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ABSTRACT: The spectroscopic properties of a Dy3+-doped aluminophosphate glass containing silver and tin were reported. Different oxidation and aggregation states of silver
were obtained by varying silver concentration and glass thermal history. The addition of silver and tin at the lowest concentration
studied results in Dy3+ ions emission under nonresonant UV excitation in connection with the appearance of an excitation band around 270nm, which
is associated to isolated Ag+ ions and twofold-coordinated Sn centers. The increase in silver and tin concentration leads to a broadening of aforementioned
band and to the presence of charged silver dimers as evidenced by the appearance of an excitation band around 330nm. The
data indicated that light absorption might take place at ionic silver species and twofold-coordinated Sn centers, followed
by energy transfer to Dy3+ ions. After heat treatment, ionic silver species were reduced to atomic Ag by tin with the subsequent formation of Ag nanoparticles
(NPs) inside the dielectric host. A quenching effect in Dy3+ ions luminescence was shown with the presence of the Ag NPs, most notably for excitation of 6H15/2→4F9/2, 4I15/2, 4G11/2 transitions, which were in resonance with the dipole absorption mode of the particles. The silver NPs were believed to provide
radiationless pathways for excitation energy loss in Dy3+ ions.
Journal of Materials Science 05/2012; 45(11):2983-2990. · 2.02 Impact Factor
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ABSTRACT: The nonequilibrium carrier dynamics in Ag nanoparticles was explored by femtosecond optical pump-probe spectroscopy. Metal-dielectric
nanocomposite materials containing Ag nanoparticles of different sizes were prepared by thermal treatment of aluminophosphate
glasses. The ultrafast relaxation dynamics of the electron scattering on the 10−13–10−12 sec scale shows size-and pump power-dependent properties. The two-photon absorption process in nanoparticles of different
sizes is discussed. The laser-induced coherent vibrations of nanoparticles were observed in transient transmission experiments.
Journal of Electronic Materials 04/2012; 35(9):1715-1721. · 1.47 Impact Factor
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ABSTRACT: An aluminophosphate glass system containing silver and tin was prepared by the melt-quenching technique in which spherical
silver nanoparticles (NPs) of different sizes were embedded upon heat treatment. Optical absorption was used in the assessment
of particle growth and for particle size estimation yielding mean radii in the 10–40nm range. Measurements in the UV region
revealed absorption features indicative of the occurrence of silver ions and twofold-coordinated tin centers. Photoluminescence
spectroscopy excited at 355nm showed a broadband emission around 420nm for the non-heat-treated glass, which shows a thermal
quenching effect in temperature dependence measurements. Heat-treated glass shows a dip in the emission spectrum ascribed
to absorption by NPs. The luminescence is attributed to single Ag+ ions. The nature of the silver emitting states is discussed.
Journal of Electronic Materials 04/2012; 36(7):812-820. · 1.47 Impact Factor
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ABSTRACT: Silver-doped aluminophosphate glasses were prepared by the melt-quenching technique in which silver nanoparticles (NP) of
different sizes were embedded upon heat treatment. Optical absorption and photoluminescence spectroscopy were used to study
the optical properties of the material before and after thermal processing. Photoluminescence (PL) experiments revealed a
broadband emission observed around 420nm for the non-heat treated samples with a luminescence decay showing a bi-exponential
behavior. Temperature dependence PL studies showed a thermal quenching effect on the broadband emission. Our data suggests
that the emission is due to single Ag+ ions. Optical absorption measurements performed on the heat treated samples allowed for particle size estimation and the
evaluation of the thermal stability of the glass system and its attributes as a host for NP inclusion. The nanocomposite showed
a dip in the broadband emission of silver ions ascribed to absorption of Ag+ ions luminescence by surface plasmons in the silver particles.
