N.V. Unnikrishnan

Sensitized fluorescence of Ce3+/Mn2+ system in phosphate glass

Abstract The preparation of sodium phosphate glasses singly and doubly doped with rare earth ion Ce 3+ and transition metal ion Mn 2+ by a melt quench method is described. The spectroscopic characterizations of the samples are conducted by absorption, excitation, and emission spectra. The orange red emission of divalent manganese sensitized by trivalent cerium ions in a phosphate glassy matrix has been investigated. Energy transfer (ET) from optically excited Ce 3+ to Mn 2+ in sodium phosphate glass, by nonradiative process is confirmed by fluorescence studies with various activator concentrations. The mechanism of ET is mainly electric dipole–dipole in nature.

Energy transfer in Sm3+:Eu3+ system in zinc sodium phosphate glasses

The mechanism of nonradiative energy transfer process in zinc sodium phosphate glass system co-doped with samarium and europium ion has been examined under cw laser excitation. Donor–acceptor distance and quantum efficiency of transfer have been evaluated using the relevant theoretical expressions. Transfer probabilities have been determined using the overlap integral and relative fluorescence methods. The Forster–Dexter theoretical predictions are found to be in excellent agreement with experimental results. The nonresonant energy transfer assisted by phonons is the dominant transfer mechanism in the concentration range taken. Excitation spectra and the decay profile of the samarium ion also support the energy transfer from samarium to europium.

A sensitive fibre optic pH sensor using multiple sol-gel coatings

The fabrication and characterization of a fibre optic pH sensor based on evanescent wave absorption is presented. The unclad portion of a multi-mode optical fibre is coated with a pH sensitive dye, which is immobilized by the sol-gel route. The sensitivity of the device has been found to increase when multiple sol-gel coatings are used as the sensing region. The dynamic range and the temporal response of the sensor are investigated for two different dyes, namely bromocresol purple and bromocresol green. The performance of the device is evaluated in terms of the results obtained during actual measurements.

Judd–Ofelt intensity parameters and laser analysis of Pr3+ doped phosphate glasses sensitized by Mn2+ ions

Abstract In this paper, we report the preparation and optical characterization of Mn2+ activated Pr3+ doped phosphate glasses in four different chemical compositions. The optical absorption spectra of Pr3+ ions in phosphate glasses have been recorded in the UV–VIS–NIR region. From the data available in the optical absorption spectra, various spectroscopic parameters such as Slater–Condon (F2, F4, F6), Racah (E1, E2, E3), spin-orbit interaction (ξ4f) and Judd–Ofelt (J–O) (Ω2, Ω4, Ω6) parameters are derived. The calculated values of the J–O parameters are utilized in evaluating the various radiative parameters such as electric dipole line strengths (Sed), magnetic dipole line strengths (Smd), radiative transition probabilities (ARAD), radiative lifetimes (τRAD), fluorescence branching ratios (βR) and the integrated absorption cross-sections (σa) for stimulated emission from various excited states of Pr3+ ion. The principal fluorescence transitions of interest are identified by recording the fluorescence spectrum and measuring their stimulated emission cross-section and optical gain. Quantum efficiencies of the prominent lasing transitions are estimated by measuring their radiative and fluorescence lifetimes. The dependence of effective radiative lifetime on Pr3+ ion concentration is discussed in the light of non-radiative processes such as multiphonon relaxation and energy transfer between like and unlike ions.

Structural, spectroscopic and electrical studies of nanostructured porous ZnO thin films prepared by pulsed laser deposition

ZnO thin films are grown on quartz substrates at various substrate temperatures (ranging from 573 to 973 K) under an oxygen ambience of 0.02 mbar by using pulsed laser ablation. Influence of substrate temperature on the structural, morphological, optical and electrical properties of the ZnO thin films are investigated. The XRD and micro-Raman spectra reveal the presence of hexagonal wurtzite structure of ZnO with preferred orientation (002). The particle size is calculated using Debye-Scherer equation and the average size of the crystallites are found to be in the range 17-29 nm. The AFM study reveals that the surface morphology of the film depends strongly on the substrate temperature. UV-Visible transmittance spectra show highly transparent nature of the films in visible region. The calculated optical band gap energy is found to be decrease with increase in substrate temperatures. The complex dielectric constant, the loss factor and the distribution of the volume and surface energy loss of the ZnO thin films prepared at different substrate temperatures are calculated. All the films are found to be highly porous in nature. The PL spectra show very strong emission in the blue region for all the films. The dc electrical resistivity of the film decreases with increase in substrate temperature. The temperature dependent electrical measurements done on the film prepared at substrate temperature 573 K reveals that the electric conduction is thermally activated and the activation energy is found to be 0.03911 eV which is less than the reported values for ZnO films.

NIR to UV absorption spectra and the optical constants of phthalocyanines in glassy medium

Optical absorption studies of phthalocyanines (Pc-s) in borate glass matrix have been reported for the first time. Measurements have been done corresponding to photon energies between 1.1 and 6.2 eV for free base, manganese, iron, nickel, molybdenum, cobalt and copper phthalocyanines. Several new discrete transitions are observed in the UV-vis region of the spectra in addition to a strong continuum component of absorption in the IR region. Values of some of the important optical constants viz. absorption coefficient (alpha), molar extinction coefficient (epsilon), absorption cross-section (sigma(a)), band width (delta lambda), electric dipole strength (q2) and oscillator strength (f) for the relevant electronic transitions are also presented. All the data reported for Pc-s in the new matrix have been compared with those corresponding to solution, vapor and thin film media.

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.