Search Results

Advanced search parameters have been applied.
open access

Research on micro-sized acoustic bandgap structures.

Description: Phononic crystals (or acoustic crystals) are the acoustic wave analogue of photonic crystals. Here a periodic array of scattering inclusions located in a homogeneous host material forbids certain ranges of acoustic frequencies from existence within the crystal, thus creating what are known as acoustic (or phononic) bandgaps. The vast majority of phononic crystal devices reported prior to this LDRD were constructed by hand assembling scattering inclusions in a lossy viscoelastic medium, predominantly air, water or epoxy, resulting in large structures limited to frequencies below 1 MHz. Under this LDRD, phononic crystals and devices were scaled to very (VHF: 30-300 MHz) and ultra (UHF: 300-3000 MHz) high frequencies utilizing finite difference time domain (FDTD) modeling, microfabrication and micromachining technologies. This LDRD developed key breakthroughs in the areas of micro-phononic crystals including physical origins of phononic crystals, advanced FDTD modeling and design techniques, material considerations, microfabrication processes, characterization methods and device structures. Micro-phononic crystal devices realized in low-loss solid materials were emphasized in this work due to their potential applications in radio frequency communications and acoustic imaging for medical ultrasound and nondestructive testing. The results of the advanced modeling, fabrication and integrated transducer designs were that this LDRD produced the 1st measured phononic crystals and phononic crystal devices (waveguides) operating in the VHF (67 MHz) and UHF (937 MHz) frequency bands and established Sandia as a world leader in the area of micro-phononic crystals.
Date: January 1, 2010
Creator: Fleming, James Grant; McCormick, Frederick Bossert; Su, Mehmet F.; El-Kady, Ihab Fathy; Olsson, Roy H., III & Tuck, Melanie R.
Partner: UNT Libraries Government Documents Department
open access

Effective dielectric constants of photonic crystal of aligned anisotropic cylinders and the optical response of a periodic array of carbon nanotubes

Description: This article discusses effective dielectric constants of photonic crystal of aligned anisotropic cylanders and the optical response of a periodic array of carbon nanotubes.
Date: October 25, 2005
Creator: Reyes, E.; Krokhin, Arkadii A. & Roberts, James A.
Partner: UNT College of Arts and Sciences
open access

Survey of the Solid State Conformation of Calix[4]arenes

Description: The characteristics of seventy-six calix[4]arene crystal structures derived from the Cambridge Crystallographic Database are presented. This survey is a discussion of the inter and intramolecular effects on the solid state cavity shape and molecular recognition ability of the compounds. In addition to this survey, four new calix[4]arene crystal structures are presented. The conformational characteristics of these four calixarenes are determined by a complicated array of inter and intramolecular interactions in the crystal packing.
Date: August 1997
Creator: Obrey, Stephen J. (Stephen James)
Partner: UNT Libraries
open access

Computer Simulations of Mechanical Behavior of Polymer Liquid Crystals

Description: In this dissertation molecular dynamics simulations of behavior of polymer liquid crystals (PLC's) under tensile deformation have been performed. PLC's composed of random or block copolymers of rigid and flexible segments have been studies. Systems of fully flexible chains have been simulated for comparison. Stress-strain relations and fracture mechanics have been investigated.
Date: December 1991
Creator: Blonski, Slawomir
Partner: UNT Libraries
open access

Calibrating Curved Crystals Used for Plasma Spectroscopy

Description: The throughput and resolving power of an X-ray spectrometer that uses a curved crystal as the diffraction element is determined primarily by the crystal X-ray reflectivity properties. This poster presents a measurement technique for these crystal parameters using a simple diode source to produce a narrow spectral band. The results from measurements on concave elliptical polyethylene terephthalate (PET) crystals and convex potassium acid phthalate (KAP) crystals show large variations in the key parameters compared to those from the flat crystal.
Date: October 29, 2012
Creator: Haugh, M. J., Jacoby, K. D., Ross, P. W., Rochau, G. Wu, M., Regan, S. P., Barrios, M. A.
Partner: UNT Libraries Government Documents Department
open access

Broadband light-matter interaction due to resonance cavities in graded photonic super-crystals

