magnetic moments remain unchanged, the HM property can be preserved for significant strains. When the electric field reaches -0.6 V/nm, the half-metallic property of this compound will be destroyed. It affects the energy gap and eliminates the HM trait since the magnetic moment of the K grew significantly greater than the moment of the N, and the N played a significant role in the realization of the half-metallic characteristic.

Abstract: ZnO films were deposited by magnetron sputtering using RF power supply, in order to study the effect of substrates on quality of the prepared films. Then, growth of the ZnO films on thin AlN buffer layer and Si(100) substrates were characterized using different techniques. The surface morphology was investigated by means of scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The structural properties were investigated via X-ray diffraction (XRD) patterns, Rocking Curve as well as Pole figures. The ZnO films were textured and they had preferred orientation (002) and the crystallinity was better for ZnO/Si in the used growth conditions. The XRD results were confirmed by HRTEM. Optical properties were analyzed by photoluminescence (PL), as well as electrical characteristics were performed by C-V and I-V measurements. The dispersion orientation of these films, as indicated via the FWHM (rocking curves), is small for thin ZnO/Si. These results are considered as hopeful for piezoelectric applications.

Abstract: The discovery of translucent concrete some years ago, opened a new horizon in Civil Engineering. Many researchers have looked for applications in matter of structural mechanics in order to replace classical concrete by this new and challenging material. However, the scientific and technical communities working in Civil Engineering seem to be still far away from clear ideas and strategies to take profit of one of the main properties of this material: its possibility to transmit light. The reason is complex: on one hand, most of the researchers are mainly interested in the mechanical properties and other researchers, more interested on lighting, maybe find it as a decorative element, but not as a functional one from the point of view of lighting. In this work, two lines concerning its optical characterization (which must be always the first step) and some applications to road tunnels are presented. We will see that more active and multidisciplinary research is still needed before fully understanding and accurately use this material for lighting purposes.

Abstract: The TiN ceramic films were deposited on Si (100) substrates with Al and Ti buffer layers by direct current reactive magnetron sputtering in the mixture gas atmosphere of argon and nitrogen. X-ray diffraction (XRD), Raman spectroscopy, scanning electronic microscope (SEM), nanohardness test and ultra violet-visible-near infrared (UV-VIS-NIR) spectrophotometer were employed to analyze the structure, mechanics and optical characteristics of the TiN films respectively. Analyses of XRD and Raman showed that all the TiN films deposited on Al and Ti buffer layers with TiN (111)-preferred orientations had polycrystalline structures, and the TiN film deposited on Ti buffer layer had a better crystallinity, smaller surface roughness, higher hardness and larger elastic modulus than those of the TiN film deposited on Al buffer layer. Moreover, the reflectivity of the Ti/TiN film became higher than Al/TiN film above λ=645 nm. At λ=1200 nm, the Ti/TiN film showed the maximum reflectivity of 81.8%. These conclusions also showed that the metal multilayer TiN films have important application and research prospect in terms of solar control coatings or resistance to high temperature in building coating material.

Abstract: Time-resolved photoluminescence spectra of vertical cavity surface emitting laser (VCSEL) structures under different excitation intensity are investigated. The effect of the distributed Bragg reflectors (DBR) on the laser emission and the mechanism of multi-longitudinal-mode are analysed. A broad peak around 2.80eV is observed from the structure without DBR cavity when pumped under low excitation intensity. At higher excitation density up to 21.4kW/cm², a lasing peak appears at 2.86eV, and exhibits a rapid growth and red shift with the increase of the excitation density. The decay time of the peak is about 66ps. When the spectrum is measured after the deposition of DBR on the top side of the grown nitride structure, the number of peaks increases to 7, and the free spectral range 0.21×10¹⁴ Hz is close to the value that calculated by the Fabry-Perot cavity length. After depositing the second DBR, multi PL peaks around 2.87eV are observed. The relationship of laser emission intensity and angle of the polarizer shows nearly a cosine square variation. However, the polarization characteristics of the four main peaks are different, which indicates that these peaks occurr by the oscillation of different optical cavities.