Journal of Materials Science 04/2012; 42(5):1856-1863. · 2.02 Impact Factor
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ABSTRACT: Relaxation dynamics of silver nanoparticles (NPs) embedded in glass have been studied by picosecond time-resolved transient
grating (TG) spectroscopy. Phosphate-based glasses were prepared by melting and heat-treatment processes by which two different
nanometal-glass composites were produced. The first is a glass system containing silver and tin in which Ag NPs are embedded
in the matrix upon heat treatment. The second is a heat-treated silver-doped glass with spectroscopic indications of Ag+–Ag0 pairs located at or near the surface of the NPs. The time evolution of the light-induced TG for the Ag/Sn-doped glass shows
an uncommon relaxation on the nanosecond time scale. Such behavior is explained in terms of energy transfer processes between
polaronic and/or excitonic states in the near-interface region of the glass matrix and the NPs. In contrast, a faster monotonic
relaxation is observed for the Ag-doped nanocomposite. This result is attributed to Ag NP→Ag+–Ag0 plasmon resonance energy transfer.
Journal of Electronic Materials 04/2012; 39(1):138-143. · 1.47 Impact Factor
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ABSTRACT: Europium-doped aluminophosphate glasses prepared by the melt-quenching technique have been studied by photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). The effects of silver and tin doping, and of further thermal processing on Eu(3+) ions luminescence have been assessed. For the glass system containing only europium, Eu(3+) PL observed under UV excitation is suggested to occur through energy transfer from the excited glass host. After silver and tin doping, an enhanced UV excited Eu(3+) PL has been indicated to occur essentially due to radiative energy transfer from isolated Ag(+) ions and/or two fold-coordinated Sn centers. Since thermal processing of the material leads to a quenching effect on Eu(3+) PL and Ag nanoparticles (NPs) formation due to reduction of silver ions by tin, XPS was employed in order to investigate the possibility for Eu(3+) -> Eu(2+) reduction during HT as a potential source of the PL decrease. The data points towards Ag NPs as main responsible for the observed weakening of Eu(3+) PL (C) 2009 Elsevier B.V. All rights reserved.
Journal of Luminescence. 01/2010; 130(1):163-167.
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ABSTRACT: The degenerate-four-wave-mixing, ultrafast optical pump-probe reflection, and scattering techniques were applied to study the nonlinear optical properties of VO <sub>2</sub> in insulating and metallic phases. The third-order nonlinear susceptibility was measured for thin films at different excitation regimes. The VO <sub>2</sub> recovery dynamics after light-induced phase transition (PT) shows strong sensitivity to optical pump energy and could be governed by pure electronic relaxation excluding thermal contribution at sufficiently low excitation. Increased light scattering during thermally and light-induced PT demonstrates significant VO <sub>2</sub> heterogeneity which appears as a coexistence of insulating and metallic phases accompanied by fluctuations of dielectric constants. Different desorption activity was monitored for insulating and metallic VO <sub>2</sub> thin solid films under femtosecond optical excitation.
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
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ABSTRACT: This paper reports the first spectroscopic demonstration of photoluminescence (PL) owing to plasmon resonance energy transfer (PRET) from silver nanoparticles (NPs) to luminescent species in glass. Optical absorption and PL spectroscopy experiments performed on the melt-quenched silver-doped glass indicate the presence of single Ag <sup>+</sup> ions, Ag <sup>+</sup>– Ag <sup>+</sup> and Ag <sup>+</sup>– Ag <sup>0</sup> pairs, and Ag NPs. After thermal processing of the material, nonradiative energy transfer from the Ag <sup>+</sup>– Ag <sup>0</sup> luminescent centers to Ag NPs is observed by uniform suppression of band emission and the vanishing of the excitation band associated. Furthermore, evidence for PRET is observed after glass heat treatment by the appearance of a new excitation band near the surface plasmon resonance peak of Ag NPs at about 420 nm, when emission of silver pairs is monitored around 550 nm. In fact, excitation at 420 nm leads to a band emission centered around 530 nm indicating that the excitation of luminescent silver species indeed takes place via PRET. The luminescence is interpreted as arising from a Ag NP → Ag <sup>+</sup>– Ag <sup>0</sup>→ Ag <sup>+</sup>– Ag <sup>+</sup> energy transfer scheme.
Journal of Applied Physics 10/2008; · 2.17 Impact Factor
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ABSTRACT: Light-induced insulator-to-metal phase transition of vanadium dioxide films was studied by ultrafast optical pump-probe spectroscopy.
