Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B15: Focus Session: Inorganic Glasses |
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Sponsoring Units: FIAP Chair: John Kieffer, University of Michigan Room: LACC 405 |
Monday, March 21, 2005 11:15AM - 11:51AM |
B15.00001: Ultrafast Laser Modification of Inorganic Glasses Invited Speaker: Consider the interaction between visible or near-infrared laser light with a ``transparent'' material, such as glass. Under normal conditions, i.e continuous low power illumination, the glass will transmit the light. However, when ultra-short pulses of modest energy of this same light (for example 100 fs, 1 $\mu $J laser pulses) are tightly focused into a micron-size spot, very high, localized (in space and time) intensities, on the order of 100 TW cm$^{-2}$ are obtained. Under these conditions the laser-materials interaction becomes highly nonlinear and can result in permanent changes in the structure of the glass. Since these high intensities are only achieved at the focal point of the laser beam, the effective nonlinear interaction and subsequent modification only occurs locally, i.e. within a confined region limited by the size of the focal volume. By moving the sample with respect to the laser focus it is possible to ``write'' 3-D patterns inside the glass. The structure of the modification patterns can differ from the unmodified material in a wide variety of properties including refractive index, absorption coefficient, nonlinear optical susceptibility, crystal structure, morphology etc.\textbf{ }Applications of this so-called fs laser writing technique are in optical data storage, telecommunications and bio-sensing and --imaging. The experimental tools that are used for modifying the material can in a different implementation also be used to optically and spectroscopically characterize a material with high spatial resolution. In this talk I will review the basics of ultrafast laser modification of glass and present our results on optical waveguide fabrication in a variety of inorganic oxide glasses. I will discuss the behavior of the different glass systems and show how confocal laser fluorescence and Raman spectroscopy help us understand the relationship between the index changes and the associated changes in the atomic scale structure of the glass. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:27PM |
B15.00002: Novel methods for first principles modeling of glasses Invited Speaker: Recently, we have explored new methods to form models of binary glasses that offer significant computational advantage and enable inclusion of \textit{a priori} information about structure from experiments or other considerations. For glassy SiO$_{2}$ and some other IV-VI binary glasses, we find that placement of column VI atoms at bond centers in a good quality tetrahedral amorphous network followed by rescaling and relaxation leads to a highly realistic and large (648 atom) \textit{ab initio} model of silica and other glasses. The problem of ``freezing in" too much disorder from the liquid state, familiar from quench from melt simulations, appears to be ameliorated. Next, we show that Reverse Monte Carlo (RMC) may be used to produce chemically and topologically realistic models \textit{if} appropriate constraints are employed in addition to forcing models to agree with diffraction measurements. The method has a high degree of flexibility that enables us to include information from Fluctuation Electron Microscopy (FEM) experiments, which have recently shown that some samples of a-Si:H exhibit medium range order. We have successfully fit diffraction and FEM data and thus formed models reproducing the experimental MRO. It seems unlikely that melt quench or continuous random network models would reproduce such complex ordering. By adopting an information theoretic viewpoint, we have merged RMC with first principles simulation to build models which simultaneously reproduce experimental data \textit{and} are a minimum energy configuration of a density functional prescription for interatomic interactions. The method is applied to a 648 atom model of g-GeSe$_{2}$ and produces an essentially perfect neutron static structure factor (including first sharp diffraction peak), electronic density of states and vibrational spectrum. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B15.00003: Structural transitions in vitreous B2O3 under pressure Liping Huang, John Kieffer We have studied the nature of structural transitions in B2O3 glass under pressure using molecular dynamics (MD) simulations, based on a coordination-dependent charge transfer potential. This potential model allows the charges on atoms to re-distribute upon the formation and rupture of chemical bonds, and accommodates multiple coordination states during the course of simulations. Our MD simulations show that the boroxol rings in vitreous B2O3 break up when subject to pressure, which explains the disappearance of the breathing mode in the experimental Raman spectrum of B2O3 glass under pressure. Excellent agreement is found between simulations and Brillouin scattering results in terms of the sequence of polyamorphic transitions during compression-decompression circle and the amount of permanent densification in vitreous B2O3 after pressure release. The polyamorphic transitions in three-coordinated B2O3 glass involve transitory four-coordinated boron atoms at high pressures, while coordination change is not necessary for compaction of four-coordinated silica glass. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B15.00004: Dynamic Light Scattering in Strong Glass-forming Liquids David Sidebottom Analysis is presented of photon correlation spectroscopy (PCS) measurements in several glass-forming liquids of varying fragility including alkali-modified boron trioxide. The measured dynamic structure factor of these liquids exhibits a two step relaxation that indicates a bimodal pattern in the distribution of relaxation times corresponding to regions of the fluid that are fast and slow, respectively. In a carefully calibrated experiment, the amplitude of the slow decay in the structure factor obtained by PCS is attributed to the volume fraction of slowly relaxing material in the liquid. A correlation between this amplitude and the fragility of the glass-forming liquid is demonstrated. This correlation then suggests that the distinction between strong and fragile behavior is directly related to volume fraction of slow material in the liquid; a quantity that should exhibit systematic trends with respect to network polymerization. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B15.00005: Elastic phase diagrams of ternary Ge-P-S bulk glasses U. Vempati, P. Boolchand Elastic phases of network glasses can be identified from a measurement of the non-reversing enthalpy ($\Delta $H$_{nr})$ near T$_{g}$ in T-modulated DSC. Glasses at low mean coordination number \textbf{\textit{r}} $<$ \textbf{\textit{r}}$_{c}$(1) possess a narrow $\Delta $H$_{nr}$(T) term that generally increases by an order of magnitude upon aging, which is characteristic of mechanically \textit{floppy phases}. Glasses in a \textbf{\textit{r}}$_{c}$(1) $<$\textbf{\textit{ r }}$<$ \textbf{\textit{r}}$_{c}$(2) range possess a $\Delta $H$_{nr}$ term that is minuscule and does not age, which is characteristic of \textit{intermediate} or \textit{self-organized phases }(IP)$.$ And glasses at high \textbf{\textit{r}} $>$ \textbf{\textit{r}}$_{c}$(2) possess a broad and asymmetric $\Delta $H$_{nr}$(T) term that ages, which is characteristic of \textit{stressed rigid phases}. Raman scattering and MDSC measurements on ternary Ge$_{x}$P$_{x}$S$_{1-2x}$ glasses have now been performed$^{1}$ and show \textbf{\textit{r}}$_{c}$(1) = 2.270 and \textbf{\textit{r}}$_{c}$(2) = 2.405, yielding an IP width ($\Delta $\textbf{\textit{r}}) of 0.135. Here \textbf{\textit{r}} = 2 +3x. In corresponding selenide glasses the IP width is found$^{2}$ at 0.210. The reduced width of the IP in sulfide glasses is attributed to S$_{8}$, P$_{4}$S$_{7}$ and P$_{4}$S$_{10}$ molecules demixing. By combining the present results with those on binary Ge-S and P-S glasses, elastic phase diagrams of ternary Ge-P-S glasses have now been constructed, and provide a global view of the three elastic