Search for: All records

In the context of anti-de Sitter/conformal field theory , gravitational shockwaves serve as a geometric manifestation of boundary quantum chaos. We study this connection in general diffeomorphism-invariant theories involving an arbitrary number of bosonic fields. Specifically, we demonstrate that theories containing spin-2 or higher-spin fields generally admit classical localized shockwave solutions on black hole backgrounds, whereas spin-0 and spin-1 theories do not. As in the gravitational case, these higher-spin shockwaves provide a means to compute the out-of-time-order correlator. Both the Lyapunov exponent and the butterfly velocity are found to universally agree with predictions from pole skipping. In particular, higher-spin fields lead to a Lyapunov exponent that violates the chaos bound and a butterfly velocity that may exceed the speed of light. 

Abstract Exotic tree species, though widely used in forestry and restoration projects, pose great threats to local ecosystems. They need to be replaced with native species from natural forests. We hypothesized that natural forests contain large, fast-growing, dominant native tree species that are suitable for specific topographic conditions in forestry. We tested this hypothesis using data from a 50-ha forest dynamics plot in subtropical China. We classified the plot into the ridge, slope, and valley habitats and found that 34/87 species had significant associations with at least one topographic habitat. There were 90 tree species with a maximum diameter ≥ 30 cm, and their abundances varied widely in all habitat types. In all habitat types, for most species, rate of biomass gain due to recruitment was < 1% of its original biomass, and rate of biomass gain due to tree growth was between 1 and 5% of its original biomass. For most species, biomass loss due to tree mortality was not significantly different than biomass gain due to recruitment, but the resulting net biomass increment rates did not significantly differ from zero. The time required to reach a diameter of 30 cm from 1 cm diameter forAltingia chinensisin the slope habitat, forQuercus chungiiandMorella rubrain the ridge habitat and forCastanopsis carlesiiin all habitats could be as short as 30 years in our simulations based on actual distributions of tree growth observed in the forest. Principal component analyses of maximum diameter, abundance and net biomass increment rates suggested several species were worthy of further tests for use in forestry.Our study provides an example for screening native tree species from natural forests for forestry. Because native tree species are better for local ecosystems, our study will also contribute to biodiversity conservation in plantations. 

Abstract Dielectric metasurfaces, composed of planar arrays of subwavelength dielectric structures that collectively mimic the operation of conventional bulk optical elements, have revolutionized the field of optics by their potential in constructing high-efficiency and multi-functional optoelectronic systems on chip. The performance of a dielectric metasurface is largely determined by its constituent material, which is highly desired to have a high refractive index, low optical loss and wide bandgap, and at the same time, be fabrication friendly. Here, we present a new material platform based on tantalum pentoxide (Ta₂O₅) for implementing high-performance dielectric metasurface optics over the ultraviolet and visible spectral region. This wide-bandgap dielectric, exhibiting a high refractive index exceeding 2.1 and negligible extinction coefficient across a broad spectrum, can be easily deposited over large areas with good quality using straightforward physical vapor deposition, and patterned into high-aspect-ratio subwavelength nanostructures through commonly-available fluorine-gas-based reactive ion etching. We implement a series of high-efficiency ultraviolet and visible metasurfaces with representative light-field modulation functionalities including polarization-independent high-numerical-aperture lensing, spin-selective hologram projection, and vivid structural color generation, and the devices exhibit operational efficiencies up to 80%. Our work overcomes limitations faced by scalability of commonly-employed metasurface dielectrics and their operation into the visible and ultraviolet spectral range, and provides a novel route towards realization of high-performance, robust and foundry-manufacturable metasurface optics. 

Ear wearables (earables) are emerging platforms that are broadly adopted in various applications. There is an increasing demand for robust earables authentication because of the growing amount of sensitive information and the IoT devices that the earable could access. Traditional authentication methods become less feasible due to the limited input interface of earables. Nevertheless, the rich head-related sensing capabilities of earables can be exploited to capture human biometrics. In this paper, we propose EarSlide, an earable biometric authentication system utilizing the advanced sensing capacities of earables and the distinctive features of acoustic fingerprints when users slide their fingers on the face. It utilizes the inward-facing microphone of the earables and the face-ear channel of the ear canal to reliably capture the acoustic fingerprint. In particular, we study the theory of friction sound and categorize the characteristics of the acoustic fingerprints into three representative classes, pattern-class, ridge-groove-class, and coupling-class. Different from traditional fingerprint authentication only utilizes 2D patterns, we incorporate the 3D information in acoustic fingerprint and indirectly sense the fingerprint for authentication. We then design representative sliding gestures that carry rich information about the acoustic fingerprint while being easy to perform. It then extracts multi-class acoustic fingerprint features to reflect the inherent acoustic fingerprint characteristic for authentication. We also adopt an adaptable authentication model and a user behavior mitigation strategy to effectively authenticate legit users from adversaries. The key advantages of EarSlide are that it is resistant to spoofing attacks and its wide acceptability. Our evaluation of EarSlide in diverse real-world environments with intervals over one year shows that EarSlide achieves an average balanced accuracy rate of 98.37% with only one sliding gesture. 

We demonstrated an insertion loss of 2.2 dB and a back-excitation suppression of 5.1 dB over the 35 nm bandwidth with on-chip asymmetric metasurface.