The transient optical reflection measurement shows that both heating and laser illumination contribute to the phase transition
of VO2. Within 10−11–10−9 sec, these two mechanisms are competitive. Excited-state dynamics were found to be strongly dependent on the concentration
of structural defects and the pump laser power as well. Comparison of the transient reflection of VO2 films deposited on different substrates suggests that the excitonic-controlled light-induced insulator-to-metal phase transition
in VO2 proceeded through an intermediate state. The transient reflection measurement of VO2 in metallic phase shows a three-stage relaxation process. A polaron excitation model is introduced to describe the dynamical
process for metallic VO2.
Journal of Electronic Materials 09/2006; 35(10):1866-1872. · 1.47 Impact Factor
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ABSTRACT: Ultrafast light-induced insulator-metal phase transitions (IM PT) in VO2 thin films was studied with use of a pump-probe technique. These studies show that the IM PT could be realized via an intermediate state. The relaxation processes after optical pumping are dependent on pump energy. The excitonic controlled model for such type of IM PT is proposed on this basis. The main channel for the ultrafast light-induced IM PT is the resonant transition between excited states of correlated vibronic Wannier-Mott excitons (WME) in insulator phase and the unoccupied excited states in metallic phase. The experiment observation, such as pump power drastic dependences of relaxation could be interpreted in the framework of the model.
Journal of Physics Conference Series 09/2005; 21(1):44.
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ABSTRACT: Laser-induced ultrafast nonlinear optical (NLO) response was observed in transient absorption, reflection and NLO measurements in VO2 thin films. The films were prepared using pulsed laser deposition technique with metallic vanadium and V2O5 powder employed as target material. The obtained thin films exhibit a thermal, as well as a laser-induced phase transition (PT) from semiconductor state at room temperature to metallic state. A typical hysterisis was observed for electric resistivity and optical reflectance verses temperature T. For the laser-induced PT in femtosecond pump–probe experiment, the PT was found to be ultrafast, immediately upon laser excitation. Kinetics study suggests that the PT is realized via an intermediate state, presumably a Wannier–Mott exciton (WME) state, followed by a resonant transition to the metallic phase state. The presence of the intermediate state is responsible for an extremely large third-order susceptibility observed in a nonlinear holographic experiment. In a 80 nm thick VO2 thin amorphous film, the value of χ(3) was measured to be 1.3×10−8 esu which is four orders of magnitude greater than a standard reference sample of CS2. In addition, the nonlinear response was further enhanced with diffraction signal increased by a factor of 4 after the laser excitation. A signal dip at ∼1.2 ns was repeatedly observed, which is presumably due to the excitation process associated with the intermediate state. Finally, the resonant transition from the intermediate state to the metallic state, giving rise to a maximum diffraction signal intensity from the formed transient grating.
Journal of Luminescence.
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ABSTRACT: Vanadium dioxide shows a passive and reversible change from a monoclinic insulator phase to a metallic tetragonal rutile structure when the sample temperature is close to and over 68 °C. As a kind of functional material, VO2 thin films deposited on fused quartz substrates were successfully prepared by the pulsed laser deposition (PLD) technique. With laser illumination at 400 nm on the obtained films, the phase transition (PT) occurred. The observed light-induced PT was as fast as the laser pulse duration of 100 fs. Using a femtosecond laser system, the relaxation processes in VO2 were studied by optical pump–probe spectroscopy. Upon a laser excitation an instantaneous response in the transient reflectivity and transmission was observed followed by a relatively longer relaxation process. The alteration is dependent on pump power. The change in reflectance reached a maximum value at a pump pulse energy between 7 and 14 mJ/cm2. The observed PT is associated with the optical interband transition in VO2 thin film. It suggests that with a pump laser illuminating on the film, excitation from the dθ,ɛ – state of valence band to the unoccupied excited mixed dθ,ɛ–π* – state of the conduction band in the insulator phase occurs, followed by a resonant transition to an unoccupied excited mixed dθ,ɛ–π* – state of the metallic phase band.
Applied Surface Science.
