Short Wavelength Side Research Articles

Fabrication and optical properties of platinum nanowire arrays on anodic aluminium oxide templates

Arrays of Pt nanowires, fabricated by electrodepositing Pt metal into nanoporous anodic aluminium oxide (AAO) templates, exhibit a preferable optical absorption band in the ultraviolet-visible (UV-VIS) spectra and present a blueshift as the wire aspect ratio increases or its radius decreases. This type of optical property of Pt nanowire/porous alumina composites has been theoretically explored using Maxwell-Garnett (MG) effective medium theory. The MG theory, however, is only applicable to nanowires with an infinitesimally small radius relative to the wavelength of an incident light. The nanowire radius is controlled by the pore radius of the host alumina, which depends on anodizing conditions such as the selected electrolyte, anodizing time, temperature and voltage. The nanowire aspect ratios depend on the amount of Pt deposited into the nanopores of AAO films. The optical absorption properties of the arrays of Pt nanowires with diameters of 24, 55 and 90 nm have been investigated by the UV-VIS spectra, which show that the extinction maximum (λmax ) shifts to shorter wavelength side as the wire aspect ratio increases or its radius decreases. The results are qualitatively consistent with those calculated based on the MG theory.

Influence of post-deposition annealing on the properties of transparent conductive nanocrystalline ZAO thin films prepared by RF magnetron sputtering with highly conductive ceramic target

Using a highly conductive ZnO (ZnAl 2O 4) ceramic target, c axis-oriented and oxygen-deficient Al-doped ZnO (ZAO) thin films were prepared on glass sheet substrates by radio frequency (RF) planar magnetron sputtering. The structural, electrical and optical properties of the films (deposited at room temperature and annealed at different conditions) were characterized with various techniques. The experimental results show that the electrical resistivity of as-grown films can be decreased to 10 −4 Ω cm level with post-deposition annealing at 400 °C for 2 h in a vacuum pressure of 10 −5 torr. By increasing the annealing temperature, the oxygen vacancies and carrier concentration of ZAO thin film increase, and the absorption edge in the transmission spectra shifts toward the shorter wavelength side (blue shift). The oxygen vacancy plays an important role in the electrical conductivity. The more the oxygen vacancies exist in ZAO thin films, the higher the electrical conductivity will be.

Spectral evolution of cooperative luminescence in an Yb 3+-doped silica optical fiber

We have analyzed the cooperative luminescence of an ytterbium-doped silica fiber in which the ytterbium concentration is quite large, i.e., ∼ 10 21 cm −3 . The presence of a fast decaying component in a decay profile of 2 F 5/2→ 2 F 7/2 transition indicates the formation of ytterbium pairs in the fiber. The ytterbium cooperative luminescence spectrum is characterized by two peaks at 488 nm and at ∼512 nm, respectively. Overall intensity of the visible cooperative upconversion luminescence is proportional to excitation power at 980 nm. However, as fiber length increases, intensity of the shorter-wavelength side of the cooperative luminescence spectrum becomes smaller than the other portion, which results in a narrower spectral bandwidth.

Emission from a transverse volume discharge in neon with small admixtures of water vapor and air

Emission from the plasma of a pulsed discharge in neon with small admixtures of water vapor and air in the wavelength range 210–620 nm is investigated. A transverse volume discharge with spark UV preionization is ignited in neon at a pressure of 100–200 kPa and charging voltage of Uch≤20 kV. It is shown that the discharge acts as a source of pulsed UV radiation on OH(A-X) (λ=308–314 nm) and NO(B-X) (247.8 nm) transitions, which is of interest for the use in an ecologically safe lamp based on the mixtures of neon with water vapor and air. In the visible spectral region, plasma emission consists of the NeI (3s, 3s′-3p, 3p′) band and Hβ 486.1-nm spectral line. On the short-wavelength side of the spectrum, a broadband emission (the third continuum of neon) is observed, whose intensity increases with increasing neon pressure and decreasing emission wavelength.

