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All-optical control of high-purity trions in nanoscale waveguide
No full textThe generation of high-purity localized trions, dynamic exciton–trion interconversion, and their spatial modulation in two-dimensional (2D) semiconductors are building blocks for the realization of trion-based optoelectronic devices. Here, we present a method for the all-optical control of the exciton-to-trion conversion process and its spatial distributions in a MoS₂ monolayer. We induce a nanoscale strain gradient in a 2D crystal transferred on a lateral metal–insulator–metal (MIM) waveguide and exploit propagating surface plasmon polaritons (SPPs) to localize hot electrons. These significantly increase the electrons and efficiently funnel excitons in the lateral MIM waveguide, facilitating complete exciton-to-trion conversion even at ambient conditions. Additionally, we modulate the SPP mode using adaptive wavefront shaping, enabling all-optical control of the exciton-to-trion conversion rate and trion distribution in a reversible manner. Our work provides a platform for harnessing excitonic quasiparticles efficiently in the form of trions at ambient conditions, enabling high-efficiency photoconversion
A Secondary Zone of Uplift Measured After Megathrust Earthquakes: Caused by Early Downdip Afterslip?
No full textA secondary zone of surface uplift (SZU), located from 200 to 400 km landward of the trench, has been measured after several megathrust earthquakes. The SZU reached a few centimeters hours to days after the 2011 M_w 9.1 Tohoku (Japan) and 2010 M_w 8.8 Maule (Chile) earthquakes. Published coseismic finite-fault models for these events do not reproduce the measured SZU. One interpretation is that this SZU is universal, driven by volume deformation around the slab interface (van Dinther et al., 2019, https://doi.org/10.1007/s00024-019-02250-z). In contrast, we demonstrate the SZU may instead result from slip on the slab interface, and suggest it might be caused by rapid afterslip. We can reproduce the SZU with fault slip if elastic heterogeneities associated with the subducting slab are accounted for, as opposed to assuming homogeneous or layered elastic lithospheric structures
The Evolution of Activity and Chemical Composition in Rosetta's Comet Targets across Multiple Apparitions: Complications for CS₂ as the CS Parent in Comet Nuclei
No full textJupiter-family comets are ephemeral small bodies injected into the inner solar system from the Kuiper Belt, doomed to either sublimate all their volatiles and become inert or violently shatter from the activity. We investigate two target candidates of the ESA Rosetta mission, comets 46P/Wirtanen and 67P/Churyumov-Gerasimenko, which had favorable apparitions for Earth-based observations in 2018–2019 and 2021, respectively. Using the Hubble Space Telescope STIS and COS instruments, we observed OH and CS emissions to characterize production rates of H₂O and CS, established Afρ values, and placed upper limits on the production rate of C₂ and its parent. We find CS/H₂O relative abundances that are significantly (5σ–7σ) larger than previous remote near-UV (NUV) measurements of 46P and 67P at similar heliocentric distances and CS/H2O values larger than those obtained via contemporaneous submillimeter observations for the same apparitions. We also find that for 67P the remote derivations of CS₂/H₂O ratios are substantially (∼50×) higher than the values measured by the ROSINA mass spectrometer on board the Rosetta spacecraft for all NUV-derived CS₂ production rates. The discrepancy points toward an unidentified CS parent or parents with contributing factors from uncertainties with the fluorescence efficiencies of the CS (0,0) band of the A¹Π–X¹Σ⁺ system around 2580 Å. Given the significance of understanding the chemistry and dissociation physics of sulfur-bearing molecules in comets for tracing planetesimal formation environments, as well as the limited studies in this area, we propose several hypotheses to explain this discrepancy and outline future studies to address these issues
Error Analysis of Convolutional Beamspace Algorithms
No full textBeamspace processing for DOA estimation offers low computational complexity and high DOA resolution. Moreover, unlike classical beamspace methods, convolutional beamspace (CBS) preserves the Vandermonde structure of uniform linear array output, so no additional preparation is needed to apply root-MUSIC. In this paper, theoretical MSE of CBS is given when MUSIC or root-MUSIC is used. Error variance can be derived from the asymptotic probability distribution of the eigenvectors of an average finite-snapshot covariance matrix. Meanwhile, the bias due to the filtered stopband sources is given by first-order perturbation analysis. Known advantages of CBS are confirmed by the MSE analysis. For example, CBS yields smaller MSE for correlated sources than element-space. The theoretical results are verified by simulations
