分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Considering the huge computational resources required by smoothed particle hydrodynamics (SPH) simulations and the overestimation of post-collision materials from perfect merging, we develop a statistical method to deal with collisions during the formation of planetary systems by introducing random material loss. In this method the mass and water content lost by the sole outcome from every merger vary randomly within a range dependent on the total mass and water content of colliding bodies. The application of the random loss method to the planet formation in the solar system shows a good consistency with existing SPH results. We also apply this method to the extrasolar planetary system 55 Cancri which hosts (at least) five planets and study the formation of terrestrial planets between the outermost two planets. A disk with 500 Mars mass embryos in dynamically cold orbits before the late-stage accretion phase is assumed. Scenarios with different amounts of planetary embryos and different loss parameters are adopted in our simulations. The statistical result from hundreds of simulations shows that an Earth-like planet with water inventory of roughly 6 Earth ocean could form between 55 Cnc f and d. It may reside between 1.0 and 2.6 AU but the most likely region extends from 1.5 to 2.1 AU. Thus the probability of this planet being in the potentially habitable zone (0.59--1.43 AU) is relatively low, only around 10\%. Planets 55 Cnc f and d could also be shaped and gain some water from giant impacts and consequently the orbits of them may also change accordingly.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The long-standing co-orbital asteroids have been thought to be the possible source of zodiacal dust ring around the orbit of Venus, but inconsistent conclusions on the orbital stability thus existence of Venus Trojans are found in literature. We present in this paper a systematic survey of the orbital stability of Venus Trojans, taking into account the dynamical influences from the general relativity and the Yarkovsky effect. The orbits of thousands of fictitious Venus Trojans are numerically simulated. Using the method of frequency analysis, their orbital stabilities and the dynamical mechanisms behind are described in detail. The influences of general relativity and of Yarkovsky effect, which were either neglected or oversimplified previously, are investigated by long-term numerical simulations. The stability maps on the $(a_0,i_0)$ plane and $(a_0,e_0)$ plane are depicted, and the most stable Venus Trojans are found to occupy the low-inclination horseshoe orbits with low eccentricities. The resonances that carve the fine structures in the stability map are determined. The general relativity decreases the stability of orbits by little but the Yarkovsky effect may drive nearly all Venus Trojans out of the Trojan region in a relatively short time. The Venus Trojans have poor orbital stability and cannot survive the age of the Solar system. The zodiacal dust ring found around the orbit of Venus is more likely a sporadic phenomenon, as the result of temporarily capture into the 1:1 mean motion resonance of dust particles produced probably from passing comets or asteroids, but not Venus Trojans.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
Changqin Liu
Sheng Zhang
Shunjia Wang
Qingnan Cai
Peng Wang
Chuanshan Tian
Lei Zhou
Yizheng Wu
Zhensheng Tao
摘要： The ability to manipulate the electric-field vector of broadband terahertz waves is essential for applications of terahertz technologies in many areas, and can open up new possibilities for nonlinear terahertz spectroscopy and coherent control. Here, we propose a novel laser-driven terahertz emitter, consisting of metasurface-patterned magnetic multilayer heterostructures. Such hybrid terahertz emitters can combine the advantages of spintronic emitters for being ultrabroadband, efficient and flexible, as well as those of metasurfaces for the unique capability to manipulate terahertz waves with high precision and degree of freedom. Taking a stripe-patterned metasurface as an example, we demonstrate the generation of broadband terahertz waves with tunable chirality. Based on experimental and theoretical studies, the interplay between the laser-induced spintronic-origin currents and the metasurface-induced transient charges/currents are investigated, revealing the strong influence on the device functionality originated from both the light-matter interactions in individual metasurface units and the dynamic coupling between them. Our work not only offers a flexible, reliable and cost-effective solution for chiral terahertz wave generation and manipulation, but also opens a new pathway to metasurface-tailored spintronic devices for efficient vector-control of electromagnetic waves in the terahertz regime.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Reflecting light to a pre-determined non-specular direction is an important ability of metasurfaces, which is the basis for a wide range of applications (e.g., beam steering/splitting and imaging). However, anomalous reflection with 100% efficiency has not been achieved at optical frequencies in conventional metasurfaces, due to losses and/or insufficient nonlocal control of light waves. Here, we propose a new type of all-dielectric quasi-three-dimensional subwavelength structures, consisting of multilayer films and specifically designed meta-gratings, to achieve perfect anomalous reflections at optical frequencies. A complex multiple scattering process was stimulated by effectively coupling different Bloch waves and propagating waves in the proposed meta-system, thus offering the whole meta-system the desired nonlocal control on light waves required to achieve perfect anomalous reflections. Two perfect anomalous reflectors were designed to reflect normally incident 1550 nm light to the 40{\deg} and 75{\deg} directions with absolute efficiencies higher than 99%, and were subsequently fabricated and experimentally demonstrated to exhibit efficiencies 98% and 88%, respectively. Our results pave the way towards realizing optical meta-devices with desired high efficiencies in realistic applications.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Skin effect that all eigenmodes within a frequency range become edge states is dictated by the topological properties of complex eigenvalues unique in non-Hermitian systems. The prevailing attempts to realize such a fascinating effect are confined to either one-dimensional or nonreciprocal systems exhibiting asymmetric couplings. Here, inspired by a recent model Hamiltonian theory, we propose a realistic reciprocal two-dimensional (2D) photonic crystal (PhC) system that shows the desired skin effect. Specifically, we establish a routine for designing such non-Hermitian systems via revealing the inherent connections between the non-trivial eigenvalue topology of order-2 exceptional points (EPs) and the skin effects. Guided by the proposed strategy, we successfully design a 2D PhC that possesses the EPs with nonzero eigenvalue winding numbers. The spectral area along a specific wavevector direction is then formed by leveraging the symmetry of the macroscopic geometry and the unit cell. The projected-band-structure calculations are performed to demonstrate that the desired skin effect exists at the specific crystalline interfaces. We finally employ time-domain simulations to vividly illustrate this phenomenon by exciting a pulse at the center of a finite-sized PhC. Our results form a solid basis for further experimental confirmations and applications of the skin effect.
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