22 research outputs found
Manipulating infrared photons using plasmons in transparent graphene superlattices
Superlattices are artificial periodic nanostructures which can control the
flow of electrons. Their operation typically relies on the periodic modulation
of the electric potential in the direction of electron wave propagation. Here
we demonstrate transparent graphene superlattices which can manipulate infrared
photons utilizing the collective oscillations of carriers, i.e., plasmons of
the ensemble of multiple graphene layers. The superlattice is formed by
depositing alternating wafer-scale graphene sheets and thin insulating layers,
followed by patterning them all together into 3-dimensional
photonic-crystal-like structures. We demonstrate experimentally that the
collective oscillation of Dirac fermions in such graphene superlattices is
unambiguously nonclassical: compared to doping single layer graphene,
distributing carriers into multiple graphene layers strongly enhances the
plasmonic resonance frequency and magnitude, which is fundamentally different
from that in a conventional semiconductor superlattice. This property allows us
to construct widely tunable far-infrared notch filters with 8.2 dB rejection
ratio and terahertz linear polarizers with 9.5 dB extinction ratio, using a
superlattice with merely five graphene atomic layers. Moreover, an unpatterned
superlattice shields up to 97.5% of the electromagnetic radiations below 1.2
terahertz. This demonstration also opens an avenue for the realization of other
transparent mid- and far-infrared photonic devices such as detectors,
modulators, and 3-dimensional meta-material systems.Comment: under revie
Photoconductivity of biased graphene
Graphene is a promising candidate for optoelectronic applications such as
photodetectors, terahertz imagers, and plasmonic devices. The origin of
photoresponse in graphene junctions has been studied extensively and is
attributed to either thermoelectric or photovoltaic effects. In addition, hot
carrier transport and carrier multiplication are thought to play an important
role. Here we report the intrinsic photoresponse in biased but otherwise
homogeneous graphene. In this classic photoconductivity experiment, the
thermoelectric effects are insignificant. Instead, the photovoltaic and a
photo-induced bolometric effect dominate the photoresponse due to hot
photocarrier generation and subsequent lattice heating through electron-phonon
cooling channels respectively. The measured photocurrent displays polarity
reversal as it alternates between these two mechanisms in a backgate voltage
sweep. Our analysis yields elevated electron and phonon temperatures, with the
former an order higher than the latter, confirming that hot electrons drive the
photovoltaic response of homogeneous graphene near the Dirac point
Mid-infrared plasmons in scaled graphene nanostructures
Plasmonics takes advantage of the collective response of electrons to
electromagnetic waves, enabling dramatic scaling of optical devices beyond the
diffraction limit. Here, we demonstrate the mid-infrared (4 to 15 microns)
plasmons in deeply scaled graphene nanostructures down to 50 nm, more than 100
times smaller than the on-resonance light wavelength in free space. We reveal,
for the first time, the crucial damping channels of graphene plasmons via its
intrinsic optical phonons and scattering from the edges. A plasmon lifetime of
20 femto-seconds and smaller is observed, when damping through the emission of
an optical phonon is allowed. Furthermore, the surface polar phonons in SiO2
substrate underneath the graphene nanostructures lead to a significantly
modified plasmon dispersion and damping, in contrast to a non-polar
diamond-like-carbon (DLC) substrate. Much reduced damping is realized when the
plasmon resonance frequencies are close to the polar phonon frequencies. Our
study paves the way for applications of graphene in plasmonic waveguides,
modulators and detectors in an unprecedentedly broad wavelength range from
sub-terahertz to mid-infrared.Comment: submitte
Graphene photodetectors for high-speed optical communications
While silicon has dominated solid-state electronics for more than four
decades, a variety of new materials have been introduced into photonics to
expand the accessible wavelength range and to improve the performance of
photonic devices. For example, gallium-nitride based materials enable the light
emission at blue and ultraviolet wavelengths, and high index contrast
silicon-on-insulator facilitates the realization of ultra dense and CMOS
compatible photonic devices. Here, we report the first deployment of graphene,
a two-dimensional carbon material, as the photo-detection element in a 10
Gbits/s optical data link. In this interdigitated metal-graphene-metal
photodetector, an asymmetric metallization scheme is adopted to break the
mirror symmetry of the built-in electric-field profile in conventional graphene
field-effect-transistor channels, allowing for efficient photo-detection within
the entire area of light illumination. A maximum external photo-responsivity of
6.1 mA/W is achieved at 1.55 {\mu}m wavelength, a very impressive value given
that the material is below one nanometer in thickness. Moreover, owing to the
unique band structure and exceptional electronic properties of graphene, high
speed photodetectors with an ultra-wide operational wavelength range at least
from 300 nm to 6 {\mu}m can be realized using this fascinating material.Comment: 20 pages, 3 figure
Strong light-matter coupling in two-dimensional atomic crystals
Two dimensional (2D) atomic crystals of graphene, and transition metal
dichalcogenides have emerged as a class of materials that show strong
light-matter interaction. This interaction can be further controlled by
embedding such materials into optical microcavities. When the interaction is
engineered to be stronger than the dissipation of light and matter entities,
one approaches the strong coupling regime resulting in the formation of
half-light half-matter bosonic quasiparticles called microcavity polaritons.
Here we report the evidence of strong light-matter coupling and formation of
microcavity polaritons in a two dimensional atomic crystal of molybdenum
disulphide (MoS2) embedded inside a dielectric microcavity at room temperature.
A Rabi splitting of 46 meV and highly directional emission is observed from the
MoS2 microcavity owing to the coupling between the 2D excitons and the cavity
photons. Realizing strong coupling effects at room temperature in a disorder
free potential landscape is central to the development of practical polaritonic
circuits and switches.Comment: 25 pages, 7 figure
Giant resonant light forces in microspherical photonics
Resonant light pressure effects can open new degrees of freedom in optical manipulation with microparticles, but they have been traditionally considered as relatively subtle effects. Using a simplified two-dimensional model of surface electromagnetic waves evanescently coupled to whispering gallery modes (WGMs) in transparent circular cavities, we show that under resonant conditions the peaks of the optical forces can approach theoretical limits imposed by the momentum conservation law on totally absorbing particles. Experimentally, we proved the existence of strong peaks of the optical forces by studying the optical propulsion of dielectric microspheres along tapered microfibers. We observed giant optical propelling velocities ∼0.45 mm s−1 for some of the 15-20 µm polystyrene microspheres in water for guided powers limited at ∼43 mW. Such velocities exceed previous observations by more than an order of magnitude, thereby providing evidence for the strongly enhanced resonant optical forces. We analyzed the statistical properties of the velocity distribution function measured for slightly disordered (∼1% size variations) ensembles of microspheres with mean diameters varying from 3 to 20 µm. These results demonstrate a principal possibility of optical sorting of microspheres with the positions of WGM resonances overlapped at the wavelength of the laser source. They can be used as building blocks of the lossless coupled resonator optical waveguides and various integrated optoelectronics devices