Polarization-insensitive perfect absorption in van der waals hyper-structure.

Infrared perfect absorption has been widely investigated due to its potential applications in photodetectors, photovoltaics and medical diagnostics. In this report, we demonstrate that at particular infrared frequencies, a simple planar structure made up of graphene-hexagonal Boron Nitride (hBN) hyper-structure is able to nearly perfectly absorb incident light irrespective of its polarization (Transverse-Magnetic TM, or Transverse-Electric TE). By using this interferenceless technique, the hyper-structure achieves nearly zero reflectance at a wide range of angles in a narrow frequency band. We analytically predict the condition of achieving such an important feature of perfect absorption for both TM and TE polarizations. Interestingly, the infrared perfect absorption can be redshifted by increasing the thickness of the hBN layers and blueshifted by increasing the graphene's chemical potential. Such flexible control of infrared perfect absorption offers a new tool for controlling electromagnetic waves and has potential applications in photodetection and other light control applications.

Demonstration of negative refraction induced by synthetic gauge fields.

Negative refraction is a counterintuitive wave phenomenon that has inspired the development of metamaterials and metasurfaces with negative refractive indices and surface phase discontinuities, respectively. Recent theories have proposed an alternative mechanism for negative refraction: Synthetic gauge fields, induced by either dynamical modulation or motion, can shift a material's dispersion in momentum space, forcing a positive refractive index medium to exhibit negative refraction above a certain threshold. However, this phenomenon has not previously been observed. Here, we report on the experimental demonstration of gauge field­induced negative refraction in a twisted bilayer acoustic metamaterial. The synthetic gauge fields arise in a projected two-dimensional geometry and can be continuously tuned by varying the wave number along the third dimension. Gauge field­induced waveguiding with backward-propagating modes is also demonstrated in a trilayer configuration. These results introduce a mechanism for performing wave manipulation in artificially engineered materials.

Ideal type-II Weyl points in topological circuits.

Weyl points (WPs), nodal degenerate points in three-dimensional (3D) momentum space, are said to be 'ideal' if they are symmetry-related and well-separated, and reside at the same energy and far from nontopological bands. Although type-II WPs have unique spectral characteristics compared with type-I counterparts, ideal type-II WPs have not yet been reported because of a lack of an experimental platform with enough flexibility to produce strongly tilted dispersion bands. Here, we experimentally realize a topological circuit that hosts only topological bands with a minimal number of four ideal type-II WPs. By stacking two-dimensional (2D) layers of inductor-capacitor (LC) resonator dimers with the broken parity inversion symmetry (P), we achieve a strongly tilted band structure with two group velocities in the same direction, and topological surface states in an incomplete bandgap. Our results establish an ideal system for the further study of Weyl physics and other exotic topological phenomena.

Existence, symmetry breaking bifurcation and stability of two-dimensional optical solitons supported by fractional diffraction.

We study existence, bifurcation and stability of two-dimensional optical solitons in the framework of fractional nonlinear Schrödinger equation, characterized by its Lévy index, with self-focusing and self-defocusing saturable nonlinearities. We demonstrate that the fractional diffraction system with different Lévy indexes, combined with saturable nonlinearity, supports two-dimensional symmetric, antisymmetric and asymmetric solitons, where the asymmetric solitons emerge by way of symmetry breaking bifurcation. Different scenarios of bifurcations emerge with the change of stability: the branches of asymmetric solitons split off the branches of unstable symmetric solitons with the increase of soliton power and form a supercritical type bifurcation for self-focusing saturable nonlinearity; the branches of asymmetric solitons bifurcates from the branches of unstable antisymmetric solitons for self-defocusing saturable nonlinearity, featuring a convex shape of the bifurcation loops: an antisymmetric soliton loses its stability via a supercritical bifurcation, which is followed by a reverse bifurcation that restores the stability of the symmetric soliton. Furthermore, we found a scheme of restoration or destruction the symmetry of the antisymmetric solitons by controlling the fractional diffraction in the case of self-defocusing saturable nonlinearity.

Ultralow loss graphene-based hybrid plasmonic waveguide with deep-subwavelength confinement.

In this paper, we theoretically propose a novel graphene-based hybrid plasmonic waveguide (GHPW) consisting of a low-index rectangle waveguide between a high-index cylindrical dielectric waveguide and the substrate with coated graphene on the surface. The geometric dependence of the mode characteristics on the proposed structure is analyzed in detail, showing that the proposed GHPW has a low loss and consequently a relatively long propagation distance. For TM polarization, highly confined modes guided in the low-index gap region between the graphene and the high-index GaAs and the normalized modal area is as small as 0.0018 (λ2/4) at 3 THz. In addition to enabling the building of high-density integration of the proposed structure are examined by analyzing crosstalk in a directional coupler composed of two GHPWs. This structure also exhibits ultra-low crosstalk when a center-to-center separation between adjacent GHPWs is 32µm, which shows great promise for constructing various terahertz integrated devices.