phases. \newline 1. U. Vempati and P. Boolchand, J. Phys.: Cond. Matter, \textbf{16} S5121 (2004) \newline 2. S. Chakravarty et al. J. Phys.: Cond. Matter (in press) \newline Supported by NSF grant DMR 04-56472 [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B15.00006: Raman pressure effects and internal stress in network glasses Fei Wang, S. Mamedov, P. Boolchand, B. Goodman, Meera Chandrasekhar Intermediate phases are predicted to be unstressed elastic phases of network glasses. The case of binary Ge$_{x}$Se$_{1-x}$ glasses reveal the intermediate phase to reside in the 0.20 $<$ x $<$ 0.25 range$^{1}$. We have now performed$^{2}$ Raman scattering on Ge$_{x}$Se$_{1-x}$ glasses under pressure and find a steady increase in the frequency of modes of corner-sharing GeSe$_{4}$ tetrahedra when the external pressure P exceeds a threshold value P$_{c}$. The threshold pressure P$_{c}$(x) decreases with x to nearly zero for 0.20 $<$ x $<$ 0.25, then increases up to x = 1/3. P$_{c}$ indicates the presence of local stress at the Raman active units; so its vanishing suggests that these units are part of an isostatically rigid backbone. Isostaticity also accounts for the non-aging behavior of glasses observed in this same composition range$^{2}$ that is identified with the intermediate phase in this binary glass system. \newline \newline 1. P.Boolchand et al. JNCS \textbf{293-295}, 348 (2001). \newline 2. Fei Wang et al. Cond Matt 0408502 (to appear in Phys. Rev. B) \newline \newline Supported by NSF grant DMR-0456472 \newline [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B15.00007: The self-organization window in (Na$_{2}$O)$_{x}$(B$_{2}$O$_{3}$)$_{1-x}$ glasses Vamsikrishna Rompicharla, T. Qu, P. Boolchand, W. Huff T-modulated DSC measurements on dry sodium borate glasses show T$_{g}$(x) to display a global maximum near x $\sim \quad \raise.5ex\hbox {$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $ , and the non-reversing enthalpy ($\Delta $H$_{nr}$(x)) to show a square- well like deep and flat global minimum ($\sim $0) in the 0.27 $<$ x $<$ 0.36 range-the \textit{reversibility window}. In analogy to the work on chalcogenide glasses, we identify borate glasses at x $<$ 0.27 to be stressed-rigid, those in the \textit {reversibility window} to be intermediate and those at x $>$ 0.36 to be floppy. The \textit{reversibility window} correlates well with a maximum in packing fraction$^{1}$ deduced from mass density results and with a maximum in concentration of isostatically rigid diborate structural groupings in NMR$^{2}$. Optical transmission of polished platelets examined with a polarizing microscope show \textit{window compositions} to be optically transparent and clear, while those outside the window at x $<$ 0.27 and at x $>$ 0.36 to show dark spots of increasing density as one goes away from the window compositions. The spotting reflects birefringence due to stress accumulation. These results confirm that intermediate phases occur in oxide- as in chalcogenide- glasses and form space filling and stress-free or self-organized networks. \newline \newline 1. Steve Feller (private communication). \newline 2. G.E.Gellison and P.J.Bray, JNCS 29, 187(1978). \newline Supported by NSF grant DMR 04-56472. \newline [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B15.00008: Molecular structure of (AgPO3)$_{1-x}$ (AgI)$_{x}$ glasses D. Novita, U. Vempati, P. Boolchand Melt-quenched AgPO$_{3}$ glasses were synthesized by dry ( Ag$_{3}$PO$_{4}$ + P$_{2}$O$_{5}$, prep. 1) and wet (NH$_{4}$H$_{2}$PO$_{4}$ + AgNO$_{3}$, prep. 2) routes. Glass transitions were examined in MDSC at a scan rate of 3$^{o}$C/min. Prep. 1 samples display \textit{bimodal} glass transition temperatures, with T$_{g}^{low}$ = 220$^{o}$C and T$_{g}^{high}$ = 238$^{o}$C and with the T$_{g}^{low}$ endotherm higher in strength than the T$_{g}^{high}$ one. In contrast, prep. 2 samples show a single T$_{g}$ = 203$^{o}$C that is significantly lower in temperature. These results are consistent with the notion that prep. 2 probably yields samples with bonded water while prep 1 gives pure AgPO$_{3}$ glasses that are intrinsically \textit{phase