Photophysical and Photochemical Processes of an Unsymmetrical Fullerene Dimer, C121

Photophysical and photochemical processes of C121, an unsymmetrical fullerene dimer, have been investigated by using laser flash photolysis mainly. The triplet excited state of C121 showed a transient absorption band at 750 nm, whose deactivation process was governed by mixed-order kinetics of first and second order, indicating the contribution of triplet−triplet annihilation process. The intrinsic lifetime of the triplet excited state was estimated to be 13 μs. The singlet and triplet excited states of C121 are considered to be localized on one fullerene moiety of the two-fullerene cages. The one- and two-electron reductions of C121 were observed in the presence of tetrakis(dimethylamino)ethylene in the ground state. The dianion of C121 showed an absorption band in shorter-wavelength side due to the interaction of the transition dipole of anion radical on each fullerene cage of C121. Photoinduced one-electron reduction of C121 was confirmed in the presence of N,N,N‘,N‘-tetramethyl-1,4-phenylenediamine as...

Ultraviolet Photon Induced Bulk, Surface and Interface Modifications in n-Hg0.8Cd0.2Te in Hydrogen Environment

Ultraviolet photon induced bulk, surface and interface modifications in n-Hg0.8Cd0.2Te (MCT) and metal insulator semiconductor (MIS) structures fabricated from CdTe/n-Hg0.8Cd0.2Te have been investigated in a hydrogen environment. The shift of the absorption edge towards the short wavelength side and the enhancement of the IR transmission in n-Hg0.8Cd0.2Te are thought to be due to the reduction in the density of gap states due to residual impurities or defects. A pre-CdTe deposition surface treatment prevents the surface inversion of n-MCT during CdTe deposition and MIS devices after UV photon excitation show a considerable lowering of the interface charges. X-ray photoelectron spectroscopy profiles of annealed CdTe/n-Hg0.8Cd0.2Te show evidence of interface grading which accounts for the stability of CdTe passivation.

Effect of Vacuum Annealing on the Properties of Transparent Conductive AZO Thin Films Prepared by DC Magnetron Sputtering

Using highly conductive Al doped ZnO (AZO) ceramic target, (001)-oriented transparent conductive AZO (Al doped ZnO) thin films are prepared by dc planar magnetron sputtering deposition on glass sheet substrates. The structural, electrical, and optical properties of the films (deposited at room temperature and annealed at different temperatures in vacuum) are characterized with various techniques. The experimental results show that electrical resistivity of the as-grown films (not intentionally heated) can be decreased to 10 -4 Ω cm level with post-deposition annealing at 400 °C for 2 h in vacuum pressure of 10 -5 Torr. AZO thin films deposited at room temperature (RT) and subsequently annealed at 450 °C for 2 h in vacuum have nano-crystalline structure with even crystal size distribution. With the increase of annealing temperature, the (002) preferred orientation of AZO thin film grows, the carrier concentration of AZO thin film increases, and the absorption edge in the transmission spectrum shifts toward the shorter wavelength side (blue shift). The oxygen vacancies have played an important role in the improvement of electrical conductivity of these AZO thin films with the vacuum annealing process.

Broad-band and extremely flat super-continuum generation via optical parametric gain extended spectrally by fourth-order dispersion in anomalous-dispersion-flattened fibers

Broad-band and extremely flat super-continuum (SC) generation phenomenon was found out in propagation of sub-100 fs and pJ optical pulse along very short anomalous-dispersion-flattened fibers (0.6/spl sim/3.4 m). In contrast to the usual SC cases, fiber-optic anomalous-dispersion characteristics are essential. Results of numerical and experimental analyzes suggest that its origin stays in the broad-band optical parametric gain generation whose spectrum is extended by fourth-order dispersion characteristics. The remarkable features of SC spectra are (1) excellent static flatness of /spl plusmn/1 dB over 150 nm (or /spl plusmn/0.5 dB over 100 nm) and (2) asymmetric spectral expansion toward the shorter wavelength side overwhelming S-band.

