Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The thermal conductivity of single crystal silicon was engineered using lithographically formed phononic crystals. Specifically, sub-micron periodic through-holes were patterned in 500nm-thick silicon membranes to construct phononic crystals, and through phonon scattering enhancement, heat transfer was significantly reduced. The thermal conductivity of silicon phononic crystals was measured as low as 32.6W/mK, which is a ∼75% reduction compared to bulk silicon thermal conductivity [1]. This corresponds to a 37% reduction even after taking into account the contributions of the thin-film and volume reduction effects, while the electrical conductivity was reduced only by as much as the volume reduction effect. The demonstrated method uses conventional lithography-based technologies that are directly applicable to diverse micro/nano-scale devices, leading toward huge performance improvements where heat management is important. © 2011 IEEE.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Thermal detection has made extensive progress in the last 40 years, however, the speed and detectivity can still be improved. The advancement of silicon photonic microring resonators has made them intriguing for detection devices due to their small size and high quality factors. Implementing silicon photonic microring or microdisk resonators as a means of a thermal detector gives rise to higher speed and detectivity, as well as lower noise compared to conventional devices with electrical readouts. This LDRD effort explored the design and measurements of silicon photonic microdisk resonators used for thermal detection. The characteristic values, consisting of the thermal time constant ({tau} {approx} 2 ms) and noise equivalent power were measured and found to surpass the performance of the best microbolometers. Furthermore the detectivity was found to be D{sub {lambda}} = 2.47 x 10{sup 8} cm {center_dot} {radical}Hz/W at 10.6 {mu}m which is comparable to commercial detectors. Subsequent design modifications should increase the detectivity by another order of magnitude. Thermal detection in the terahertz (THz) remains underdeveloped, opening a door for new innovative technologies such as metamaterial enhanced detectors. This project also explored the use of metamaterials in conjunction with a cantilever design for detection in the THz region and demonstrated the use of metamaterials as custom thin film absorbers for thermal detection. While much work remains to integrate these technologies into a unified platform, the early stages of research show promising futures for use in thermal detection.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Interaction between metamaterial elements and intersubband transitions in GaAs/AlGaAs quantum wells is observed in the mid-infrared. Transmission measurements were performed through metamaterial arrays, each having a different resonance frequency. © 2011 OSA.

We examine the long-wave infrared (LWIR) optical characteristics of heavily-doped silicon and explore engineering of surface plasmons polaritons (SPP) in this spectral region. Both phosphorus (n-type Si) and boron (p-type Si) implants are evaluated and various cap layers and thermal annealing steps are examined. The optical properties are measured using ellipsometry and fit to a Drude model for the infrared (IR) permittivity. The predicted metallic behavior for Si in the thermal IR and its impact on the spatial confinement and dispersion for surface plasmons is studied. We find that the transverse spatial confinement for a surface plasmon on highly doped Si is strongly sub-wavelength near the plasma edge, and the confinement to the surface is enhanced to greater than 10 × that of the metal confined SPP over the entire LWIR spectrum. © 2011 American Institute of Physics.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Measurements of the electrical and thermal transport properties of one-dimensional nanostructures (e.g.nanotubes and nanowires) are typically obtained without detailed knowledge of the specimen's atomic-scale structure or defects. To address this deficiency, we have developed a microfabricated, chip-based characterization platform that enables both transmission electron microscopy (TEM) of the atomic structure and defects as well as measurement of the thermal transport properties of individual nanostructures. The platform features a suspended heater line that physically contacts the center of a suspended nanostructure/nanowire that was placed using insitu scanning electron microscope nanomanipulators. Suspension of the nanostructure across a through-hole enables TEM characterization of the atomic and defect structure (dislocations, stacking faults, etc) of the test sample. This paper explains, in detail, the processing steps involved in creating this thermal property measurement platform. As a model study, we report the use of this platform to measure the thermal conductivity and defect structure of a GaN nanowire. © 2011 IOP Publishing Ltd.

