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A key concept underlying the specific functionalities of metasurfaces, i.e. arrays of subwavelength nanoparticles, is the use of constituent components to shape the wavefront of the light, on-demand. Metasurfaces are versatile and novel platforms to manipulate the scattering, colour, phase or the intensity of the light. Currently, one of the typical approaches for designing a metasurface is to optimize one or two variables, among a vast number of fixed parameters, such as various materials' properties and coupling effects, as well as the geometrical parameters. Ideally, it would require a multi-dimensional space optimization through direct numerical simulations. Recently, an alternative approach became quite popular allowing to reduce the computational cost significantly based on a deep-learning-assisted method. In this paper, we utilize a deep-learning approach for obtaining high-quality factor (high-Q) resonances with desired characteristics, such as linewidth, amplitude and spectral position. We exploit such high-Q resonances for the enhanced light-matter interaction in nonlinear optical metasurfaces and optomechanical vibrations, simultaneously. We demonstrate that optimized metasurfaces lead up to 400+ folds enhancement of the third harmonic generation (THG); at the same time, they also contribute to 100+ folds enhancement in optomechanical vibrations. This approach can be further used to realize structures with unconventional scattering responses., Comment: 29 pages, 7 figures

High index dielectric nanostructure supports different types of resonant modes. However, it is very challenging to achieve high-Q factor in a single subwavelength dielectric nanoresonator due to non-hermtian property of the open system. Here, we present a universal approach of finding out a series of high-Q resonant modes in a single nonspherical dielectric nanocavity by exploring quasi-bound state in the continuum. Unlike conventional method relying on heavy computation (ie, frequency scanning by FDTD), our approach is built upon leaky mode engineering, through which many high-Q modes can be easily achieved by constructing avoid-crossing (or crossing) of the eigenvalue for pair leaky modes. The Q-factor can be up to 2.3*10^4 for square subwavelength nanowire (NW) (n=4), which is 64 times larger than the highest Q-factor (Q=360) reported so far in single subwavelength nanodisk. Such high-Q modes can be attributed to suppressed radiation in the corresponding eigenchannels and simultaneously quenched electric(magnetic) at momentum space. As a proof of concept, we experimentally demonstrate the emergence of the high-Q resonant modes (Q=380) in the scattering spectrum of a single silicon subwavelength nanowire., Comment: 8 pages, 5 figures

The microstructural features and high-temperature oxidation resistance of hybrid (TiC+TiB) networks reinforced Ti-6Al-4V composites were investigated after fabricated with reaction hot pressing technique. The inhomogeneous distribution of hybrid reinforcers resulted in a sort of stress-induced grain refinement for {\alpha}-Ti matrix phase, which was further facilitated by heterogeneous nucleation upon additive interfaces. HRTEM analyses revealed the crystallographic orientation relation between TiB and alpha-Ti phases as (201)TiB//(-1100)alpha-Ti plus [11-2]//[0001] alpha-Ti, while TiC and {\alpha}-Ti phases maintained the interrelation of (-200)TiC//(-2110) {\alpha}-Ti and [001]TiC//[01-10] alpha-Ti. The hybridly reinforced Ti-6Al-4V/(TiC+TiB) composites displayed superior oxidation resistance to both the sintered matrix alloy and the two composites reinforced solely with TiC or TiB addition during the cyclic oxidation at 873, 973 and 1073 K respectively for 100 h. The hybrid reinforcers volume fraction was a more influential factor to improve oxidation resistance than the matrix alloy powder size. As temperature rose from 873 to 1073 K, the oxidation kinetics transferred from the nearly parabolic type through qusilinear tendency into the finally linear mode. This corresponded to the morphological transition of oxide scales from a continuous protective film to a partially damaged layer and ended up with the complete spallation of alternating alumina and rutile multilayers. A phenomenological model was proposed to elucidate the growth process of oxides scales. The release of thermal stress, the suppression of oxygen diffusion and the fastening of oxide adherence were found as the three major mechanisms to enhance the oxidation resistance of hybrid reinforced composites., Comment: 20 pages, 15 figures

It has been widely accepted that the introduction of titanium carbides into titanium-based alloys can significantly enhance the oxidation resistance due to their superior physicochemical stability at elevated temperatures. The present study reported for the first time that the ordered C vacancies within titanium carbides could lead to an uncommon phenomenon particularly at the very initial stage of oxidation. The intrinsic micro-to-macro oxidation mechanisms were systematically clarified with the aids of transmission electron microscope and ab-initio molecular dynamics simulation., Comment: 8 pages, 3 figures, 1 graphical abstract

We report that the refractive index of transition metal dichacolgenide (TMDC) monolayers, such as MoS2, WS2, and WSe2, can be substantially tuned by > 60% in the imaginary part and > 20% in the real part around exciton resonances using CMOS-compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers and the strong susceptibility of the excitons to the influence of injected charge carriers. The tunability mainly results from the effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge carriers, such as renormalizing bandgap and changing exciton binding energy, only play negligible roles. We also demonstrate that the atomically thin monolayers, when combined with photonic structures, can enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80% (20%) due to the giant tunability of refractive index. This work may pave the way towards the development of field-effect photonics in which the optical functionality can be controlled with CMOS circuits.

Transition metal dichalcogenide (TMDC) monolayers present a remarkable multifunctional material with potential to enable the development of a wide range of novel devices. However, the functionalities observed often fall short of the expectation, which hinders the device development. Here we demonstrate that the optical, catalytic, and thermal functionalities of TMDC monolayers can all be substantially enhanced by up to orders of magnitude with the intercalation of water molecules or small cations (H+ and Li+) between the monolayers and underlying substrates. In contrast, the same molecules or cations adsorbed on top of the monolayers show negligible effects. We also discover two major roles of the intercalated species in the enhancement: doping the monolayers and modifying the interaction of the monolayers with the substrate. The result points out a versatile and convenient strategy of using the intercalation of molecules or ions to enhance the functionalities of TMDC monolayers., Comment: This paper has been withdrawn due to the incomplete introduction information and reference

Enabling perfect light absorption in ultrathin materials promises the development of exotic photonic devices. Here we demonstrate new strategies that can provide capabilities to rationally design ultrathin (thickness < {\lambda}/10~{\lambda}/5) semiconductor perfect absorbers for arbitrary wavelengths, including those at which the intrinsic absorption of the semiconductor is weak, e.g. Si for near-IR wavelengths. This is in stark contrast with the existing studies on ultrathin perfect absorbers, which have focused on metallic materials or highly-absorptive semiconductors. Our design strategies are built upon an intuitive model, coupled leaky mode theory that we recently developed and can turn the design for perfect absorbers to the design for leaky modes. The designed absorber is featured with extraordinary absorption enhancement, miniaturized dimension, and high selectivity for the wavelength, polarization, and angle of incident light. It can enable the development of flexible, light-weight, high-performance, cost-effective, and multifunctional optoelectronic devices that are difficult with current light absorbers., Comment: This paper has been withdrawn due to the incomplete introduction information and reference