478 research outputs found
Negative refraction in hyperbolic hetero-bicrystals
We visualized negative refraction of phonon-polaritons in hetero-bicrystals of two hyperbolic van der Waals materials: molybdenum oxide (MoO3) and isotopically pure hexagonal boron nitride (h11BN). The polaritons - hybrids of infrared photons and lattice vibrations - form collimated rays that display negative refraction when passing through a planar interface between the h11BN and MoO3 crystals. At a special frequency ω0, these rays can circulate along closed diamond-shaped trajectories. We show that polariton eigenmodes display regions of both positive and negative dispersion interrupted by multiple gaps resulting from polaritonic level repulsion
Negative refraction in hyperbolic hetero-bicrystals
We visualized negative refraction of phonon-polaritons in hetero-bicrystals
of two hyperbolic van der Waals materials: molybdenum oxide () and
isotopically pure hexagonal boron nitride (). The polaritons -
hybrids of infrared photons and lattice vibrations - form collimated rays that
display negative refraction when passing through a planar interface between the
and crystals. At a special frequency , these rays
can circulate along closed diamond-shaped trajectories. We show that polariton
eigenmodes display regions of both positive and negative dispersion interrupted
by multiple gaps resulting from polaritonic level repulsion
Programmable hyperbolic polaritons in van der Waals semiconductors
Collective electronic modes or lattice vibrations usually prohibit propagation of electromagnetic radiation through the bulk of common materials over a frequency range associated with these oscillations. However, this textbook tenet does not necessarily apply to layered crystals. Highly anisotropic materials often display nonintuitive optical properties and can permit propagation of subdiffractional waveguide modes, with hyperbolic dispersion, throughout their bulk. Here, we report on the observation of optically induced electronic hyperbolicity in the layered transition metal dichalcogenide tungsten diselenide (WSe2). We used photoexcitation to inject electron-hole pairs in WSe2 and then visualized, by transient nanoimaging, the hyperbolic rays that traveled along conical trajectories inside of the crystal. We establish here the signatures of programmable hyperbolic electrodynamics and assess the role of quantum transitions of excitons within the Rydberg series in the observed polaritonic response
Metamaterial-Enhanced Nonlinear Terahertz Spectroscopy
We demonstrate large nonlinear terahertz responses in the gaps of metamaterial split ring resonators in several materials and use nonlinear THz transmission and THz-pump/THz-probe spectroscopy to study the nonlinear responses and dynamics. We use the field enhancement in the SRR gaps to initiate high-field phenomena at lower incident fields. In vanadium dioxide, we drive the insulator-to-metal phase transition with high-field THz radiation. The film conductivity increases by over two orders of magnitude and the phase transition occurs on a several picosecond timescale. In gallium arsenide, we observe high-field transport phenomena, including mobility saturation and impact ionization. The carrier density increases by up to ten orders of magnitude at high fields. At the highest fields, we demonstrate THz-induced damage in both vanadium dioxide and gallium arsenide.United States. Dept. of Energy (DOE-BES, grant DE-FG02- 09ER46643)United States. Office of Naval Research (ONR Grant No. N00014-09-1-1103
Femtosecond exciton dynamics in WSe2 optical waveguides
Van-der Waals (vdW) atomically layered crystals can act as optical waveguides over a broad range of the electromagnetic spectrum ranging from Terahertz to visible. Unlike common Si-based waveguides, vdW semiconductors host strong excitonic resonances that may be controlled using non-thermal stimuli including electrostatic gating and photoexcitation. Here, we utilize waveguide modes to examine photo-induced changes of excitons in the prototypical vdW semiconductor, WSe2, prompted by femtosecond light pulses. Using time-resolved scanning near-field optical microscopy we visualize the electric field profiles of waveguide modes in real space and time and extract the temporal evolution of the optical constants following femtosecond photoexcitation. By monitoring the phase velocity of the waveguide modes, we detect incoherent A-exciton bleaching along with a coherent optical Stark shift in WSe2
Electronic interactions in Dirac fluids visualized by nano-terahertz spacetime mapping
Ultraclean graphene at charge neutrality hosts a quantum critical Dirac fluid
of interacting electrons and holes. Interactions profoundly affect the charge
dynamics of graphene, which is encoded in the properties of its collective
modes: surface plasmon polaritons (SPPs). The group velocity and lifetime of
SPPs have a direct correspondence with the reactive and dissipative parts of
the tera-Hertz (THz) conductivity of the Dirac fluid. We succeeded in tracking
the propagation of SPPs over sub-micron distances at femto-second (fs) time
scales. Our experiments uncovered prominent departures from the predictions of
the conventional Fermi-liquid theory. The deviations are particularly strong
when the densities of electrons and holes are approximately equal. Our imaging
methodology can be used to probe the electromagnetics of quantum materials
other than graphene in order to provide fs-scale diagnostics under
near-equilibrium conditions
Failed back surgeries and minnesota multiphasic personality inventory (MMPI) profiles
MMPI profiles were evaluated for 105 prospective surgical patients who had previously undergone surgery or other procedures for treatment of back pain. Patients were classified into groups having undergone zero, one, two, three, or four or more previous surgeries. While all groups demonstrated a characteristic somatogenic profile, none of the MMPI validity or clinical scales significantly differentiated the groups and there was no relationship between increased number of surgeries and MMPI scale characteristics. These results support the nonoptimistic prognostication of the somatogenic MMPI profile for surgical intervention for back pain but show no clear relationship of MMPI profile characteristics to degree of experience of previously failed surgery.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44850/1/10880_2005_Article_BF01999744.pd
Infrared plasmons propagate through a hyperbolic nodal metal
Metals are canonical plasmonic media at infrared and optical wavelengths, allowing one to guide and manipulate light at the nanoscale. A special form of optical waveguiding is afforded by highly anisotropic crystals revealing the opposite signs of the dielectric functions along orthogonal directions. These media are classified as hyperbolic and include crystalline insulators, semiconductors, and artificial metamaterials. Layered anisotropic metals are also anticipated to support hyperbolic waveguiding. However, this behavior remains elusive, primarily because interband losses arrest the propagation of infrared modes. Here, we report on the observation of propagating hyperbolic waves in a prototypical layered nodal-line semimetal ZrSiSe. The observed waveguiding originates from polaritonic hybridization between near-infrared light and nodal-line plasmons. Unique nodal electronic structures simultaneously suppress interband loss and boost the plasmonic response, ultimately enabling the propagation of infrared modes through the bulk of the crystal
The impact of EEG in the diagnosis and management of patients with acute impairment of consciousness
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