70 research outputs found
Valley-Hall photonic topological insulators with dual-band kink states
Extensive researches have revealed that valley, a binary degree of freedom
(DOF), can be an excellent candidate of information carrier. Recently, valley
DOF has been introduced into photonic systems, and several valley-Hall photonic
topological insulators (PTIs) have been experimentally demonstrated. However,
in the previous valley-Hall PTIs, topological kink states only work at a single
frequency band, which limits potential applications in multiband waveguides,
filters, communications, and so on. To overcome this challenge, here we
experimentally demonstrate a valley-Hall PTI, where the topological kink states
exist at two separated frequency bands, in a microwave substrate-integrated
circuitry. Both the simulated and experimental results demonstrate the
dual-band valley-Hall topological kink states are robust against the sharp
bends of the internal domain wall with negligible inter-valley scattering. Our
work may pave the way for multi-channel substrate-integrated photonic devices
with high efficiency and high capacity for information communications and
processing
Realization of a three-dimensional photonic topological insulator
Confining photons in a finite volume is in high demand in modern photonic
devices. This motivated decades ago the invention of photonic crystals,
featured with a photonic bandgap forbidding light propagation in all
directions. Recently, inspired by the discoveries of topological insulators
(TIs), the confinement of photons with topological protection has been
demonstrated in two-dimensional (2D) photonic structures known as photonic TIs,
with promising applications in topological lasers and robust optical delay
lines. However, a fully three-dimensional (3D) topological photonic bandgap has
never before been achieved. Here, we experimentally demonstrate a 3D photonic
TI with an extremely wide (> 25% bandwidth) 3D topological bandgap. The sample
consists of split-ring resonators (SRRs) with strong magneto-electric coupling
and behaves as a 'weak TI', or a stack of 2D quantum spin Hall insulators.
Using direct field measurements, we map out both the gapped bulk bandstructure
and the Dirac-like dispersion of the photonic surface states, and demonstrate
robust photonic propagation along a non-planar surface. Our work extends the
family of 3D TIs from fermions to bosons and paves the way for applications in
topological photonic cavities, circuits, and lasers in 3D geometries
Topological triply-degenerate point with double Fermi arcs
Unconventional chiral particles have recently been predicted to appear in
certain three dimensional (3D) crystal structures containing three- or
more-fold linear band degeneracy points (BDPs). These BDPs carry topological
charges, but are distinct from the standard twofold Weyl points or fourfold
Dirac points, and cannot be described in terms of an emergent relativistic
field theory. Here, we report on the experimental observation of a topological
threefold BDP in a 3D phononic crystal. Using direct acoustic field mapping, we
demonstrate the existence of the threefold BDP in the bulk bandstructure, as
well as doubled Fermi arcs of surface states consistent with a topological
charge of 2. Another novel BDP, similar to a Dirac point but carrying nonzero
topological charge, is connected to the threefold BDP via the doubled Fermi
arcs. These findings pave the way to using these unconventional particles for
exploring new emergent physical phenomena
LIBERO: Benchmarking Knowledge Transfer for Lifelong Robot Learning
Lifelong learning offers a promising paradigm of building a generalist agent
that learns and adapts over its lifespan. Unlike traditional lifelong learning
problems in image and text domains, which primarily involve the transfer of
declarative knowledge of entities and concepts, lifelong learning in
decision-making (LLDM) also necessitates the transfer of procedural knowledge,
such as actions and behaviors. To advance research in LLDM, we introduce
LIBERO, a novel benchmark of lifelong learning for robot manipulation.
Specifically, LIBERO highlights five key research topics in LLDM: 1) how to
efficiently transfer declarative knowledge, procedural knowledge, or the
mixture of both; 2) how to design effective policy architectures and 3)
effective algorithms for LLDM; 4) the robustness of a lifelong learner with
respect to task ordering; and 5) the effect of model pretraining for LLDM. We
develop an extendible procedural generation pipeline that can in principle
generate infinitely many tasks. For benchmarking purpose, we create four task
suites (130 tasks in total) that we use to investigate the above-mentioned
research topics. To support sample-efficient learning, we provide high-quality
human-teleoperated demonstration data for all tasks. Our extensive experiments
present several insightful or even unexpected discoveries: sequential
finetuning outperforms existing lifelong learning methods in forward transfer,
no single visual encoder architecture excels at all types of knowledge
transfer, and naive supervised pretraining can hinder agents' performance in
the subsequent LLDM. Check the website at https://libero-project.github.io for
the code and the datasets
LivelySpeaker: Towards Semantic-Aware Co-Speech Gesture Generation
Gestures are non-verbal but important behaviors accompanying people's speech.
While previous methods are able to generate speech rhythm-synchronized
gestures, the semantic context of the speech is generally lacking in the
gesticulations. Although semantic gestures do not occur very regularly in human
speech, they are indeed the key for the audience to understand the speech
context in a more immersive environment. Hence, we introduce LivelySpeaker, a
framework that realizes semantics-aware co-speech gesture generation and offers
several control handles. In particular, our method decouples the task into two
stages: script-based gesture generation and audio-guided rhythm refinement.
Specifically, the script-based gesture generation leverages the pre-trained
CLIP text embeddings as the guidance for generating gestures that are highly
semantically aligned with the script. Then, we devise a simple but effective
diffusion-based gesture generation backbone simply using pure MLPs, that is
conditioned on only audio signals and learns to gesticulate with realistic
motions. We utilize such powerful prior to rhyme the script-guided gestures
with the audio signals, notably in a zero-shot setting. Our novel two-stage
generation framework also enables several applications, such as changing the
gesticulation style, editing the co-speech gestures via textual prompting, and
controlling the semantic awareness and rhythm alignment with guided diffusion.
Extensive experiments demonstrate the advantages of the proposed framework over
competing methods. In addition, our core diffusion-based generative model also
achieves state-of-the-art performance on two benchmarks. The code and model
will be released to facilitate future research.Comment: Accepted by ICCV 202
Ideal Unconventional Weyl Point in a Chiral Photonic Metamaterial
Unconventional Weyl points (WPs), carrying topological charge 2 or higher,
possess interesting properties different from ordinary charge-1 WPs, including
multiple Fermi arcs that stretch over a large portion of the Brillouin zone.
Thus far, such WPs have been observed in chiral materials and acoustic
metamaterials, but there has been no clean demonstration in photonics in which
the unconventional photonic WPs are separated from trivial bands. We
experimentally realize an ideal symmetry-protected photonic charge-2 WP in a
three-dimensional topological chiral microwave metamaterial. We use field
mapping to directly observe the projected bulk dispersion, as well as the two
long surface arcs that form a noncontractible loop wrapping around the surface
Brillouin zone. The surface states span a record-wide frequency window of
around 22.7% relative bandwidth. We demonstrate that the surface states exhibit
a novel topological self-collimation property and are robust against disorder.
This work provides an ideal photonic platform for exploring fundamental physics
and applications of unconventional WPs.Comment: 6 pages, 4 figure
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