496 research outputs found
Non-Paraxial Accelerating Beams
We present the spatially accelerating solutions of the Maxwell equations.
Such non-paraxial beams accelerate in a circular trajectory, thus generalizing
the concept of Airy beams. For both TE and TM polarizations, the beams exhibit
shape-preserving bending with sub-wavelength features, and the Poynting vector
of the main lobe displays a turn of more than 90 degrees. We show that these
accelerating beams are self-healing, analyze their properties, and compare to
the paraxial Airy beams. Finally, we present the new family of periodic
accelerating beams which can be constructed from our solutions
On the Development and Pre-Flight Testing of the Affordable Guided Airdrop System for G-12 Cargo Parachute
16th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar, Boston, MA, May 21-24, 2001
meV resolution in laser-assisted energy-filtered transmission electron microscopy
The electronic, optical, and magnetic properties of quantum solids are
determined by their low-energy (< 100 meV) many-body excitations. Dynamical
characterization and manipulation of such excitations relies on tools that
combine nm-spatial, fs-temporal, and meV-spectral resolution. Currently,
phonons and collective plasmon resonances can be imaged in nanostructures with
sub-nm and 10s meV space/energy resolution using state-of-the-art
energy-filtered transmission electron microscopy (TEM), but only under static
conditions, while fs-resolved measurements are common but lack spatial or
energy resolution. Here, we demonstrate a new method of spectrally resolved
photon-induced near-field electron microscopy (SRPINEM) that allows us to
obtain nm-fs-resolved maps of nanoparticle plasmons with an energy resolution
determined by the laser linewidth (20 meV in this work), and not limited by
electron beam and spectrometer energy spreading. This technique can be extended
to any optically-accessible low-energy mode, thus pushing TEM to a previously
inaccessible spectral domain with an unprecedented combination of space, energy
and temporal resolution.Comment: 19 pages, 7 figure
All-angle negative refraction of highly squeezed plasmon and phonon polaritons in graphene-boron nitride heterostructures
A fundamental building block for nanophotonics is the ability to achieve
negative refraction of polaritons, because this could enable the demonstration
of many unique nanoscale applications such as deep-subwavelength imaging,
superlens, and novel guiding. However, to achieve negative refraction of highly
squeezed polaritons, such as plasmon polaritons in graphene and phonon
polaritons in boron nitride (BN) with their wavelengths squeezed by a factor
over 100, requires the ability to flip the sign of their group velocity at
will, which is challenging. Here we reveal that the strong coupling between
plasmon and phonon polaritons in graphene-BN heterostructures can be used to
flip the sign of the group velocity of the resulting hybrid
(plasmon-phonon-polariton) modes. We predict all-angle negative refraction
between plasmon and phonon polaritons, and even more surprisingly, between
hybrid graphene plasmons, and between hybrid phonon polaritons. Graphene-BN
heterostructures thus provide a versatile platform for the design of
nano-metasurfaces and nano-imaging elements.Comment: 16 pages; 3 figure
From attosecond to zeptosecond coherent control of free-electron wave functions using semi-infinite light fields
Light-electron interaction in empty space is the seminal ingredient for
free-electron lasers and also for controlling electron beams to dynamically
investigate materials and molecules. Pushing the coherent control of free
electrons by light to unexplored timescales, below the attosecond, would enable
unprecedented applications in light-assisted electron quantum circuits and
diagnostics at extremely small timescales, such as those governing
intramolecular electronic motion and nuclear phenomena. We experimentally
demonstrate attosecond coherent manipulation of the electron wave function in a
transmission electron microscope, and show that it can be pushed down to the
zeptosecond regime with existing technology. We make a relativistic pulsed
electron beam interact in free space with an appropriately synthesized
semi-infinite light field generated by two femtosecond laser pulses reflected
at the surface of a mirror and delayed by fractions of the optical cycle. The
amplitude and phase of the resulting coherent oscillations of the electron
states in energymomentum space are mapped via momentum-resolved ultrafast
electron energy-loss spectroscopy. The experimental results are in full
agreement with our theoretical framework for light-electron interaction, which
predicts access to the zeptosecond timescale by combining semi-infinite X-ray
fields with free electrons.Comment: 22 pages, 6 figure
Sending femtosecond pulses in circles: highly non-paraxial accelerating beams
We use caustic beam shaping on 100 fs pulses to experimentally generate
non-paraxial accelerating beams along a 60 degree circular arc, moving
laterally by 14 \mum over a 28 \mum propagation length. This is the highest
degree of transverse acceleration reported to our knowledge. Using diffraction
integral theory and numerical beam propagation simulations, we show that
circular acceleration trajectories represent a unique class of non-paraxial
diffraction-free beam profile which also preserves the femtosecond temporal
structure in the vicinity of the caustic
Maximal Spontaneous Photon Emission and Energy Loss from Free Electrons
Free electron radiation such as Cerenkov, Smith--Purcell, and transition
radiation can be greatly affected by structured optical environments, as has
been demonstrated in a variety of polaritonic, photonic-crystal, and
metamaterial systems. However, the amount of radiation that can ultimately be
extracted from free electrons near an arbitrary material structure has remained
elusive. Here we derive a fundamental upper limit to the spontaneous photon
emission and energy loss of free electrons, regardless of geometry, which
illuminates the effects of material properties and electron velocities. We
obtain experimental evidence for our theory with quantitative measurements of
Smith--Purcell radiation. Our framework allows us to make two predictions. One
is a new regime of radiation operation---at subwavelength separations, slower
(nonrelativistic) electrons can achieve stronger radiation than fast
(relativistic) electrons. The second is a divergence of the emission
probability in the limit of lossless materials. We further reveal that such
divergences can be approached by coupling free electrons to photonic bound
states in the continuum (BICs). Our findings suggest that compact and efficient
free-electron radiation sources from microwaves to the soft X-ray regime may be
achievable without requiring ultrahigh accelerating voltages.Comment: 7 pages, 4 figure
Controlling Cherenkov angles with resonance transition radiation
Cherenkov radiation provides a valuable way to identify high energy particles
in a wide momentum range, through the relation between the particle velocity
and the Cherenkov angle. However, since the Cherenkov angle depends only on
material's permittivity, the material unavoidably sets a fundamental limit to
the momentum coverage and sensitivity of Cherenkov detectors. For example, Ring
Imaging Cherenkov detectors must employ materials transparent to the frequency
of interest as well as possessing permittivities close to unity to identify
particles in the multi GeV range, and thus are often limited to large gas
chambers. It would be extremely important albeit challenging to lift this
fundamental limit and control Cherenkov angles as preferred. Here we propose a
new mechanism that uses constructive interference of resonance transition
radiation from photonic crystals to generate both forward and backward
Cherenkov radiation. This mechanism can control Cherenkov angles in a flexible
way with high sensitivity to any desired range of velocities. Photonic crystals
thus overcome the severe material limit for Cherenkov detectors, enabling the
use of transparent materials with arbitrary values of permittivity, and provide
a promising option suited for identification of particles at high energy with
enhanced sensitivity.Comment: There are 16 pages and 4 figures for the manuscript. Supplementary
information with 18 pages and 5 figures, appended at the end of the file with
the manuscript. Source files in Word format converted to PDF. Submitted to
Nature Physic
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