Model for quantitative tip-enhanced spectroscopy and the extraction of nanoscale-resolved optical constants

Near-field infrared spectroscopy by elastic scattering of light from a probe tip resolves optical contrasts in materials at dramatically sub-wavelength scales across a broad energy range, with the demonstrated capacity for chemical identification at the nanoscale. However, current models of probe-sample near-field interactions still cannot provide a sufficiently quantitatively interpretation of measured near-field contrasts, especially in the case of materials supporting strong surface phonons. We present a model of near-field spectroscopy derived from basic principles and verified by finite-element simulations, demonstrating superb predictive agreement both with tunable quantum cascade laser near-field spectroscopy of SiO$_2$ thin films and with newly presented nanoscale Fourier transform infrared (nanoFTIR) spectroscopy of crystalline SiC. We discuss the role of probe geometry, field retardation, and surface mode dispersion in shaping the measured near-field response. This treatment enables a route to quantitatively determine nano-resolved optical constants, as we demonstrate by inverting newly presented nanoFTIR spectra of an SiO$_2$ thin film into the frequency dependent dielectric function of its mid-infrared optical phonon. Our formalism further enables tip-enhanced spectroscopy as a potent diagnostic tool for quantitative nano-scale spectroscopy.Comment: 19 pages, 9 figure

The Chromo-Dielectric Soliton Model: Quark Self Energy and Hadron Bags

The chromo-dielectric soliton model (CDM) is Lorentz- and chirally-invariant. It has been demonstrated to exhibit dynamical chiral symmetry breaking and spatial confinement in the locally uniform approximation. We here study the full nonlocal quark self energy in a color-dielectric medium modeled by a two parameter Fermi function. Here color confinement is manifest. The self energy thus obtained is used to calculate quark wave functions in the medium which, in turn, are used to calculate the nucleon and pion masses in the one gluon exchange approximation. The nucleon mass is fixed to its empirical value using scaling arguments; the pion mass (for massless current quarks) turns out to be small but non-zero, depending on the model parameters.Comment: 24 pages, figures available from the author

Detection of High Energy Ionizing Radiation using Deeply Depleted Graphene-Oxide-Semiconductor Junctions

Graphene's linear bandstructure and two-dimensional density of states provide an implicit advantage for sensing charge. Here, these advantages are leveraged in a deeply depleted graphene-oxide-semiconductor (D2GOS) junction detector architecture to sense carriers created by ionizing radiation. Specifically, the room temperature response of the silicon-based D2GOS junction is analyzed during irradiation with 20 MeV Si4+ ions. Detection was demonstrated for doses ranging from 12-1200 ions with device functionality maintained with no substantive degradation. To understand the device response, D2GOS pixels were characterized post-irradiation via a combination of electrical characterization, Raman spectroscopy, and photocurrent mapping. This combined characterization methodology underscores the lack of discernible damage caused by irradiation to the graphene while highlighting the nature of interactions between the incident ions and the silicon absorber.Comment: 15 pages, 4 figure

Relative efficiency of polariton emission in two-dimensional materials

We investigated emission and propagation of polaritons in a two dimensional van der Waals material hexagonal boron nitride (hBN). Our specific emphasis in this work is on hyperbolic phonon polariton emission that we investigated by means of scattering-type scanning near-field optical microscopy. Real-space nano-images detail how the polaritons are launched in several common arrangements including: light scattering by the edges of the crystal, metallic nanostructures deposited on the surface of hBN crystals, as well as random defects and impurities. Notably, the scanned tip of the near-field microscope is itself an efficient polariton launcher. Our analysis reveals that the scanning tips are superior to other types of emitters we have investigated. Furthermore, the study of polariton emission and emission efficiency may provide insights for development of polaritonic devices and for fundamental studies of collective modes in other van der Waals materials.Comment: 19 pages, 3 figure

Hadronization of a Quark-Gluon Plasma in the Chromodielectric Model

We have carried out simulations of the hadronization of a hot, ideal but effectively massive quark-gluon gas into color neutral clusters in the framework of the semi-classical SU(3) chromodielectric model. We have studied the possible quark-gluon compositions of clusters as well as the final mass distribution and spectra, aiming to obtain an insight into relations between hadronic spectral properties and the confinement mechanism in this model.Comment: 34 pages, 37 figure

Phase transition in bulk single crystals and thin films of VO2 by nanoscale infrared spectroscopy and imaging

We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (∼20nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations, but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain- and symmetry-dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches

Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial

Hexagonal boron nitride (h-BN) is a natural hyperbolic material1, in which the dielectric constants are the same in the basal plane (ε[superscript t] ≡ ε[superscript x] = ε[superscript y]) but have opposite signs (ε[superscript t] ε[superscript z ]< 0) in the normal plane (ε[superscript z]). Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons—collective modes that originate from the coupling between photons and electric dipoles in phonons. However, control of these hyperbolic phonon polaritons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene with hyperbolic phonon polaritons in h-BN so that the eigenmodes of the graphene/h-BN heterostructure are hyperbolic plasmon–phonon polaritons. The hyperbolic plasmon–phonon polaritons in graphene/h-BN suffer little from ohmic losses, making their propagation length 1.5–2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon–phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial as the resulting properties of these devices are not present in its constituent elements alone