Gijo Jose

Energy transfer in Rh 6G:Rh B system in PMMA matrix under cw laser excitation

Energy transfer of Rh 6G:Rh B dye–mixtures in a poly methyl methacrylate (PMMA) matrix was studied by investigating the emission spectral characteristics under argon ion laser excitation. The fluorescence behaviour of the acceptor dye (Rh B) was investigated for various concentration of the donor (Rh 6G) and excitation powers. The energy transfer rate constants (kf ), transfer probability ( Pda) and the transfer efficiency ( η) have been evaluated and their concentration dependence on donor molecules was studied. The observed decrement in the emission peak wavelength of the acceptor dye with donor concentration is attributed to the formation of complexes in the dye–mixture and also to the result of cross-relaxation taking place in the donor dye system. The blue shift in the emission wavelength of the acceptor was also investigated for various donor concentrations. A significant enhancement of fluorescence intensity of the acceptor (Rh B) of the order of 226% and a transfer efficiency of 90% were observed in the present dye–mixture system as compared with pure Rh B, both in PMMA matrices. Further the experimental results revealed the non-radiative type energy transfer for the dye system in the PMMA matrix.

Application of a modified Judd–Ofelt theory to Pr3+ doped phosphate glasses and the evaluation of radiative properties

Abstract Kornieko et al.’s [Phys. Stat. Sol. (B) 157 (1990) 267] modified version of the Judd–Ofelt (J–O) theory has been used to evaluate the oscillator strengths and radiative lifetimes of Pr 3+ in a series of phosphate glasses. The results are compared with those obtained from the original J–O theory in our previous work [J. Non-Cryst. Solids 275 (2000) 93]. Experimental and theoretical oscillator strengths are presented and J–O parameters are determined by both methods for all glasses. The calculated radiative lifetimes are compared with the experimentally measured values for the 1 D 2 energy level and the suitability of the two models are compared. Results show that the modified theory provides better agreement with the experimentally measured lifetimes of the 1 D 2 transition.

Optical characterization of Eu3+ ions in CdSe nanocrystal containing silica glass.

Silica glass samples containing CdSe/Eu3+ ions were prepared by sol-gel route. Size distribution and optical band gap of the nanoparticles were calculated from absorption spectrum. It is observed that the presence of CdSe nanocrystallites enhances the fluorescence of europium in silica glass. The phonon sideband spectrum associated with the excitation transition 7F0−5D2 is used to analyze the electron–phonon coupling and nonradiative deexcitiation of the rare earth ions in the glass host. The observed fluorescence enhancement is discussed on the basis of phonon assisted energy transfer from electron-hole recombination of the CdSe nanocrystallites to the rare earth ion and multiphonon relaxation.

Spectroscopic studies of Cu2+ ions in sol-gel derived silica matrix

The Cu2+ ion doped silica gel matrices in monolithic shape were prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS). The absorption, transmittance and fluorescence spectra of the gel matrices heat treated at different temperatures were monitored. The loss of water and hydroxyl group from silica network changes the optical properties of the Cu2+ ions in the host, noted by the change in colour of monolith and spectral characteristics. The pronounced blue shift observed (700–900 nm to 600–850 nm) for the broad band of the absorption spectra of the samples heated up to 700°C is attributed to the ligand field splitting and partial removal of hydroxyl group from the silica matrices. The results indicate broadband filtering effects of the samples in the wavelength region 400–600 nm. Absorption and fluorescence spectra of the glass matrices heated to 1000°C confirms the conversion of Cu2+ ion to Cu+ ion.

Fluorescence enhancement from Eu3+ ions in CdSe nanocrystal containing silica matrix hosts

Abstract Semiconductor cadmium selenide particles together with europium ions were incorporated into the silica matrix using sol–gel method. Here, the effect of nanocrystals on the absorption and fluorescence features of europium ions is discussed. The fluorescence spectra reveal that the intensity of characteristic emission of europium increases considerably in the presence of CdSe particles. This phenomenon can be explained as due to the energy transfer resulting from electron–hole recombination in the CdSe to the rare earth ion. These zero-dimensional materials, along with the effect of matrix incorporating europium ions, are found to have increased the optical gain.

Upconversion fluorescence in Sm3+-doped zinc phosphate glassy matrix

The mechanism governing the upconversion fluorescence from the 4G5/2 level of Sm3+ ion has been investigated in zinc phosphate glass matrices under infrared (1.06 µm) laser excitation. This visible emission is centred at 590 nm and corresponds to the 4G5/2 → 6H7/2 transition. The dependence of the integrated intensity on the excitation power confirms that this emission is due to the three-photon absorption process and is in agreement with the existing theory. The intensity of the upconversion fluorescence has a cubic dependence on the excitation power. An absolute upconversion efficiency of about 10−7 was obtained for the sample doped with 0.5 wt% Sm3+ ions. A quantitative estimate of the number of photons leaving the sample was also determined.

Structural Evolution and Fluorescence Properties of Dy3+: Silica Matrix

The Dy3+ doped silica glasses were prepared by sol-gel process with appropriate heat treatment. The structural changes during the gel to glass transition of the silica xerogels and the effect on fluorescence properties of Dy3+ were studied. For Dy3+ two dominant emissions occur at about 575nm(4F9/2→6H15/2) in the yellow region and 480nm (4F9/2→6H13/2) in the blue region. Both transitions are electric dipole in nature out of which (4F9/2→6H13/2) is hypersensitive. The intensity ratio (yellow to blue (Y/B)) of Dy3+ emission follows a trend parallel to the red to orange (R/O) ratio of Eu3+ in silica matrices. These ratios are influenced by the site symmetry and electro negativity of ligand atoms. The fluorescence intensity of Dy3+ ions increases remarkably when the doped xerogel is heat-treated. The fluorescence intensity ratio (Y/B) is used as a measure of the symmetry of Dy3+ environment during the gel - glass conversion, the higher the ratio, the more asymmetric is the environment. The high value of the intensity ratio of the 1000°C heated gel showed that the Dy3+ was embedded in the glassy silica network with an asymmetric environment. The FTIR and thermo gravimetric analysis (TGA) clearly showed the complete densification (density=2.2 gm/cm3) of the gels around 1000°C.