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Search Results: 1 - 10 of 194642 matches for " Joshua D. Caldwell "
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Perfect interference-less absorption at infrared frequencies by a van der Waal's crystal
D. G. Baranov,J. H. Edgar,Tim Hoffman,Nabil Bassim,Joshua D. Caldwell
Physics , 2015, DOI: 10.1103/PhysRevB.92.201405
Abstract: Traditionally, efforts to achieve perfect absorption have required the use of complicated metamaterial-based structures as well as relying on destructive interference to eliminate back reflections. Here, we have demonstrated both theoretically and experimentally that such perfect absorption can be achieved using a naturally occurring material, hexagonal boron nitride (hBN) due to its high optical anisotropy without the requirement of interference effects to absorb the incident field. This effect was observed for p-polarized light within the mid-infrared spectral range, and we provide the full theory describing the origin of the perfect absorption as well as the methodology for achieving this effect with other materials. Furthermore, while this is reported for the uniaxial crystal hBN, this is equally applicable to biaxial crystals and more complicated crystal structures. Interference-less absorption is of fundamental interest to the field of optics; moreover, such materials may provide additional layers of flexibility in the design of frequency selective surfaces, absorbing coatings and sensing devices operating in the infrared.
High growth rate 4H-SiC epitaxial growth using dichlorosilane in a hot-wall CVD reactor
Iftekhar Chowdhury,MVS Chandrasekhar,Paul B Klein,Joshua D. Caldwell,Tangali Sudarshan
Physics , 2010, DOI: 10.1016/j.jcrysgro.2010.11.128
Abstract: Thick, high quality 4H-SiC epilayers have been grown in a vertical hot-wall chemical vapor deposition system at a high growth rate on (0001) 80 off-axis substrates. We discuss the use of dichlorosilane as the Si-precursor for 4H-SiC epitaxial growth as it provides the most direct decomposition route into SiCl2, which is the predominant growth species in chlorinated chemistries. A specular surface morphology was attained by limiting the hydrogen etch rate until the system was equilibrated at the desired growth temperature. The RMS roughness of the grown films ranged from 0.5-2.0 nm with very few morphological defects (carrots, triangular defects, etc.) being introduced, while enabling growth rates of 30-100 \mum/hr, 5-15 times higher than most conventional growths. Site-competition epitaxy was observed over a wide range of C/Si ratios, with doping concentrations < 1x1014 cm-3 being recorded. X-ray rocking curves indicated that the epilayers were of high crystallinity, with linewidths as narrow as 7.8 arcsec being observed, while microwave photoconductive decay (\muPCD) measurements indicated that these films had high injection (ambipolar) carrier lifetimes in the range of 2 \mus.
Temperature dependence and mechanism of electrically detected ESR at the n=1 filling factor of a two-dimensional electron system in GaAs quantum wells
Eugene Olshanetsky,Manyam Pilla,Joshua D. Caldwell,Shu-chen Liu,Clifford R. Bowers,Jerry A. Simmons,John L. Reno
Physics , 2002, DOI: 10.1103/PhysRevB.67.165325
Abstract: Electrically detected electron spin resonance (EDESR) signals were acquired as a function of temperature in the 0.3-4.2 K temperature range in a AlGaAs/GaAs multiple quantum well sample at the filling factor at 5.7 T. In the particular sample studied, the line width is approximately temperature independent, while the amplitude exhibits a maximum at about 2.2 K and vanishes with increased or decreased temperature. To explain the observed temperature dependence of the signal amplitude, the signal amplitude temperature dependence is calculated assuming a model based on heating. The model ascribes the resonant absorption of microwave power of the 2DES to the uniform mode of the electron spin magnetization where the elementary spin excitations at filling factor are taken to be spin waves, while the short wavelength spin wave modes serve as a heat sink for the absorbed energy. Due to the finite thermal conductance to the surroundings, the temperature of the 2DES spin wave system is increased, resulting in a thermal activation of the longitudinal magnetoconductance. The proposed heating model correctly predicts the location of the maximum in the experimentally observed temperature dependence of the EDESR amplitude. It also correctly predicts that the signal should vanish as the temperature is increased or decreased. The results of the present study demonstrate how experimental EDESR studies can, under appropriate conditions, provide data that can be used to discriminate between competing theories for the magnetic ordering and magnetic excitations of a 2DES in the regime of the quantum Hall effect.
