13 research outputs found
Hamiltonian Optics of Hyperbolic Polaritons in Nanogranules
Semiclassical
quantization rules and numerical calculations are applied to study
polariton modes of materials whose permittivity tensor has principal
values of opposite sign (so-called hyperbolic materials). The spectra
of volume- and surface-confined polaritons are computed for spheroidal
nanogranules of hexagonal boron nitride, a natural hyperbolic crystal.
The field distribution created by polaritons excited by an external
dipole source is predicted to exhibit raylike patterns due to classical
periodic orbits. Near-field infrared imaging and Purcell-factor measurements
are suggested to test these predictions
IR Near-Field Spectroscopy and Imaging of Single Li<sub><i>x</i></sub>FePO<sub>4</sub> Microcrystals
This study demonstrates the unique
capability of infrared near-field nanoscopy combined with Fourier
transform infrared spectroscopy to map phase distributions in microcrystals
of Li<sub><i>x</i></sub>FePO<sub>4</sub>, a positive electrode
material for Li-ion batteries. Ex situ nanoscale IR imaging provides
direct evidence for the coexistence of LiFePO<sub>4</sub> and FePO<sub>4</sub> phases in partially delithiated single-crystal microparticles.
A quantitative three-dimensional tomographic reconstruction of the
phase distribution within a single microcrystal provides new insights
into the phase transformation and/or relaxation mechanism, revealing
a FePO<sub>4</sub> shell surrounding a diamond-shaped LiFePO<sub>4</sub> inner core, gradually shrinking in size and vanishing upon delithiation
of the crystal. The observed phase propagation pattern supports recent
functional models of LiFePO<sub>4</sub> operation relating electrochemical
performance to material design. This work demonstrates the remarkable
potential of near-field optical techniques for the characterization
of electrochemical materials and interfaces
Ultrafast Dynamics of Surface Plasmons in InAs by Time-Resolved Infrared Nanospectroscopy
We report on time-resolved mid-infrared
(mid-IR) near-field spectroscopy
of the narrow bandgap semiconductor InAs. The dominant effect we observed
pertains to the dynamics of photoexcited carriers and associated surface
plasmons. A novel combination of pumpāprobe techniques and
near-field nanospectroscopy accesses high momentum plasmons and demonstrates
efficient, subpicosecond photomodulation of the surface plasmon dispersion
with subsequent tens of picoseconds decay under ambient conditions.
The photoinduced change of the probe intensity due to plasmons in
InAs is found to exceed that of other mid-IR or near-IR media by 1ā2
orders of magnitude. Remarkably, the required control pulse fluence
is as low as 60 Ī¼J/cm<sup>2</sup>, much smaller than fluences
of ā¼1ā10 mJ/cm<sup>2</sup> previously utilized in ultrafast
control of near-IR plasmonics. These low excitation densities are
easily attained with a standard 1.56 Ī¼m fiber laser. Thus, InAsīøa
common semiconductor with favorable plasmonic properties such as a
low effective massīøhas the potential to become an important
building block of optically controlled plasmonic devices operating
at infrared frequencies
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Imaging the Localized Plasmon Resonance Modes in Graphene Nanoribbons
We report a nanoinfrared
(IR) imaging study of the localized plasmon
resonance modes of graphene nanoribbons (GNRs) using a scattering-type
scanning near-field optical microscope (s-SNOM). By comparing the
imaging data of GNRs that are aligned parallel and perpendicular to
the in-plane component of the excitation laser field, we observed
symmetric and asymmetric plasmonic interference fringes, respectively.
Theoretical analysis indicates that the asymmetric fringes are formed
due to the interplay between the localized surface plasmon resonance
(SPR) mode excited by the GNRs and the propagative surface plasmon
polariton (SPP) mode launched by the s-SNOM tip. With rigorous simulations,
we reproduce the observed fringe patterns and address quantitatively
the role of the s-SNOM tip on both the SPR and SPP modes. Furthermore,
we have seen real-space signatures of both the dipole and higher-order
SPR modes by varying the ribbon width
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Ultraconfined Plasmonic Hotspots Inside Graphene Nanobubbles
We report on a nanoinfrared
(IR) imaging study of ultraconfined plasmonic hotspots inside graphene
nanobubbles formed in graphene/hexagonal boron nitride (hBN) heterostructures.
