37 research outputs found
Highly confined low-loss plasmons in graphene-boron nitride heterostructures
Graphene plasmons were predicted to possess ultra-strong field confinement
and very low damping at the same time, enabling new classes of devices for deep
subwavelength metamaterials, single-photon nonlinearities, extraordinarily
strong light-matter interactions and nano-optoelectronic switches. While all of
these great prospects require low damping, thus far strong plasmon damping was
observed, with both impurity scattering and many-body effects in graphene
proposed as possible explanations. With the advent of van der Waals
heterostructures, new methods have been developed to integrate graphene with
other atomically flat materials. In this letter we exploit near-field
microscopy to image propagating plasmons in high quality graphene encapsulated
between two films of hexagonal boron nitride (h-BN). We determine dispersion
and particularly plasmon damping in real space. We find unprecedented low
plasmon damping combined with strong field confinement, and identify the main
damping channels as intrinsic thermal phonons in the graphene and dielectric
losses in the h-BN. The observation and in-depth understanding of low plasmon
damping is the key for the development of graphene nano-photonic and
nano-optoelectronic devices
Electrical detection of hyperbolic phonon-polaritons in heterostructures of graphene and boron nitride
Light properties in the mid-infrared can be controlled at a deep
subwavelength scale using hyperbolic phonons-polaritons (HPPs) of hexagonal
boron nitride (h-BN). While propagating as waveguided modes HPPs can
concentrate the electric field in a chosen nano-volume. Such a behavior is at
the heart of many applications including subdiffraction imaging and sensing.
Here, we employ HPPs in heterostructures of h-BN and graphene as new
nano-optoelectronic platform by uniting the benefits of efficient hot-carrier
photoconversion in graphene and the hyperbolic nature of h-BN. We demonstrate
electrical detection of HPPs by guiding them towards a graphene pn-junction. We
shine a laser beam onto a gap in metal gates underneath the heterostructure,
where the light is converted into HPPs. The HPPs then propagate as confined
rays heating up the graphene leading to a strong photocurrent. This concept is
exploited to boost the external responsivity of mid-infrared photodetectors,
overcoming the limitation of graphene pn-junction detectors due to their small
active area and weak absorption. Moreover this type of detector exhibits
tunable frequency selectivity due to the HPPs, which combined with its high
responsivity paves the way for efficient high-resolution mid-infrared imaging
Thermoelectric detection and imaging of 1 propagating graphene plasmons
Controlling, detecting and generating propagating plasmons by all-electrical means is at the heart of on-chip nano-optical processing1, 2, 3. Graphene carries long-lived plasmons that are extremely confined and controllable by electrostatic fields4, 5, 6, 7; however, electrical detection of propagating plasmons in graphene has not yet been realized. Here, we present an all-graphene mid-infrared plasmon detector operating at room temperature, where a single graphene sheet serves simultaneously as the plasmonic medium and detector. Rather than achieving detection via added optoelectronic materials, as is typically done in other plasmonic systems8, 9, 10, 11, 12, 13, 14, 15, our device converts the natural decay product of the plasmon—electronic heat—directly into a voltage through the thermoelectric effect16, 17. We employ two local gates to fully tune the thermoelectric and plasmonic behaviour of the graphene. High-resolution real-space photocurrent maps are used to investigate the plasmon propagation and interference, decay, thermal diffusion, and thermoelectric generation.Peer ReviewedPostprint (author's final draft
Near-field photocurrent nanoscopy on bare and encapsulated graphene
Opto-electronic devices utilizing graphene have already demonstrated unique
capabilities, which are much more difficult to realize with conventional
technologies. However, the requirements in terms of material quality and
uniformity are very demanding. A major roadblock towards high-performance
devices are the nanoscale variations of graphene properties, which strongly
impact the macroscopic device behaviour. Here, we present and apply
opto-electronic nanoscopy to measure locally both the optical and electronic
properties of graphene devices. This is achieved by combining scanning
near-field infrared nanoscopy with electrical device read-out, allowing
infrared photocurrent mapping at length scales of tens of nanometers. We apply
this technique to study the impact of edges and grain boundaries on spatial
carrier density profiles and local thermoelectric properties. Moreover, we show
that the technique can also be applied to encapsulated graphene/hexagonal boron
nitride (h-BN) devices, where we observe strong charge build-up near the edges,
and also address a device solution to this problem. The technique enables
nanoscale characterization for a broad range of common graphene devices without
the need of special device architectures or invasive graphene treatment
Dissociation of two-dimensional excitons in monolayer WSe<sub>2</sub>
In two-dimensional semiconductors excitons are strongly bound, suppressing the creation of free carriers. Here, the authors investigate the main exciton dissociation pathway in p-n junctions of monolayer WSe2 by means of time and spectrally resolved photocurrent measurements