Kenneth L. Marshall

Electrical modulation of silicon-based two-dimensional photonic bandgap structures

Electrically tunable photonic band gap (PBG) structures hold the potential to become a versatile and compact backbone for optical signal processing. In this letter we report electrical tuning of silicon-based two-dimensional PBG structures infiltrated with liquid crystals. An improved electrode configuration is used to avoid electric field screening by the conductive silicon walls. Electrical tuning using fields well below 1V∕μm is demonstrated experimentally using both polarized light microscopy and reflectance PBG measurements. The structures can be operated with any electro-optic materials and lead to fast and efficient modulators, routers, and tunable filters.

Doping of organic semiconductors induced by lithium fluoride/aluminum electrodes studied by electron spin resonance and infrared reflection-absorption spectroscopy

We report our investigations on the chemical doping mechanisms induced by LiF|Al electrodes evaporated onto fullerene thin films. Electron spin resonance (ESR) and infrared reflection-absorption spectroscopy (IRRAS) are utilized to characterize C60|Al and C60|LiF|Al interfaces. ESR spectra show that deposition of LiF followed by Al generates C60 radical anions and also the presence of an additional paramagnetic species of lower concentration that is present in all C60 films regardless of LiF. IRRAS clarifies the mechanism occurring at the C60|LiF|Al interface, showing that interaction between LiF and C60 followed by deposition of Al causes LiF clusters to chemically dissociate.

NOVEL GLASS-FORMING LIQUID CRYSTALS. IV. EFFECTS OF CENTRAL CORE AND PENDANT GROUP ON VITRIFICATION AND MORPHOLOGICAL STABILITY

Abstract To unravel the effects of the volume-excluding central core and the mesogenic pendant group on both the glass-forming ability and morphological stability of the thermally quenched glass, nine model compounds were synthesized that contain various nematogenic and cholesteryl pendant groups. The glass-forming ability of the melt and morphological stability of the thermally quenched glass were assessed using the DSC, XRD, and hot-stage POM techniques. With cyanobiphenyl as the pendant group, the following descending order in morphological stability against thermally activated recrystallization was established: trans-cyclohexane < all-exo-bicyclo [2.2.2] oct-7-ene < cubane < cis-cyclohexane < benzene. While the cyclohexane compound containing three cyanoterphenyl groups showed a strong tendency to crystallize upon quenching, the chiral nematic system in which one of the cyanoterphenyl groups is substituted by a cholesteryl group showed superior glass-forming ability and morphological stability. Additi...

Room temperature source of single photons of definite polarization

A definite polarization in fluorescence from single emitters (dye molecules) at room temperature is demonstrated. A planar-aligned, nematic liquid-crystal host provides definite alignment of single dye molecules in a preferred direction. Well-defined polarized fluorescence from single emitters (single photon source) is important for applications in photonic quantum information. Polarized single-photon sources based on single emitters, for example, are key hardware elements both for absolutely secure quantum communication and quantum computation systems.

Electric-field-induced motion of polymer cholesteric liquid-crystal flakes in a moderately conductive fluid.

Polymer cholesteric liquid-crystal flakes suspended in a fluid with nonnegligible conductivity can exhibit motion in the presence of an ac electric field. The plateletlike particles with a Grandjean texture initially lie parallel to the cell substrates and exhibit a strong selective reflection that is diminished or extinguished as the flakes move. Flake motion was seen within a specific frequency bandwidth in an electric field as low as 5 mV(rms)/microm. Flakes reoriented about their longest axis to align parallel to theapplied field, and a rise time of 560 ms was seen in a 40-mV(rms)/microm field at 100 Hz.

High-damage-threshold static laser beam shaping using optically patterned liquid-crystal devices

Beam shaping of coherent laser beams is demonstrated using liquid crystal (LC) cells with optically patterned pixels. The twist angle of a nematic LC is locally set to either 0 or 90° by an alignment layer prepared via exposure to polarized UV light. The two distinct pixel types induce either no polarization rotation or a 90° polarization rotation, respectively, on a linearly polarized optical field. An LC device placed between polarizers functions as a binary transmission beam shaper with a highly improved damage threshold compared to metal beam shapers. Using a coumarin-based photoalignment layer, various devices have been fabricated and tested, with a measured single-shot nanosecond damage threshold higher than 30 J/cm2.

Improving the performance of high-laser-damage-threshold, multilayer dielectric pulse-compression gratings through low-temperature chemical cleaning

A low-temperature chemical cleaning approach has been developed to improve the performance of multilayer dielectric pulse-compressor gratings for use in the OMEGA EP laser system. X-ray photoelectron spectroscopy results guided the selection of targeted cleaning steps to strip specific families of manufacturing residues without damaging the gratings fragile 3D profile. Grating coupons that were cleaned using the optimized method consistently met OMEGA EP requirements on diffraction efficiency and 1054 nm laser-damage resistance at 10 ps. The disappearance of laser-conditioning effects for the highest-damage-threshold samples suggests a transition from a contamination-driven laser-damage mechanism to defect-driven damage for well-cleaned components.

