Study of two-dimensional hole gas concentration and hole mobility in zinc delta-doped GaAs and pseudomorphic GaAs/In0.2Ga0.8As heterostructures

Zinc delta-doped GaAs and pseudomorphic GaAs/In}_0.2Ga_0.8As heterostructures grown by low-pressure metalorganic chemical vapour deposition have been demonstrated. The influence of delta-doping period and spacer thickness on two-dimensional hole gas concentrations and hole mobility was studied. From secondary-ion mass spectroscopy and Hall measurement, we conclude that zinc delta-doping can form an excellent abrupt profile (full-width at half maximum is of 10 nm) and offer a high two-dimensional hole gas sheet density (as high as 1 × 10¹³cm⁻²) By adopting a strained InGaAs material as the active channel and by carefully modulating the spacer layer thickness, one can obtain a significantly enhanced hole mobility.     

Synthesis and characterization of Eu2+ activated X12Al10.6Si3.4O32Cl5.4 (X=Sr,Ca) phosphors

In this article, we have reported emission characteristics of Eu²⁺ activated Sr₁₂Al_10.6Si_3.4O₃₂Cl_5.4 and Ca₁₂Al_10.6Si_3.4O₃₂Cl_5.4 phosphors, prepared by combustion synthesis. The PL excitation was monitored at two different wavelengths 254 nm (mercury excitation) and 354 nm (mercury free excitation), which shows broad-blue emission bands at 432 nm, and 438 nm for 354 nm excitation wavelength. Under mercury excitation at 254 nm it shows emission wavelengths at 470 nm, 589 nm, 592 nm, and 617 nm for the prepared Sr₁₂Al_10.6Si_3.4O₃₂Cl_5.4 and Ca₁₂Al_10.6Si_3.4O₃₂Cl_5.4 phosphors. Further it was characterized by XRD, for the investigation of phase and purity of the phosphors. Thermal degradation of the phosphors was also studied at different temperatures.     

Photo-induced effects on structural and optical properties of Ga15Se81Ag4 chalcogenide thin films

Thin films with thickness of 400 nm have been obtained from the Ga₁₅Se₈₁Ag₄ ternary chalcogenide glass prepared by the melt quenching technique. The behavior of several optical constants has been studied from absorption and reflection spectra as a function of photon energy in the wavelength region 400–1200 nm. The amorphous nature of the sample was examined by X-ray diffraction and non-isothermal DSC measurements. Thin films were illuminated by shining white light using 1500 W tungsten lamp with different exposure time. The ambient temperature during the illumination process was controlled and kept at 348 K, selected by DSC thermogram. Analysis of the optical absorption data shows that the rule of non-direct transition predominates. It is found that the optical band gap decreases by increasing the illumination time. It has also been observed that the value of absorption and extinction coefficients increases while the refractive index decreases by increasing the illumination time from 0 to 150 min. The decrease in optical band gap is explained on the basis of the change in nature of the films, from amorphous to crystalline state, with increase of the illumination time.     

Blue emission in Eu2+ activated MgXAl10O17 (X = Sr,Ca) phosphors

Here we reported photoluminescence properties of Eu²⁺ activated in novel and existing MgXAl₁₀O₁₇ (X = Sr, Ca) phosphor which has been prepared by combustion synthesis at 550 °C under UV and near UV excitation wavelength. The PL emission properties of MgSrAl₁₀O₁₇:Eu²⁺ were monitored at 254 nm and 354 nm respectively keeping emission wavelength at 469 nm. Whereas novel MgCaAl₁₀O₁₇:Eu²⁺ exhibit emission band at 452 nm keeping excitation at 378 nm. These blue emission corresponds to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ions. Further phosphor was analyzed by XRD for the confirmation of desired phase and purity.     

Influence of the ZnO nanoparticle sizes and morphology on the photoinduced light reflectivity

We have established a principal possibility of changes of the light reflectivity at the wavelength of 633 nm (He–Ne laser) under influence of the external laser light. The changes are very sensitive to the wavelength of the photoinduced laser. We have chosen two types of the photoinduced lasers: UV nitrogen 7 ns laser at wavelength 371 nm heating near the absorption edge and the 10 ns 1064 nm Nd:YAG laser with wavelength 1064 nm. The power dependences of the reflectivity were studied. Possible explanation of the observed effects is presented following the conception of the nano-trapping levels. These results have been obtained from two ZnO thin films prepared from principally different deposition parameters leading to different particle features and morphologies.     