Phononic crystals (PnCs) are acoustic devices composed of a periodic arrangement of scattering centers embedded in a homogeneous background matrix with a lattice spacing on the order of the acoustic wavelength. When properly designed, a superposition of Bragg and Mie resonant scattering in the crystal results in the opening of a frequency gap over which there can be no propagation of elastic waves in the crystal, regardless of direction. In a fashion reminiscent of photonic lattices, PnC patterning results in a controllable redistribution of the phononic density of states. This property makes PnCs a particularly attractive platform for manipulating phonon propagation. In this communication, we discuss the profound physical implications this has on the creation of novel thermal phenomena, including the alteration of the heat capacity and thermal conductivity of materials, resulting in high-ZT materials and highly-efficient thermoelectric cooling and energy harvesting. © 2011 SPIE.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Measurements of the electrical and thermal transport properties of one-dimensional nanostructures (e.g., nanotubes and nanowires) typically are obtained without detailed knowledge of the specimen's atomic-scale structure or defects. To address this deficiency we have developed a microfabricated, chip-based characterization platform that enables both transmission electron microscopy (TEM) of atomic structure and defects as well as measurement of the thermal transport properties of individual nanostructures. The platform features a suspended heater line that contacts the center of a suspended nanostructure/nanowire that was placed using in-situ scanning electron microscope nanomanipulators. One key advantage of this platform is that it is possible to measure the thermal conductivity of both halves of the nanostructure (on each side of the central heater), and this feature permits identification of possible changes in thermal conductance along the wire and measurement of the thermal contact resistance. Suspension of the nanostructure across a through-hole enables TEM characterization of the atomic and defect structure (dislocations, stacking faults, etc.) of the test sample. As a model study, we report the use of this platform to measure the thermal conductivity and defect structure of GaN nanowires. The utilization of this platform for the measurements of other nanostructures will also be discussed.

A tandem interferometer system measuring the absolute phase and amplitude of planar split-ring resonators fabricated on a BaF2 substrate with a designed resonance at 10.5 μm is presented. © 2010 Optical Society of America.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The mid-infrared (mid-IR, 3 {micro}m -12 {micro}m) is a highly desirable spectral range for imaging and environmental sensing. We propose to develop a new class of mid-IR devices, based on plasmonic and metamaterial concepts, that are dynamically controlled by tunable semiconductor plasma resonances. It is well known that any material resonance (phonons, excitons, electron plasma) impacts dielectric properties; our primary challenge is to implement the tuning of a semiconductor plasma resonance with a voltage bias. We have demonstrated passive tuning of both plasmonic and metamaterial structures in the mid-IR using semiconductors plasmas. In the mid-IR, semiconductor carrier densities on the order of 5E17cm{sup -3} to 2E18cm{sup -3} are desirable for tuning effects. Gate control of carrier densities at the high end of this range is at or near the limit of what has been demonstrated in literature for transistor style devices. Combined with the fact that we are exploiting the optical properties of the device layers, rather than electrical, we are entering into interesting territory that has not been significantly explored to date.

We present the results of a three year LDRD project that focused on understanding the impact of defects on the electrical, optical and thermal properties of GaN-based nanowires (NWs). We describe the development and application of a host of experimental techniques to quantify and understand the physics of defects and thermal transport in GaN NWs. We also present the development of analytical models and computational studies of thermal conductivity in GaN NWs. Finally, we present an atomistic model for GaN NW electrical breakdown supported with experimental evidence. GaN-based nanowires are attractive for applications requiring compact, high-current density devices such as ultraviolet laser arrays. Understanding GaN nanowire failure at high-current density is crucial to developing nanowire (NW) devices. Nanowire device failure is likely more complex than thin film due to the prominence of surface effects and enhanced interaction among point defects. Understanding the impact of surfaces and point defects on nanowire thermal and electrical transport is the first step toward rational control and mitigation of device failure mechanisms. However, investigating defects in GaN NWs is extremely challenging because conventional defect spectroscopy techniques are unsuitable for wide-bandgap nanostructures. To understand NW breakdown, the influence of pre-existing and emergent defects during high current stress on NW properties will be investigated. Acute sensitivity of NW thermal conductivity to point-defect density is expected due to the lack of threading dislocation (TD) gettering sites, and enhanced phonon-surface scattering further inhibits thermal transport. Excess defect creation during Joule heating could further degrade thermal conductivity, producing a viscous cycle culminating in catastrophic breakdown. To investigate these issues, a unique combination of electron microscopy, scanning luminescence and photoconductivity implemented at the nanoscale will be used in concert with sophisticated molecular-dynamics calculations of surface and defect-mediated NW thermal transport. This proposal seeks to elucidate long standing material science questions for GaN while addressing issues critical to realizing reliable GaN NW devices.