Symmetric and asymmetric solitons supported by a 𝒫𝒯-symmetric potential with saturable nonlinearity: bifurcation, stability and dynamics.

The symmetry breaking bifurcation of solitons in an optical waveguide with focusing saturable nonlinearity and parity-time (𝒫𝒯)-symmetric complex-valued external potentials is investigated. As the soliton power increases, it is found that the branches of asymmetric solitons split off from the base branches of 𝒫𝒯-symmetric fundamental soliton. The bifurcation diagrams, consisting essentially of the propagation constants of optical solitons, indicate that symmetric fundamental and multipole solitons, as well as asymmetric solitons can exist. The stabilities and the dynamics characteristics of solitons are comprehensively investigated. We find the different instability scenarios of the symmetric solitons, but the symmetry breaking bifurcation is caused only by the onset of instability of the symmetric fundamental solitons. This result is further confirmed by the numerical examples with the different saturable nonlinearity parameters. In particular, we find that the soliton power and the stability of soliton at the bifurcation points are significantly changed by varying the strength of the saturable nonlinearities. These results provide additional way to control symmetry breaking bifurcations in 𝒫𝒯-symmetric optical waveguide.

Dynamical manipulation of Cosine-Gauss beams in a graphene plasmonic waveguide.

In this paper, we theoretically propose for the first time that graphene monolayer can be used to manipulate the Cosine-Gauss beams (CGBs). We show that both the transverse oscillation period and propagation length of a CGB can be dynamically manipulated by utilizing the tunability of the graphene's chemical potential. The graphene-based planar plasmonic waveguide provides a good platform to investigate the propagation properties of CGBs, which is potentially compatible to the microelectronic technology.

Hyperbolic-polaritons-enabled dark-field lens for sensitive detection.

Sensitive detection of features in a nanostructure may sometimes be puzzled in the presence of significant background noise. In this regard, background suppression and super-resolution are substantively important for detecting weakly scattering nanoscale features. Here, we present a lens design, termed hyperbolic-polaritons-enabled dark-field lens (HPEDL), which has the ability to accomplish straightforward sensitive detection. This HPEDL structure consists of type I and type II hyperbolic media that support high-k field waves via hyperbolic polaritons (HPs). We show that the cone-like characteristics of the HPs could be manipulated while the influence of the low-k field waves would be removed. Numerical simulations demonstrate that this proposed structure can successfully realize straightforward sensitive detection by modifying its thickness under the phase compensation condition. Besides, the minimum resolvable length and angular-dependent performance for sensitive detection are also demonstrated by simulations. Remarkably, these findings are very promising for propelling nanophotonics technologies and constitute a further important step towards practical applications of optical microscopy.

Bistable scattering in graphene-coated dielectric nanowires.

In nonlinear plasmonics, the switching threshold of optical bistability is limited by the weak nonlinear responses from the conventional Kerr dielectric media. Considering the giant nonlinear susceptibility of graphene, here we develop a nonlinear scattering model under the mean field approximation and study the bistable scattering in graphene-coated dielectric nanowires based on the semi-analytical solutions. We find that the switching intensities of bistable scattering can be smaller than 1 MW cm-2 at the working frequency. To further decrease the switching intensities, we show that the most important factor that restricts the bistable scattering is the relaxation time of graphene. Our work not only reveals some general characteristics of graphene-based bistable scattering, but also provides a guidance to further applications of optical bistability in the high speed all-optical signal processing.

The impact of exposure to environmental contaminant on hepatocellular lipid metabolism.

Increasing evidences show that ubiquitous perfluorooctanoic acid (PFOA), a representative environmental pollutant, is found to be linked to lipid dysmetabolism. However, the biological mechanism behind this outcome remains uninvestigated. In the present study, we established the PFOA-injured liver in mice to explore the underlying mechanism associated with PFOA-induced lipid disturbance in the liver via a group of biochemical and molecular assays. As results, PFOA-exposed mice showed increased transaminase (ALT), reduced triglyceride and free fatty acid contents in serum, as well as elevated level of hepatic triglyceride. Morphologically, PFOA-exposed mice displayed visible vacuolation in cytoplasm and abnormal cytoarchitecture in liver. In addition, PFOA-exposed liver showed up-regulated expressions of lipid-uptake associated mRNA of hepatic lipoprotein lipase (LPL) and fatty acid translocase (CD36) and down-regulated expression of lipid-uptake associated mRNA of apolipoprotein-B100 (APOB). Moreover, validated data from immunohistochemistry and immunoblotting found that hepatocellular LPL and CD36 proteins were increased dose-dependently, and lowered expression of hepatic APOB was observed. In conclusion, our current findings reveal that PFOA-induced lipid dysmetabolism in the liver is involved to dysregulation of fatty acid trafficking.

