90 research outputs found
Manipulation of Giant Faraday Rotation in Graphene Metasurfaces
Faraday rotation is a fundamental magneto-optical phenomenon used in various
optical control and magnetic field sensing techniques. Recently, it was shown
that a giant Faraday rotation can be achieved in the low-THz regime by a single
monoatomic graphene layer. Here, we demonstrate that this exceptional property
can be manipulated through adequate nano-patterning, notably achieving giant
rotation up to 6THz with features no smaller than 100nm. The effect of the
periodic patterning on the Faraday rotation is predicted by a simple physical
model, which is then verified and refined through accurate full-wave
simulations.Comment: 4 pages, 5 figures, submitted to Applied Physics Letter
Tunable plasmon-enhanced birefringence in ribbon array of anisotropic 2D materials
We explore the far-field scattering properties of anisotropic 2D materials in
ribbon array configuration. Our study reveals the plasmon-enhanced linear
birefringence in these ultrathin metasurfaces, where linearly polarized
incident light can be scattered into its orthogonal polarization or be
converted into circular polarized light. We found wide modulation in both
amplitude and phase of the scattered light via tuning the operating frequency
or material's anisotropy and develop models to explain the observed scattering
behavior
High Efficiency Terahertz Generation in a Multi-Stage System
We describe a robust system for laser-driven narrowband terahertz generation
with high conversion efficiency in periodically poled Lithium Niobate (PPLN).
In the multi-stage terahertz generation system, the pump pulse is recycled
after each PPLN stage for further terahertz generation. By out-coupling the
terahertz radiation generated in each stage, extra absorption is circumvented
and effective interaction length is increased. The separation of the terahertz
and optical pulses at each stage is accomplished by an appropriately designed
out-coupler. To evaluate the proposed architecture, the governing 2-D coupled
wave equations in a cylindrically symmetric geometry are numerically solved
using the finite difference method. Compared to the 1-D calculation which
cannot capture the self-focusing and diffraction effects, our 2-D numerical
method captures the effects of difference frequency generation, self-phase
modulation, self-focusing, beam diffraction, dispersion, and terahertz
absorption. We found that the terahertz generation efficiency can be greatly
enhanced by compensating the dispersion of the pump pulse after each stage.
With a two-stage system, we predict the generation of a mJ terahertz
pulse with total conversion efficiency at THz
using a 1.1 J pump laser with a two-line spectrum centered at 1 m. The
generation efficiency of each stage is above with the out-coupling
efficiencies above
Nonlocal Electromagnetic Response of Graphene Nanostructures
Nonlocal electromagnetic effects of graphene arise from its naturally
dispersive dielectric response. We present semi-analytical solutions of
nonlocal Maxwell's equations for graphene nano-ribbons array with features
around 100 nm, where we found prominent departures from its local response.
Interestingly, the nonlocal corrections are stronger for light polarization
parallel to the ribbons, which manifests as additional broadening of the Drude
peak. For the perpendicular polarization case, nonlocal effects lead to
blue-shifts of the plasmon peaks. These manifestations provide a physical
measure of nonlocal effects, and we quantify their dependence on ribbon width,
doping and wavelength
Laser-Induced Linear Electron Acceleration in Free Space
Linear acceleration in free space is a topic that has been studied for over
20 years, and its ability to eventually produce high-quality, high energy
multi-particle bunches has remained a subject of great interest. Arguments can
certainly be made that such an ability is very doubtful. Nevertheless, we chose
to develop an accurate and truly predictive theoretical formalism to explore
this remote possibility in a computational experiment. The formalism includes
exact treatment of Maxwell's equations, exact relativistic treatment of the
interaction among the multiple individual particles, and exact treatment of the
interaction at near and far field. Several surprising results emerged. For
example, we find that 30 keV electrons (2.5% energy spread) can be accelerated
to 7.7 MeV (2.5% spread) and to 205 MeV (0.25% spread) using 25 mJ and 2.5 J
lasers respectively. These findings should hopefully guide and help develop
compact, high-quality, ultra-relativistic electron sources, avoiding
conventional limits imposed by material breakdown or structural constraints.Comment: Supplementary Information starts on pg 1
Terahertz-driven linear electron acceleration
The cost, size and availability of electron accelerators is dominated by the
achievable accelerating gradient. Conventional high-brightness radio-frequency
(RF) accelerating structures operate with 30-50 MeV/m gradients. Electron
accelerators driven with optical or infrared sources have demonstrated
accelerating gradients orders of magnitude above that achievable with
conventional RF structures. However, laser-driven wakefield accelerators
require intense femtosecond sources and direct laser-driven accelerators and
suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing
requirements due to the short wavelength of operation. Here, we demonstrate the
first linear acceleration of electrons with keV energy gain using
optically-generated terahertz (THz) pulses. THz-driven accelerating structures
enable high-gradient electron or proton accelerators with simple accelerating
structures, high repetition rates and significant charge per bunch. Increasing
the operational frequency of accelerators into the THz band allows for greatly
increased accelerating gradients due to reduced complications with respect to
breakdown and pulsed heating. Electric fields in the GV/m range have been
achieved in the THz frequency band using all optical methods. With recent
advances in the generation of THz pulses via optical rectification of slightly
sub-picosecond pulses, in particular improvements in conversion efficiency and
multi-cycle pulses, increasing accelerating gradients by two orders of
magnitude over conventional linear accelerators (LINACs) has become a
possibility. These ultra-compact THz accelerators with extremely short electron
bunches hold great potential to have a transformative impact for free electron
lasers, future linear particle colliders, ultra-fast electron diffraction,
x-ray science, and medical therapy with x-rays and electron beams
Terahertz-driven, all-optical electron gun
Ultrashort electron beams with narrow energy spread, high charge, and low
jitter are essential for resolving phase transitions in metals, semiconductors,
and molecular crystals. These semirelativistic beams, produced by
phototriggered electron guns, are also injected into accelerators for x-ray
light sources. The achievable resolution of these time-resolved electron
diffraction or x-ray experiments has been hindered by surface field and timing
jitter limitations in conventional RF guns, which thus far are <200 MV/m and
>96 fs, respectively. A gun driven by optically-generated single-cycle THz
pulses provides a practical solution to enable not only GV/m surface fields but
also absolute timing stability, since the pulses are generated by the same
laser as the phototrigger. Here, we demonstrate an all-optical THz gun yielding
peak electron energies approaching 1 keV, accelerated by 300 MV/m THz fields in
a novel micron-scale waveguide structure. We also achieve quasimonoenergetic,
sub-keV bunches with 32 fC of charge, which can already be used for
time-resolved low-energy electron diffraction. Such ultracompact, easy to
implement guns driven by intrinsically synchronized THz pulses that are pumped
by an amplified arm of the already present photoinjector laser provide a new
tool with potential to transform accelerator based science.Comment: 24 pages, 9 figure
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