The volume of these plasmonic hotspots is more than one-million-times
smaller than what could be achieved by free-space IR photons, and
their real-space distributions are controlled by the sizes and shapes
of the nanobubbles. Theoretical analysis indicates that the observed
plasmonic hotspots are formed due to a significant increase of the
local plasmon wavelength in the nanobubble regions. Such an increase
is attributed to the high sensitivity of graphene plasmons to its
dielectric environment. Our work presents a novel scheme for plasmonic
hotspot formation and sheds light on future applications of graphene
nanobubbles for plasmon-enhanced IR spectroscopy
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Tuning and Persistent Switching of Graphene Plasmons on a Ferroelectric Substrate
We characterized plasmon propagation
in graphene on thin films of the high-Īŗ dielectric PbZr<sub>0.3</sub>Ti<sub>0.7</sub>O<sub>3</sub> (PZT). Significant modulation
(up to Ā±75%) of the plasmon wavelength was achieved with application
of ultrasmall voltages (< Ā±1 V) across PZT. Analysis of the
observed plasmonic fringes at the graphene edge indicates that carriers
in graphene on PZT behave as noninteracting Dirac Fermions approximated
by a semiclassical Drude response, which may be attributed to strong
dielectric screening at the graphene/PZT interface. Additionally,
significant plasmon scattering occurs at the grain boundaries of PZT
from topographic and/or polarization induced graphene conductivity
variation in the interior of graphene, reducing the overall plasmon
propagation length. Lastly, through application of 2 V across PZT,
we demonstrate the capability to persistently modify the plasmonic
response of graphene through transient voltage application
Graphene-Based Platform for Infrared Near-Field Nanospectroscopy of Water and Biological Materials in an Aqueous Environment
Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful nanoscale spectroscopic tool capable of characterizing individual biomacromolecules and molecular materials. However, applications of scattering-based near-field techniques in the infrared (IR) to native biosystems still await a solution of how to implement the required aqueous environment. In this work, we demonstrate an IR-compatible liquid cell architecture that enables near-field imaging and nanospectroscopy by taking advantage of the unique properties of graphene. Large-area graphene acts as an impermeable monolayer barrier that allows for nano-IR inspection of underlying molecular materials in liquid. Here, we use s-SNOM to investigate the tobacco mosaic virus (TMV) in water underneath graphene. We resolve individual virus particles and register the amide I and II bands of TMV at <i>ca</i>. 1520 and 1660 cm<sup>ā1</sup>, respectively, using nanoscale Fourier transform infrared spectroscopy (nano-FTIR). We verify the presence of water in the graphene liquid cell by identifying a spectral feature associated with water absorption at 1610 cm<sup>ā1</sup>
Faraday Rotation Due to Surface States in the Topological Insulator (Bi<sub>1ā<i>x</i></sub>Sb<sub><i>x</i></sub>)<sub>2</sub>Te<sub>3</sub>
Using
magneto-infrared spectroscopy, we have explored the charge dynamics
of (Bi,Sb)<sub>2</sub>Te<sub>3</sub> thin films on InP substrates.
From the magneto-transmission data we extracted three distinct cyclotron
resonance (CR) energies that are all apparent in the broad band Faraday
rotation (FR) spectra. This comprehensive FR-CR data set has allowed
us to isolate the response of the bulk states from the intrinsic surface
states associated with both the top and bottom surfaces of the film.
The FR data uncovered that electron- and hole-type Dirac Fermions
reside on opposite surfaces of our films, which paves the way for
observing many exotic quantum phenomena in topological insulators
Active Optical Metasurfaces Based on Defect-Engineered Phase-Transition Materials
Active, widely tunable optical materials have enabled
rapid advances in photonics and optoelectronics, especially in the
emerging field of meta-devices. Here, we demonstrate that spatially
selective defect engineering on the nanometer scale can transform
phase-transition materials into optical metasurfaces. Using ion irradiation
through nanometer-scale masks, we selectively defect-engineered the
insulator-metal transition of vanadium dioxide, a prototypical correlated
phase-transition material whose optical properties change dramatically
depending on its state. Using this robust technique, we demonstrated
several optical metasurfaces, including tunable absorbers with artificially
induced phase coexistence and tunable polarizers based on thermally
triggered dichroism. Spatially selective nanoscale defect engineering
represents a new paradigm for active photonic structures and devices
Active Optical Metasurfaces Based on Defect-Engineered Phase-Transition Materials
Active, widely tunable optical materials have enabled
rapid advances in photonics and optoelectronics, especially in the
emerging field of meta-devices. Here, we demonstrate that spatially
selective defect engineering on the nanometer scale can transform
phase-transition materials into optical metasurfaces. Using ion irradiation
through nanometer-scale masks, we selectively defect-engineered the
insulator-metal transition of vanadium dioxide, a prototypical correlated
phase-transition material whose optical properties change dramatically
depending on its state. Using this robust technique, we demonstrated
several optical metasurfaces, including tunable absorbers with artificially
induced phase coexistence and tunable polarizers based on thermally
triggered dichroism. Spatially selective nanoscale defect engineering
represents a new paradigm for active photonic structures and devices