Solvent and concentration effects on the visible spectra of tri- para-dialkylamino-substituted triarylmethane dyes in liquid solutions

We have characterized the spectroscopy properties of crystal violet (CV +) and ethyl violet (EV +) in liquid solutions as a function of the solvent type and dye concentration. The analysis of how solvent properties and dye concentration affects the electronic spectra of these tri- para-dialkylamino substituted tryarylmethane (TAM +) dyes was performed on the basis of two spectroscopic parameters, namely the difference in wavenumber (Δ ν) between the maximum and the shoulder that appears in the short-wavelength side of the respective maximum visible band (Δ ν=1/ λ shoulder−1/ λ max, cm −1), and the wavelength of the maximum absorption ( λ max). The solvent and the concentration effects on λ max and Δ ν have indicated that both solute/solute (ion-pairing and dye aggregation) and solute/solvent (H-bonding type) interactions modulate the shape of the visible electronic spectra of these dyes in solution. In solvent with small dieletric constant ( ε<∼10), the formation of ion-pairs represents a major contribution to the shaping of these spectra. Upon increasing dye concentration the formation of ion-pairs was characterized by an increase in Δ ν observed concomitantly with a red shift in λ max. In chloroform and chlorobenzene the ion-pair association constant of CV + and EV + with Cl − ions were found to be in the order of 10 6 and 10 5 M −1, respectively. In trichloroethylene the association constant for the CV +Cl − pair was 10 8 M −1. In water, dye aggregation instead of ion-pairing represents a major contribution to the shaping of the visible spectra of CV + and EV +. Dye aggregation was indicated by an increase in Δ ν observed concomitantly with a blue shift in λ max upon increasing dye concentration. The distinct behavior of λ max for dye aggregation and ion-pairing as a function of dye concentration can therefore assist in the characterization of these two distinct phenomena. The solute/solvent interactions were studied in a series of polar solvents in which solute/solute interactions do not occur in any detectable extent. The dependence found for Δ ν as a function of the Kamlet–Tafts solvatochromic parameters ( α, β and π*) is in keeping with previous inferences indicating that the splitting in the overlapped absorption band of CV + and EV + in hydroxilated solvents arises from a perturbation in the molecular symmetry induced by hydrogen bonding (donor–acceptor) type interactions with solvent molecules. A distinction between the effects of solute/solute and solute/solvent interactions on the visible spectra of these dyes is provided.

Directional phonon-assisted cascading of photoexcited carriers in stepped In x(Al 0.17Ga 0.83) 1− xAs/Al 0.17Ga 0.83As multiple quantum wells

Perpendicular motion of photoexcited electron and hole pairs assisted by phonon scattering is investigated in a novel step-graded staircase heterostructure consisting of strained In x (Al 0.17Ga 0.83) 1− x As multiple quantum wells (QWs) with similar widths but five different x values by cw and time-resolved photoluminescence (PL) experiments. From the temperature dependence of PL spectral intensity of distinct five peaks corresponding to the QW layers, we find that, as temperature increases, the PL peaks decrease their relative intensities progressively from shorter wavelength sides after increasing the signal amplitude in the intermediate temperature range. These variations reveal that the photoexcited carriers directionally move from shallower to deeper QWs via phonon-assisted activation above the barrier band edge state. The PL dynamics directly indicate the perpendicular flowing of photoexcited carriers and the capture by the deeper QW, thus providing firm evidences for the dynamical carrier flow and capture processes.

Epitaxial Growth for Solar-Blind AlGaN Photodetector Imaging Arrays by Metalorganic Chemical Vapor Deposition

ABSTRACTSolar-blind imaging arrays based on AlGaN p-i-n structures are of high interest for defense applications. We have studied the material issues involved in development of such imaging arrays and have developed discrete photodetector devices with a high external quantum efficiency (EQE) and imaging arrays of high operability.For the discrete devices, a record EQE of 58.1% peaking at 274 nm under zero volt bias was obtained without using an anti-reflecting (AR) coating. The EQE was seen to have a slight voltage dependence: going up to 64.5% at –5V reverse bias. The responsivity had a drop-off by one order of magnitude for a wavelength change of 4 nm on both the shorter and longer wavelength side. The material quality and uniformity was found to be very good leading to the development of 256 × 256 arrays. A high yield along with uniform, high EQE was obtained for the detector devices in the array leading to a high operability of 99.8%.

E-Beam Longitudinally Pumped Laser Based on ZnCdSe/ZnSe MQW Structure Grown by MBE on ZnSe(001) Substrate

Electron beam longitudinally pumped laser based on 15 ZnCdSe/ZnSe QW periodic-gain structure grown by molecular beam epitaxy on ZnSe(001) substrate was studied. An output power of 0.3 W was achieved. The laser wavelength was in 518-536 nm range, being at the short wavelength side of the QW emission line at low excitation level. Such unusual feature was explained by the participation of excited QW levels in the creation of the optical gain.

