36 research outputs found
Spectral and spatial shaping of Smith Purcell Radiation
The Smith Purcell effect, observed when an electron beam passes in the
vicinity of a periodic structure, is a promising platform for the generation of
electromagnetic radiation in previously-unreachable spectral ranges. However,
most of the studies of this radiation were performed on simple periodic
gratings, whose radiation spectrum exhibits a single peak and its higher
harmonics predicted by a well-established dispersion relation. Here, we propose
a method to shape the spatial and spectral far-field distribution of the
radiation using complex periodic and aperiodic gratings. We show, theoretically
and experimentally, that engineering multiple peak spectra with controlled
widths located at desired wavelengths is achievable using Smith-Purcell
radiation. Our method opens the way to free-electron driven sources with
tailored angular and spectral response, and gives rise to focusing
functionality for spectral ranges where lenses are unavailable or inefficient
Smith-Purcell Radiation from Low-Energy Electrons
Recent advances in the fabrication of nanostructures and nanoscale features
in metasurfaces offer a new prospect for generating visible, light emission
from low energy electrons. In this paper, we present the experimental
observation of visible light emission from low-energy free electrons
interacting with nanoscale periodic surfaces through the Smith-Purcell (SP)
effect. SP radiation is emitted when electrons pass in close proximity over a
periodic structure, inducing collective charge motion or dipole excitations
near the surface, thereby giving rise to electromagnetic radiation. We
demonstrate a controlled emission of SP light from nanoscale gold gratings with
periodicity as small as 50 nm, enabling the observation of visible SP radiation
by low energy electrons (1.5 to 6 keV), an order of magnitude lower than
previously reported. We study the emission wavelength and intensity dependence
on the grating pitch and electron energy, showing agreement between experiment
and theory. Further reduction of structure periodicity should enable the
production of SP-based devices that operate with even slower electrons that
allow an even smaller footprint and facilitate the investigation of quantum
effects for light generation in nanoscale devices. A tunable light source
integrated in an electron microscope would enable the development of novel
electron-optical correlated spectroscopic techniques, with additional
applications ranging from biological imaging to solid-state lighting.Comment: 16 pages, 4 figure
Monochromatic X-ray source based on scattering from a magnetic nanoundulator
We present a novel design for an ultra-compact, passive light source capable
of generating ultraviolet and X-ray radiation, based on the interaction of free
electrons with the magnetic near-field of a ferromagnet. Our design is
motivated by recent advances in the fabrication of nanostructures, which allow
the confinement of large magnetic fields at the surface of ferromagnetic
nanogratings. Using ab initio simulations and a complementary analytical
theory, we show that highly directional, tunable, monochromatic radiation at
high frequencies could be produced from relatively low-energy electrons within
a tabletop design. The output frequency is tunable in the extreme ultraviolet
to hard X-ray range via electron kinetic energies from 1 keV-5 MeV and
nanograting periods from 1 {\mu}m-5 nm. Our design reduces the scale, cost, and
complexity of current free-electron-driven light schemes, bypassing the need
for lengthy acceleration stages in conventional synchrotrons and free-electron
lasers and driving lasers in other compact designs. Our design could help
realize the next generation of tabletop or on-chip X-ray sources.Comment: 8 pages, 4 figure
End-to-End Optimization of Metasurfaces for Imaging with Compressed Sensing
We present a framework for the end-to-end optimization of metasurface imaging
systems that reconstruct targets using compressed sensing, a technique for
solving underdetermined imaging problems when the target object exhibits
sparsity (i.e. the object can be described by a small number of non-zero
values, but the positions of these values are unknown). We nest an iterative,
unapproximated compressed sensing reconstruction algorithm into our end-to-end
optimization pipeline, resulting in an interpretable, data-efficient method for
maximally leveraging metaoptics to exploit object sparsity. We apply our
framework to super-resolution imaging and high-resolution depth imaging with a
phase-change material: in both situations, our end-to-end framework
computationally discovers optimal metasurface structures for compressed sensing
recovery, automatically balancing a number of complicated design
considerations. The optimized metasurface imaging systems are robust to noise,
significantly improving over random scattering surfaces and approaching the
ideal compressed sensing performance of a Gaussian matrix, showing how a
physical metasurface system can demonstrably approach the mathematical limits
of compressed sensing
Transcending shift-invariance in the paraxial regime via end-to-end inverse design of freeform nanophotonics
Traditional optical elements and conventional metasurfaces obey
shift-invariance in the paraxial regime. For imaging systems obeying paraxial
shift-invariance, a small shift in input angle causes a corresponding shift in
the sensor image. Shift-invariance has deep implications for the design and
functionality of optical devices, such as the necessity of free space between
components (as in compound objectives made of several curved surfaces). We
present a method for nanophotonic inverse design of compact imaging systems
whose resolution is not constrained by paraxial shift-invariance. Our method is
end-to-end, in that it integrates density-based full-Maxwell topology
optimization with a fully iterative elastic-net reconstruction algorithm. By
the design of nanophotonic structures that scatter light in a
non-shift-invariant manner, our optimized nanophotonic imaging system overcomes
the limitations of paraxial shift-invariance, achieving accurate, noise-robust
image reconstruction beyond shift-invariant resolution
Fullwave Maxwell inverse design of axisymmetric, tunable, and multi-scale multi-wavelength metalenses
We demonstrate new axisymmetric inverse-design techniques that can solve
problems radically different from traditional lenses, including
\emph{reconfigurable} lenses (that shift a multi-frequency focal spot in
response to refractive-index changes) and {\emph{widely separated}}
multi-wavelength lenses (m and m). We also present
experimental validation for an axisymmetric inverse-designed monochrome lens in
the near-infrared fabricated via two-photon polymerization. Axisymmetry allows
fullwave Maxwell solvers to be scaled up to structures hundreds or even
thousands of wavelengths in diameter before requiring domain-decomposition
approximations, while multilayer topology optimization with degrees
of freedom can tackle challenging design problems even when restricted to
axisymmetric structures.Comment: 13 pages, 6 figure
Biasing the quantum vacuum to control macroscopic probability distributions
One of the most important insights of quantum field theory is that
electromagnetic fields must fluctuate. Even in the vacuum state, the electric
and magnetic fields have a nonzero variance, leading to ubiquitous effects such
as spontaneous emission, the Lamb shift, the Casimir effect, and more. These
"vacuum fluctuations" have also been harnessed as a source of perfect
randomness, for example to generate perfectly random photonic bits. Despite
these achievements, many potential applications of quantum randomness in fields
such as probabilistic computing rely on controllable probability distributions,
which have not yet been realized on photonic platforms. In this work, we show
that the injection of vacuum-level "bias" fields into a multi-stable optical
system enables a controllable source of "biased" quantum randomness. We
demonstrate this concept in an optical parametric oscillator (OPO). Ordinarily,
an OPO initiated from the ground state develops a signal field in one of two
degenerate phase states (0 and ) with equal probability. By injecting bias
pulses which contain less than one photon on average, we control the
probabilities associated with the two output states, leading to the first
controllable photonic probabilistic bit (p-bit). We shed light on the physics
behind this process, showing quantitative agreement between theory and
experiment. Finally, we demonstrate the potential of our approach for sensing
sub-photon level fields by showing that our system is sensitive to the temporal
shape of bias field pulses far below the single photon level. Our results
suggest a new platform for the study of stochastic quantum dynamics in
nonlinear driven-dissipative systems, and point toward possible applications in
ultrafast photonic probabilistic computing, as well as the sensing of extremely
weak fields
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