Relationship between the optical gap and the optical-absorption tail breadth in amorphous GaAs

We study the relationship between the optical gap and the optical-absorption tail breadth for the case of amorphous gallium arsenide (a-GaAs). In particular, we analyze the optical-absorption spectra corresponding to some recently prepared a-GaAs samples. The optical gap and the optical-absorption tail breadth corresponding to each sample is determined. Plotting the optical gap as a function of the corresponding optical-absorption tail breadth, we note that a trend, similar to that found for the cases of the hydrogenated amorphous silicon and hydrogenated amorphous germanium, is also found for the case of a-GaAs. The impact of alloying on the optical-absorption spectrum associated with a-GaAs is also briefly examined. (C) 2004 American Institute of Physics.

Femtosecond nonlinear optical properties of lead-germanium oxide amorphous films

The nonlinear (NL) response of lead-germanium oxide amorphous films was investigated using a Ti:saphire laser delivering pulses of approximate to 150 fs at 800 nm. The Kerr shutter technique was employed to reveal the time response of the nonlinearity that is smaller than 150 fs. The sign and magnitude of the nonlinearity were obtained using a novel technique called thermally managed eclipse Z scan which allows the simultaneous characterization of cumulative and noncumulative NL effects. The NL refractive index of electronic origin, n(2)approximate to 2x10(-17) m(2)/W, and the NL absorption coefficient, alpha(2)approximate to 3x10(3) cm/GW, were determined. (c) 2007 American Institute of Physics.

Optical properties and energy transfer processes in (Tm3+, Nd3+) doped tungstate fluorophosphate glass

We investigate the linear optical properties and energy transfer processes in tungstate fluorophosphate glass doped with thulium (Tm3+) and neodymium (Nd3+) ions. The linear absorption spectra from 370 to 3000 nm were obtained. Transitions probabilities, radiative lifetimes, and transition branching ratios were determined using the Judd-Ofelt [Phys. Rev. 127, 750 (1962); J. Chem. Phys. 37, 511 (1962)] theory. Frequency up-conversion to the blue region and fluorescence in the infrared were observed upon pulsed excitation in the range of 630-700 nm. The excitation spectra of the luminescence were obtained to understand the origin of the signals. The temporal decay of the fluorescence was measured for different concentrations of the doping ions. Energy transfer rates among the Tm3+ and Nd3+ ions were also determined.

Bulk photochromism in a tungstate-phosphate glass: A new optical memory material?

In this work, we present a new photochromic tungstate based glass which have both absorption coefficient and refractive index modified under laser exposure. The photosensitive effect is superficial under ultraviolet (UV) irradiation but occurs in the entire volume of the glass under visible irradiation. The effect can be obtained in any specific point inside the volume using an infrared femtosecond laser. In addition, the photosensitive phenomenon can be erased by specific heat treatment. This glass can be useful to substitute actual data storage supports and is a promising material for 3-dimensional (3D) and holographic optical storage.

Annealing effects on optical properties of natural alexandrite

Natural alexandrite (BeAl2O4:Cr3+) crystals are investigated as regards the effects of annealing on their optical properties. Optical absorption spectra are measured from the ultraviolet (190 nm) to the near infrared (900 nm), for a sample subjected to consecutive annealing processes, where time and temperature are varied. Besides this, luminescence spectra are simultaneously obtained for this sample, excited with a Kr+ laser source, tuned on an ultraviolet multi-line mode (337.5, 350.7 and 356.4 nm). We observe from absorption as well as from emission data that annealing mainly influences the distribution of Cr3+ and Fe3+ ions, located on sites of a mirror plane (C-s symmetry), which are responsible for the optical properties of alexandrite. The results obtained lead to the conclusion that annealing induces a modification of the population of Cr3+ on C-s sites as well as on sites located on an inversion plane (C-i). Annealing could improve the optical properties of this material, as regards its application as a tunable laser.

RELATIONSHIPS BETWEEN THE PLASMA ENVIRONMENT AND THE COMPOSITION AND OPTICAL-PROPERTIES OF PLASMA-POLYMERIZED THIN-FILMS PRODUCED IN RF DISCHARGES OF C2H2-SF6 MIXTURES

Polymer films were grown in rf discharges containing different proportions of C2H2 and SF6. Quantitative optical emission spectrometry (actinometry) was used to follow the trends in the plasma concentrations of the species H and F, and more tentatively, of CH, CF, and CF2, as a function of the feed composition. Infrared spectroscopy revealed the density of CH and CF bonds in the deposited material. As the partial pressure of SF6 in the feed was increased, the degree of fluorination of the polymer also rose. The form of the dependency of the deposition rate on the proportion of SF6 in the feed was in good qualitative agreement with the activated growth model. From transmission ultraviolet visible spectroscopy data the refractive index and the absorption coefficient of the polymers were calculated as a function of the deposition parameters. Since the optical gap depended to some extent upon the degree of fluorination, it could, within limits, be determined by a suitable choice of the proportion of SF6 in the feed. A qualitative explanation of this relationship is given.