E-Beam Longitudinally Pumped Laser Based on ZnCdSe/ZnSe MQW Structure Grown by MBE on ZnSe(001) Substrate

Electron beam longitudinally pumped laser based on 15 ZnCdSe/ZnSe QW periodic-gain structure grown by molecular beam epitaxy on ZnSe(001) substrate was studied. An output power of 0.3 W was achieved. The laser wavelength was in 518–536 nm range, being at the short wavelength side of the QW emission line at low excitation level. Such unusual feature was explained by the participation of excited QW levels in the creation of the optical gain.

Theoretical Study on the Nonadiabatic Transitions in the Photodissociation Processes of Cl2

Photodissociation of Cl2 on the shorter wavelength side of the first absorption band has been known to yield a small but significant amount of Cl(2P1/2) from the C 1Πu state, despite its adiabatic correlation to the two Cl(2P3/2) atoms. We calculated some potential energy curves of the ground and excited states of Cl2 by the spin−orbit configuration interaction method and examined the possibilities of several nonadiabatic transition mechanisms. It was found that the radial Rosen−Zener−Demkov (RZD)-type nonadiabatic transition from the C 1Πu to the third Ω = 1u (3Σ+1u (σg → σu*)) state is responsible for the production of Cl(2P1/2), and the rotational nonadiabatic transition probability from the C 1Πu to the B 3Π0+u state is negligibly small. The wavelength dependence of the product branching ratio Cl(2P1/2)/Cl(2P3/2) and the anisotropy parameter β(Cl(2P1/2)), which was calculated from the electronic transition moments to the A 3Π1u, B 3Π0+u, and C 1Πu states with the RZD transition mechanism, was in good ...

Luminescence of magnetic excitons in orthorhombic MnF 2 films

Photoluminescence has been measured from 100 to 300 nm epitaxial thin films of MnF 2 prepared on Si(1 1 1) substrates and having α-PbO 2 orthorhombic structural type. The dominant luminescence consists of a broad band peaking around 575 nm with a gradual tail extending to longer wavelengths. Weak spectral line features are observed on the short-wavelength side of the main band and are associated with magnon sidebands of Mn 2+ excitons that are perturbed by Mg and Ca impurities. Of note is a blue shift (∼12 nm) of these sidebands when compared to their positions in bulk MnF 2. The shift can be associated with different crystal field for Mn 2+ ions in orthorhombic phase.

A novel optical device with wide-bandwidth wavelength conversion and an optical sampling experiment at 200 Gbit/s

This paper reports the characteristics of our proposed prototype optical parametric diffuser (OPD). An OPD is based on the theory of four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). However, to improve the conversion bandwidth and FWM efficiency, the gain bandwidth is spread and the gain peak wavelengths are set to a wavelength near the FWM light on the short-wavelength side by combining different MQW active layers. We measured the optical gain characteristics; the fiber-to fiber gain was 16.1 dB and the gain bandwidth over 8 dB was 117 nm when driven at 200 mA dc, and 190 nm when driven by an 800 mA pulse current. In a wavelength-conversion experiment, a high conversion efficiency of /spl ges/-20 dB was obtained across a detuning wavelength bandwidth of 43 nm. A clear waveform was obtained in an optical sampling experiment to measure 200 Gbit/s optical data sequences.

Effect of third-order dispersion on the phases of solitonlike Cr^4+ : YAG-laser pulses characterized by the second-harmonic generation frequency-resolved optical gating method

The effect of third-order dispersion on solitonlike operation of the L-fold cavity Cr4+:YAG laser is studied. Dispersion equations are derived to the fourth order for an L-fold cavity, and dispersion values for a Cr4+:YAG laser are calculated. Third-order dispersion is found to be comparable with the second-order dispersion in a 60-fs-class Cr4+:YAG laser, and fourth-order dispersion is negligible. The temporal and spectral phases of a 60-fs pulse are calculated from the dispersion properties of the cavity. The spectral phase is constant, despite considerable third-order dispersion. A second-harmonic generation (SHG) frequency-resolved optical gating (FROG) trace is calculated for the simulated pulse. The SHG spectrum has a tail to the short-wavelength side at zero delay, consistent with experimental results. The calculated phase and the FROG trace should be recognized as characteristic of the third-order-dispersion effect on solitonlike lasers.

