6 research outputs found
Electrical Switching of Infrared Light Using Graphene Integration with Plasmonic Fano Resonant Metasurfaces
Graphene
has emerged as a promising optoelectronic material because
its optical properties can be rapidly and dramatically changed using
electric gating. Graphene’s weak optical response, especially
in the infrared part of the spectrum, remains the key challenge to
developing practical graphene-based optical devices such as modulators,
infrared detectors, and tunable reflect-arrays. Here it is experimentally
and theoretically demonstrated that a plasmonic metasurface with two
Fano resonances can dramatically enhance the interaction of infrared
light with single layer graphene. Graphene’s plasmonic response
in the Pauli blocking regime is shown to cause strong spectral shifts
of the Fano resonances without inducing additional nonradiative losses.
It is shown that such electrically controllable spectral shift, combined
with the narrow spectral width of the metasurface’s Fano resonances,
enables reflectivity modulation by nearly an order of magnitude. We
also demonstrate that metasurface-based enhancement of the interaction
between graphene and infrared light can be utilized to extract one
of the key optical parameters of graphene: the free carrier scattering
rate. Numerical simulations demonstrate the possibility of strong
active modulation of the phase of the reflected light while keeping
the reflectivity nearly constant, thereby paving the way to tunable
infrared lenses and beam steering devices based on electrically controlled
graphene integrated with resonant metasurfaces
Interplay Between Optical Bianisotropy and Magnetism in Plasmonic Metamolecules
The smallness of
natural molecules and atoms with respect to the
wavelength of light imposes severe limits on the nature of their optical
response. For example, the well-known argument of Landau and Lifshitz
and its recent extensions that include chiral molecules show that
the electric dipole response dominates over the magneto-electric (bianisotropic)
and an even smaller magnetic dipole optical response for all natural
materials. Here, we experimentally demonstrate that both these responses
can be greatly enhanced in plasmonic nanoclusters. Using atomic force
microscopy nanomanipulation technique, we assemble a plasmonic metamolecule
that is designed for strong and simultaneous optical magnetic and
magneto-electric excitation. Angle-dependent scattering spectroscopy
is used to disentangle the two responses and to demonstrate that their
constructive/destructive interplay causes strong directional scattering
asymmetry. This asymmetry is used to extract both magneto-electric
and magnetic dipole responses and to demonstrate their enhancement
in comparison to ordinary atomistic materials
Supplement 1: Ultrathin gradient nonlinear metasurface with a giant nonlinear response
Supplemental document Originally published in Optica on 20 March 2016 (optica-3-3-283
Scaling on-chip photonic neural processors using arbitrarily programmable wave propagation
On-chip photonic processors for neural networks have potential benefits in both speed and energy efficiency but have not yet reached the scale at which they can outperform electronic processors. The dominant paradigm for designing on-chip photonics is to make networks of relatively bulky discrete components connected by one-dimensional waveguides. A far more compact alternative is to avoid explicitly defining any components and instead sculpt the continuous substrate of the photonic processor to directly perform the computation using waves freely propagating in two dimensions. We propose and demonstrate a device whose refractive index as a function of space, , can be rapidly reprogrammed, allowing arbitrary control over the wave propagation in the device. Our device, a 2D-programmable waveguide, combines photoconductive gain with the electro-optic effect to achieve massively parallel modulation of the refractive index of a slab waveguide, with an index modulation depth of and approximately programmable degrees of freedom. We used a prototype device with a functional area of to perform neural-network inference with up to 49-dimensional input vectors in a single pass, achieving 96% accuracy on vowel classification and 86% accuracy on -pixel MNIST handwritten-digit classification. This is a scale beyond that of previous photonic chips relying on discrete components, illustrating the benefit of the continuous-waves paradigm. In principle, with large enough chip area, the reprogrammability of the device's refractive index distribution enables the reconfigurable realization of any passive, linear photonic circuit or device. This promises the development of more compact and versatile photonic systems for a wide range of applications, including optical processing, smart sensing, spectroscopy, and optical communications
Improved Electrical Conductivity of Graphene Films Integrated with Metal Nanowires
Polycrystalline graphene grown by chemical vapor deposition
(CVD)
on metals and transferred onto arbitrary substrates has line defects
and disruptions such as wrinkles, ripples, and folding that adversely
affect graphene transport properties through the scattering of the
charge carriers. It is found that graphene assembled with metal nanowires
(NWs) dramatically decreases the resistance of graphene films. Graphene/NW
films with a sheet resistance comparable to that of the intrinsic
resistance of graphene have been obtained and tested as a transparent
electrode replacing indium tin oxide films in electrochromic (EC)
devices. The successful integration of such graphene/NW films into
EC devices demonstrates their potential for a wide range of optoelectronic
device applications
Contrast between Surface Plasmon Polariton-Mediated Extraordinary Optical Transmission Behavior in Epitaxial and Polycrystalline Ag Films in the Mid- and Far-Infrared Regimes
In this Letter we report a comparative study, in the
infrared regime,
of surface plasmon polariton (SPP) propagation in epitaxially grown
Ag films and in polycrystalline Ag films, all grown on Si substrates.
Plasmonic resonance features are analyzed using extraordinary optical
transmission (EOT) measurements, and SPP band structures for the two
dielectric/metal interfaces are investigated for both types of film.
At the Si/Ag interface, EOT spectra show almost identical features
for epitaxial and polycrystalline Ag films and are characterized by
sharp Fano resonances. On the contrary, at the air/Ag interface, dramatic
differences are observed: while the epitaxial film continues to exhibit
sharp Fano resonances, the polycrystalline film shows only broad spectral
features and much lower transmission intensities. In corroboration
with theoretical simulations, we find that surface roughness plays
a critical role in SPP propagation for this wavelength range