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Hexagonal boron nitride encapsulation is the method of choice for protecting graphene from environmental doping and impurity scattering. It was previously demonstrated that metal-organic vapor phase epitaxy (MOVPE) grows epitaxially ordered, uniform BN layers on epigraphene (graphene grown on SiC). Due to graphene non-wetting properties, h-BN growth starts preferentially from the graphene ledges. We use this fact here to selectively promote growth of high-quality flat h-BN on epigraphene by patterning epigraphene microstructures prior to BN growth. Thin h-BN films (down to 6 nm) grown by MOVPE show smooth and pleated surface morphology on epigraphene, while crumpled BN is observed on the SiC. Cross-sectional high-resolution transmission electron microscopy images and fluorescence imaging confirm the higher BN quality grown on the epigraphene. Transport measurements reveal p-doping as expected from hydrogen intercalation of epigraphene and regions of high and low mobility. This method can be used to produce structurally uniform high-quality h-BN/epigraphene micro/nano scale heterostructure.

In this article, we investigate through numerical simulation the reduction of self-heating effects (SHEs) in GaN HEMT via the integration of hexagonal boron nitride (h-BN) as a passivation layer and as a release layer to transfer GaN HEMT to diamond substrate. The obtained devices exhibit improved thermal performance compared to SiO2/GaN/sapphire HEMT. The lattice temperature was reduced from 507 K in SiO2/GaN/sapphire to 372 K in h-BN/GaN/diamond HEMT. The temperature decrease enhances the drain current and transconductance to 900 mA/mm and 250 mS/mm, corresponding to a 47 % improvement. In addition, the total thermal resistance Rth is reduced by a factor of 5 from 27 K.mm/W in GaN/sapphire HEMT to 5.5 K.mm/W in GaN/diamond HEMT. This study indicates that h-BN integration in GaN HEMT as a top heat spreader and a release layer for transfer to diamond substrate can be a promising solution to reduce self-heating effects and extend the device lifetime and reliability., Comment: 19 pages, 9 figures

We report the growth of high-quality triangular GaN nanomesas, 30-nm thick, on the C-face of 4H-SiC using nano selective area growth (NSAG) with patterned epitaxial graphene grown on SiC as an embedded mask. NSAG alleviates the problems of defective crystals in the heteroepitaxial growth of nitrides, and the high mobility graphene film can readily provide the back low-dissipative electrode in GaN-based optoelectronic devices. The process consists in first growing a 5-8 graphene layers film on the C-face of 4H- SiC by confinement-controlled sublimation of silicon carbide. The graphene film is then patterned and arrays of 75-nanometer-wide openings are etched in graphene revealing the SiC substrate. 30-nanometer-thick GaN is subsequently grown by metal organic vapor phase epitaxy. GaN nanomesas grow epitaxially with perfect selectivity on SiC, in openings patterned through graphene, with no nucleation on graphene. The up-or-down orientation of the mesas on SiC, their triangular faceting, and cross-sectional scanning transmission electron microscopy show that they are biphasic. The core is a zinc blende monocrystal surrounded with single-crystal hexagonal wurtzite. The GaN crystalline nanomesas have no threading dislocations, and do not show any V-pit. This NSAG process potentially leads to integration of high-quality III-nitrides on the wafer scalable epitaxial graphene / silicon carbide platform., Comment: 7 pages, 5 figures

Patterning of graphene is key for device fabrication. We report a way to increase or reduce the number of layers in epitaxial graphene grown on the C-face (000-1) of silicon carbide by the deposition of a 120 nm to 150nm-thick silicon nitride (SiN) mask prior to graphitization. In this process we find that areas covered by a Si-rich SiN mask have one to four more layers than non-masked areas. Conversely N-rich SiN decreases the thickness by three layers. In both cases the mask decomposes before graphitization is completed. Graphene grown in masked areas show good quality as observed by Raman spectroscopy, atomic force microscopy (AFM) and transport data. By tailoring the growth parameters selective graphene growth and sub-micron patterns have been obtained.

We use a quantum-dynamical model to investigate the optical response of graphene under low excitation power. Ultrafast carrier relaxation processes, which play an important role for understanding the optical response of graphene, are included phenomenologically into the model. We obtain analytical solutions for the linear and third-order nonlinear optical response of graphene, and four-wave mixing in particular. This theory shows agreement with recently reported experimental data on linear complex optical conductivity and four-wave mixing, providing evidence for ultrafast quantum-dephasing times of approximately 1 fs.

We report a continuous variable key distribution system that achieves a final secure key rate of 3.45 kb/sec over a distance of 24.2 km of optical fiber. The protocol uses discrete signaling and post-selection to improve reconciliation speed and quantifies security by means of quantum state tomography. Polarization multiplexing and a frequency translation scheme permit transmission of a continuous wave local oscillator and suppression of noise from guided acoustic wave Brillouin scattering by more than 27 dB.

