Curcumin was complexed with β-CD using co-precipitation, freeze-drying and solvent evaporation methods. Co-precipitation enabled complex formation, as indicated by the FT-IR and FT-Raman techniques via the shifts in the peaks that were assigned to the aromatic rings of curcumin. In addition, photoacoustic spectroscopy and X-ray diffraction, with the disappearance of the band related to aromatic rings, by Gaussian fitting, and modifications in the spectral lines, respectively, also suggested complex formation. The possible complexation had an efficiency of 74% and increased the solubility of the pure colourant 31-fold. Curcumin-β-CD complex exhibited a sunlight stability 18% higher than the pure colourant. This material was stable to pH variations and storage at -15 and 4°C. With an isothermal heating at 100 and 150°C for 2h, the material exhibited a colour retention of approximately 99%. The application of curcumin-β-CD complex in vanilla ice creams intensified the colour of the products and produced a great sensorial acceptance.

The optical band-gap energy of a nanostructured tungsten trioxide film is determined using the photoacoustic spectroscopy method under continuous light excitation. The mechanism of the photoacoustic signal generation is discussed. The band-gap energy is also computed by other methods. The absorption coefficient as well as the band-gap energy of three different crystal structures of tungsten trioxide is calculated by a first-principles Green's function approach using the projector augmented wave method. The theoretical study indicates that the cubic crystal structure shows good agreement with the experimental data. © 2010 American Institute of Physics. ͓doi:10.1063/1.3313945͔Tungsten trioxide ͑WO 3 ͒ films have attracted much interest during the last decade due to their potential applications. Nanostructured WO 3 films have been used in eletrochromic ͑EC͒ devices such as displays and smart windows.1-3 For this reason, a detailed understanding of the optical processes responsible for the EC effect would greatly facilitate the optimization of EC devices.4 WO 3 is a wideband-gap semiconductor. Its band-gap energy has been mainly measured by optical absorption, varying from about 2.6 to 3.0 eV. 2,5 The band gap of WO 3 is certainly of interest for both applied and fundamental aspects. The literature is however somewhat confusing. Values below 3.0 eV have mostly been obtained assuming an indirect band gap.Taking into account that the understanding of the optical processes responsible for the EC effect is an important parameter in design and optimization of EC devices, and that the band gap energy is one of the most important parameter of semiconductors, we investigate the optical absorption in the region of the fundamental band edge by the photoacoustic spectroscopy ͑PAS͒ technique. PAS has been extensively used as a nondestructive method for measuring the optical properties of semiconductors and many other materials. 6-10The nonradiative relaxation processes-which are associated with the band structure, defect-related energy loss mechanism, etc.-can be directly and very accurately obtained from the analysis of the PAS spectra. 10The optical band-gap energy ͑E g ͒ has been determined by the PAS technique using mainly two methods. In the first, the E g value is adopted as the absorption edge obtained from a linear fitting in the plot of the square of the product between the absorption coefficient and the photon energy versus the photon energy for direct band gap, or the plot of the square root of the product between the absorption coefficient and the photon energy versus the photon energy for indirect band gap. 11 In the second, E g is estimated by the changing of the derivative near the fundamental absorption edge. 7In this letter, we analyze the PA-signal behavior of a nanostructured WO 3 film under continuous laser excitation, using an experimental procedure similar to that described in Ref. 12. The influence of the continuous excitation in the mechanisms responsible for the generation of the PA signal is discussed ...

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The time-resolved and steady-state mode-mismatched thermal lens technique has been used to determine the temperature coefficient of optical path length ds/dT at different wavelengths of soda lime glasses (72 wt % SiO2, 18 wt % Na2O, 10 wt % CaO, and 70.5 wt % SiO2, 17.5 wt % Na2O, 10 wt % CaO, 2 wt % Fe2O3). The aberrant model, which takes into account the thickness change of the sample and is more realistic than the parabolic treatment when used to describe the thermal lens effect, was used as the theoretical model. The results showed that ds/dT is (2.1±0.1)×10−6 K−1 at 632.8 nm and (2.4±0.1)×10−6 K−1 at 442 nm for the undoped sample and (4.66±0.03)×10−6 K−1 at 632.8 nm and (6.1±0.1)×10−6 K−1 at 442 nm for the iron-doped sample. The greater value of this parameter found for the doped sample at the absorption band (442 nm) was associated with the bigger value of the temperature coefficient of the electronic polarizability caused by the presence and environment of Fe3+ iron in the structure of the glass.

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The effect of radiation forces at the interface between dielectric materials has been a long-standing debate for over a century. Yet there has been so far only limited experimental verification in complete accordance with the theory. Here we measure the surface deformation at the air–water interface induced by continuous and pulsed laser excitation and match this to rigorous theory of radiation forces. We demonstrate that the experimental results are quantitatively described by the numerical calculations of radiation forces. The Helmholtz force is used for the surface radiation pressure. The resulting surface pressure obtained is consistent with the momentum conservation using the Minkowski momentum density expression assuming that the averaged momentum per photon is given by the Minkowski momentum. Considering the total momentum as a sum of that propagating with the electromagnetic wave and that deposited locally in the material, the Abraham momentum interpretation also appears to be appropriate.

An absolute thermal lens method to determine fluorescence quantum efficiency and concentration quenching of solids is described in this work. The quantum efficiency of low silica calcium aluminate glasses doped with different concentrations of neodymium dioxide and melted under vacuum conditions to remove water has been measured by using mode-mismatched thermal lens spectrometry. It has been shown that the thermal lens signal amplitude is linearly dependent on neodymium concentrations up to 4.0 wt %, changing significantly from 4.5 to 5.0 wt %, indicating that there was quenching of the fluorescence only above 4.0 wt % neodymium dioxide. The quantitative treatment for the thermal lens effect provided the absolute value of the sample's fluorescence quantum efficiency. The technique is simple to perform and can be applied for a wide range of fluorescent materials. ͓S0163-1829͑98͒00714-0͔

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