Problems in searching for new solid-state UV- and VUV-active media: The role of photodynamic processes

The impact of photodynamic processes induced by pump radiation in crystals doped with rare-earth (RE) ions on the possibility of generating stimulated radiation in the UV and VUV spectral ranges is analyzed. It is shown that, in addition to objective factors, one of the main causes preventing the lasing effect from being obtained using the interconfigurational 4f n - 15d-4fn transitions of RE ions is insufficient consideration of the photodynamic processes involved in experiments. It is proposed to correct laser test techniques and "pump-probe" experiments aimed at investigating active media appropriate for lasing in the UV and VUV ranges. Extended criteria oriented to search for new solid-state UV and VUV active media are formulated. © 2005 Pleiades Publishing, Inc

Excited-state absorption spectra of Pr³⁺ ions doped into LiY<inf>0.3</inf>Lu<inf>0.7</inf>F<inf>4</inf> mixed crystal

© 2016 Optical Society of America.The UV/visible polarized excited-state absorption (ESA) spectra from the 1D2 and 3Pj manifolds of 4f2-configuration of Pr3+ ions doped into LiY0.3Lu0.7F4 mixed crystal were studied at room temperature. The data about ESA cross-sections are necessary to estimate an efficiency of stepwise excitation of 4f5d-states of Pr3+ ions in these crystals

EPR study of nanocrystalline CeO<inf>2</inf> exhibiting ferromagnetism at room temperature

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Electron paramagnetic resonance (EPR) spectroscopy complemented with X-ray diffraction, X-ray fluorescence, and optical spectroscopy was used to study nanocrystalline CeO2 powder samples that exhibit weak room-temperature ferromagnetism. EPR lines assigned to the Ce3+ trigonal sites were found for the first time in cerium dioxide that contains a trace impurity of Mn2+. This finding indicates that manganese dopant facilitates the conversion of the oxidation state of Ce4+ to Ce3+ in nanocrystalline CeO2. Our results support the view that Ce3+/Ce4+ pairs along with defects on the surface of nanoparticles are responsible for the ferromagnetism in CeO2. The EPR study reveals that the charge-transfer mechanism proposed recently is more suitable to explain the origin of room-temperature ferromagnetism in CeO2 than the F+-centers exchange interactions

Microwave resonant technique in studies of photodielectric properties of bulk, thin film and nanoparticle materials

© 2016 IOP Publishing Ltd Printed in the UK.An enhanced contactless microwave technique allows us to study the photoconductivity of materials. The transient response of the complex permittivity of matter (ϵ = ϵ1 - jϵ 2) under optical irradiation is measured with nanosecond time resolution. The main advantage of the novel methodology is the elimination of the polarization effect in evaluating photoconductivity. The potential of the methodology was demonstrated by photoconductivity measurements in Si [1 0 0] crystal, CeO2 nanocrystalline powder and Ce-doped LiYF4 single crystal. The variations of complex permittivity (δϵ1 and δϵ 2) of Si [1 0 0] crystal, CeO2 nanocrystalline powder and Ce-doped LiYF4 single crystal under optical irradiation was measured and accurate values for crystalline band gaps were extracted. Finally, quantum confinement effects were observed in nano-size crystalline powders

Room-temperature ferromagnetism in cerium dioxide powders

© 2015, Pleiades Publishing, Inc. Room-temperature ferromagnetism is detected in a CeO2 powder with a grain size of about 35 nm and a low (2 sample with a submicron crystallite size and the same manganese and iron impurity content is lower than in the nanocrystalline sample by an order of magnitude. Apart from ferromagnetism, both samples exhibit EPR spectra of localized paramagnetic centers, the concentration of which is lower than 0.01 at %. A comparative analysis of these results shows that the F-center exchange (FCE) mechanism cannot cause ferromagnetism. This conclusion agrees with the charge-transfer ferromagnetism model proposed recently

Photoaging of methyl methacrylate-methacrylic acid copolymer doped by laser dyes and diphenylthiourea

