6 research outputs found
Rubik's Optical Neural Networks: Multi-task Learning with Physics-aware Rotation Architecture
Recently, there are increasing efforts on advancing optical neural networks (ONNs), which bring significant advantages for machine learning (ML) in terms of power efficiency, parallelism, and computational speed. With the considerable benefits in computation speed and energy efficiency, there are significant interests in leveraging ONNs into medical sensing, security screening, drug detection, and autonomous driving. However, due to the challenge of implementing reconfigurability, deploying multi-task learning (MTL) algorithms on ONNs requires re-building and duplicating the physical diffractive systems, which significantly degrades the energy and cost efficiency in practical application scenarios. This work presents a novel ONNs architecture, namely, \textit{RubikONNs}, which utilizes the physical properties of optical systems to encode multiple feed-forward functions by physically rotating the hardware similarly to rotating a \textit{Rubik's Cube}. To optimize MTL performance on RubikONNs, two domain-specific physics-aware training algorithms \textit{RotAgg} and \textit{RotSeq} are proposed. Our experimental results demonstrate more than 4 improvements in energy and cost efficiency with marginal accuracy degradation compared to the state-of-the-art approaches
Boron NitrideâGraphene Nanocapacitor and the Origins of Anomalous Size-Dependent Increase of Capacitance
Conventional
wisdom suggests that decreasing dimensions of dielectric
materials (e.g., thickness of a film) should yield increasing capacitance.
However, the quantum capacitance and the so-called âdead-layerâ
effect often conspire to decrease the capacitance of extremely small
nanostructures, which is in sharp contrast to what is expected from
classical electrostatics. Very recently, first-principles studies
have predicted that a nanocapacitor made of graphene and hexagonal
boron nitride (h-BN) films can achieve superior capacitor properties.
In this work, we fabricate the thinnest possible nanocapacitor system,
essentially consisting of only monolayer materials: h-BN with graphene
electrodes. We experimentally demonstrate an increase of the h-BN
filmsâ permittivity in different stack structures combined
with graphene. We find a significant increase in capacitance below
a thickness of âŒ5 nm, more than 100% of what is predicted by
classical electrostatics. Detailed quantum mechanical calculations
suggest that this anomalous increase in capacitance is due to the
negative quantum capacitance that this particular materials system
exhibits
An Atomically Layered InSe Avalanche Photodetector
Atomically thin photodetectors based
on 2D materials have attracted great interest due to their potential
as highly energy-efficient integrated devices. However, photoinduced
carrier generation in these media is relatively poor due to low optical
absorption, limiting device performance. Current methods for overcoming
this problem, such as reducing contact resistances or back gating,
tend to increase dark current and suffer slow response times. Here,
we realize the avalanche effect in a 2D material-based photodetector
and show that avalanche multiplication can greatly enhance the device
response of an ultrathin InSe-based photodetector. This is achieved
by exploiting the large Schottky barrier formed between InSe and Al
electrodes, enabling the application of a large bias voltage. Plasmonic
enhancement of the photosensitivity, achieved by patterning arrays
of Al nanodisks onto the InSe layer, further improves device efficiency.
With an external quantum efficiency approaching 866%, a dark current
in the picoamp range, and a fast response time of 87 ÎŒs, this
atomic layer device exhibits multiple significant advances in overall
performance for this class of devices
Carbon Nanotube Terahertz Detector
Terahertz (THz) technologies are
promising for diverse areas such
as medicine, bioengineering, astronomy, environmental monitoring,
and communications. However, despite decades of worldwide efforts,
the THz region of the electromagnetic spectrum still continues to
be elusive for solid state technology. Here, we report on the development
of a powerless, compact, broadband, flexible, large-area, and polarization-sensitive
carbon nanotube THz detector that works at room temperature. The detector
is sensitive throughout the entire range of the THz technology gap,
with responsivities as high as âŒ2.5 V/W and polarization ratios
as high as âŒ5:1. Complete thermoelectric and opto-thermal characterization
together unambiguously reveal the photothermoelectric origin of the
THz photosignal, triggered by plasmonic absorption and collective
antenna effects, and suggest that judicious design of thermal management
and quantum engineering of Seebeck coefficients will lead to further
enhancement of device performance
3D Band Diagram and Photoexcitation of 2Dâ3D Semiconductor Heterojunctions
The emergence of a rich variety of
two-dimensional (2D) layered semiconductor materials has enabled the
creation of atomically thin heterojunction devices. Junctions between
atomically thin 2D layers and 3D bulk semiconductors can lead to junctions
that are fundamentally electronically different from the covalently
bonded conventional semiconductor junctions. Here we propose a new
3D band diagram for the heterojunction formed between n-type monolayer
MoS<sub>2</sub> and p-type Si, in which the conduction and valence
band-edges of the MoS<sub>2</sub> monolayer are drawn for both stacked
and in-plane directions. This new band diagram helps visualize the
flow of charge carriers inside the device in a 3D manner. Our detailed
wavelength-dependent photocurrent measurements fully support the diagrams
and unambiguously show that the band alignment is type I for this
2D-3D heterojunction. Photogenerated electronâhole pairs in
the atomically thin monolayer are separated and driven by an external
bias and control the âon/offâ states of the junction
photodetector device. Two photoresponse regimes with fast and slow
relaxation are also revealed in time-resolved photocurrent measurements,
suggesting the important role played by charge trap states
Tailoring the Physical Properties of Molybdenum Disulfide Monolayers by Control of Interfacial Chemistry
We
demonstrate how substrate interfacial chemistry can be utilized
to tailor the physical properties of single-crystalline molybdenum
disulfide (MoS<sub>2</sub>) atomic-layers. Semiconducting, two-dimensional
MoS<sub>2</sub> possesses unique properties that are promising for
future optical and electrical applications for which the ability to
tune its physical properties is essential. We use self-assembled monolayers
with a variety of end termination chemistries to functionalize substrates
and systematically study their influence on the physical properties
of MoS<sub>2</sub>. Using electrical transport measurements, temperature-dependent
photoluminescence spectroscopy, and empirical and first-principles
calculations, we explore the possible mechanisms involved. Our data
shows that combined interface-related effects of charge transfer,
built-in molecular polarities, varied densities of defects, and remote
interfacial phonons strongly modify the electrical and optical properties
of MoS<sub>2</sub>. These findings can be used to effectively enhance
or modulate the conductivity, field-effect mobility, and photoluminescence
in MoS<sub>2</sub> monolayers, illustrating an approach for local
and universal property modulations in two-dimensional atomic-layers