Observing the transient buildup of a superscatterer in the time domain.

Superscatterer is an intriguing electromagnetic device, which can enhance the wave scattering of a given object with an arbitrary magnification factor in principle. However, observing the transient buildup of a superscatterer in numerical time domain still has not been investigated yet. In this paper, by using the dispersive finite difference time domain method, the transient response of a dispersive superscatterer created with monotonic optical transformation function is studied. We find that the time delay grows dramatically when the magnification factor increases. In addition, we notice an interesting phenomenon that, placing a scattering body with more complicated structure leads to longer time delays. These findings are very useful to reveal the physics behind the superscatterer.

Magnetic Fano resonances by design in symmetry broken THz meta-foils.

Magnetic Fano resonances in there-dimensional symmetry broken meta-foils at THz frequencies are theoretically and experimentally studied. Sharp Fano resonances occur due to the interference between different resonances and can be designed by choosing geometric parameters of the meta-foil. At the Fano resonances, the meta-foil supports antisymmetric modes, whereas, at the main resonance, only a symmetric mode exists. The meta-foil is left-handed at the Fano resonances and shows sharp peaks of the real part of the refractive index in transmission with small effective losses opening a way to very sensitive high-speed sensing of dielectric changes in the surrounding media and of mechanical configuration.

Unidirectional invisibility induced by parity-time symmetric circuit.

Parity-time (PT) symmetric structures present the unidirectional invisibility at the spontaneous PT-symmetry breaking point. In this paper, we propose a PT-symmetric circuit consisting of a resistor and a microwave tunnel diode (TD) which represent the attenuation and amplification, respectively. Based on the scattering matrix method, the circuit can exhibit an ideal unidirectional performance at the spontaneous PT-symmetry breaking point by tuning the transmission lines between the lumped elements. Additionally, the resistance of the reactance component can alter the bandwidth of the unidirectional invisibility flexibly. Furthermore, the electromagnetic simulation for the proposed circuit validates the unidirectional invisibility and the synchronization with the input energy well. Our work not only provides an unidirectional invisible circuit based on PT-symmetry, but also proposes a potential solution for the extremely selective filter or cloaking applications.

Panoramic lens designed with transformation optics.

The panoramic lens is a special kind of lens, which is applied to observe full view. In this letter, we theoretically present a panoramic lens (PL) using transformation optics method. The lens is designed with inhomogeneous and anisotropic constitutive parameters, which has the ability to gather light from all directions and confine light within the visual angle of observer. Simulation results validate our theoretical design.

Hybrid Airy plasmons with dynamically steerable trajectories.

With their intriguing diffraction-free, self-accelerating, and self-healing properties, Airy plasmons show promise for use in the trapping, transporting, and sorting of micro-objects, imaging, and chip scale signal processing. However, high dissipative loss and lack of dynamical steerability restrict the implementation of Airy plasmons in these applications. Here we reveal hybrid Airy plasmons for the first time by taking a hybrid graphene-based plasmonic waveguide in the terahertz (THz) domain as an example. Due to coupling between optical modes and plasmonic modes, the hybrid Airy plasmons can have large propagation lengths and effective transverse deflections, where the transverse waveguide confinements are governed by the hybrid modes with moderate quality factors. Meanwhile, the propagation trajectories of the hybrid Airy plasmons are dynamically steerable by changing the chemical potential of graphene. These hybrid Airy plasmons may promote the further discovery of non-diffracting beams along with the emerging developments of optical tweezers and tractor beams.

Transient response of a signal through a dispersive invisibility cloak.

The transient response of the invisibility cloak has long been an interesting research topic, since it is valuable to further understand the steady-state process and to design more effective cloaks. Here we investigate the transient response of a set of dispersive invisibility cloaks impinged on by a sinusoidal signal or a modulated Gaussian pulse using the finite difference time domain method. Cylindrical cloaks with linear, convex, and concave transformation functions are studied. We find that their time to reach a steady state is different and they grow significantly when the thickness of the cloak decreases. Moreover, a centrally depressed ladder-like spatial time delay distribution is observed with a modulated Gaussian pulse. We show that the central frequency of the Gaussian pulse suffers a blue-shift in the forward scattering direction in agreement with previous theoretical predictions.