Parameter Estimation in a System of Integro-Differential Equations with Time-Delay
No full textThis brief presents a technique for estimating the parameters of a system of integro-differential equations with a time-delay component using some given samples. The considered system is reduced to a set of linear equations by expanding the involved functions in the triangular orthogonal polynomials. While the unknowns (system parameters) can now be obtained by solving the linear equations, however, it is an overdetermined system that is usually inconsistent. In addition, there is no guarantee that the estimated parameters will predict the states nicely outside the time range from which the samples are given. Hence, the upper bound on the error (in the state prediction) is minimized, which depends only on the system parameters and is obtained using the Parseval-Plancherel identity, Cauchy-Schwarz inequality, and the Mean Value theorem. Consequently, an optimization problem is set up to minimize the accumulated error subject to the set of linear equations. The proposed technique is validated numerically where the estimated parameters are found to be close to the true values
Causality constraints on corrections to Einstein gravity
No full textWe study constraints from causality and unitarity on 2 → 2 graviton scattering in four-dimensional weakly-coupled effective field theories. Together, causality and unitarity imply dispersion relations that connect low-energy observables to high-energy data. Using such dispersion relations, we derive two-sided bounds on gravitational Wilson coefficients in terms of the mass M of new higher-spin states. Our bounds imply that gravitational interactions must shut off uniformly in the limit G → 0, and prove the scaling with M expected from dimensional analysis (up to an infrared logarithm). We speculate that causality, together with the non-observation of gravitationally-coupled higher spin states at colliders, severely restricts modifications to Einstein gravity that could be probed by experiments in the near future
Algorithms for Competitive Division of Chores
No full textWe study the problem of allocating divisible bads (chores) among multiple agents with additive utilities when monetary transfers are not allowed. The competitive rule is known for its remarkable fairness and efficiency properties in the case of goods. This rule was extended to chores by Bogomolnaia, Moulin, Sandomirskiy, and Yanovskaya. For both goods and chores, the rule produces Pareto optimal and envy-free allocations. In the case of goods, the outcome of the competitive rule can be easily computed. Competitive allocations solve the Eisenberg-Gale convex program; hence the outcome is unique and can be approximately found by standard gradient methods. An exact algorithm that runs in polynomial time in the number of agents and goods was given by Orlin. In the case of chores, the competitive rule does not solve any convex optimization problem; instead, competitive allocations correspond to local minima, local maxima, and saddle points of the Nash social welfare on the Pareto frontier of the set of feasible utilities. The Pareto frontier may contain many such points and, consequently, the outcome of the competitive rule is no longer unique. In this paper, we show that all the outcomes of the competitive rule for chores can be computed in strongly polynomial time if either the number of agents or the number of chores is fixed. The approach is based on a combination of three ideas: all consumption graphs of Pareto optimal allocations can be listed in polynomial time; for a given consumption graph, a candidate for a competitive utility profile can be constructed via an explicit formula; each candidate can be checked for competitiveness and the allocation can be reconstructed using a maximum flow computation. Our algorithm immediately gives an approximately-fair allocation of indivisible chores by the rounding technique of Barman and Krishnamurthy
Evidence of a Decreased Binary Fraction for Massive Stars within 20 milliparsecs of the Supermassive Black Hole at the Galactic Center
