Ab initio calculation of optical absorption and reflectivity of Si(0 0 1)/SiO2 superlattices with varying interfaces

Using first-principles calculations we investigate the influence of interface modification and layer thicknesses on the optical properties of Si/SiO₂ superlattices. Four interface models with different dangling-bond passivation are considered. The results demonstrate confinement effects not only for the fundamental band gaps but also for the optical properties. While for a large Si layer thickness of the Si/SiO₂ superlattices the interface dependence is small, the calculations show a significant structure dependence for thin Si layers. © 2007 Elsevier Science. All rights reserved.     

The influence of growth parameters on the optical properties and morphology of UHV deposited Ni thin films

Nickel films of different thickness ranging from 15 nm to 350 nm were deposited on glass substrates, at different substrate temperatures (313-600 K) under UHV condition. The nano-structure of the films was obtained, using X-ray diffraction (XRD) and atomic force microscopy (AFM). The nano-strain in these films was obtained using the Warren-Averbach method. Their optical properties were measured by spectrophotometry in the spectral range of 190-2500 nm. Kramers-Kronig method was used for the analysis of the reflectivity curves. The absorption peaks of Ni thin films at ∼1.4 eV (transition between the bands near W and K symmetry points) and ∼5.0 eV (transition from L_2^′ to L₁ upper) are observed, with an additional bump at about 2 eV. The over-layer thickness was calculated to be less than 3.0 nm, using the Transfer Matrix method. The changes in optical data are related to different phenomena, such as different crystallographic orientations of the grains in these polycrystalline films (film texture), nano-strain, and film surface roughness.     

Investigation of energy band gap and optical properties of cubic CdS epilayers

High quality cubic CdS epilayers were grown on GaAs (1 0 0) substrates by the hot-wall epitaxy method. The crystal structure of the grown epilayers was confirmed to be the cubic structure by X-ray diffraction patterns. The optical properties of the epilayers were investigated in a wide photon energy range between 2.0 and 8.5 eV using spectroscopic ellipsometry (SE) and were studied in the transmittance spectra at a wavelength range of 400-700 nm at room temperature. The data obtained by SE were analyzed to find the critical points of the pseudodielectric function spectra, 〈?(E)〉 = 〈?₁(E)〉 + i〈?₂(E)〉, such as E₀, E₁, E₂, E^′₀, and E^′₁ structures. In addition, the optical properties related to the pseudodielectric function of CdS, such as the absorption coefficient α(E), were investigated. All the critical point structures were observed, for the first time, at 300 K by ellipsometric measurements for the cubic CdS epilayers. Also, the energy band gap was determined by the transmittance spectra of the free-standing film, and the results were compared with the E₀ structure obtained by SE measurement.     

Growth and optical property of ZnWO4: Er crystal

Good quality crystals ZnWO₄ activated with Er³⁺ have been grown by means of Czochralski method and characterized using optical spectroscopy techniques. XRD, absorption spectra, fluorescence spectrum are presented and the Judd-Ofelt intensity parameters Ω₂, Ω₄, and Ω₆ are obtained to be 6.76×10^-20, 0.37×10^-20, and 0.50×10^-20 cm², respectively. Along crystallographic axes, refractive indices are presented. The fluorescence decay time of the ⁴I_13/2 level has also been investigated and shows an exponential behavior with a lifetime value of 5.52 ms. The crystal is potentially used for green and infrared eye-safe lasers.     

Antireflective structure imprinted on the surface of optical glass by SiC mold

An antireflective structure with two-dimensional 300-nm periodicity was fabricated on a phosphate glass surface using an imprinting process with a SiC mold. The optimized structure designed using RCWA calculation was a convex circular cone sharing the ridge line of adjacent cones. The SiC mold was fabricated using electron beam drawing and subsequent reactive ion etching with CHF₃ and O₂ gases. The glass’ surface reflectance was estimated as 0.2% at 530 nm wavelength, which was approximately 1/20 that of the optically polished surface.     

Effects of hydrogen etching process on the structural and optical properties of nano-crystalline diamond films

In this work, hydrogen etching method is applied to improve the quality of nano-crystalline diamond (NCD) films grown from hot-filament assisted chemical vapor deposition (HFCVD) system. From the characteristics of the structure and optical property, the grain size and surface roughness decrease while the optical transmission increase obviously under certain deposition parameters (gas pressure and substrate temperature) and longer etching time. Soft X-ray transmission measurements by synchrotron radiation are also carried out on the NCD films. The result shows that the X-ray transmission has an obvious improvement when the NCD film is fabricated from the hydrogen etching method. And the transmittance reaches 53.3% at X-ray photon energy of 258 eV, which has met the requirement for X-ray mask materials.     

