95 research outputs found
Comparison of gold- and graphene-based resonant nano-structures for terahertz metamaterials and an ultra-thin graphene-based modulator
Graphene exhibits unique material properties and in electromagnetic wave
technology, it raises the prospect of devices miniaturized down to the atomic
length scale. Here we study split-ring resonator metamaterials made from
graphene and we compare them to gold-based metamaterials. We find that
graphene's huge reactive response derived from its large kinetic inductance
allows for deeply subwavelength resonances, although its resonance strength is
reduced due to higher dissipative loss damping and smaller dipole coupling.
Nevertheless, tightly stacked graphene rings may provide for negative
permeability and the electric dipole resonance of graphene meta-atoms turns out
to be surprisingly strong. Based on these findings, we present a terahertz
modulator based on a metamaterial with a multi-layer stack of alternating
patterned graphene sheets separated by dielectric spacers. Neighbouring
graphene flakes are biased against each other, resulting in modulation depths
of over 75% at a transmission level of around 90%.Comment: 16 pages, 5 figure
Nonlinearity in the Dark: Broadband Terahertz Generation with Extremely High Efficiency
Plasmonic metamaterials and metasurfaces offer new opportunities in
developing high performance terahertz emitters and detectors beyond the
limitations of conventional nonlinear materials. However, simple meta-atoms for
second-order nonlinear applications encounter fundamental trade-offs in the
necessary symmetry breaking and local-field enhancement due to radiation
damping that is inherent to the operating resonant mode and cannot be
controlled separately. Here we present a novel concept that eliminates this
restriction obstructing the improvement of terahertz generation efficiency in
nonlinear metasurfaces based on metallic nanoresonators. This is achieved by
combining a resonant dark-state metasurface, which locally drives nonlinear
nanoresonators in the near field, with a specific spatial symmetry that enables
destructive interference of the radiating linear moments of the nanoresonators,
and perfect absorption via simultaneous electric and magnetic critical coupling
of the pump radiation to the dark mode. Our proposal allows eliminating linear
radiation damping, while maintaining constructive interference and effective
radiation of the nonlinear components. We numerically demonstrate a giant
second-order nonlinear susceptibility around Hundred-Billionth m/V, a one order
improvement compared with the previously reported split-ring-resonator
metasurface, and correspondingly, a 2 orders of magnitude enhanced terahertz
energy extraction should be expected with our configuration under the same
conditions. Our study offers a paradigm of high efficiency tunable nonlinear
metadevices and paves the way to revolutionary terahertz technologies and
optoelectronic nanocircuitry.Comment: 6 pages, 4 figure
Achieving a high-Q response in metamaterials by manipulating the toroidal excitations
The excitation of toroidal multipoles in metamaterials is investigated for a high- Q response at a subwavelength scale. In this paper, we explore the optimization of toroidal excitations in a planar metamaterial comprised of asymmetric split ring resonators (ASRRs). It is found that the scattering power of a toroidal dipole can be remarkably strengthened by adjusting the characteristic parameter of ASRRs: an asymmetric factor. Interestingly, the improvement in toroidal excitation accompanies an increment of the Q factor of the toroidal metamaterial; it is shown that both the scattering power of the toroidal dipole and the Q factor increase more than one order by changing the asymmetric factor of ASRRs. The optimization in the excitation of a toroidal multipole provides an opportunity to further increase the Q factor of the metamaterial and boost light-matter interactions at the subwavelength scale for potential applications in low-power nonlinear processing and sensitive photonic applications
Experimental Realization of an Extreme-Parameter Omnidirectional Cloak
An ideal transformation-based omnidirectional cloak always relies on metamaterials with extreme parameters, which were previously thought to be too difficult to realize. For such a reason, in previous experimental proposals of invisibility cloaks, the extreme parameters requirements are usually abandoned, leading to inherent scattering. Here, we report on the first experimental demonstration of an omnidirectional cloak that satisfies the extreme parameters requirement, which can hide objects in a homogenous background. Instead of using resonant metamaterials that usually involve unavoidable absorptive loss, the extreme parameters are achieved using a nonresonant metamaterial comprising arrays of subwavelength metallic channels manufactured with 3D metal printing technology. A high level transmission of electromagnetic wave propagating through the present omnidirectional cloak, as well as significant reduction of scattering field, is demonstrated both numerically and experimentally. Our work may also inspire experimental realizations of the other full-parameter omnidirectional optical devices such as concentrator, rotators, and optical illusion apparatuses
One- and two-dimensional photo-imprinted diffraction gratings for manipulating terahertz waves
Emerging technology based on artificial materials containing metallic
structures has raised the prospect for unprecedented control of terahertz waves
through components like filters, absorbers and polarizers. The functionality of
these devices is static by the very nature of their metallic or polaritonic
composition, although some degree of tunability can be achieved by
incorporating electrically biased semiconductors. Here, we demonstrate a
photonic structure by projecting the optical image of a metal mask onto a thin
GaAs substrate using a femtosecond pulsed laser source. We show that the
resulting high-contrast pattern of photo- excited carriers can create
diffractive elements operating in transmission. With the metal mask replaced by
a digital micromirror device, our photo-imprinted photonic structures provide a
route to terahertz components with reconfigurable functionality.Comment: 12 pages, 5 figure
Graphene Plasmonics: A Platform for 2D Optics
2D optics is gradually emerging as a frontier in modern optics. Plasmons in graphene provide a prominent platform for 2D optics in which the light is squeezed into atomic scale. This report highlights some recent progresses in graphene plasmons toward the 2D optics. The launch, observation, and advanced manipulation of propagating graphene plasmons for 2D optical circuits are described. Representative achievements associated with graphene metasurfaces, challenges, recent progresses like photoexcited graphene metasurfaces, and the transformation optics linking 2D to bulk optics with singularity are investigated
Robustness of Optical Response for SelfâAssembled Plasmonic Metamaterials with Morphological Disorder and Surface Roughness
Bottomâup fabrication of metallized biotemplated nanostructures to form specific plasmonic nanoresonators holds promise as a means of achieving largeâscale optical metamaterials. However, in contrast to topâdown methods, the stochastic growth of selfâassembled nanoresonators is prone to significant disorder and surface roughness, which naturally raise an important question about the robustness of their resonant properties in terms of structural imperfections. An aggregatedârandomâsphere model is developed to mimic the nucleated growth of metallized DNA origami assembly, leading to metaâatoms with realistic, experimentally observed morphological disorder and surface roughness. Using the wellâknown splitâringâresonator (SRR) motif as an example, the resonant properties of metaâatoms under different levels of roughness are investigated and a strong tolerance of optical response against morphological disorder is revealed. It is found that in SRRs, even with dramatic roughness introduced, the expected resonances are still observed, despite broadening line shapes compared to ideal smooth structure. Only for extreme disorder, which causes drastic segmentation of SRRs, does the resonant response disappear. The demonstrations are very encouraging for the prospects of bottomâup fabrication toward versatile functional metamaterials and metadevices
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