Near-field microwave microscopy can be used as an alternative to atomic-force microscopy or Raman microscopy in determination of graphene thickness. We evaluated the values of AC impedance for few layer graphene. The impedance of mono and few-layer graphene at 4GHz was found predominantly active. Near-field microwave microscopy allows simultaneous imaging of location, geometry, thickness, and distribution of electrical properties of graphene without device fabrication. Our results may be useful for design of future graphene-based microwave devices.

Abstract not provided.

Graphene has emerged as a promising material for high speed nano-electronics due to the relatively high carrier mobility that can be achieved. To further investigate electronic transport in graphene and reveal its potential for microwave applications, we employed a near-field scanning microwave microscope with the probe formed by an electrically open end of a 4 GHz half-lambda parallel-strip transmission line resonator. Because of the balanced probe geometry, our microscope allows for truly localized quantitative characterization of various bulk and low-dimensional materials, with the response region defined by the one micron spacing between the two metallic strips at the probe tip. The single- and few-layer graphene flakes were fabricated by a mechanical cleavage method on 300-nm-thick silicon dioxide grown on low resistivity Si wafer. The flake thickness was determined using both AFM and Raman microscopies. We observe clear correlation between the near-field microwave and far-field optical images of graphene produced by the probe resonant frequency shift and thickness-defined color gradation, respectively. We show that the microwave response of graphene flakes is determined by the local sheet impedance, which is found to be predominantly active. Furthermore, we apply a quantitative electrodynamic model relating the probe resonant frequency shift to 2D conductivity of single- and few-layer graphene. From fitting a model to the experimental data we evaluate graphene sheet resistance as a function of thickness. Near-field scanning microwave microscopy can simultaneously image location, geometry, thickness, and distribution of electrical properties of graphene without a need for device fabrication. The approach may be useful for design of graphene-based microwave transistors, quality control of large area graphene sheets, or investigation of chemical and electrical doping effects on graphene transport properties. We acknowledge support from the DOE Center for Integrated Nanotechnologies user support program (grant No.U2008A061), from the NASA NM Space Grant Consortium program, and from the LANL-NMT MOU program supported by UCDRD.

Abstract not provided.

Abstract not provided.

Resonant plasmonic detectors are potentially important for terahertz (THz) spectroscopic imaging. We have fabricated and characterized antenna coupled detectors that integrate a broad-band antenna, which improves coupling of THz radiation. The vertex of the antenna contains the tuning gates and the bolometric barrier gate. Incident THz radiation may excite 2D plasmons with wave-vectors defined by either a periodic grating gate or a plasmonic cavity determined by ohmic contacts and gate terminals. The latter approach of exciting plasmons in a cavity defined by a short micron-scale channel appears most promising. With this short-channel geometry, we have observed multiple harmonics of THz plasmons. At 20 K with detector bias optimized we report responsivity on resonance of 2.5 kV/W and an NEP of 5 x 10{sup -10} W/Hz{sup 1/2}.

Abstract not provided.

Near-field scanning microwave microscopy is employed for quantitative imaging at 4 GHz of the local impedance for monolayer and few-layer graphene. The microwave response of graphene is found to be thickness dependent and determined by the local sheet resistance of the graphene flake. Calibration of the measurement system and knowledge of the probe geometry allows evaluation of the AC impedance for monolayer and few-layer graphene, which is found to be predominantly active. The use of localized evanescent electromagnetic field in our experiment provides a promising tool for investigations of plasma waves in graphene with wave numbers determined by the spatial spectrum of the near-field. By using near-field microwave microscopy one can perform simultaneous imaging of location, geometry, thickness, and distribution of electrical properties of graphene without a need for device fabrication.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Metal films perforated with subwavelength hole arrays have been show to demonstrate an effect known as Extraordinary Transmission (EOT). In EOT devices, optical transmission passbands arise that can have up to 90% transmission and a bandwidth that is only a few percent of the designed center wavelength. By placing a tunable dielectric in proximity to the EOT mesh, one can tune the center frequency of the passband. We have demonstrated over 1 micron of passive tuning in structures designed for an 11 micron center wavelength. If a suitable midwave (3-5 micron) tunable dielectric (perhaps BaTiO{sub 3}) were integrated with an EOT mesh designed for midwave operation, it is possible that a fast, voltage tunable, low temperature filter solution could be demonstrated with a several hundred nanometer passband. Such an element could, for example, replace certain components in a filter wheel solution.

Abstract not provided.