Light transmission through polyethylene samples

A spectral calibration Hg(Ar) lamp and a pulsed Nd:YAG laser were employed to study the UV, VIS and IR light transmission through polyethylene samples, as pure and doped with Fe₂O₃ and carbon nanotubes. Optical measurements were performed in the spectral range between 200 nm and 800 nm. Measurements with 0.1% and 1% in weight of filler concentrations were investigated. The polymers' optical absorption coefficients, calculated from the experimental transmitted component, were tabulated at different wavelengths as a function of the analyzed samples. The radiation depth penetration was also evaluated.     

Using high haze (> 90%) light-trapping film to enhance the efficiency of a-Si:H solar cells

The high haze light-trapping (LT) film offers enhanced scattering of light and is applied to a-Si:H solar cells. UV glue was spin coated on glass, and then the LT pattern was imprinted. Finally, a UV lamp was used to cure the UV glue on the glass. The LT film effectively increased the Haze ratio of glass and decreased the reflectance of a-Si:H solar cells. Therefore, the photon path length was increased to obtain maximum absorption by the absorber layer. High Haze LT film is able to enhance short circuit current density and efficiency of the device, as partial composite film generates broader scattering light, thereby causing shorter wave length light to be absorbed by the P layer so that the short circuit current density decreases. In case of lab-made a-Si:H thin film solar cells with v-shaped LT films, superior optoelectronic performances have been found (V_oc = 0.74 V, J_sc = 15.62 mA/cm², F.F. = 70%, and η = 8.09%). We observed ~ 35% enhancement of the short-circuit current density and ~ 31% enhancement of the conversion efficiency.     

Efficient cw violet-light generation in a ring cavity with a periodically poled KTP

In this work, we experimentally demonstrate an efficient cw second harmonic generation (SHG) at 780 nm wavelength with a first-order type-I phase matching periodically poled KTP (PPKTP) crystal in a ring cavity, the wavelength corresponds to the D₂ line of Rb atom transition. The fundamental laser used is a grating-stabilized external cavity diode laser and its frequency is precisely locked to Rb atom transition frequency using the saturated absorption technique. About maximal 6.9 mW UV radiation of 390 nm with a net conversion of 9.5% at an input mode-matched power of 73 mW is generated with one crystal, and about maximal 8.8 mW with the net conversion of 12% is obtained with another crystal; the powers in stable operation are about 1.7 mW and 3.4 mW, respectively. This is, to our best knowledge, the first SHG experiment at 780 nm wavelength with the PPKTP in a ring cavity.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

Tunable dual-wavelength fiber laser based on an opto-VLSI processor and four-wave mixing in a photonic crystal fiber

A stable wavelength and wavelength spacing tunable dual-wavelength fiber laser based on an Opto-very-large-scale-integration (Opto-VLSI) processor and four-wave mixing (FWM) in a high-nonlinear photonic crystal fiber is experimentally demonstrated. The results show that the line width of the tunable dual-wavelength fiber laser is 0.02 nm, and the wavelength spacing can be tuned from 0.8 nm to 4 nm with a 0.15 nm step. Under the influence of the FWM, the uniformity is below 0.6 dB and the measured side mode suppression ratio (SMSR) is above 45 dB.     

Low-cost micro-lens arrays fabricated by photosensitive sol–gel and multi-beam laser interference

In this paper, we introduce a low-cost approach for fabricating micro-lens arrays that is based on photosensitive sol–gel and multi-beam laser interference. UV photosensitive ZrO₂ gel films are prepared with Zr(O(CH₂)₃CH₃)₄ and BzAcH as the precursor and chemical modifier, respectively. With UV laser irradiation via different dose, nonlinear photodecomposition occurs in this film. Large scale 2D micro lens arrays with the sizes of 830 nm × 830 nm and 280 nm × 280 nm are fabricated by four-beam laser interference. The surface profile modeling shows that the micro lens is plano convex lens, and the effective focal lengths are 812.0 nm and 317.6 nm, respectively.     

The effect of spacer layer thickness on vertical alignment of InGaAs/GaNAs quantum dots grown on GaAs(3 1 1)B substrate

We fabricated multiple stacked self-organized InGaAs quantum dots (QDs) on GaAs (3 1 1)B substrate by atomic hydrogen-assisted molecular beam epitaxy (H-MBE) to realize an ordered three-dimensional QD array. High quality stacked QDs with good size uniformity were achieved by using strain-compensation growth technique, in which each In_0.35Ga_0.65As QD layer was embedded by GaNAs strain-compensation layer (SCL). In order to investigate the effect of spacer layer thickness on vertical alignment of InGaAs/GaNAs QDs, the thickness of GaNAs SCL was varied from 40 to 20 nm. We observed that QDs were vertically aligned in [3 1 1] direction when viewed along [0 1 −1], while the alignment was inclined when viewed along [−2 3 3] for all samples with different SCL thickness. This is due to their asymmetric shape along [−2 3 3] with two different dominant facets thereby the local strain field around QD extends further outward from the lower-angle facet. Furthermore, the inclination angle of vertical alignment QDs became monotonously smaller from 22° to 2° with decreasing SCL thickness from 40 to 20 nm. These results suggest that the strain field extends asymmetrically resulting in vertically tilted alignment of QDs for samples with thick SCLs, while the propagated local strain field is strong enough to generate the nucleation site of QD formation just above lower QD in the sample with thinner SCLs.     