Transition Metal Dithiolene Near-IR Dyes and Their Applications in Liquid Crystal Devices

Numerous commercial and military applications exist for guest–host liquid crystal (LC) devices operating in the near- to mid-IR region. Progress in this area has been hindered by the severe lack of near-IR dyes with good solubility in the LC host, low impact on the inherent order of the LC phase, good thermal and chemical stability, and a large absorbance maximum tunable by structural modification over a broad range of the near-IR region. Transition metal complexes based on nickel, palladium, or platinum dithiolene cores show substantial promise in meeting these requirements. These new dye complexes are extraordinarily stable, possess liquid crystalline phases in their own right with the proper terminal functional groups, and can have melting points below room temperature. The latter property is especially significant for producing liquid crystal/dye mixtures with both high dye concentration and good resistance to phase separation. Because transition metal dithiolenes are zerovalent, they can exhibit high solubility in LC hosts (up to 10 wt%). The λmax in these materials can range from 600 to 1500 nm, depending on structure. Depending on their overall molecular geometry and the choice of terminal functional groups, transition metal dithiolenes that show either positive or negative dichroism from the same basic core structure can be readily synthesized. When enantiomerically enriched terminal substituents are employed, nickel dithiolenes can induce a chiral mesophase in a nonchiral nematic host. This finding opens the possibility of generating novel LC mixtures with two degrees of tunability—an electronic absorbance band tunable by synthesis, and a selective reflection band tunable by temperature or applied electric field. In this paper, we overview our past and present activities in the design and synthesis of transition metal dithiolene dyes, show some specific applications examples for these materials as near-IR dyes in LC electro-optical devices, and present our most recent results in the computational modeling of physical and optical properties of this interesting class of organometallic optical materials.

Engineering architecture of the neutron Time-of-Flight (nToF) diagnostic suite at the National Ignition Facility

This paper describes the engineering architecture and function of the neutron Time-of-Flight (nToF) diagnostic suite installed on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). These instruments provide key measures of neutron yield, ion temperature, drift velocity, neutron bang-time, and neutron downscatter ratio. Currently, there are five nToFs on three collimated lines-of-site (LOS) from 18m to 27m from Target Chamber Center, and three positioned 4.5m from TCC, within the NIF Target Chamber but outside the vacuum and confinement boundary by use of re-entrant wells on three other LOS. NIF nToFs measure the time history and equivalent energy spectrum of reaction generated neutrons from a NIF experiment. Neutrons are transduced to electrical signals, which are then carried either by coaxial or Mach-Zehnder transmission systems that feed divider assemblies and fiducially timed digitizing oscilloscopes outside the NIF Target Bay (TB) radiation shield wall. One method of transduction employs a two-stage process wherein a neutron is converted to scintillation photons in hydrogen doped plastic (20x40mm) or bibenzyl crystals (280x1050mm), which are subsequently converted to an electrical signal via a photomultiplier tube or a photo-diode. An alternative approach uses a single-stage conversion of neutrons-to-electrons by use of a thin (0.25 to 2 mm) Chemical Vapor Deposition Diamond (CVDD) disc (2 to 24mm radius) under high voltage bias. In comparison to the scintillator method, CVDDs have fast rise and decay times (

Manufacture and development of multilayer diffraction gratings

The OMEGA EP Facility includes two high-energy, short-pulse laser beams that will be focused to high intensity in the OMEGA target chamber, providing backlighting of compressed fusion targets and investigating the fast-ignition concept. To produce 2.6-kJ output energy per beam, developments in grating compressor technology are required. Gold-coated diffraction gratings limit on-target energy because of their low damage fluence. Multilayer dielectric (MLD) gratings have shown promise as high-damage-threshold, high-efficiency diffraction gratings suitable for use in high-energy chirped-pulse amplification [ B. W. Shore et al., J. Opt. Soc. Am. A 14, 1124 (1997).] Binary 100-mm-diam MLD gratings have been produced at the Laboratory for Laser Energetics (LLE) using large-aperture, holographic exposure and reactive ion-beam etching systems. A diffraction efficiency of greater than 99.5% at 1053 nm has been achieved for gratings with 1740 grooves/mm, with a 1:1 damage threshold of 5.49 J/cm2 diffracted beam fluence at 10 ps. To demonstrate the ability to scale up to larger substrates, several 100-mm substrates have been distributed over an aperture of 47 × 43 cm and successfully etched, resulting in high efficiency over the full aperture. This paper details the manufacture and development of these gratings, including the specifics of the MLD coating, holographic lithography, reactive ion etching, reactive ion-beam cleaning, and wet chemical cleaning.