Dependence of film thickness on the structural and optical properties of ZnO thin films

ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10⁻⁴ Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.     

The optical absorption edge of brookite TiO2

The optical absorption edge of brookite TiO₂ was measured at room temperature, using natural crystals. The measurements extend up to 3.54 eV in photon energy and 2000 cm⁻¹ in absorption coefficient. The observed absorption edge is broad and extends throughout the visible, quite different from the steep edges of rutile and anatase. No evidence of a direct gap is seen in the range measured. The spectral dependence of the absorption strongly suggests that the brookite form of TiO₂ is an indirect-gap semiconductor with a bandgap of about 1.9 eV.     

Line patterning with large refractive index changes in the deep inside of glass by nanosecond pulsed YAG laser irradiation

Nanosecond (∼100 ns) pulsed (10 Hz) Nd:YAG laser operating at the wavelength (λ) of 1064 nm with pulse energies of 0.16-1.24 mJ/cm² has irradiated 10Sm₂O₃·40BaO·50B₂O₃ glass. It is demonstrated for the first time that the structural modification resulting the large decease (∼3.5%) in the refractive index is induced by the irradiation of YAG laser with λ=1064 nm. The lines with refractive index changes are written in the deep inside of 100-1000 μm depths by scanning laser. The line width is 1-13 μm, depending on laser pulse energy and focused beam position. It is proposed that the samarium atom heat processing is a novel technique for inducing structural modification (refractive index change) in the deep interior of glass.     

Influence of size effect and sputtering conditions on the crystallinity and optical properties of ZnO thin films

The effects of the thickness variation, substrate type and annealing on the crystallinity parameters, luminescent and optical properties of the zinc oxide (ZnO) thin films were reported. The thin films were deposited on the glass and the amorphous quartz substrates by the standard RF-magnetron sputtering method using ZnO targets in the argon atmosphere. It has been found that the films deposited on the glass substrate manifest a clear size effect. Both the structural and the optical parameters show clearly minima on their thickness dependences. It has been shown that annealing of the comparatively thick ZnO films leads to increase of the crystallite sizes that are followed by a considerable rise of the cathodoluminescence intensity. The corresponding model of the crystallite growth is proposed.     

Numerical study on gain and optical properties of AlGaInAs, InGaNAs, and InGaAsP material systems for 1.3-μm semiconductor lasers

In reference to real devices fabricated in laboratories, the optical properties of AlGaInAs, InGaNAs, and InGaAsP semiconductor material systems for 1.3-μm semiconductor lasers are systematically studied. Simulation results show that both the AlGaInAs/InP and InGaNAs/GaAs material systems have better gain performance and smaller transparency carrier density than the InGaAsP/InP material system. For the AlGaInAs/InP material system, the characteristic temperature is improved by using compensating tensile strain in barrier. Specifically, for a 250-μm-long short-cavity AlGaInAs/InP laser, when the barrier is with a compensating tensile strain of 0.39%, the characteristic temperatures in 290-330 K and 330-350 K can be enhanced to 121.7 K and 58.9 K, respectively. For the InGaNAs/GaAs material system, simulation results suggest that the laser performance can be significantly improved when the laser is with strain-compensated GaNAs barriers.     

Quantitative analysis of optical power budget of bismuth oxide-based erbium-doped fiber

We investigated optical power budget of Bi₂O₃-based erbium-doped fiber (BIEDF). Lateral spontaneous emissions and scattering laser powers in the BIEDF were measured quantitatively by using an integrating sphere. Compared with the power of amplified spontaneous emission and signal detected at the output fiber end, it was found that considerable powers were consumed by the laterally emitting lights. As an optically undetected loss limits power conversion efficiency (PCE) of the fiber amplifier, the effect of nonradiative decay from the termination level of pump excited state absorption (pump ESA) was estimated from decay rate analyses of the relevant levels. The nonradiative loss was comparable to amplified signal power in the BIEDF when pumped with a 980-nm LD. Nonradiative decay following cooperative upconversion (CUP) process is also discussed using rate equations analysis.     

Femtosecond Z-scan measurement of third-order optical nonlinearities in anatase TiO2 thin films

Anatase phase TiO₂ films have been grown on fused silica substrate by pulsed laser deposition technique at substrate temperature of 750 °C under the oxygen pressure of 5 Pa. From the transmission spectra, the optical band gap and linear refractive index of the TiO₂ films were determined. The third-order optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. The real and imaginary parts of third-order nonlinear susceptibility χ⁽³⁾ were determined to be −7.1 × 10⁻¹¹esu and −4.42 × 10⁻¹²esu, respectively. The figure of merit, T, defined by T=βλ/n₂, was calculated to be 0.8, which meets the requirement of all-optical switching devices. The results show that the anatase TiO₂ films have great potential applications for nonlinear optical devices.     