Luminescent features of sol–gel derived rare-earth multi-doped oxyfluoride nano-structured phosphors for white LED application

Rare-earth doped oxyfluoride 75SiO₂:25PbF₂ nano-structured phosphors for white-light-emitting diodes were synthesized by thermal treatment of precursor sol–gel derived glasses. Room temperature luminescence features of Eu³⁺, Sm³⁺, Tb³⁺, Eu³⁺/Tb³⁺, and Sm³⁺/Tb³⁺ ions incorporated into low-phonon-energy PbF₂ nanocrystals dispersed in the aluminosilicate glass matrix and excited with UV light emitting diode were investigated. The luminescence spectra exhibited strong emission signals in the red (600, 610, 625, and 646 nm), green (548 and 560 nm), and blue (485 nm) wavelength regions. White-light emission was observed in Sm/Tb and Eu/Tb double-doped activated phosphors employing UV-LED excitation at 395 nm. The dependence of the luminescence emission intensities upon annealing temperature and rare-earth concentration was also examined. The results indicated that there exist optimum annealing temperature and activator ion concentration in order to obtain intense visible emission light with high color rendering index. The study suggests that the nanocomposite phosphor based upon 75SiO₂:25PbF₂ host herein reported is a promising contender for white-light LED applications.     

Effects of morphology on photocatalytic performance of Zinc oxide nanostructures synthesized by rapid microwave irradiation methods

In this study, two different chemical solution methods were used to synthesize Zinc oxide nanostructures via a simple and fast microwave assisted method. Afterwards, the photocatalytic performances of the produced ZnO powders were investigated using methylene blue (MB) photodegradation with UV lamp irradiation. The obtained ZnO nanostructures showed spherical and flower-like morphologies. The average crystallite size of the flower-like and spherical nanostructures were determined to be about 55 nm and 28 nm, respectively. X-ray diffraction (XRD), scanning electronic microscopy (SEM), Brunauer–Emmett–Teller (BET), room temperature photoluminescence (RT-PL) and UV–vis analysis were used for characterization of the synthesized ZnO powders. Using BET N₂-adsorption technique, the specific surface area of the flower-like and spherical ZnO nanostructures were found to be 22.9 m²/gr and 98 m²/gr, respectively. Both morphologies show similar band gap values. Finally, our results depict that the efficiency of photocatalytic performance in the Zinc oxide nanostructures with spherical morphology is greater than that found in the flower-like Zinc oxide nanostructures as well as bulk ZnO.     

Photo-induced isomerization of poly 4′-(6-acryloxy) hexyloxy-4-methoxyazobenzene ultra-thin solid film prepared by Langmuir–Blodgett technique

The photo-induced response of an ultra thin polymeric film of poly 4′-(6-acryloxy) hexyloxy-4-methoxyazobenzene (P5) is investigated. A monolayer of P5 at a gas–water interface possesses a mean molecular area of 28.0 Å²/monomer-repeat. Multilayer films of P5 were prepared by horizontal deposition at a surface pressure of 25 mN/m². The uniformity of the transfer process is shown by UV–vis absorption spectra where a linear relationship between the absorption maxima and the number of transferred layer was observed. The average layer thickness of the transferred film determined by XRD measurements is 34.0 Å. This is longer than the length of the azobenzene side group. The transferred film shows a blue shift of the π–π¹ transition from 357 nm for the P5 in solution to 340 nm for the P5 in the film. This suggests the formation of H-aggregate with a head-to-head arrangement of the dipole within the film. The optical property of the transferred film is changed by the irradiation of the film with the UV light at 385 nm. An irreversible change in its molecular packing in the film is seen in the shift of the UV–vis absorption maxima and the change in morphology as observed by AFM. The film morphology changes from being a smooth film into an island-like surface when exposed to the UV irradiation. The layer structure in the film is destroyed. A mass transport is observed during the cis–trans thermal back isomerization process. This suggests that movement of the P5 took place in both the trans–cis isomerization process and the cis–trans back isomerization process. The first movement leads to a molecular expansion while the second, to a molecular contraction.     