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ABSTRACT: The optical properties of silver species in various oxidation and aggregation states and of tin centers in melt-quenched phosphate glasses have been assessed by optical absorption and photoluminescence (PL) spectroscopy. Glasses containing silver and tin, or either dopant, were studied. Emission and excitation spectra along with time-resolved and temperature-dependent PL measurements were employed in elucidating the different emitting centers observed and investigating on their interactions. In regard to silver, the data suggests the presence of luminescent single Ag+ ions, Ag+–Ag+ and Ag+–Ag0 pairs, and nonluminescent Ag nanoparticles (NPs), where Ag+–Ag0→Ag+–Ag+ energy transfer is indicated. Tin optical centers appear as twofold-coordinated Sn centers displaying PL around 400 nm ascribed to triplet-to-singlet electronic transitions. The optically active silver centers were observed in glasses where 8 mol% of both Ag2O and SnO, and 4 mol% of Ag2O were added. Heat treatment (HT) of the glass with the high concentration of silver and tin leads to chemical reduction of ionic silver species resulting in a large volume fraction of silver NPs and the vanishing of silver PL features. Further characterization of such heat-treated glass by transmission electron microscopy and X-ray photoelectron spectroscopy appears consistent with silver being present mainly in nonoxidized form after HT. On the other hand, HT of the glass containing only silver results in the quenching of Ag+–Ag0 pairs emission that is ascribed to nonradiative energy transfer to Ag NPs due to the positioning of the pairs near the surface of NPs during HT. In this context, an important finding is that a faster relaxation was observed for this nanocomposite in relation to a heat-treated glass containing both silver and tin (no silver pairs) as revealed by degenerate four-wave mixing spectroscopy. Such result is attributed to Ag NP→Ag+–Ag0 plasmon resonance energy transfer. The data thus indicates that energy transfer between Ag+–Ag0 pairs and NPs is bi-directional.
Journal of Luminescence.
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ABSTRACT: The light-induced insulator-to-metal phase transition was investigated by femtosecond pump-probe tech-niques in nanocrystalline VO 2 films. The size of VO 2 nanoparticles and the VO 2 film morphology were found to be critical for the ultrafast light-induced phase-transition dynamics. Experimental measurements of the third-order nonlinear susceptibility 3 for the insulating phase at different excitation levels and transient grating experiments show a size-dependent threshold behavior which is related to the light-induced transition process. Optical properties of VO 2 nanoparticles, 3 data, and transition dynamics at subnanosecond time scale also demonstrate a pronounced size dependence. The rate of structural phase transition increases due to confinement effect as particle size decreases. Nucleation of the metallic phase shows a critical behavior with formation of a metastable state in VO 2 . A kinetic model for the metallic phase growth and transient grating signal evolution upon ultrafast laser excitation is developed for VO 2 thin film.
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ABSTRACT: Glasses containing silver, tin and europium were prepared by the melt-quenching technique with silver nanoparticles (NPs) being embedded upon heat treatment (HT). An intensification of Eu3+ ions emission was observed for non-resonant excitation around 270 nm, corresponding to UV absorption in the material. Optical measurements suggest that light absorption occurs at single Ag+ ions and/or twofold-coordinated Sn centers followed by energy transfer to europium which results in populating the 5D0 emitting state in Eu3+. After HT at 843 K, a quenching effect is observed on Eu3+ luminescence with increasing holding time in the 350–550 nm excitation range. The quenching effect shows with the presence of Ag NPs which may provide multipole radiationless pathways for excitation energy loss in europium ions.
Journal of Luminescence. 128:831-833.
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ABSTRACT: Ultrafast light-induced insulator-metal phase transitions (PT) in VO2 thin films was studied with use of a pump-probe technique. The theoretical and experimental study of PT kinetics shows that the PT could be realized via an intermediate state. The relaxation processes after optical pumping are dependent on pump energy. The excitonic controlled model for such type of PT is proposed. The main channel for the ultrafast light-induced PT is the resonant transition between excited states of correlated vibronic Wannier–Mott excitons (WME) in insulator phase and the unoccupied excited states in metallic phase. During this process an equilibrium local distortion occurred. According to the proposed model the experimental observation of the drastic temperature- and pump power- dependent relaxation processes could be interpreted.
Solid State Communications.