No full textWe present the results of the first systematic search for spectroscopic binaries within the central 2 × 3 arcsec² around the supermassive black hole at the center of the Milky Way galaxy. This survey is based primarily on over a decade of adaptive optics-fed integral-field spectroscopy (R ∼ 4000), obtained as part of the Galactic Center Orbits Initiative at Keck Observatory, and it has a limiting K'-band magnitude of 15.8, which is at least 4 mag deeper than previous spectroscopic searches for binaries at larger radii within the central nuclear star cluster. From this primary data set, over 600 new radial velocities are extracted and reported, increasing by a factor of 3 the number of such measurements. We find no significant periodic signals in our sample of 28 stars, of which 16 are massive, young (main-sequence B) stars and 12 are low-mass, old (M and K giant) stars. Using Monte Carlo simulations, we derive upper limits on the intrinsic binary star fraction for the young star population at 47% (at 95% confidence) located ∼20 mpc from the black hole. The young star binary fraction is significantly lower than that observed in the field (70%). This result is consistent with a scenario in which the central supermassive black hole drives nearby stellar binaries to merge or be disrupted, and it may have important implications for the production of gravitational waves and hypervelocity stars
Time-of-flight anemometry using a displacement plate-beamsplitter
No full textWe propose the use of a second surface mirror as a displacement plate-beamsplitter to provide significant simplification and cost reduction of time-of-flight anemometry (ToFA), without sacrificing precision and accuracy. These benefits are most pronounced for long-range applications. Our method’s principle benefits are due to the few and simple components it requires as well as low sensitivity to both temperature effects and light source incoherence. We found that precise and accurate results are possible using a common consumer mirror as the main optical element and an inexpensive diode laser as the light source, which could broaden access to laser anemometry and make many industry applications economically feasible. The nature of the design also permits an increase in range for a given laser power since the method can utilize the entire optical area of the focusing lens/mirror independent of other design considerations and the cost of a flat second-surface mirror is usually negligible. To characterize the performance of this method, we develop a Cramer–Rao bound (CRB) for a general class of ToFA’s with multiple Gaussian beams under signal-independent Gaussian white noise. For a given measurement volume, the lowest velocity uncertainty is achieved by creating a standard two-sheet geometry: power-matching the first two beams by adjusting the beamsplitter and blocking the rest of the beams is optimal. However, keeping the higher order beams permits determination of flow direction. Conditions to achieve beam power-matching are given. An anemometer is built using a diode laser with 12 mw 405 nm beam using a total of just three transmitting optical components. Our setup has an accuracy of 99.1%. The worst-case precision of 96.7% nearly achieves the CRB, although optimizing the setup more can lower the bound, and therefore allow increase in the performance by an order of magnitude or more
Tsunami Genesis of Strike-Slip Earthquakes Revealed in the 2018 Indonesian Palu Event
No full textThe devastating tsunami after the 2018 Indonesian Sulawesi-Palu strike-slip earthquake was a surprise because strike-slip faulting was a known phenomenon of primarily lateral movement of land, while tsunamis were believed to be caused by vertical movements of seafloor or landslides. Here we demonstrated how the strike-slip faulting could have pushed waters from outside and inside the Palu Bay to form a powerful tsunami in the Palu Bay. We constructed three earthquake inversions from seismographs, satellite radar and optical imagery, and used an open-source ocean circulation model to replicate the tsunami. Our experiments revealed that: (1) the southward horizontal displacement of deeper-water slopes along the Makassar coast generated a long-wave tsunami of 40 km, propagating southward into the Palu Bay and consisting with the two distinguished tsunami-peaks in the Pantoloan tide-record, twice higher than the local resonance waves; (2) the two types of tsunamis in the Mamuju tide-record—the “early arrival” tsunami and the late larger tsunami—were originated from the outside and inside sources; and (3) the eyewitness account of the whirlpool circulation in the Palu Bay could be explained by the horizontal strike-slip forcing of the two involved tectonic plates. The east plate was largely responsible for pushing the long-wave tsunami southward that inundated the Palu City and resulted in the devastation. Our findings suggest that the tsunami’s behavior of strike-slip earthquakes is more complex than previously thought and should be considered in future tsunami early warnings