Abstract: In this paper, diffuser with taper microstructure has been put forward, and the transmission of the light in the taper microstructure has been analyzed. The software of Lighttools has been used to analyze the influence on light intensity distribution of the vertex angle θ, the space between the microstructure L, the diameter ratio of up to down of the microstructure A/B, the cutting output of the microstructure H and the refractive index of the diffuser n1.As a result, the light intensity of the optical axis will increase and visual angle will decrease when the vertex angle θ, the refractive index of the diffuser n1 and the cutting output of the microstructure H increase; the light intensity of the optical axis will increase and visual angle will decrease when the space between the microstructure decrease; with the increasing of the ratio A/B, the light intensity of the optical axis would first increase and then decrease, visual angle would first decrease and then increase. When A/B=0.4, the light intensity gets the maximum value, the visual angle gets the minimum value. The result of the simulation is significant to manufacture and research.

Abstract: Transparent conducting gallium-titanium codoped zinc oxide thin films were grown onto glass substrates with magnetron sputtering technique in ambient argon gas, and the influence of argon pressure on optical properties of the films were investigated. The refractive indexes of the films were calculated. It is found that the refractive index tends to increase with increasing the photon energy. Meanwhile, the dispersion behavior of the refractive index was analyzed in terms of the single-oscillator model and the oscillator parameters were obtained. Furthermore, the optical energy gaps were evaluated by single-oscillator model and Tauc’s relation, respectively. The values of optical energy gap estimated from single-oscillator model are in agreement with those determined from Tauc’s relation.

Abstract: 4H-SiC based semicircular electrode metal-semiconductor-metal (SEMSM), triangular electrode MSM (TEMSM) and conventional electrode MSM (CEMSM) ultraviolet (UV) sensors have been modeled, investigated and characterized with numerical simulator ISE-DESSIS. By comparing with relevant experimental data, the model correctness is verified. The electrical and optical features of these sensors are simulated and calculated to character the effect of the novel electrode on performance enhancement. In contrast to CEMSM device, the SEMSM and TEMSM sensors show an outstanding superiority in terms of higher photocurrent, comparable low dark current and excellent quantum efficiency. At a bias of 30 V, the dark currents of SEMSM and TEMSM sensors are below 3.5 pA and the photocurrents are 20.7 nA and 23.7 nA under 310 nm UV illumination, respectively. Furthermore, the peak responsivity is estimated to be larger than 0.135 A/W, 0.156 A/W at 290 nm and maximum quantum efficiency at 280 nm is 58.8% and 67.7% for semicircular and triangular electrode structure, respectively.

Abstract: Aluminum-doped zinc oxide (AZO) thin films with highly (002)-preferred orientation were grown on glass substrates by rf magnetron sputtering. The effect of thickness on structural and optical characteristics of the deposited films were investigated by X-ray diffractometer and spectrophotometer. The results show that the polycrystalline AZO films consist of the hexagonal crystal structures with c-axis as the preferred growth orientation normal to the substrate, and that the thickness significantly affects the crystal structure and optical properties of the thin films. With the increase of thickness, the crystallite size of the films increases, the lattice spacing, dislocation density, micro strain and optical energy gap decrease, and the average transmitance in the wavelength range of the visible spectrum also slightly decreases.

Abstract: The ultimate goals of this study are to investigate numerically nonequilibrium energy transfer between electrons and phonons, and to predict the crater formation shapes of gold thin film structures irradiated by femtosecond pulse lasers. In particular, the present article expands the onedimensional two-temperature model (1DTTM) to the two-dimensional model (2DTTM) considering wave interference, and it involves the quantum effect to predict thermal and optical properties. The predictions by using 2DTTM are extensively compared with those of 1DTTM, and the influence of film thickness on radiation heat transfer and optical characteristics are also examined. From the results, it is found that the predictions of 2DTTM are in good agreement with those of 1DTTM. As the gold film thickness decreases, the reflectivity decreases dramatically and the absorbed laser intensities at the top surface increases substantially because of wave interference in thin films.