Nonlinear optical properties of tungstate fluorophosphate glasses

The optical nonlinearity of tungstate fluorophosphate glasses, synthesized in the NaPO3-BaF2-WO3 system, was investigated through experiments based on the third-order susceptibility, chi((3)). Nonlinear (NL) refraction and NL absorption measurements in the picosecond regime were performed using the Z-scan technique at 532 nm. NL refractive index, n(2)proportional toRe chi((3)), ranging from 0.4x10(-14) cm(2)/W to 0.6x10(-14) cm(2)/W were determined. The two-photon absorption coefficient, alpha(2)proportional toIm chi((3)), for excitation at 532 nm, vary from 0.3 to 0.5 cm/GW. Light induced birefringence experiments performed in the femtosecond regime indicate that the response time of the nonlinearity at 800 nm is faster than 100 fs. The experiments show that chi((3)) is enhanced when the WO3 concentration increases and this behavior is attributed to the hyperpolarizabilities associated to W-O bonds. (C) 2004 American Institute of Physics.

Optical recording in Se(Te)/As2S3 multilayers

The development of Te containing nanoscaled, compositionally modulated Te/As2S3 multilayer structures enables the realisation of optical recording in real time with a CW He-Ne laser illumination (lambda=633 nm) with improved recording parameters in comparison with known Se/As2S3 structures. It is established that the efficiency of the recording process depends on the light stimulated interdiffusion and thickness of sub-layers.

Efficient energy upconversion emission in Tm3+/Yb3+-codoped TeO2-based optical glasses excited at 1.064 mu m

Efficient energy upconversion of cw radiation at 1.064 mum into blue, red, and near infrared emission in Tm3+-doped Yb3+-sensitized 60TeO(2)-10GeO(2)-10K(2)O-10Li(2)O-10Nb(2)O(5) glasses is reported. Intense blue upconversion luminescence at 485 nm corresponding to the Tm3+ (1)G(4)--> H-3(6) transition with a measured absolute power of 0.1 muW for 800 mW excitation power at room temperature is observed. The experimental results also revealed a sevenfold enhancement in the upconversion efficiency when the sample was heated from room temperature to 235 degreesC yielding 0.7 muW of blue absolute fluorescence power for 800 mW pump power. High brightness emission around 800 nm (F-3(4)--> H-3(6)) in addition to a less intense 655 nm ((1)G(4)--> H-3(4) and F-3(2,3)--> H-3(6)) fluorescence is also recorded. The energy upconversion excitation mechanism for thulium emitting levels is assigned to multiphonon-assisted anti-Stokes excitation of the ytterbium-sensitizer followed by multiphonon-assisted sequential energy-transfer processes. (C) 2001 American Institute of Physics.

Femtosecond nonlinear optical properties of tellurite glasses

The third-order nonlinear optical properties of tellurite glasses with different compositions were investigated in the femtosecond regime at 810 nm. Using the I-scan technique, positive nonlinear refractive indices of similar to 10(-15) cm(2)/W were measured. The authors also determined that nonlinear absorption was negligible for all studied samples. This result, added to their good chemical stability, indicates that tellurite glasses are promising materials for ultrafast photonic applications. (c) 2006 American Institute of Physics.

An ultra stable glass system for optical fiber devices

In this paper a modified chalcogenide glass was studied by X-ray powder diffraction, differential thermal analysis, infrared and Raman scattering spectroscopies. The study of this new matrix opens new perspectives to fabricate Pr3+-doped fibers to operate as an optical amplifier in the 1.3 mum telecommunications window. The Pr3+-doped 70Ga(2)S(3)-30La(2)S(3) glass was modified through the substitution of La2S3 by La2O3, which improves the thermal stability of these glasses without any modification of phonon energy. The possibility to pull a fiber from this glass system without any devitrification is easily achieved.

Temperature and wavelength dependence of the thermo-optical properties of tellurite and chalcogenide glasses

The refractive index and the temperature coefficient of the optical path length change of tellurite (80TeO(2):20Li(2)O) and chalcogenide glasses (72.5Ga(2)S(3):27.5La(2)O(3)) were determined as a function of temperature (up to 150 degrees C) and wavelength (in the range between 454 and 632.8 nm). The tellurite glass exhibits the usual refractive index dispersion in the wavelength range analyzed, while anomalous refractive index dispersion was observed for the chalcogenide glass between 454 and 530 nm. The dispersion parameters were determined by means of the single-effective oscillator model. In addition, a strong dependence of the temperature coefficient of the optical path length on the photon energy and temperature was found for the chalcogenide glass. The latter was correlated to the shift of the optical band gap (or electronic edge) with temperature, which was interpreted by the electron-phonon interaction model. (C) 2007 American Institute of Physics.

Effect of temperature on the optical properties of GaAsSbN/GaAs single quantum wells grown by molecular-beam epitaxy

GaAsSbN/GaAs strained-layer single quantum wells grown on a GaAs substrate by molecular-beam epitaxy with different N concentrations were studied using the photoluminescence (PL) technique in the temperature range from 9 to 296 K. A strong redshift in optical transition energies induced by a small increase in N concentration has been observed in the PL spectra. This effect can be explained by the interaction between a narrow resonant band formed by the N-localized states and the conduction band of the host semiconductor. Excitonic transitions in the quantum wells show a successive red/blue/redshift with increasing temperature in the 2-100 K range. The activation energies of nonradiative channels responsible for a strong thermal quenching are deduced from an Arrhenius plot of the integrated PL intensity. (C) 2003 American Institute of Physics.

Tungstate fluorophosphate glasses as optical limiters

Nonlinear absorption measurements were performed on fluorophosphate glasses with high concentrations of tungsten oxide. Large two-photon absorption coefficients, alpha(2), were determined at 660 nm using nanosecond laser pulses. It was observed that alpha(2) increases for increasing tungsten oxide concentrations and, hence, the optical limiting performance of this glass composition can be controlled. (C) 2002 American Institute of Physics.

Optical and electrical diagnostics of a spark-plug discharge in air

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)