Synthesis and luminescent properties of europium-activated Ca2SnO4 phosphors by sol-gel method

In this paper, the Ca₂SnO₄:Eu³⁺ phosphor was prepared by low-temperature sol-gel method. The influence of calcined temperature and time on structure of Ca₂SnO₄:Eu³⁺ was investigated by using X-ray powder diffraction (XRD). The experimental results show that the dried gel was crystallized to the pure orthorhombic phase after calcination at 900 °C in air for 6 h. These phosphors have displayed bright red color under a UV source. The richness of the red color has been verified by determining their color coordination from the CIE standard charts, and this red emission has been assigned to ⁵D₀→⁷F₂ electric dipole transition at 616 and 620 nm. The excellent luminescence properties make it possible as a good candidate for plasma display panel (PDP) application.     

Effect of spontaneous and piezoelectric polarization on optical characteristics of ultraviolet AlGaInN light-emitting diodes

Polarization is a crucial issue for electrical and optical characteristics of the UV AlGaInN light-emitting diodes. The effect of spontaneous and piezoelectric polarization on optical characteristics of the UV AlGaInN light-emitting diodes is investigated numerically. The simulation results indicate that the polarization-related effect for the UV AlGaInN light-emitting diodes is dominated by the piezoelectric polarization. When the UV AlGaInN light-emitting diodes are without piezoelectric polarization, the optical performance is enhanced effectively due to improved overlap of electron and hole wavefunctions, reduced electron leakage current, and increased density of holes in the active region.     

The dependence of the structural and optical properties on the Te mole fraction in ZnS1−xTex/GaAs heterostructures

Lattice-mismatched ZnS_1−xTe_x epilayers with various Te mole fractions on GaAs (100) substrates were grown by double well temperature gradient vapor deposition. X-ray diffraction patterns showed that the grown ZnS_1−xTe_x layers were epitaxial films. The photoluminescence spectra showed that the peak position of the acceptor-bound exciton (A⁰, X) varied dramatically with changing the Te mole fraction and that the behavior of the (A⁰, X) peak position of the ZnS_1−xTe_x epilayers with a small amount of the Te mole fraction was attributed to a bowing effect. The reflectivity and ellipsometry spectra showed that the absorption energy peak was significantly affected due to the Stoke's effect. These results provide important information on the structural and optical properties of ZnS_1−xTe_x/GaAs heterostructures for improving optoelectronic device efficiencies operating in the spectral range between near ultraviolet and visible regions.     

Potential PDP phosphors with strong absorption around 172 nm: Rare earth doped NaLaP2O7 and NaGdP2O7

NaLaP₂O₇ and NaGdP₂O₇ powder samples are prepared by solid-state reactions at 750 and 600 °C, respectively, and the VUV-excited luminescence properties of Ln³⁺ (Ln=Ce, Pr, Tb, Tm, Eu) in both diphosphates are studied. Ln³⁺ ions in both hosts show analogous luminescence. For Ce³⁺-doped samples, the five Ce³⁺ 5d levels can be clearly identified. As for Pr³⁺ and Tb³⁺-doped samples, strong 4f-5d absorption band around 172 nm is observed, which matches well with Xe-He excimer in plasma display panel (PDP) devices. As a result, Pr³⁺ can be utilized as sensitizer to absorb 172 nm VUV photon and transfer energy to appropriate activators, and Tb³⁺-doped NaREP₂O₇(RE=La, Gd) are potential 172 nm excited green PDP phosphors. For Tm³⁺ and Eu³⁺-doped samples, the Tm³⁺-O²⁻ charge transfer band (CTB) is observed to be at 177 nm, but the CTB of Eu³⁺ is observed at abnormally low energy position, which might originate from multi-position of Eu³⁺ ions. The similarity in luminescence properties of Ln³⁺ in both hosts indicates certain structural resemblance of coordination environment of Ln³⁺ in the two sodium rare earth diphosphates.     

Magnetic properties,electronic structure,and optical properties of the filled skutterudite BaFe4Sb12

The magnetic properties, electronic structure, and optical properties of the filled skutterudite BaFe₄Sb₁₂ are calculated by the first-principles full-potential linearized augmented plane wave (FPLAPW) plus local orbital method. It is found that the local spin density approximation (LSDA) method appears more accurate than the generalized gradient approximation (GGA) method in calculating the electronic structures and optical properties of this compound. Furthermore, our calculated lattice constant and spin magnetic moments with the LSDA method are in overall better agreement with experiment. In contrast with recent experiment, our calculations are in good agreement with experimental reflectivity spectra and optical conductivity spectrum.