We demonstrate an optical-fiber based source of polarization entangled photon-pairs with improved quality and efficiency, which has been integrated with off-the-shelf telecom components and is, therefore, well suited for quantum communication applications in the 1550 nm telecom band. Polarization entanglement is produced by simultaneously pumping a loop of standard dispersion-shifted fiber with two orthogonally-polarized pump pulses, one propagating in the clockwise and the other in the counter-clockwise direction. We characterize this source by investigating two-photon interference between the generated signal-idler photon-pairs under various conditions. The experimental parameters are carefully optimized to maximize the generated photon-pair correlation and to minimize contamination of the entangled photon-pairs from extraneously scattered background photons that are produced by the pump pulses for two reasons: i) spontaneous Raman scattering causes uncorrelated photons to be emitted in the signal/idler bands and ii) broadening of the pump-pulse spectrum due to self-phase modulation causes pump photons to leak into the signal/idler bands. We obtain two-photon interference with visibility $>90$% without subtracting counts caused by the background photons (only dark counts of the detectors are subtracted), when the mean photon number in the signal (idler) channel is about 0.02/pulse, while no interference is observed in direct detection of either the signal or the idler photons., Comment: 4 figures, submitted to Phys. Rev. A

We present a quantum theory of nondegenerate phase-sensitive parametric amplification in a chi^3 nonlinear medium. The non-zero response time of the Kerr chi^3 nonlinearity determines the quantum-limited noise figure of chi^3 parametric amplification, as well as the limit on quadrature squeezing. This non-zero response time of the nonlinearity requires coupling of the parametric process to a molecular-vibration phonon bath, causing the addition of excess noise through spontaneous Raman scattering. We present analytical expressions for the quantum-limited noise figure of frequency non-degenerate and frequency degenerate chi^3 parametric amplifiers operated as phase-sensitive amplifiers. We also present results for frequency non-degenerate quadrature squeezing. We show that our non-degenerate squeezing theory agrees with the degenerate squeezing theory of Boivin and Shapiro as degeneracy is approached. We have also included the effect of linear loss on the phase-sensitive process., Comment: Now final submitted version. Added references, added some content, edited language

We present a fiber based source of polarization-entangled photon pairs that is well suited for quantum communication applications in the 1550nm band of standard fiber-optic telecommunications. Polarization entanglement is created by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed orthogonally-polarized pump pulses and subsequently removing the time distinguishability by passing the parametrically scattered signal-idler photon pairs through a piece of birefringent fiber. Coincidence detection of the signal-idler photons yields biphoton interference with visibility greater than 90%, while no interference is observed in direct detection of either the signal or the idler photons. All four Bell states can be prepared with our setup and we demonstrate violations of CHSH form of Bell's inequalities by up to 10 standard deviations of measurement uncertainty., Comment: 12 pages, 4 figures, to be submitted to Phys. Rev. Lett. See also paper QTuB4 in QELS'03 Technical Digest (OSA, Washington, D.C., 2003). This is a more complete version

In this paper we return to an issue often discussed in the literature regarding the relationship between highway expansion and population change. Typically it simply is assumed that this relationship is well established and understood. We argue, following a thorough review of the relevant literature, that the notion that highway expansion leads to increased population growth in the vicinity of the improved infrastructure finds only weak and often conflicting support. Using data on all major highway expansions in Wisconsin covering the period from the late-1960s through the 1990s from the Wisconsin Department of Transportation (WisDOT), and census data at the minor civil division (MCD) level covering the period 1970 to 2000, we deploy the analytical tools of geographic information system (GIS) software, and theory from the expanding literature in spatial analysis and modeling, to take a fresh look at this relationship. Our analysis reveals that there is a modest relationship between highway expansion and population growth among MCDs within 10-20 miles of the expanded major highway. The causal structure, however, is complex. Our starting hypothesis argues that population growth precedes highway expansion as frequently as population growth results from highway expansion, but the data show otherwise. The dominant causal influence appears to flow from highway expansion to population growth. (Contains 5 figures and 4 tables.)

Utilizing 1970 census data on a 45-county area in the northern Upper Great Lakes Region, the following questions were addressed: (1) In what ways do recent migrants to this nonmetropolitan region differ from those "nonmigrants" who resided in the region in both 1965 and 1970? (2) To what extent do the recent migrants from metropolitan counties differ from migrants from other nonmetropolitan counties? (3) Does the origin of migrants influence the character of their location in 1970? The populations represented were 985,000 residents of the region in 1970 who were 5 years or older and had reported U.S. residence in 1965 and a sub-population of 322,000 white heads of households in 1970. Migrants and non-migrants were compared in terms of sex differences, location of residence (urban, rural, rural nonfarm, and group quarters), age differences, and selected characteristics of white household heads. Results indicated that: inmigrants were considerably younger than the non-migrant residents (due to a higher propensity to migrate among the young in general and due to substantial outmigration among the young of this region); the majority of inmigrants had metropolitan origins and urban destinations, although they were attracted disproportionately to rural nonfarm locations (especially the older migrants seeking retirement); a substantial proportion of the inmigrants were enumerated in group quarters in 1970, having changed residence to attend an academic institution. (JC)