Compact resonantly intra-cavity pumped tunable Ho:Sc2SiO5 laser

A compact intra-cavity pumped low threshold continuous-wave Ho:Sc₂SiO₅ laser is reported. The characteristics of output wavelength tuning are investigated by use a intra-cavity briefringent (BF) filter. A wavelength tunable range of 140 nm from 2020 to 2160 nm is achieved. For the free-running mode, the laser slope efficiency is 24.8%, when the output central wavelength is 2110 nm. The laser threshold is about 820 mW of incident pump power. With the BF filter, a maximum output power of 870 mW is obtained at the incident pump power of 5 W, corresponding to a slope efficiency of 20.3%. The characteristics of output wavelength verse the crystal temperature are also investigated.     

Photocatalytic water splitting of La2AlTaO7 and the effect of aluminum on the electronic structure

A new photocatalyst La₂AlTaO₇ with orthorhombic structure was synthesized by the solid-state reaction method. The formation rate of H₂ evolution from CH₃OH/H₂O solution under the irradiation of a 350 W high-pressure Hg lamp is about 108.9 μmol h^?1 for La₂AlTaO₇ (0.1 g). It also showed activity leading to the decomposition of pure water into H₂ and O₂ even in the absence of co-catalysts under UV light irradiation. The photocatalyst loaded with 0.2 wt% NiO co-catalyst was found to have the highest activity. It was found from the electronic band structure study, using the density functional theory (DFT) with plane-wave basis, that the valence band top mainly consists of O 2p orbitals and the conduction band bottom is mainly constructed of Al 3s3p. The effect of aluminum on electronic structure was discussed in close connection with the UV–vis absorption spectrum.     

Green synthesis of nanocomposites consisting of silver and protease alpha chymotrypsin

The synergy of ultrasonication and the exposure to light radiation was found to be necessary in the formation of nanocomposites of silver and a protease alpha chymotrypsin. The reaction was carried out in aqueous medium and the process took just less than 35 min. Ultrasonication alone formed very negligible number of nanoparticles of <100 nm size whereas light alone produced enough number but the size of the particles was >100 nm.The effects of pH (in the range of 3–5, 9–10), ultrasonication time periods (0–30 min), ultrasonication intensity (33–83 W cm^?2), energy of light radiation (short UV, long UV and Fluorescent light) and time period of exposure (5–60 min) to different light radiations were studied.The formation of nanocomposites under these effects was followed by surface plasmon resonance (SPR) spectra, dynamic light scattering (DLS), transmission electron microscopy (TEM). Ag–chymotrypsin nanocomposites of sizes ranging from 13 to 72 nm were formed using the synergy of ultrasonication and exposure to short UV radiation. Results show that ultrasonication promoted nuclei formation, growth and reduction of polydispersity by Ostwald ripening.     

Chromatic aberration free liquid crystal adaptive optics for flood illuminated retinal camera

A high-resolution, flood-illumination retinal camera using liquid crystal (LC) adaptive optics (AO) is presented. The retinal camera uses light at 780 nm for ocular aberration measurement while light at 655 nm and 593 nm for retinal imaging. In order to avoid chromatic aberrations due to wavelength dependence of LC, we adopt an open-loop technique, in which dynamic correction of aberrations is applied only to the imaging light. A compensation pattern projected on the LC wavefront corrector is adjusted to provide phase wrapping of 2 π for illumination light. We confirmed feasibility of this technique by performing in vivo retinal imaging experiments. Photoreceptors were clearly revealed at both imaging light at 655 nm and 593 nm. Feasibility of the technique was also supported by comparison of the retinal images taken by the present open-loop technique with those taken by the conventional closed-loop one and by analysis of the spatial distribution of the photoreceptors.     

Optical investigation of vacuum evaporated Se80−xTe20Sbx (x = 0, 6, 12) amorphous thin films

Amorphous thin films of Se_80−xTe₂₀Sb_x (x = 0, 6, 12) chalcogenide glasses has been deposited onto pre-cleaned glass substrate using thermal evaporation technique under a vacuum of 10⁻⁵ Torr. The absorption and transmission spectra of these thin films have been recorded using UV spectrophotometer in the spectral range 400–2500 nm at room temperature. Swanepoel envelope method has been employed to obtain film thickness and optical constants such as refractive index, extinction coefficient and dielectric constant. The optical band gap of the samples has been calculated using Tauc relation. The study reveals that optical band gap decreases on increase in Sb content. This is due to decrease in average single bond energy calculated using chemical bond approach. The values of urbach energy has also been computed to support the above observation. Variation of refractive index has also been studies in terms of wavelength and energy using WDD model and values of single oscillator energy and dispersion energy has been obtained.