Thomas L. Reinecke

Properties of Fluorinated Graphene Films

Graphene films grown on Cu foils have been fluorinated with xenon difluoride (XeF(2)) gas on one or both sides. When exposed on one side the F coverage saturates at 25% (C(4)F), which is optically transparent, over 6 orders of magnitude more resistive than graphene, and readily patterned. Density functional calculations for varying coverages indicate that a C(4)F configuration is lowest in energy and that the calculated band gap increases with increasing coverage, becoming 2.93 eV for one C(4)F configuration. During defluorination, we find hydrazine treatment effectively removes fluorine while retaining graphenes carbon skeleton. The same films may be fluorinated on both sides by transferring graphene to a silicon-on-insulator substrate enabling XeF(2) gas to etch the Si underlayer and fluorinate the backside of the graphene film to form perfluorographane (CF) for which calculated the band gap is 3.07 eV. Our results indicate single-side fluorination provides the necessary electronic and optical changes to be practical for graphene device applications.

Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons

Abstract The excitation of surface-phonon-polariton (SPhP) modes in polar dielectric crystals and the associated new developments in the field of SPhPs are reviewed. The emphasis of this work is on providing an understanding of the general phenomenon, including the origin of the Reststrahlen band, the role that optical phonons in polar dielectric lattices play in supporting sub-diffraction-limited modes and how the relatively long optical phonon lifetimes can lead to the low optical losses observed within these materials. Based on this overview, the achievements attained to date and the potential technological advantages of these materials are discussed for localized modes in nanostructures, propagating modes on surfaces and in waveguides and novel metamaterial designs, with the goal of realizing low-loss nanophotonics and metamaterials in the mid-infrared to terahertz spectral ranges.

Chemical Gradients on Graphene To Drive Droplet Motion

This work demonstrates the production of a well-controlled, chemical gradient on the surface of graphene. By inducing a gradient of oxygen functional groups, drops of water and dimethyl-methylphosphonate (a nerve agent simulant) are pulled in the direction of increasing oxygen content, while fluorine gradients push the droplet motion in the direction of decreasing fluorine content. The direction of motion is broadly attributed to increasing/decreasing hydrophilicity, which is correlated to high/low adhesion and binding energy. Such tunability in surface chemistry provides additional capabilities in device design for applications ranging from microfluidics to chemical sensing.

Atomic-scale photonic hybrids for mid-infrared and terahertz nanophotonics

The field of nanophotonics focuses on the ability to confine light to nanoscale dimensions, typically much smaller than the wavelength of light. The goal is to develop light-based technologies that are impossible with traditional optics. Subdiffractional confinement can be achieved using either surface plasmon polaritons (SPPs) or surface phonon polaritons (SPhPs). SPPs can provide a gate-tunable, broad-bandwidth response, but suffer from high optical losses; whereas SPhPs offer a relatively low-loss, crystal-dependent optical response, but only over a narrow spectral range, with limited opportunities for active tunability. Here, motivated by the recent results from monolayer graphene and multilayer hexagonal boron nitride heterostructures, we discuss the potential of electromagnetic hybrids--materials incorporating mixtures of SPPs and SPhPs--for overcoming the limitations of the individual polaritons. Furthermore, we also propose a new type of atomic-scale hybrid--the crystalline hybrid--where mixtures of two or more atomic-scale (∼3 nm or less) polar dielectric materials lead to the creation of a new material resulting from hybridized optic phonon behaviour of the constituents, potentially allowing direct control over the dielectric function. These atomic-scale hybrids expand the toolkit of materials for mid-infrared to terahertz nanophotonics and could enable the creation of novel actively tunable, yet low-loss optics at the nanoscale.

Quantum control of a spin qubit coupled to a photonic crystal cavity

Using a long-lived quantum-dot spin qubit coupled to a GaAs-based photonic crystal cavity, researchers demonstrate complete quantum control of an electron spin qubit. By cleverly controlling the charge state of the InAs quantum dot using laser pulses, optical initialization, control and readout of an electron spin are achieved.

Engineering Graphene Mechanical Systems

We report a method to introduce direct bonding between graphene platelets that enables the transformation of a multilayer chemically modified graphene (CMG) film from a paper mache-like structure into a stiff, high strength material. On the basis of chemical/defect manipulation and recrystallization, this technique allows wide-range engineering of mechanical properties (stiffness, strength, density, and built-in stress) in ultrathin CMG films. A dramatic increase in the Youngs modulus (up to 800 GPa) and enhanced strength (sustainable stress ≥1 GPa) due to cross-linking, in combination with high tensile stress, produced high-performance (quality factor of 31,000 at room temperature) radio frequency nanomechanical resonators. The ability to fine-tune intraplatelet mechanical properties through chemical modification and to locally activate direct carbon-carbon bonding within carbon-based nanomaterials will transform these systems into true materials-by-design for nanomechanics.

Van der Waals screening by single-layer graphene and molybdenum disulfide

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

Ultralow-loss polaritons in isotopically pure boron nitride

Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called flat optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength. Although high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of PhP-based nanophotonic devices.Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called flat optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength1,2,3,4,5,6,7,8. Although high optical losses still plague many approaches9, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes10,11,12,13 in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of PhP-based nanophotonic devices.

Disorder in ferroelectrics

Abstract A model of a displacive ferroelectric in which the order parameter phonon is coupled to modes corresponding to local disorder is considered, and it is shown that the coupling can cause the order parameter to become a discontinuous function of temperature. Such a model having disorder modes with a distribution of excitation energies is shown to lead to a temperature dependent refractive index with the same features as that observed in “dirty” ferroelectrics.

Anion antisite defects in GaAs and GaP

Abstract The electronic properties of anion antisite defects and the related ideal cation vacancies are calculated based on tight-binding Hamiltonians and using a novel recursion method. For the antisite defects symmetric A1 states are found in the upper part of the fundamental gaps, and for the ideal vacancies T2 states are found in the lower part of the gaps. These results for antisite defects compare favorably with recent ESR experiments.