Description: This article reports the formation of resonance cavities within graded photonic super-crystals (GPSCs) with unit cells formed via a near-uniform central region with eight side graded regions.
Date: August 29, 2019
Creator: Lin, Yuankun; Hassan, Safaa; Sale, Oliver; Alnasser, Khadijah; Hurley, Noah; Zhang, Hualiang et al.
Partner: University of North Texas
open access

Large-amplitude motion in polymer crystals and mesophases

Description: Large-amplitude motion of macromolecules involves mainly rotation about bonds (conformational motion). In the liquid phases, the large- amplitude motion is coupled with disorder and accounts for the flow and viscoelastic behavior. Perfectly ordered crystals, in contrast, permit only little large-amplitude motion. The mesophases are intermediate in order and mobility. In crystals, large-amplitude motion leads initially to gauche defects and kinks (conformational defects), and ultimately may produce conformationally disordered crystals (conis crystals). Molecular dynamics simulations of crystals with up to 30,000 atoms have been carried out and show the mechanism of defect formation, permit the study of the distribution of defects, and the visualization of hexagonal crystals. Distinction between main-chain liquid-crystalline macromolecules and condis crystals, the two mesophases of polymers, can be done on basis of analysis of phase separation (partial crystallinity), present in condis crystals and not in liquid crystals. Solid state NMR is the tool of choice for detecting mobile and rigid phases. In highly drawn fibers one can find four different states of order and mobility. Besides the (defect) crystalline phase and the isotropic amorphous phase, an intermediate oriented phase and a rigid amorphous phase exists.
Date: December 31, 1994
Creator: Wunderlich, B.
Partner: UNT Libraries Government Documents Department
open access

Discontinuous Thermal Expansions and Phase Transformations in Crystals at Higher Temperatures

Description: The purpose of this investigation is to make more detailed studies of transformations. Fourteen compounds have been examined by high temperature X-ray diffraction for this purpose. The investigations have been carried out in such a way as to reveal: 1. the existence of transformations, 2. the influence of polarizability on thermal expansion, 3. the anisotropy of expansion, and 4. the discontinuity of thermal expansion.
Date: 1967
Creator: Hsu, Yuan Tsun
Partner: UNT Libraries
open access

Generation of over 1000 Diffraction Spots from 2D Graded Photonic Super-Crystals

Description: Discusses a project in which researchers were able to generate over 1000 diffraction spots from a graded photonic super-crystal with a unit super-cell size of 12a × 12a where a is the lattice constant and hole radii are gradually changed in dual directions.
Date: April 10, 2020
Creator: Hassan, Safaa; Jiang, Yan; Alnasser, Khadijah; Hurley, Noah; Zhang, Hualiang; Philipose, Usha et al.
Partner: University of North Texas
open access

A Classical Theory of the Dielectric Susceptibility of Anharmonic Crystals

Description: An expression for the dielectric susceptibility tensor of a cubic ionic crystal has been derived using the classical Liouville operator. The effect of cubic anharmonic forces is included as a perturbation on the harmonic crystal solution, and a series expansion for the dielectric susceptibility is developed. The most important terms in the series are identified and summed, yielding an expression for the complex susceptibility with an anharmonic contribution which is linearly dependent on temperature. A numerical example shows that both the real and imaginary parts of the susceptibility are continuous, finite functions of frequency.
Date: May 1976
Creator: Kennedy, Howard V.
Partner: UNT Libraries
open access

Highly Efficient Single Frequency Blue Laser Generation by Second Harmonic Generation of Infrared Lasers Using Quasi Phase Matching in Periodically Poled Ferroelectric Crystals

Description: Performance and reliability of solid state laser diodes in the IR region exceeds those in the visible and UV part of the light spectrum. Single frequency visible and UV laser diodes with higher than 500 mW power are not available commercially. However we successfully stabilized a multi-longitudinal mode IR laser to 860 mW single frequency. This means high efficiency harmonic generation using this laser can produce visible and UV laser light not available otherwise. In this study we examined three major leading nonlinear crystals: PPMgO:SLN, PPKTP and PPMgO:SLT to generate blue light by second harmonic generation. We achieved record high net conversion efficiencies 81.3% using PPMgO:SLT (~500 mW out), and 81.1% using PPKTP (~700 mW out). In both these cases an external resonance buildup cavity was used. We also studied a less complicated single pass waveguide configuration (guided waist size of ~ 5 um compared to ~60 um) to generate blue. With PPMgO:SLN we obtained net 40.4% and using PPKT net 6.8% (110mW and 10.1 mW respectively).
Date: August 2014
Creator: Khademian, Ali
Partner: UNT Libraries
open access

