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Singh A.K.,Indian Institute of Technology BHU Varanasi | Kumar K.,Indian School of Mines | Kumar K.,Allahabad University | Pandey A.C.,Allahabad University | And 2 more authors.
Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy | Year: 2013

LaF3:Er3+ phosphor is synthesized through chemical precipitation method and its upconversion (UC) emission studies have carried out using 532-nm excitation. Phosphor has shown two-photon absorption UV bands at the 325 nm, 342 nm, 383 nm, 403 nm and 411 nm wavelengths. At relatively higher excitation powers multi-phonon assisted energy migration from 2H 11/2 (4S3/2) level to the upper 4F3/2, 4F5/2 and 4F 7/2 levels has observed and this energy migration opened new channel of emission at 440 nm, 453 nm and 488 nm due to the 4F3/2 → 4I15/2, 4F5/2 → 4I15/2 and 4F7/2 → 4I15/2 transitions, respectively. Temperature dependent UC measurement is also done and observed emission pattern is correlated with the power dependence studies. Upconversion bands at 411 and 488 nm wavelengths have shown reversal in intensity as either excitation power or sample temperature is increased and hence these two bands are found to act as optical switch. Planck blackbody like continuum emission has also observed at higher excitation powers. © 2013 Elsevier B.V. All rights reserved. Source


Singh A.K.,IT | Kumar K.,Allahabad University | Pandey A.C.,Allahabad University | Parkash O.,IT | And 2 more authors.
Applied Physics B: Lasers and Optics | Year: 2011

Hexagonal LaF3:Er3+/Yb3+ phosphor material has been synthesized by chemical precipitation method to obtain high near-infrared to green upconversion (UC) efficiency. Its thermal, structural and fluorescence properties have been studied. UC emission bands have been observed up to 315 nm in UV region. The effect of input pump power on the intensities of various emission bands has been studied in detail and photon avalanche UC mechanism has been identified. On increasing the excitation power, some bands have shown saturation in intensity. Also, at higher pump intensities two new UC bands were observed and their origin has been discussed. The phosphor has also been tested for possible UC-based fingerprint detection. © 2011 Springer-Verlag. Source


Singh S.K.,Laser and Spectroscopy Laboratory | Singh A.K.,Institute of Technology | Kumar D.,Institute of Technology | Prakash O.,Institute of Technology | Rai S.B.,Laser and Spectroscopy Laboratory
Applied Physics B: Lasers and Optics | Year: 2010

A nano-crystalline La2O3: Er3+/Yb 3+ phosphor sample has been synthesized through the solution combustion route using urea as a reducing agent. Thermal, structural and optical characterizations have been carried out to explore several of its properties. By thermal analysis one concludes to the presence of moisture and hydroxide phases [La(OH)3 and LaOOH] of lanthanum in the as-synthesized sample, which further changes to La2O3 phase above 600°C temperature. Up-conversion (UC) study shows the intense emission bands in the UV, blue, green and red regions. This paper also reports the first observation of UC emission bands extending up to the UV (240 nm) region on excitation with 976 nm wavelength. Heat treatment of the samples shows a change in the crystallite phase along with crystallite growth and relative UC luminescence intensities. The input pump power dependence shows the involvement of up to four photons. © 2009 Springer-Verlag. Source


Singh D.,Laser and Spectroscopy Laboratory | Popp J.,Friedrich - Schiller University of Jena | Singh R.K.,Laser and Spectroscopy Laboratory
Zeitschrift fur Physikalische Chemie | Year: 2011

Diazines form various types of hydrogen bonds with water, wherein C-H groups are sometimes involved directly and sometimes indirectly. On hydrogen bond formation, the wavenumber of C-H stretching vibration are usually red shifted (normal hydrogen bond) but blue shift of C-H modes (anomalous hydrogen bond) is also possible in some cases. The Raman spectra of the C-H stretching bands of three diazines; pyrimidine, pyridazine, and pyrazine in pure form and at many concentrations in mole fractions of diazines in the mixture of diazines + water have been measured to analyze the wavenumber shifts experimentally. Theoretical wavenumber shifts have been calculated and NBO analysis has been performed using DFT methods to understand the cause of the shifts. All the four C-H stretching bands of diazines are blue shifted on dilution with water both experimentally and theoretically. The NBO calculations reveal that the cause of the shift is decrease in the charge density in the antibonding orbital of C-H bond on complex formation. © by Oldenbourg Wissenschaftsverlag, München. Source


Giri N.K.,Laser and Spectroscopy Laboratory | Giri N.K.,Allahabad University | Mishra K.,Laser and Spectroscopy Laboratory | Rai S.B.,Laser and Spectroscopy Laboratory
Journal of Fluorescence | Year: 2011

We report the generation of efficient white light based on upconversion (UC) in Tm 3+/Er 3+/Yb 3+:Y 2O 3 nanocrystalline phosphor synthesized by simple and cost effective solution combustion technique on 976 nm laser excitation. The calculated color coordinates (using 1931 CIE standard) for samples annealed at different temperatures vary from (0.16, 0.30) to (0.32, 0.33) with dopant concentration, annealing temperature and the pump power; thus providing a wide color tunability including the white one. White emission is observed even at a very low laser power (60 mW). The maximum upconversion efficiency obtained for white emission is 2.79% with the color coordinates (0.30, 0.32) at laser power of 420 mW which is quite close to the standard white color coordinates. © 2011 Springer Science+Business Media, LLC. Source

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