Temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence for PLIF applications

The temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence were studied in support of planar laser-induced fluorescence (PLIF) imaging of temperature and mixture fraction in flows of practical interest. The temperature dependencies (300–875 K) of absorption and fluorescence were measured for gaseous 3-pentanoneat atmospheric pressure in a nitrogen bath gas using 248, 266, and 308 nm excitation. The results indicate that the fluorescence signal per unit mole fraction using 248 nm excitation is highly temperature-sensitive below 600 K, while the signal from 308 nm excitation is not temperature sensitive below 500 K. For quantitative measurements over a broad range of temperatures, one must choose excitation schemes carefully to balance the trade-off between measurement sensitivity and the amount of signal at the expected conditions. As an example of such a choice and to show the capabilities of ketone PLIF techniques, we include temperature and mixture fraction images of a 300–650 K heated air jet using near-simultaneous 308 and 266 nm excitation. Received: 29 May 2002 / Revised version: 5 November 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-650/723-1748, E-mail: jkoch@stanford.edu RID="**" ID="**"E-mail: hanson@me.stanford.edu     

Spectroscopic measurements and electrical diagnostics of microhollow cathode discharges in argon flow at atmospheric pressure

This paper is dedicated to the study of the electrical and optical characteristics of direct current microhollow cathode discharges (MHCD) in argon flow. Experiments have been carried out in order to determine the so-called Paschen’s curves in a static open MHCD. Current-voltage characteristics were obtained as a function of the pressure and hole diameter. MHCD enable stable direct current discharge operation, which could be ignited for pressures ranging from 12 to 800 Torr, in a very wide range of current densities and electrodes materials. Optical emission spectroscopy and analysis of the spectral line broadening of plasma line emissions were performed in order to measure parameters such as electron number density (2–4 × 10¹⁴ cm^-3)^{-3}), gas temperature (460–640 K), excitation temperature (~ 7000 K) and electron temperature (~ 8500 K), for current ranging from 7 to 15 mA. Lower gas temperature was measured compared to the static MHCD ones.     

Investigation of temperature-dependent electrical properties of p-VOPc/n-si heterojunction under dark conditions

An organic/inorganic heterojunction p-VOPc/n-Si was fabricated and its electrical properties were investigated. Temperature-dependent dark current–voltage (I–V) characteristics of the heterojunction exhibited rectification behaviour with a rectification ratio of 405 at ±1 V and room temperature. The current–voltage characteristics of the cell showed ohmic conduction at low voltages followed by a space charge-limited current (SCLC) conduction dominated by an exponential trap distribution at higher voltages. At room temperature, the series and shunt resistances were found to be approximately 1.4 and 100 kΩ, respectively. Diode ideality factor n was found to be 3.2 at room temperature and dropped to 1.9 at 363 K. Room temperature mobility of vanadyl phthalocyanine (VOPc) was extracted from the I–V characteristics in the SCLC region and was found approximately 15.5 × 10⁻³ cm² V⁻¹ s⁻¹. The effective barrier height, Ф_B, was estimated as 0.77 eV. The effect of temperature, on various heterojunction parameters was recorded under dark conditions and at temperatures ranging from 300 to 363 K.     

Optical gain and upconversion luminescence in LaF3: Er, Yb nanoparticles-doped organic–inorganic hybrid materials waveguide amplifier

A LaF₃: Er, Yb nanoparticle-doped organic–inorganic hybrid materials waveguide amplifier is demonstrated using reactive ion etching. A maximum gain of approximately 6.8 dB is observed in a 20-mm-long waveguide. Under excitation at 976 nm, the waveguides emit a strong green upconversion luminescence. The possible upconversion mechanisms are discussed. The dependence of upconversion emission intensity on excitation power confirms a three-photon process contributes to the upconversion of the emission band 405 nm and two-photon processes for the green and red emission bands. The temperature behavior by the measurement of the fluorescence intensity ratio of the signals at 520 nm and 544 nm as a function of the pump power demonstrates a fast thermalization between the ² H _11/2 and ⁴ S _3/2 levels. The influence of upconversion emission on the gain performance of the waveguide amplifier is analyzed.     

Ultraviolet and visible upconversion emission in Tb3+/Yb3+ co-doped fluorophosphate glasses

Ultraviolet and visible upconversion emissions in Tb³⁺/Yb³⁺ co-doped YF₃–BaF₂–Ba(PO₃)₂ glasses were observed under 980-nm laser diode excitation. The dependence of the emission intensities of Tb³⁺ on the pump power reveals that two-photon processes account for blue cooperative emission of Yb³⁺ at 476 nm and green upconversion emission of Tb³⁺ at 543 nm, and three-photon processes for ultraviolet emission of Tb³⁺ in the wavelength range of 379–435 nm. The effects of Tb³⁺ concentration on the emission intensity and the lifetime of Tb³⁺ and Yb³⁺ are investigated in detail. It is found that the cooperative energy transfer from a pair of excited Yb³⁺ ions to a ground Tb³⁺ ion is responsible for the appearance of blue and green upconversion emissions due to the ⁵D₄→⁷F _J (J=6,5,4,3) transitions of Tb³⁺, and the resonance energy transfer from Yb³⁺ to Tb³⁺ accounts for the population on the ⁵D₃,⁵G₆ level and ultraviolet upconversion emission.     

Temporal dynamics of upconversion luminescence in Er3+, Yb3+ co-doped crystalline KY(WO4)2 thin films

Crystalline Er³⁺ and Yb³⁺ singly and doubly doped KY(WO₄)₂ thin films were grown by low-temperature liquid-phase epitaxy. Absorption, luminescence, excitation and temporal evolution measurements were carried out for both Er³⁺ and Yb³⁺ transitions from 10 K to room temperature. Green Er³⁺ upconversion luminescence was observed after Yb³⁺ and Er³⁺ excitation. The mechanisms responsible for the upconversion phenomena detected in each case were identified.     

Analysis of the optical properties of Er3+-doped strontium barium niobate nanocrystals using time-resolved laser spectroscopy

This paper reports the results obtained in strontium barium niobate (SBN) nanocrystals in glasses doped with 1, 2.5 and 5 mol% of Er³⁺ ions. The melt-quenching method was applied to fabricate the glasses with composition SrO–BaO–Nb₂O₅–B₂O₃ and further thermal treatment was used to obtain glass ceramic samples from the glass precursor. X-ray diffraction patterns confirmed the formation of SBN nanocrystals with an average size of about 50 nm in diameter. Time-resolved fluorescence spectra for the emission of Er³⁺ ions at 1550 nm have been analyzed in order to confirm the incorporation of the Er³⁺ ions into the nanocrystals. Green frequency upconversion emission under excitation at 975 nm coming from the ions in the nanocrystals has been obtained. This intense upconversion is about a factor of 500 higher than that obtained from the ions which reside in the glassy phase. Moreover, temporal evolution studies have been carried out with the purpose of determining the involved upconversion mechanism and the importance of these processes as a source of losses for the optical amplification at 1550 nm.     

Laser induced breakdown spectroscopy of germane plasma induced by IR CO2 pulsed laser

Laser-induced breakdown spectroscopy (LIBS) in germane (GeH₄), initially at room temperature and pressures ranging from 2 to 10 kPa, was studied using a high-power transverse excitation atmospheric (TEA) CO₂ laser (λ=10.653 μm, τ _FWHM=64 ns and power densities ranging from 0.28 to 5.52 GW cm⁻²). The strong emission spectrum of the generated plasma is mainly due to electronic relaxation of excited Ge, H and ionic fragments Ge⁺, Ge²⁺ and Ge³⁺. The weak emission is due to molecular bands of H₂. Excitation temperatures of 8100±300 K and 23,500±2500 K were estimated by Ge atomic and Ge⁺ singly ionized lines, respectively. Electron number densities of the order of (0.7–6.2)×10¹⁷ cm⁻³ were deduced from the Stark broadening of several atomic Ge lines. The characteristics of the spectral emission intensities from different species have been investigated as functions of the germane pressure and laser irradiance. Optical breakdown threshold intensities in germane at 10.653 μm have been determined. The mechanism of initiation of the laser-induced plasma in germane has been analyzed.     

The study of two-color excitation upconversion of Pr(0.5)Yb(3):ZBLAN

The excited state absorption upconversion of Pr(0.5)Yb(3):ZBLAN glass material, under two-color excitation of the 960 nm semiconductor laser and the Xe lamp light simultaneously, is reported in this article. It was found that the upconversion emission spectra of 480.1, 519.0, 601.9 and 631.8 nm coincide with the common emission spectra. Meanwhile, the upconversion-excitation spectrum has three obvious peaks under two-color excitation, and they respectively correspond to the 856.0 nm upconversion excitation transition [¹G₄(Pr³⁺)→¹I₆(Pr³⁺) and ¹G₄(Pr³⁺)→³P₁(Pr³⁺)], the 789.0 nm upconversion excitation transition ¹G₄(Pr³⁺)→³P₂(Pr³⁺), and the 803.7 nm upconversion excitation transition ³H₆(Pr³⁺)→¹D₂(Pr³⁺). The upconversion excitation transition ¹G₄(Pr³⁺)→¹I₆(Pr³⁺) is strong because its oscillator strength f = 23.040×10⁻⁶ is large, which results in a large peak appearing in the upconversion excitation spectrum. That is just the new interesting two-color excitation upconversion luminescence phenomenon of Pr(0.5)Yb(3):ZBLAN induced by one laser and one continuous normal light simultaneously.     

Synchrotron and laser excitation of luminescence in PbWO<Subscript>4</Subscript>:Tb crystals at different temperatures

The effect of temperature on the spectral luminescence characteristics of PbWO₄:Tb³⁺ crystals with synchrotron and laser excitation is studied. If PbWO₄:Tb³⁺ is excited by synchrotron radiation with λ = 88 nm at 300 K, a faint recombination luminescence of the impurity terbium is observed against the matrix luminescence. When the temperature is reduced to 8 K, the luminescence intensity of PbWO₄:Tb³⁺ increases by roughly an order of magnitude and the characteristic luminescence of the unactivated crystal is observed. Excitation of PbWO₄:Tb³⁺ by a nitrogen laser at 300 K leads to the appearance of emission from Tb³⁺ ions. At 90 K, a faint matrix luminescence is observed in addition to the activator emission. The formation of the luminescence excitation spectra for wavelengths of 60–320 nm is analyzed and the nature of the emission bands is discussed.     

Spectroscopic analysis of Nd3+ doped (Lu x + Gd1−x )3Ga5O12 crystal

In this paper we report a complete optical investigation of Nd³⁺ doped (Lu _x  + Gd_1−x )₃Ga₅O₁₂ crystal including the absorption and emission spectroscopy at room temperature and 10 K, the excitation spectroscopy at 10 K clearly showing the multi-site feature, and the lifetime at temperatures from 10 K to 300 K. The Judd–Ofelt theory has been applied to calculate the spontaneous transition probabilities, the branching ratio, and the radiative lifetime of the ⁴F_3/2 multiplet; the calculated results are in good agreement with the experimental ones. Finally, we calculated the emission cross sections of the transitions concerned to evaluate the potential of the material in laser field application.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.     

Comparative Investigation on Nanocrystal Structure and Luminescence Properties of Gadolinium Molybdates Codoped with Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript>

This paper reports on the comparative investigation of nanocrystal structure and luminescence properties of Er³⁺/Yb³⁺-codoped gadolinium molybdate nanocrystals Gd₂(MoO₄)₃ and Gd₂MoO₆ synthesized by the Pechini method with citric acid and ethylene glycol. Their crystallization, structure transformation, and morphologies have been investigated by X-ray diffraction, thermogravimetric/differential scanning calorimetry, and transmission electron microscopy. It is noticed that Er³⁺/Yb³⁺-codoped monoclinic Gd₂(MoO₄)₃ nanocrystals have shown an intense upconversion through a sintering of the organic complex precursor at 600°C. Furthermore, it transforms to orthorhombic Gd₂(MoO₄)₃ when the precursor is sintered at 900°C. In counterpart of monoclinic Gd₂MoO₆, however, the monoclinic structure remains unchanged when the precursor is sintered at a temperature ranging from 600°C to 900°C. Intense visible emissions of Er³⁺ attributed to the transitions of ²H_11/2, ⁴S_3/2–⁴I_15/2 at 520 and 550 nm, and ⁴F_9/2–⁴I_15/2 at 650 nm have been observed upon an excitation with a UV source and a 980 nm laser diode, and the involved mechanisms have been explained. It is quite interesting to observe obvious differences both in the excitation and the upconversion emission spectra of Er³⁺/Yb³⁺-codoped Gd₂(MoO₄)₃ respectively with monoclinic and orthorhombic structure. The quadratic dependence of fluorescence on excitation laser power has confirmed that two-photons contribute to upconversion of the green–red emissions.     

Microwave microplasma sources based on microstrip-like transmission lines

In this paper, we study two microwave sources based on a planar transmission line configuration, corresponding to linear resonators. In both sources, micro-plasmas are produced within the 50–200 m\mu m gap created between two metal electrodes placed at the open end of a microstrip-like transmission line. The study of the sources follows a complementary approach that uses simulation and experiment. Simulations analyze the electromagnetic behavior of the sources, using the commercial tool CST Microwave Studio^?, and characterize the plasmas produced, using a fluid-type code to describe the dynamics of charged particles. Experimentally, the return loss of the sources (hence their quality factors) is measured without and with plasma. Plasma diagnostics (in air and in argon), based on optical emission spectroscopy measurements, enable to obtain the typical plasma temperatures and the electron density (using Stark broadening measurements of the H_b_{\beta} line-emission profile). Results reveal that the sources have similar quality factors (~15–20), yielding high-density (~10¹⁴ cm^-3)^{-3}), low-power (~10–50 W), non-equilibrium micro-plasmas (with rotational temperatures of ~950–1400 K in air and ~550–630 K in argon, vibrational temperatures of ~5200–5800 K in air and excitation temperatures of ~5800 K in argon), over volumes of ~10^-4–10^-3 cm³.     

Spectroscopy study of air plasma induced by IR CO2 laser pulses

A spectroscopic study of ambient air plasma, initially at room temperature and pressures ranging from 32 to 101 kPa, produced by high-power transverse excitation atmospheric (TEA) CO₂ laser (λ=9.621 and 10.591 μm; τ _FWHM≈64 ns; power densities ranging from 0.29 to 6.31 GW cm⁻²) has been carried out in an attempt to clarify the processes involved in laser-induced breakdown (LIB) air plasma. The strong emission observed in the plasma region is mainly due to electronic relaxation of excited N, O and ionic fragments N⁺. The medium-weak emission is due to excited species O⁺, N²⁺, O²⁺, C, C⁺, C²⁺, H, Ar and molecular band systems of N ₂⁺(_{2}^{+}( B ²\varSigma _u⁺^{2}\varSigma _{\mathrm{u}}^{+} –X ²\varSigma _g⁺)^{2}\varSigma _{\mathrm{g}}^{+}) , N₂(C³ Π _u–B³ Π _g), N ₂⁺(_{2}^{+}( D² Π _g–A² Π _u) and OH(A² Σ ⁺–X² Π). Excitation temperatures of 23400±700 K and 26600±1400 K were estimated by means of N⁺ and O⁺ ionic lines, respectively. Electron number densities of the order of (0.5–2.4)×10¹⁷ cm⁻³ and (0.6–7.5)×10¹⁷ cm⁻³ were deduced from the Stark broadening of several ionic N⁺ and O⁺ lines, respectively. Estimates of vibrational and rotational temperatures of N ₂⁺_{2}^{+} electronically excited species are reported. The characteristics of the spectral emission intensities from different species have been investigated as functions of the air pressure and laser irradiance. Optical breakdown threshold intensities in air at 10.591 μm have been measured.     

Energy transfer and upconversion in erbium–ytterbium-doped fluoride phosphate glasses

The energy-transfer rate coefficient from ytterbium to erbium (K) and upconversion rate coefficient for erbium (C) are determined by analysis of erbium fluorescence decay under different pumping conditions. A simple rate-equation model was used to describe the fluorescence dynamics. Numerical analysis of the model delivered K and C as fit parameters to the experimental curves. Values of K ranging from 0.75–1.42×10^-16 cm³/s and C from 0.8–1.7×10^-18 cm³/s were found. The results obtained are of the same magnitude as results for phosphate glasses reported in the literature. Further analysis showed that high erbium concentrations (2×10²⁰ cm^-3) need a much higher pump intensity to reach threshold due to increased losses caused by upconversion. Received: 14 January 2002 / Published online: 7 February 2002     

Multifunctional Er3+–Yb3+ codoped Gd2O3 nanocrystalline phosphor synthesized through optimized combustion route

This paper reports the synthesis of high upconversion luminescent Gd₂O₃: Er³⁺, Yb³⁺ nanophosphor through optimized combustion route using urea as a reducing agent. The paper also reports the first observation of upconversion emission bands extending upto the UV region (335, 366 and 380 nm) in Er³⁺–Yb³⁺ co-doped phosphor materials. The fuel to oxidizer ratio has been varied to obtain the maximum upconversion luminescence. Three high intensity bands are found at 408, 523–548 and 667 nm due to the ⁴G_11/2 → ⁴I_15/2, ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively, along with the other bands. Input excitation power dependence has been studied for different transitions, and the saturation effect and decrease in the slope of different transitions at higher input pump power has been explained. Heat treatments of the samples show change in crystallite phase/size and relative upconversion luminescence intensities of blue, green and red bands. The color of the phosphor emission has shown to be tunable with change in the crystal structure as well as on excitation laser power and Er³⁺–Yb³⁺ concentration. The property of color tunability of the phosphor material has been used to record the fingerprint in different colors. Also, the future prospect of the nanocrystalline phosphor material as a sensor for temperature, using FIR method, has been explored.     

Preparation and optical properties of sol–gel neodymium-doped germania/γ-glycidoxypropyltrimethoxysilane organic–inorganic hybrid thin films

Germania/γ-glycidoxypropyltrimethoxysilane organic–inorganic hybrid spin-coating thin films doped with neodymium ions are prepared by a sol–gel technique and a spin-coating process. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with germanium isopropoxide are used as matrix precursors. Thermal gravimetric analysis, UV–visible spectroscopy, and Fourier transform infrared spectroscopy are used to study the structural and optical properties of the hybrid thin films. The results indicate that films that are crack-free and have a high transparency in the visible and near-infrared range can be obtained; a strong UV absorption region at short wavelength ∼200 nm, accompanied with a shoulder peaked at ∼240 nm, due to the neutral oxygen monovacancy defects, is also identified. Upconversion emission properties of the transparent dried gel and the thin films heated at different heat treatment temperatures and doped with different neodymium ion concentrations are studied; a relatively strong room-temperature yellow to violet upconversion emission at 397 nm (⁴ D _3/2→⁴ I _13/2) is observed under a xenon lamp excitation with yellow light at the wavelength of 580 nm (⁴ I _9/2→⁴ G _5/2). The effect of Nd³⁺ doping concentration and heat treatment temperature on upconversion emission of the thin films is also studied. The mechanism of the upconversion is proposed. PACS 81.05.Kf; 81.20.Fw; 78.55.Hx     

Optical gain by upconversion in Tm–Yb oxyfluoride glass ceramic

Evidence of positive optical gain is observed in Tm³⁺–Yb³⁺-codoped oxyfluoride glass ceramic in an upconversion pump and probe experiment. The ¹G₄ level of the Tm³⁺ ions is populated by an upconversion mechanism under excitation of the Yb³⁺ ions at 975 nm with a high-power pulsed laser and give rise to an intense emission from the ¹G₄ to the ³F₄ levels. The ¹G₄→³F₄ electronic transition is stimulated with a low signal at 650 nm as a probe.     

Upconversion Based Tunable White-Light Generation in Ln:Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocrystalline Phosphor (Ln = Tm/Er/Yb)

We report the generation of efficient white light based on upconversion (UC) in Tm³⁺/Er³⁺/Yb³⁺:Y₂O₃ 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.