Photonic Band Gaps and Resonance Modes in 2D Twisted Moiré Photonic Crystal

Description: Article studying 2D twisted moiré photonic crystals without physical rotation and simulating their photonic band gaps in photonic crystals formed at different twisted angles, different gradient levels, and different dielectric filling factors.
Date: September 23, 2021
Creator: Alnasser, Khadijah; Kamau, Steve; Hurley, Noah; Cui, Jingbiao & Lin, Yuankun
Partner: University of North Texas

[Celestial crystal - Salar stone]

Description: Photograph of a circular red crystal set against a black background. The crystal has a multitude of colors within it, and the image was signed by Junebug Clark along the lower rim.
Date: [1970..1989]
Creator: Clark, Joseph Wade Junebug & Clark, Joe
Partner: UNT Libraries Special Collections
open access

Mechanical Behavior of Crystalline Solids: Proceedings of a Symposium, April 28-29, 1962

Description: From Introduction: "These first three chapters from a background of introduction and general ideas about dislocations. The next two chapters are concerned specifically with properties of ceramic materials, with emphasis on single crystals."
Date: March 25, 1963
Creator: United States. Bureau of Standards.
Partner: UNT Libraries Government Documents Department
open access

Magneto-Optical and Chaotic Electrical Properties of n-InSb

Description: This thesis investigation concerns the optical and nonlinear electrical properties of n-InSb. Two specific areas have been studied. First is the magneto-optical study of magneto-donors, and second is the nonlinear dynamic study of nonlinear and chaotic oscillations in InSb. The magneto-optical study of InSb provides a physical picture of the magneto-donor levels, which has an important impact on the physical model of nonlinear and chaotic oscillations. Thus, the subjects discussed in this thesis connect the discipline of semiconductor physics with the field of nonlinear dynamics.
Date: December 1991
Creator: Song, Xiang-Ning
Partner: UNT Libraries
open access

Fabrication and Study of the Optical Properties of 3D Photonic Crystals and 2D Graded Photonic Super-Crystals

Description: In this dissertation, I am presenting my research on the fabrication and simulation of the optical properties of 3D photonic crystals and 2D graded photonic super-crystals. The 3D photonic crystals were fabricated using holographic lithography with a single, custom-built reflective optical element (ROE) and single exposure from a visible light laser. Fully 3D photonic crystals with 4-fold, 5- fold, and 6-fold symmetries were fabricated using the flexible, 3D printed ROE. In addition, novel 2D graded photonic super-crystals were fabricated using a spatial light modulator (SLM) in a 4f setup for pixel-by-pixel phase engineering. The SLM was used to control the phase and intensity of sets of beams to fabricate the 2D photonic crystals in a single exposure. The 2D photonic crystals integrate super-cell periodicities with 4-fold, 5-fold, and 6-fold symmetries and a graded fill fraction. The simulations of the 2D graded photonic super-crystals show extraordinary properties such as full photonic band gaps and cavity modes with Q-factors of ~106. This research could help in the development of organic light emitting diodes, high-efficiency solar cells, and other devices.
Date: December 2018
Creator: Lowell, David
Partner: UNT Libraries
open access

Gamma Ray Efficiencies for Well Type Scintillation Crystals

Description: From Abstract : "Gamma ray efficiencies for various sized well type NaI(Ti) crystals have been calculated using an IBM 704 digital computer. The adjustable parameters in the program are the gamma ray energy, the heights and diameters of the crystal and the well, the thickness of the well lining, and the positions of the (point) source inside the well. Some typical results are presented in graphical form."
Date: February 8, 1963
Creator: Dingus, Ronald S. & Stewart, M. G.
Partner: UNT Libraries Government Documents Department
Back to Top of Screen