Rajesh Sharma

Lifetime shortening in doped ZnS nanophosphors

ZnS : Mn nanophosphor has been doped with quencher impurity Ni using the colloidal precipitation method with capping agent polyvinylpyrrolidone. The formation of ~2.5 nm sized nanoparticles has been confirmed by x-ray diffraction and transmission electron microscope studies. Energy and time resolved photoluminescence spectra of the synthesized nanophosphors have been studied at room temperature. Photoluminescent spectra for ZnS : Mn, Ni show three peaks at 412 nm, 433 nm and 590 nm corresponding to Ni impurity, sulphide vacancies (Vs) and Mn impurity, respectively. The ZnS : Mn nanophosphor show typical nanosecond lifetimes for defect-related emission and millisecond lifetimes for Mn impurity-related emission. However, the ZnS : Mn, Ni samples showed lifetime shortening, with variation of dopant concentration for defect-related (420 nm) as well as impurity-related emission (590 nm), which is attributed to exchange interaction between Mn2+ and nearest neighbour Ni2+ impurities.

A simple approach to non-uniform vowel normalization

In this paper, we present results of non-uniform vowel normalization and show that the frequency-warping necessary to do nonuniform vowel nonnalization is similar to the mel-scale. We compare our methods to Fants non-uniform vowel normalization method and show that with proposed frequency warping approach we can achieve similar performance without any knowledge of the spoken vowel and the fonnant number. The proposed approach is motivated by a desire to perform non-uniform speaker normalization in automatic speech recognition systems. We also present results of a more comprehensive study of our earlier work on non-uniform scaling which again shows that mel-scale is the appropriate warping function. All the results in this paper are based on data from Peterson & Barney and Hillenbrand et al. vowel databases.

Enhanced photoluminescence decay processes of doped CaS phosphors at low temperature

CaS phosphor samples singly doped with Mn impurity and doubly doped with Mn and X (X= Fe, Co and Ni) have been synthesized using a flux method. Crystal structure and phase identification of doped CaS phosphors have been carried out by X-ray diffraction (XRD) studies. A pulse excitation method has been employed to study the optical parameters of the doped phosphors. Excited state lifetime measurements at liquid nitrogen temperature were carried out with a pulsed UV laser (nitrogen laser) as the excitation source has a short pulse width (10 ns) and high peak power (200 kW per pulse). The results have been compared with room temperature lifetime studies. Enhancement in photoluminescence intensity and lifetime shortening in these synthesized doped phosphors is reported at low temperature. The lifetime values have been found to be in the microsecond time domain for CaS:Mn phosphors while on addition of a quencher impurity Ni to CaS:Mn phosphors, the lifetime values reduce to the nanosecond time domain at 77 K temperature. A thermally activated carrier transfer model has been proposed to explain the observed abnormal temperature behaviour of emission from CaS:Mn, X-doped phosphors.

Laser-induced down-conversion parameters of singly and doubly doped ZnS phosphors

Singly and doubly doped ZnS phosphors have been synthesized using flux method. Laser-induced photoluminescence has been observed in ZnS-doped phosphors when these were excited by the pulsed UV N2 laser radiation. Due to down-conversion phenomenon, fast phosphorescence emission in the visible region is recorded in milliseconds time domain for ZnS:Mn while in the case of ZnS:Mn:killer (Fe, Co and Ni) the lifetime reduces to microseconds time domain. Experimentally observed luminescent emission parameters of excited states such as, lifetimes, trap-depth values and decay constants have been reported here at room temperature. The high efficiency and fast recombination times observed in doped ZnS phosphors make these materials very attractive for optoelectronic applications.

Fast photoluminescence decay processes of doped CaS Phosphors

Singly and doubly doped CaS phosphors have been synthesized using the flux method. Excited-state lifetime measurements have been carried out for CaS doped with Mn and quencher impurities (Fe or Co or Ni) having doping concentrations 0.05–0.70% by weight, using pulsed-UV laser (nitrogen laser) as the excitation source having pulse width (10 ns) and peak power (200 kW per pulse). Laser-induced photoluminescence has been observed in CaS-doped phosphors when these phosphors were excited by the pulsed-UV N2 laser radiation. Owing to the downconversion phenomenon, fast photoluminescence emission in the visible region is recorded in microseconds time domain for CaS:Mn, whereas in case of CaS:Mn:Ni, shortest component of lifetime values reduces to nanoseconds time domain.