For methyl methacrylate-methacrylic acid copolymer colored by laser dyes (Rhodamine 6G and Coumarin 7), the mutual effect of polymeric matrix and dyes on the kinetics of their photochemical aging was studied. Diphenylthiourea was introduced into the copolymer as a modifying additive. It was established that, upon irradiation with a mercury lamp, both the dyes and diphenylthiourea exert photostabilizing effects on the copolymer. It was found that there exists a correlation between the photofading of the dyes and the processes related to the photoaging of the copolymer, such as the degradation of macrochains, the accumulation of free radicals, and a reduction in the transmission coefficient. Introduction of diphenylthiourea causes a tenfold increase in the generation photostability of the copolymer samples colored by Rhodamine 6G, and consequently, their operating life as the laser-active elements

Photoconductivity and photodielectric effect in LiY1 - X Lu x F4 crystals doped with Ce3+ and Yb 3+ ions

Time and spectral dependences of the dielectric permittivity of the LiY1 - x Lu x F4 (x = 0, 0.5, and 1) crystals doped with Ce3+ and co-doped with Yb3+ ions under UV laser excitation were studied by the 8-mm microwave resonant technique at room temperature. The obtained photoconductivity spectrum in 240-310 nm spectral range was interpreted as a stepwise photoionization spectrum of the Ce 3+ ions due to sequential 4f-5d and 5d-6s transitions. Average lifetimes of free and defect trapped (color centers) charge carriers were estimated. © 2014 Pleiades Publishing, Ltd

Photoaging of methyl methacrylate-methacrylic acid copolymer doped by laser dyes and diphenylthiourea

For methyl methacrylate-methacrylic acid copolymer colored by laser dyes (Rhodamine 6G and Coumarin 7), the mutual effect of polymeric matrix and dyes on the kinetics of their photochemical aging was studied. Diphenylthiourea was introduced into the copolymer as a modifying additive. It was established that, upon irradiation with a mercury lamp, both the dyes and diphenylthiourea exert photostabilizing effects on the copolymer. It was found that there exists a correlation between the photofading of the dyes and the processes related to the photoaging of the copolymer, such as the degradation of macrochains, the accumulation of free radicals, and a reduction in the transmission coefficient. Introduction of diphenylthiourea causes a tenfold increase in the generation photostability of the copolymer samples colored by Rhodamine 6G, and consequently, their operating life as the laser-active elements

Photoconductivity of SrAlF5 crystals doped with Ce3+ ions

© Published under licence by IOP Publishing Ltd. Temporal behaviour of the complex permittivity of Ce:SrAlF5 and Ce,Yb:SrAlF5 crystals under laser irradiation in 240-280 nm spectral range was investigated by a microwave resonance technique. The photoconductivity spectrum of these crystals was detected. It was established that photoconductivity signal of the samples in 240-280 nm spectral range is caused by two processes: excited-state absorption of Ce3+ ions and photoionization of color centers

EPR, optical, and dielectric spectroscopy of Er-doped cerium dioxide nanoparticles

Abstractauthoren Abstractauthoren The cerium dioxide nanoparticles doped at low level with Er ions and with grain sizes of about 22 and 300nm were comprehensively studied using EPR, optical and microwave dielectric spectroscopy. The EPR observation of mainly cubic sites of Er3+ dopant in CeO2 reveals that vacancies are located more distant than the nearest neighbor position. This finding does not agree with recently published results based on density functional theory calculations. Time and spectral dependences of the permittivity of Er:CeO2 nanoparticles under UV laser excitation were studied by a Q-band microwave resonance technique at the room temperature. The photoconductivity threshold for cerium dioxide nanoparticles has been estimated. The luminescence spectra for the nanocrystals in wide spectral range (λ=240-1000nm) were investigated. The anti-Stokes emission of Er3+ ions under irradiation in 545-562nm spectral range, stipulated by the thermally coupled 2H11/2 and 4S3/2 levels of Er3+ ions, has been observed. The UV irradiation (240-370nm), which is not resonant with the 4f-4f transitions of Er3+ ions, excites emission of Er3+ ions due to the charge transfer from O2- to Ce4+ host ions and the subsequent energy transfer to Er3+ dopant ions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim