Temperature dependence of thermal properties of Ag8In14Sb55Te23 phase-change memory materials

The dependence of thermal properties of Ag8In14Sb55Te23 phase-change memory materials in crystalline and amorphous states on temperature was measured and analyzed. The results show that in the crystalline state, the thermal properties monotonically decrease with the temperature and present obvious crystalline semiconductor characteristics. The heat capacity, thermal diffusivity, and thermal conductivity decrease from 0.35 J/g K, 1.85 mm(2)/s, and 4.0 W/m K at 300 K to 0.025 J/g K, 1.475 mm(2)/s, and 0.25 W/m K at 600 K, respectively. In the amorphous state, while the dependence of thermal properties on temperature does not present significant changes, the materials retain the glass-like thermal characteristics. Within the temperature range from 320 K to 440 K, the heat capacity fluctuates between 0.27 J/g K and 0.075 J/g K, the thermal diffusivity basically maintains at 0.525 mm(2)/s, and the thermal conductivity decreases from 1.02 W/m K at 320 K to 0.2 W/m K at 440 K. Whether in the crystalline or amorphous state, Ag8In14Sb55Te23 are more thermally active than Ge2Sb2Te5, that is, the Ag8In14Sb55Te23 composites bear stronger thermal conduction and diffusion than the Ge2Sb2Te5 phase-change memory materials.

Spectral anomalies of focused hollow Gaussian beams at the geometrical focal plane

On the basis of diffraction integral and the expansion of the hard-aperture function into a finite series of complex Gaussian functions, an approximate expression for spatially fully coherent polychromatic hollow Gaussian beams passing through aperture lens is obtained. Detailed numerical results indicate that remarkable spectral changes always occurs near the points where the field amplitude has zero value. The effects of truncation parameter, Fresnel number and the beam order on spectral shifts and spectral switches are investigated numerically. (C) 2008 Elsevier B.V. All rights reserved.

Passive Q-switching of short-length Tm3+-doped silica fiber lasers by polycrystalline Cr2+:ZnSe microchips

We demonstrate passive Q-switching of short-length double-clad Tm3+-doped silica fiber lasers near 2 mu m pumped by a laser diode array (LDA) at 790 nm. Polycrystalline Cr2+:ZnSe microchips with thickness from 0.3 to 1 mm are adopted as the Q-switching elements. Pulse duration of 120 ns, pulse energy over 14 mu] and repetition rate of 53 kHz are obtained from a 5-cm long fiber laser. As high as 530 kHz repetition rate is achieved from a 50-cm long fiber laser at similar to 10-W pump power. The performance of the Q-switched fiber lasers as a function of fiber length is also analyzed. (c) 2008 Elsevier B.V. All rights reserved.

Generation of 1.5-12 ns width-tunable 532 nm pulses by adopting laser-induced plasma shutter technique

The pulse-shaping technique has found widespread applications in nonlinear optics and material processing. Experimental research on laser-induced plasma shutter to control the 532 nm pulse width is conducted. The impacts of the total pulse output energy on pulse compression are investigated, and a useful conclusion can be drawn that there exists an optimal value of pulse energy at which the shortest output pulse of 3.23 ns can be obtained without a device for delay-time. Once the device for delay-time is employed to change the optical differences between two laser paths, the pulse width can be further shortened to 1.51 ns. In short, the 1.5-12 ns width-tunable 532 nm laser pulses have been obtained by adopting the laser-induced plasma shutter technique. (C) 2007 Elsevier GmbH. All rights reserved.

Effects of thermal treatment on broadband near-infrared emission from Bi-doped chalcohalide glasses

GeGaSKBr glass with Bi ions as emission centers were fabricated. An intense emission centered at around 1230 nm with the width of more than 175 nm was observed by 808 nm photo-excitation of the glass. Lower quenching rate and thermal treatment promote micro-crystallization process, thus strengthening the emission. Crown Copyright (c) 2008 Published by Elsevier Ltd. All rights reserved.

Luminescence properties of bismuth-doped lime silicate glasses

Luminescences from bismuth-doped lime silicate glasses were investigated. Luminescences centered at about 400, 650, and 1300 nm were observed, excited at 280, 532 and 808 nm, respectively. These three luminescence bands arise from three different kinds of bismuth ions in the glasses. The visible luminescences centered at 400 and 650 nm arise from Bi3+, and Bi2+, respectively. The infrared luminescences cover the wavelength range from 1000 to 1600 nm when exited by an 808 nm laser diode. The full width at half maximum (FWHM) of the infrared luminescences is more than 205 urn. The intensity of the infrared luminescence decreases with the increment in CaO content. We suggest that the infrared luminescences might arise from Bi+. Such broadband luminescences indicate that the glasses may be potential candidate material for broadband fiber amplifiers and tunable lasers. (C) 2007 Elsevier B.V. All rights reserved.

Two-photon excited red upconversion luminescence of thulium ions doped GeS2-In2S3-CsI glass

We report a novel phenomenon in GeS2-In2S3-CsI chalcohalide glass doped with Tm3+ ions. Under irradiation with an 808 nm laser diode, a bright red emission centered at 700 nm is observed for the first time in this glass. The log-log correlation between integrated emission intensity and pump power reveals that a two-photon absorption process is involved in the phenomenon, suggesting that the F-3(3,2) -> H-3(6) transition of Tm3+ ions is responsible for the appearance of the red emission. The results indicate that the indium (In) based chalcohalide glass containing Tm3+ ions is expected to find applications in visible lasers, high density optical storage and three-dimensional color displays. (C) 2009 Elsevier B.V. All rights reserved.

Upconversion Luminescence of Er3+-Yb3+ Codoped NaYF4-PVP Electrospun Nanofibers

NaYF4: 0.02Er center dot xYb-PVP composite nanofibers with the diameter of similar to 400 nm have been prepared by electrospinning. Field emission scanning electron microscope and X-ray diffraction have been utilized to characterize morphology and structure of the as-prepared electrospun nanofibers. Their up-conversion luminescence is investigated under a 980-nm excitation. Green (538 and 520 nm), red (6-55 nm), and blue (405 nm) emissions are observed in the up-conversion luminescence spectra, and the intensity of these three emissions changes differently with the variety of Yb content, which has been interpreted successfully in this letter. The color of NaYF4: 0.02Er center dot xYb-PVP nanolibers under a 980-nm excitation can be changed from green --> white --> yellow gradually via changing the Yb content.

Influence of carbon on the thermoluminescence and optically stimulated luminescence of α -Al2O3:C crystals

alpha-Al2O3:C crystal shows excellent thermoluminescence (TL) and optically stimulated luminescence (OSL) properties but the real role carbon plays in this crystal is still not clearly understood so far. In this work, alpha-Al2O3:C crystal doping with different amounts of carbon were grown by the temperature gradient technique, and TL and OSL properties of as-grown crystals were investigated. Additionally, a mechanism was proposed to explain the role of carbon in forming the TL and OSL properties of alpha-Al2O3:C. TL and OSL intensities of as-grown crystals increase with the increasing amount of carbon doping in the crystal, but no shift is found in the glow peak location at 465 K. As the amount of carbon doping in the crystals decreases, OSL decay rate becomes faster. With the increase in heating rate, the integral TL response of as-grown crystals decreases and glow peak shifts to higher temperatures. TL response decrease rate increases with the increasing amount of carbon doping in the crystals. All the TL and OSL response curves of as-grown crystals show linear-sublinear-saturation characteristic, and OSL dose response exhibits higher sensitivity and wider linear dose range than that of TL. The crystal doping with 5000 ppm carbon shows the best dosimetric properties. Carbon plays the role of a dopant in alpha-Al2O3:C crystal and four-valent carbon anions replace the two-valent anions of oxygen during the crystal growth process, and large amounts of oxygen vacancies were formed, which corresponds to the high absorption coefficient of F and F+ centers in the crystals.

Studies of magnetic interactions in Ni-doped ZnO from first-principles calculations

The magnetic interactions in Ni-doped ZnO are calculated using GGA and GGA + U method of density functional theory. The following three cases: (i) Ni-doped ZnO, (ii) (Ni, Al)-codoped ZnO, and (iii) (Ni, Li)-codoped ZnO are studied. The ferromagnetic ordering is always favorable for the three cases within GGA method. However, the ferromagnetic state is sometimes favorable after treating within the method of GGA + U. The GGA underestimates the correlated interactions especially when the Ni ions align directly to each other. (C) 2007 Elsevier B.V. All rights reserved.

Generation of 210 fs laser pulses at 1093 nm by a self-starting mode-locked Yb:GYSO laser

We report the first demonstration, to our knowledge, of the femtosecond laser operation by using a new alloyed Yb:GYSO crystal as the gain medium. With a 5 at. % Yb3+-doped sample and chirped mirrors for dispersion compensation, we obtained pulses as short as 210 fs at the center wavelength of 1093 nm. The average mode-locking power is 300 mW, and the pulse repetition frequency is 80 MHz. (C) 2008 Optical Society of America

Judd-Oflet analysis of spectrum and laser performance of Ho:YAP crystal end-pumped by 1.91-mu m Tm:YLF laser

The Ho:YAP crystal is grown by the Czochralski technique. The room-temperature polarized absorption spectra of Ho:YAP crystal was measured on a c-cut sample with 1 at% holmium. According to the obtained Judd-Ofelt intensity parameters Omega(2) = 1.42 x 10(-20) cm(2), Omega(4) = 2.92 x 10(-20) cm(2), and Omega(6) = 1.71 x 10(-20) cm(2), this paper calculated the fluorescence lifetime to be 6 ms for I-5(7) -> I-5(8) transition, and the integrated emission cross section to be 2.24 x 10(-18) cm(2). It investigates the room-temperature Ho:YAP laser end-pumped by a 1.91-mu m Tm:YLF laser. The maximum output power was 4.1 W when the incident 1.91-mu m pump power was 14.4W. The slope efficiency is 40.8%, corresponding to an optical-to-optical conversion efficiency of 28.4%. The Ho:YAP output wavelength was centred at 2118 nm with full width at half maximum of about 0.8 nm.

Characterization of m-plane ZnO thin film on gamma-LiAlO2 (100) substrate by metal-organic chemical vapor deposition

Non-polar (1 (1) over bar 00)m-plane ZnO thin film has been prepared on gamma-LiAlO2 (100)substrate via the low pressure metal organic chemical vapor deposition. Obvious intensity variation of the E-2 mode in the polarized Raman spectra and the absorption edge shift in the polarized optical transmission spectra indicate that the m-plane film exhibits optical anisotropy, which have applications in certain optical devices, such as the UV modulator and polarization-dependent beam switch. From the atomic force microscopy images, highly-oriented uniform-sized grains of rectangular shape were observed. (c) 2008 Elsevier B.V. All rights reserved.

Absence of room-temperature ferromagnetism in Al-codoped Zn0.95Co0.05O nanoparticles

Nanocrystalline Zn0.95-xCo0.05AlxO (x=0, 0.01, 0.05) diluted magnetic semiconductors have been synthesized by an auto-combustion method. X-ray diffraction measurements indicated that Al-doped Zn0.95Co0.05O samples had the pure wurtzite structure. X-ray absorption spectroscopy, high-resolution transmission electron microscope, energy dispersive spectrometer and Co 2p core-level photoemission spectroscope analyses indicated that Co2+ substituted for Zn2+ without forming any secondary phases or impurities. Resistance measurements showed that the resistance values of Co and Al codoped samples were still so large in the giga magnitude. Magnetic investigations showed that nanocrystalline Al-doped Zn0.95Co0.05O samples had no indication of room temperature ferromagnetism. (C) 2007 Elsevier B.V. All rights reserved.

Low temperature growth of c-axis oriented AlN films on gamma-LiAlO2 by radio frequency magnetron sputtering

Aluminum nitride (AlN) films were prepared on gamma-LiAlO2 substrates by radio frequency (rf) magnetron sputtering. The influence of substrate temperature (T-s) and nitrogen (N-2) concentration on film growth was investigated. The X-ray diffraction (XRD) results reveal that highly c-axis oriented AlN films can be obtained in the temperature range from room temperature (RT) to 300 degrees C. A smoother surface and a crystalline quality decrease with increasing N-2 concentration have been observed by XRD and atomic force microscopy (AFM) for the films deposited at lower substrate temperature. On the contrary, the degradation of the surface smoothness and the higher crystalline quality can be observed for the films deposited at a higher substrate temperature with N-2-rich ambient. The growth mechanism which leads to different crystalline quality of the films is discussed. (C) 2008 Elsevier B.V. All rights reserved.