Photoluminescence Study of Self-Limiting Oxidation in Nanocrystalline Silicon Quantum Dots

ABSTRACTWe have studied photoluminescence (PL) of surface oxidized nanocrystalline silicon quantum dots (QDs) for various oxidation periods and temperatures. With increasing oxidation period, the surface oxide grows and the Si QD core shrinks initially, then retardation of the oxidation process occurs which is ascribed to compressive stress at the interface between Si QD core and oxide. Upon oxidation, the PL spectrum peak shifts toward the shorter wavelength side followed by retardation of the blueshift or even manifestation of the redshift. The origin of PL is due to the localized excitons at the interface between Si QD core and oxide or amorphous SiOx (a-SiOx) formed at the interface. The blueshift is associated with the increased quantum con.nement or increased bandgap of a-SiOx. The redshift is due to the stress e.ect of the bandgap of Si QD core or a-SiOx. We have successfully confirmed the effect of compressive stress associated with the self-limiting oxidation by PL measurement.

Wavelength Tunable Ultrashort Pulse Fiber Laser.

Compact system of wavelength tunable ultrashort soliton pulse generation has been demonstratedusing modelockedfemtosecond fiber laser and polarization maintaining (PM) fiber. The wavelengths of generated pulsescan be changed continuously by merely varying fiber input power. The generated pulses are almost transformlimited ideal soliton pulses. Two-colored soliton pulses can be generated simultaneously usingthe birefringenceof PM fibers. Using the dispersion shifted PM fibers, in addition to the soliton pulse, the anti-stokespulse is generated at the shorter wavelength side. Using the diameter reduced PM fibers and dispersion shiftedPM fibers, the wavelength tunable ultrashort pulses can be generated in the wide wavelength region of 1.3-2.0μm. In this paper, the characteristics of wavelength tunable soliton pulse are described withthe principles and applications.

Changes in surface solar UV irradiances and total ozone during the solar eclipse of August 11, 1999

During the solar eclipse of August 11, 1999, intensive measurements of UV solar irradiance and total ozone were performed at a number of observatories located near the path of the Moon's shadow. At the Laboratory of Atmospheric Physics (LAP) of the Aristotle University of Thessaloniki, Greece, global and direct spectra of UV solar irradiances (285–365 nm) were recorded with a double monochromator, and erythemal irradiances were measured with broadband pyranometers. In addition, higher‐frequency measurements of global and direct irradiances at six UV wavelengths were performed with a single Brewer spectrophotometer. Total ozone measurements were also performed with Dobson and Brewer spectrophotometers at Hradec Kralove (Czech Republic), Ispra (Italy), Sestola (Italy), Hohenpeissenberg (Germany), Bucharest (Romania), Arosa (Switzerland), and Thessaloniki (Greece). From the spectral UV measurements the limb darkening effect of the solar disk was tentatively quantified from differences of measured solar spectral irradiances at the peak of the eclipse (near to limb conditions) and before the eclipse. Two blackbody curves were fit to the preeclipse and peak eclipse spectra, which have shown a difference in effective temperatures of about 165°K between the limb and the whole of the solar disk. The limb darkening effect is larger at the shorter UV wavelengths. The ratio of the diffuse to direct solar irradiances during the eclipse shows that the diffuse component is reduced much less compared to the decline of the direct solar irradiance at the shorter wavelengths. Moreover, a 20‐min oscillation of erythemal UV‐B solar irradiance was observed before and after the time of the eclipse maximum under clear skies, indicating a possible 20‐min fluctuation in total ozone, presumably caused by the eclipse‐induced gravity waves. This work also shows that routine total ozone measurements with a Brewer or a Dobson spectrophotometer should be used with caution during a solar eclipse. This is because the diffuse light increases by more than 30% with respect to the direct solar radiation, increasing more at the shorter wavelength side of the UV spectrum. This plausible mechanism introduces an artificial decrease in total ozone during solar eclipse of more than 30 Dobson units (DU), which is confirmed by all Brewer and Dobson measurements. Changes in total ozone cited earlier in the refereed literature have not been confirmed in the present study.