Photoinduced tunability of the Reststrahlen band in 4H-SiC
Bryan T. Spann,Ryan Compton,Daniel Ratchford,James P. Long,Adam D. Dunkelberger,Paul B. Klein,Alexander J. Giles,Joshua D. Caldwell,Jeffrey C. Owrutsky
Physics , 2015,
Abstract: Materials with a negative dielectric permittivity (e.g. metals) display high reflectance and can be shaped into nanoscale optical-resonators exhibiting extreme mode confinement, a central theme of nanophotonics. However, the ability to $actively$ tune these effects remains elusive. By photoexciting free carriers in 4H-SiC, we induce dramatic changes in reflectance near the "Reststrahlen band" where the permittivity is negative due to charge oscillations of the polar optical phonons in the mid-infrared. We infer carrier-induced changes in the permittivity required for useful tunability (~ 40 cm$^{-1}$) in nanoscale resonators, providing a direct avenue towards the realization of actively tunable nanophotonic devices in the mid-infrared to terahertz spectral range.
Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging
Peining Li,Martin Lewin,Andrey V. Kretinin,Joshua D. Caldwell,Kostya S. Novoselov,Takashi Taniguchi,Kenji Watanabe,Fabian Gaussmann,Thomas Taubner
Physics , 2015,
Abstract: Optical imaging beyond the diffraction limit was one of the primary motivations for negative-index metamaterials, resulting in Pendry's perfect lens and the more attainable superlens. While these approaches offer sub-diffractional resolution, they do not provide a mechanism for magnification of the image. Hyperbolic (or indefinite-permittivity) metamaterials have been theoretically considered and experimentally demonstrated to provide simultaneously subdiffractive imaging and magnification; however, they are plagued with low efficiency and complex fabrication. In this work, we present theoretical and experimental studies of near-field optical imaging through a flat slab of the low-loss, natural hyperbolic material, hexagonal boron nitride (hBN). This thin hBN layer exhibits wavelength-dependent multifunctional operations, offering both an enhanced near-field imaging of single buried objects with down to lambda/32 resolution (0.4 um at lambda=12.8 um), as well as enabling an enlarged reconstruction of the geometric outline of the investigated objects. Both the excellent resolution and the multifunctional operation can be explained based on the volume-confined, wavelength-dependent propagation angle of Type I hyperbolic polaritons. Our results provide both the understanding of near-field imaging performance through this natural hyperbolic media, as well as inspire their exciting potential for guiding and sensing of light at an extreme sub-diffractional scale.
Graphene stabilization of two-dimensional gallium nitride
Zakaria Y. Al Balushi,Ke Wang,Ram Krishna Ghosh,Rafael A. Vilá,Sarah M. Eichfeld,Paul A. DeSario,Dennis F. Paul,Joshua D. Caldwell,Suman Datta,Joan M. Redwing,Joshua A. Robinson
Physics , 2015,
Abstract: The spectrum of two-dimensional (2D) materials beyond graphene offers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (5.0-6.0 eV), has clearly established that 2D nitrides are key to advancing novel devices. A gap, however, remains between the theoretical discovery of 2D nitrides beyond hBN and experimental realization of such structures. Here we demonstrate the robust synthesis of 2D bilayer gallium nitride (GaN) via a novel migration enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene. We theoretically predict and experimentally validate using MEEG, that the atomic structure of 2D GaN is notably different from reported theory. Moreover, we establish that graphene plays a critical role in stabilizing the direct bandgap, 2D buckled structure. Our results provide a foundation for discovery and stabilization of novel 2D nitrides that are difficult to prepare via traditional synthesis.