Analysis and modeling of Fano resonances using equivalent circuit elements.

Fano resonance presents an asymmetric line shape formed by an interference of a continuum coupled with a discrete autoionized state. In this paper, we show several simple circuits for Fano resonances from the stable-input impedance mechanism, where the elements consisting of inductors and capacitors are formulated for various resonant modes, and the resistor represents the damping of the oscillators. By tuning the pole-zero of the input impedance, a simple circuit with only three passive components e.g. two inductors and one capacitor, can exhibit asymmetric resonance with arbitrary Q-factors flexiblely. Meanwhile, four passive components can exhibit various resonances including the Lorentz-like and reversely electromagnetically induced transparency (EIT) formations. Our work not only provides an intuitive understanding of Fano resonances, but also pave the way to realize Fano resonaces using simple circuit elements.

Loss induced amplification of graphene plasmons.

This Letter introduces a new mechanism to reverse and control the effect of losses in the plasmonic systems by using a coupled parity-time symmetric graphene waveguide with complex potentials. In order to explore the uncharted properties of parity-time symmetric graphene plasmons, this Letter analytically shows the plasmonic parity-time symmetry breaking in the coupled graphene waveguide by Sommerfeld integration. This phase transition leads to the distinct spatial propagation behaviors of graphene plasmons in the exact or broken parity-time symmetric phase driven by a point source. Particularly, a loss induced plasmonic amplification, as a characteristic of exceptional point behavior, is for the first time, to the best of our knowledge, revealed in the realm of graphene plasmonics.

Atomically thin spherical shell-shaped superscatterers based on a Bohr model.

Graphene monolayers can be used for atomically thin three-dimensional shell-shaped superscatterer designs. Due to the excitation of the first-order resonance of transverse magnetic (TM) graphene plasmons, the scattering cross section of the bare subwavelength dielectric particle is enhanced significantly by five orders of magnitude. The superscattering phenomenon can be intuitively understood and interpreted with a Bohr model. In addition, based on the analysis of the Bohr model, it is shown that contrary to the TM case, superscattering is hard to achieve by exciting the resonance of transverse electric (TE) graphene plasmons due to their poor field confinements.

Effect of 3,4-dihydroxyacetophenone on endothelial dysfunction in obese rats.

CONTEXT: 3,4-Dihydroxyacetophenone (DHAP) has been reported to possess cardiovascular pharmacological effects. OBJECTIVE: This study was designed to determine whether DHAP could improve endothelial function in obese rats. MATERIALS AND METHODS: Wistar rats were randomly divided into control, obesity, and DHAP groups and fed a normal, high-fat, and high-fat plus DHAP (10 mg kg(-1) d(-1)) diet, respectively, for 8 weeks. Endothelial-dependent vasodilatation was assessed. Endothelial nitric oxide synthase (eNOS) activity and nitric oxide (NO) production in endothelial cells were determined. Nuclear transcription factor kappa B (NF-κB) expression and superoxide production in aorta were evaluated. RESULTS: DHAP treatment significantly decreased plasma triglycerides (0.94 ± 0.31 mmol/l versus 1.36 ± 0.29 mmol/l, p < 0.05) and free fatty acids (0.53 ± 0.15 mmol/l versus 0.99 ± 0.24 mmol/l, p < 0.05), reduced serum tumor necrosis factor α (35.56 ± 9.28 pg/ml versus 68.3 ± 10.24 pg/ml, p < 0.05) and malondialdehyde (2.94 ± 0.58 pg/ml versus 6.45 ± 0.70 pg/ml, p < 0.05), and increased serum adiponectin levels (164.5 ± 34.5 µg/l versus 84.5 ± 20.4 µg/l, p < 0.05). DHAP enhanced endothelial-dependent vasodilatation and improved endothelial function in obese rats (p < 0.05). eNOS activity and NO production in endothelial cells significantly decreased and NF-κB activation and superoxide production in aorta significantly increased in obese rats compared with the control group (p < 0.05). However, DHAP treatment significantly up-regulated the eNOS-NO pathway and decreased NF-κB activation and superoxide production (p < 0.05). CONCLUSION: DHAP improved endothelial function in obese rats. This beneficial effect may be associated with up-regulation of the eNOS-NO pathway by improving lipid metabolism and reducing oxidative stress and inflammation activity.