The spectroscopic behavior of gadolinium gallium garnet (Gd3Ga5O12, GGG) nanocrystals codoped with 1% each of Tm3+ and Yb3+ prepared via a solution combustion synthesis procedure was investigated. Initial excitation of the codoped nanocrystals with 465.8 nm (into the 1G4 state) showed a dominant blue-green emission ascribed to the 1G4-3H6 transition as well as red and NIR emissions from the 1G4-3F4 and 1G4-3H5/3H4-3H6 transitions, respectively. Excitation at this wavelength (465.8 nm) showed the existence of a Tm3+ --> Yb3+ energy transfer process evidenced by the presence of the 2F5/2-2F7/2 Yb3+ emission in the NIR emission spectrum. The decay time constants proved that the transfer of energy occurred via the 3H4 state. Following excitation of the Yb3+ ion with 980 nm, intense upconverted emission was observed. Emissions in the UV (1D2-3H6), blue (1D2-3F4), blue-green (1G4-3H6), red (1G4-3F4), and NIR (1G4-3H5/3H4-3H6) were observed and were the direct result of subsequent transfers of energy from the Yb3+ ion to the Tm3+ ion. Power dependence studies showed a deviation from expected values for the number of photons involved in the upconversion thus indicating a saturation of the upconversion process. An energy transfer efficiency of 0.576 was determined experimentally.
Optical measurements of particle size and composition in granular samples are difficult to make due to complex light scattering from particles. These multiple scattering events bias absorption estimates and complicate the calculation of scattering and absorption coefficients used to estimate sample properties. Time series data, such as chromatograms and photon time-of-flight (TOF) profiles, contain self-repeating (fractal) characteristics. Power law analysis of photon TOF profiles allows the determination of absorption coefficients and particle sizes in a single experiment. A correlation dimension algorithm was used on photon TOF data from scattering samples. MLR models were then obtained from correlation dimension plots for the estimation of sample properties. Estimates of particle sizes and absorption coefficients were shown to agree well with theoretical values when compared using independent validation sets. Results show close to a 3-fold and up to a 5-fold decrease in the errors of estimation of dye concentration and particle size, respectively, as compared to steady-state measurements. The power law approach provides a useful means of determining sample properties in highly scattering media.
A non-invasive method has been developed for analyte quantification in fluids surrounded by optically-scattering, opaque walls. This method is based on steady state, visible wavelength reflectance measurements made simultaneously at multiple positions on the surface of a sample. Previous work has shown that reflectance measurements contain information about underlying scattering layers in layered scattering samples. We hypothesise that similar information about an absorbing layer below a scattering layer can be obtained from evanescent wave effects. Principal component analysis showed the data to be composed of three components, which were refined by a multivariate curve resolution alternating least squares (MCR-ALS) approach with non-negativity constraints. The first component is related to the scattering layer thickness, the second is associated with analyte concentration and the third is due to a minor back reflection within the sample cell. Both MCR and stagewise multi-linear regression (SMLR) approaches were taken to estimate analyte concentration and scattering layer thickness, for samples having thicknesses between 1 mm and 8 mm. Results demonstrate that a simple experimental configuration can easily predict optical properties of unknown samples. With the adoption of a multi-wavelength approach to this method, it is expected that improved absorption coefficient (µ a ) estimation accuracy can be realised in a variety of application areas such as in analysis through opaque containers, in vivo measurements and in-line monitoring of reactions.
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