separated}. The nature of the two phases in the latter is less obvious at present, but we note that upon alloying AgI, the additive selectively bonds in the T$_{g}^{low}$ phase at low x ($<$0.20) with T$_{g}^{low}$ steadily decreasing, and with the T$_{g}^{high}$ phase remaining largely unaffected. At higher x ($>$0.20) a major structural reorganization occurs, and we observe the opening of a \textit{reversibility window} in the 0.22 $<$ x $<$ 0.37 range. As in the chalcogenides, we identify the \textit{window }with the intermediate phase with glasses at x $<$ 0.20 stressed-rigid, while those at x $>$ 0.37 as floppy. A percolation threshold for electrical conduction occurs$^{1}$ near x $\sim $ 0.3 and falls in the reversibility window as expected. \newline \newline 1. M. Mangion and G.P. Johari, Phys. Rev. \underline {B36}, 8845 (1987) \newline Supported by NSF grant DMR 04-56472 [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B15.00009: Optical Properties and ESR defects of amorphous GeTe, Sb$_2$Te$_3$ and Ge$_2$Sb$_2$Te$_5$ Tong Ju, Jared Olson, Janica Whitaker, John Viner, Heng Li, P. Craig Taylor Amorphous films, which are based on mixtures of GeTe and Sb$_{2}$Te$_{3 }$,are used in re-writeable, digital versatile discs(DVD+RW). The most commonly used composition is Ge$_{2}$Sb$_{2}$Te$_{5}$. We use electron spin resonance (ESR) and photothermal deflection spectroscopy(PDS) experiments to characterize the amorphous films of the three compositions GeTe, Sb$_{2}$Te$_{3}$ and Ge$_{2}$Sb$_{2}$Te$_{5}$. In Ge$_{2}$Sb$_{2}$Te$_{5}$ thin films with large concentrations of oxygen impurities, an ESR signal exists without any optical excitation. The signal is probably due to a singly or three-fold coordinated Te. The spin density from this center is around 2$\times $10$^{19}$ cm$^{-3}$. We also observe a sharp feature due to E$'$ centers (three-fold-coordinated Si bonded to 3 oxygen atoms) at the interface between the SiO$_{2 }$substrate and the sample. The signal related to Te is greatly reduced in the films made with lower concentrations of oxygen impurities. In these films we observe an additional ESR signal that appears in GeTe, Ge$_{2}$Sb$_{2}$Te$_{5, }$and Sb$_{2}$Te$_{3}$. Since this signal scales with the number of surfaces, we attribute it to a defect in an oxide of Te on the surface. PDS measurements show that the optical band gaps depend on the oxygen concentrations. When the growth rates are slower, the sample contains higher oxygen concentrations, and the optical band gaps are also higher .The optical band gap is around 0.9 eV in films of Ge$_{2}$Sb$_{2}$Te$_{5 }$containing approximately 10$^{19}$ cm$^{-3}$oxygen impurities. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B15.00010: Photon echo Measurement of Optical Decoherence in Er$^{3+}$-doped Silicate Fiber Roger M. Macfarlane, Yongchen Sun, Peter B. Sellin, Rufus L. Cone The dephasing time T$_2$ ($\Gamma_h = 1/\pi T_2$ is the homogeneous linewidth) of the $^4$I$_{15/2}$ -- $^4$I$_{13/2}$ transition of Er$^{3+}$ in a silicate optical fiber was measured by two-pulse photon echoes vs. external magnetic field and temperature. A field of 2~T reduces the homogeneous linewidth by 1.8 MHz from its value of 3.2 MHz at zero field, an anomalously large reduction compared to that in oxide crystals with similar Er$^{3+}$ concentration. We propose that the dephasing is caused by two classes of low frequency tunneling modes: elastic ``two-level-systems'' (TLS) responsible for 1.4~MHz and coupled spin-elastic TLS modes for 1.8 MHz. The coupled modes acquire a magnetic character from an elastic Er$^{3+}$ spin-TLS interaction. The temperature dependence of the homogeneous linewidth is linear in the measured range of 1.4 to 4~K. Three- pulse photon echo decays measured from 0.5 $\mu$s to 500 $\mu$s and at a field of 5~T characterized spectral diffusion caused by the distribution of TLS tunneling rates. Our results suggest a potential for application of doped communication fibers in frequency selective optical processing, buffer memories and spatial-spectral holographic devices. [Preview Abstract] |
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