25 research outputs found
Caustic graphene plasmons with Kelvin angle
A century-long argument made by Lord Kelvin that all swimming objects have an
effective Mach number of 3, corresponding to the Kelvin angle of 19.5 degree
for ship waves, has been recently challenged with the conclusion that the
Kelvin angle should gradually transit to the Mach angle as the ship velocity
increases. Here we show that a similar phenomenon can happen for graphene
plasmons. By analyzing the caustic wave pattern of graphene plasmons stimulated
by a swift charged particle moving uniformly above graphene, we show that at
low velocities of the charged particle, the caustics of graphene plasmons form
the Kelvin angle. At large velocities of the particle, the caustics disappear
and the effective semi-angle of the wave pattern approaches the Mach angle. Our
study introduces caustic wave theory to the field of graphene plasmonics, and
reveals a novel physical picture of graphene plasmon excitation during electron
energy-loss spectroscopy measurement.Comment: 15 pages, 4 figure
Bulk-plasmon-mediated free-electron radiation beyond the conventional formation time
Free-electron radiation is a fundamental photon emission process that is
induced by fast-moving electrons interacting with optical media. Historically,
it has been understood that, just like any other photon emission process,
free-electron radiation must be constrained within a finite time interval known
as the "formation time", whose concept is applicable to both Cherenkov
radiation and transition radiation, the two basic mechanisms describing
radiation from a bulk medium and from an interface, respectively. Here we
reveal an alternative mechanism of free-electron radiation far beyond the
previously defined formation time. It occurs when a fast electron crosses the
interface between vacuum and a plasmonic medium supporting bulk plasmons. While
emitted continuously from the crossing point on the interface - thus consistent
with the features of transition radiation - the extra radiation beyond the
conventional formation time is supported by a long tail of bulk plasmons
following the electron's trajectory deep into the plasmonic medium. Such a
plasmonic tail mixes surface and bulk effects, and provides a sustained channel
for electron-interface interaction. These results also settle the historical
debate in Ferrell radiation, regarding whether it is a surface or bulk effect,
from transition radiation or plasmonic oscillation.Comment: 37 pages, 13 figure
Observation of the Stimulated Quantum Cherenkov Effect
As charged particles surpass the speed of light in an optical medium they
produce radiation - analogously to the way jet planes surpass the speed of
sound and produce a sonic boom. This radiation emission, known as the Cherenkov
effect, is among the most fundamental processes in electrodynamics. As such, it
is used in numerous applications of particle detectors, particle accelerators,
light sources, and medical imaging. Surprisingly, all Cherenkov-based
applications and experiments thus far were fully described by classical
electrodynamics even though theoretical work predicts new Cherenkov phenomena
coming from quantum electrodynamics. The quantum description could provide new
possibilities for the design of highly controllable light sources and more
efficient accelerators and detectors. Here, we provide a direct evidence of the
quantum nature of the Cherenkov effect and reveal its intrinsic quantum
features. By satisfying the Cherenkov condition for relativistic electron
wavefunctions and maintaining it over hundreds of microns, each electron
simultaneously accelerates and decelerates by absorbing and emitting hundreds
of photons in a coherent manner. We observe this strong interaction in an
ultrafast transmission electron microscope, achieving for the first time a
phase-matching between a relativistic electron wavefunction and a propagating
light wave. Consequently, the quantum wavefunction of each electron evolves
into a coherent plateau, analogous to a frequency comb in ultrashort laser
pulses, containing hundreds of quantized energy peaks. Our findings prove that
the delocalized wave nature of electrons can become dominant in stimulated
interactions. In addition to prospects for known applications of the Cherenkov
effect, our work provides a platform for utilizing quantum electrodynamics for
applications in electron microscopy and in free-electron pump-probe
spectroscopy.Comment: 15 pages, 4 figure
Electromagnetic wave excitation and propagation in novel photonic materials
The interaction of swift electrons with materials has been widely used to study the properties of materials and works as the sources for light and surface plasmon wave. A clear study of the dynamical physical process can benefit a deep understanding of the interaction and is necessary for the future development of plasmonics. We adopt the classical dielectric approach that can give the dynamical process to study the interaction between swift electrons with graphene, metal and cloak.DOCTOR OF PHILOSOPHY (SPMS
Two-dimensional graphene plasmons as analogue of two-dimensional hydrodynamic waves
Intricate and intriguing hydrodynamic wave phenomena are revealed to have counterparts in graphene plasmonics, including the plasmonic splashing generated by a fast-moving electron perpendicularly impacting upon a two-dimensional graphene monolayer and the plasmonic V-shaped ship-wake generated by a swift electron moving parallel above a graphene monolayer
Synthetic-gauge-field-induced Dirac semimetal state in an acoustic resonator system
Recently, a proposal of synthetic gauge field in reduced two-dimensional (2D) system from three-dimensional (3D) acoustic structure shows an analogue of the gapped Haldane model with fixed k z , and achieves the gapless Weyl semimetal phase in 3D momentum space. Here, extending this approach of synthetic gauge flux, we propose a reduced square lattice of acoustic resonators, which exhibits Dirac nodes with broken effective time-reversal symmetry. Protected by an additional hidden symmetry, these Dirac nodes with quantized values of topological charge are characterized by nonzero winding number and the finite structure exhibits flat edge modes that cannot be destroyed by perturbations.MOE (Min. of Education, S’pore)Published versio
Electromagnetic detection of a perfect carpet cloak
It has been shown that a spherical invisibility cloak originally proposed by Pendry et al. can be electromagnetically detected by shooting a charged particle through it, whose underlying mechanism stems from the asymmetry of transformation optics applied to motions of photons and charges [PRL 103, 243901 (2009)]. However, the conceptual three-dimensional invisibility cloak that exactly follows specifications of transformation optics is formidably difficult to implement, while the simplified cylindrical cloak that has been experimentally realized is inherently visible. On the other hand, the recent carpet cloak model has acquired remarkable experimental development, including a recently demonstrated full-parameter carpet cloak without any approximation in the required constitutive parameters. In this paper, we numerically investigate the electromagnetic radiation from a charged particle passing through a perfect carpet cloak and propose an experimentally verifiable model to demonstrate symmetry breaking of transformation optics.Published versio
Ultrathin Three-Dimensional Thermal Cloak
We report the first experimental realization of a three-dimensional thermal cloak shielding an air bubble in a bulk metal without disturbing the external conductive thermal flux. The cloak is made of a thin layer of homogeneous and isotropic material with specially designed three-dimensional manufacturing. The cloak’s thickness is 100  μm while the cloaked air bubble has a diameter of 1 cm, achieving the ratio between dimensions of the cloak and the cloaked object 2 orders smaller than previous thermal cloaks, which were mainly realized in a two-dimensional geometry. This work can find applications in novel thermal devices in the three-dimensional physical space.Published versio