Sub-diffractional, volume-confined polaritons in a natural hyperbolic material: hexagonal boron nitride
Joshua D. Caldwell,Andrey Kretinin,Yiguo Chen,Vincenzo Giannini,Michael M. Fogler,Yan Francescato,Chase T. Ellis,Joseph G. Tischler,Colin R. Woods,Alexander J. Giles,Minghui Hong,Kenji Watanabe,Takashi Taniguchi,Stefan A. Maier,Kostya S. Novoselov
Physics , 2014, DOI: 10.1038/ncomms6221
Abstract: Strongly anisotropic media where the principal components of the dielectric tensor have opposite signs are called hyperbolic. Such materials length exhibit unique nanophotonic properties enabled by the highly directional propagation of slow-light modes localized at deeply sub-diffractional scales. While artificial hyperbolic metamaterials have been demonstrated, they suffer from high plasmonic losses and require complex nanofabrication, which in turn induces the size-dependent limitations on optical confinement. The low-loss, mid-infrared, natural hyperbolic material, hexagonal boron nitride is an attractive alternative. We observe four series of multiple (up to seven) 'hyperbolic polariton' modes in boron nitride nanocones in two spectral bands. The resonant modes obey the predicted aspect ratio dependence and exhibit record-high quality factors (Q up to 283) in the strong confinement regime (lambda/86 in the smallest structures). These observations assert hexagonal boron nitride as a promising platform for studying novel regimes of light-matter interactions and nanophotonic device engineering.
An Approach to Parallel Simulation of Ordinary Differential Equations  [PDF]
Joshua D. Carl, Gautam Biswas
Journal of Software Engineering and Applications (JSEA) , 2016, DOI: 10.4236/jsea.2016.95019
Abstract: Cyber-physical systems (CPS) represent a class of complex engineered systems where functionality and behavior emerge through the interaction between the computational and physical domains. Simulation provides design engineers with quick and accurate feedback on the behaviors generated by their designs. However, as systems become more complex, simulating their behaviors becomes computation all complex. But, most modern simulation environments still execute on a single thread, which does not take advantage of the processing power available on modern multi-core CPUs. This paper investigates methods to partition and simulate differential equation-based models of cyber-physical systems using multiple threads on multi-core CPUs that can share data across threads. We describe model partitioning methods using fixed step and variable step numerical in-tegration methods that consider the multi-layer cache structure of these CPUs to avoid simulation performance degradation due to cache conflicts. We study the effectiveness of each parallel simu-lation algorithm by calculating the relative speedup compared to a serial simulation applied to a series of large electric circuit models. We also develop a series of guidelines for maximizing performance when developing parallel simulation software intended for use on multi-core CPUs.
Technique for the Dry Transfer of Epitaxial Graphene onto Arbitrary Substrates
Joshua D. Caldwell,Travis J. Anderson,James C. Culbertson,Glenn G. Jernigan,Karl D. Hobart,Fritz J. Kub,Marko J. Tadjer,Joseph L. Tedesco,Jennifer K. Hite,Michael A. Mastro,Rachael L. Myers-Ward,Charles R. Eddy Jr.,Paul M. Campbell,D. Kurt Gaskill
Physics , 2009, DOI: 10.1021/nn901585p
Abstract: In order to make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO2, GaN and Al2O3 substrates using a thermal release tape. We further report on the impact of this process on the electrical properties of the EG films. This process enables EG films to be used in flexible electronic devices or as optically transparent contacts.
A case of calciphylaxis and acute myeloid leukemia: A previously unreported association  [PDF]
Joshua D. Rosenberg, Pamela Boswell, William Miller
Case Reports in Clinical Medicine (CRCM) , 2013, DOI: 10.4236/crcm.2013.23061

Calciphylaxis is a condition of induced hypersensitivity in which tissues respond to challenge agents with local calcification. This article reports the first known case of diffuse calciphylaxis associated with acute myeloid leukemia resulting in death from hypoxic respiratory failure and refractory hypotension.

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