2D-patterned graphene metasurfaces for efficient third harmonic generation at THz frequencies.

Graphene is an attractive two-dimensional material for nonlinear applications in the THz regime, since it possesses high third order nonlinearity and the ability to support tightly confined surface plasmons. Here, we study 2D-patterned graphene-patch metasurfaces for efficient third harmonic generation. The efficiency of the nonlinear process is enhanced by spectrally aligning the fundamental and third harmonic frequencies with resonances of the metasurface, leading to spatiotemporal energy confinement in both steps of excitation at ω and radiation at 3ω. This precise resonance alignment is enabled by the 2D-patterning; it is achieved by modifying the dispersion of the underlying plasmons and, thus, the spectral positions of the supported standing wave resonances. Efficiencies as high as -20dB (1%) for input intensity 0.1 MW/cm2 are achieved. Moreover, we verify that the efficiency does not deteriorate when finite-size metasurfaces are used in place of ideal periodic systems. Our results highlight the potential of graphene-based metasurfaces for nonlinear applications.

Autosomal recessive hypotrichosis with loose anagen hairs associated with TKFC mutations.

BACKGROUND: Loose anagen hair is a rare form of impaired hair anchorage in which anagen hairs that lack inner and outer root sheaths can be gently and painlessly plucked from the scalp. This condition usually occurs in children and is often self-limiting. A genetic basis for the disorder has been suggested but not proven. A better understanding the aetiology of loose anagen hair may improve prevention and treatment strategies. OBJECTIVES: To identify a possible genetic basis of loose anagen hair using next-generation DNA sequencing and functional analysis of variants identified. METHODS: In this case study, whole-exome sequencing analysis of a pedigree with one affected individual with features of loose anagen hair was performed. RESULTS: The patient was found to be compound heterozygous for two single-nucleotide substitutions in TKFC resulting in the following missense mutations: c.574G> C (p.Gly192Arg) and c.682C> T (p.Arg228Trp). Structural analysis of human TKFC showed that both mutations are located near the active site cavity. Kinetic assays of recombinant proteins bearing either of these amino acid substitutions showed almost no dihydroxyacetone kinase or D-glyceraldehyde kinase activity, and FMN cyclase activity reduced to just 10% of wildtype catalytic activity. CONCLUSIONS: TKFC missense mutations may predispose to the development of loose anagen hairs. Identification of this new biochemical pathobiology expands the metabolic and genetic basis of hypotrichosis.

Split-cube-resonator-based metamaterials for polarization-selective asymmetric perfect absorption.

A split-cube-resonator-based metamaterial structure that can act as a polarization- and direction-selective perfect absorber for the infrared region is theoretically and experimentally demonstrated. The structure, fabricated by direct laser writing and electroless silver plating, is comprised of four layers of conductively-coupled split-cube magnetic resonators, appropriately rotated to each other to bestow the desired electromagnetic properties. We show narrowband polarization-selective perfect absorption when the structure is illuminated from one side; the situation is reversed when illuminating from the other side, with the orthogonal linear polarization being absorbed. The absorption peak can be tuned in a wide frequency range by a sparser or denser arrangement of the split cube resonators, allowing to cover the entire atmospheric transparency window. The proposed metamaterial structure can find applications in polarization-selective thermal emission at the IR atmospheric transparency window for radiative cooling, in cost-effective infrared sensing devices, and in narrowband filters and linear polarizers in reflection mode.

The effect of neuroleptic drugs on DPPC/sphingomyelin/cholesterol membranes.

The hydrophobic nature of neuroleptic drugs renders that these molecules interact not only with protein receptors, but also with the lipids constituting the membrane bilayer. We present a systematic study of the effect of seven neuroleptic drugs on a biomembrane model composed of DPPC, sphingomyelin, and cholesterol. Differential scanning calorimetry (DSC) measurements were used to monitor the gel-fluid phase transition of the lipid bilayer at three pH values and also as a function of drug concentration. The implementation of a new methodology to mix lipids homogeneously allowed us to assemble bilayers completely free of organic solvents. The seven neuroleptics were: trifluoperazine, haloperidol decanoate, clozapine, quetiapine, olanzapine, aripiprazole, and amisulpride. The DSC results show that the insertion of the drug into the bilayer produces a fluidization and a disordering of the bilayer. The bilayer perturbation is qualitatively the same for all the studied drugs, but quantitatively different. The driving force for the neuroleptic drug to place itself in the lipid bilayer is entropic in nature, signaling to the importance of the size and geometry of the drugs. The drug protonated species produce stronger effects than their non-protonated forms. At high concentrations two of the neuroleptics revert the fluidization effect and another completely abolishes the gel-fluid transition. The DSC data and the associated discussion contribute to the understanding of the interactions between neuroleptic drugs and lipid membranes.

Experimental Realization of an Extreme-Parameter Omnidirectional Cloak.

An ideal transformation-based omnidirectional cloak always relies on metamaterials with extreme parameters, which were previously thought to be too difficult to realize. For such a reason, in previous experimental proposals of invisibility cloaks, the extreme parameters requirements are usually abandoned, leading to inherent scattering. Here, we report on the first experimental demonstration of an omnidirectional cloak that satisfies the extreme parameters requirement, which can hide objects in a homogenous background. Instead of using resonant metamaterials that usually involve unavoidable absorptive loss, the extreme parameters are achieved using a nonresonant metamaterial comprising arrays of subwavelength metallic channels manufactured with 3D metal printing technology. A high level transmission of electromagnetic wave propagating through the present omnidirectional cloak, as well as significant reduction of scattering field, is demonstrated both numerically and experimentally. Our work may also inspire experimental realizations of the other full-parameter omnidirectional optical devices such as concentrator, rotators, and optical illusion apparatuses.

Phase-Modulated Scattering Manipulation for Exterior Cloaking in Metal-Dielectric Hybrid Metamaterials.

Artificially structured metamaterials with metallic or dielectric inclusions are extensively studied for exotic light manipulations via controlling the local-resonant modes in the microstructures. The coupling between these resonant modes has drawn growing interest in recent years due to the advanced functional metamaterial making the microstructures more and more complex. Here, the suppression of magnetic resonance of a dielectric cuboid, an analogue to the scattering cancellation effect or radiation control system, realized with an exterior cloaking in a hybrid metamaterial system, is demonstrated. Furthermore, the significant modulation of the absorption of the dielectric resonator in the hybrid metamaterial is also demonstrated. The physical insight of the experimental results is well illuminated with a classical double-harmonic-oscillator model, from which it is revealed that the complex coupling, i.e., the phase of coupling coefficient, plays a crucial role in the overall response of the metal-dielectric hybrid system. The proposed design strategy is anticipated to form a more straightforward and efficient paradigm for practical applications based on radiation control via versatile mode couplings.

Chiral Metamaterials with PT Symmetry and Beyond.

Optical systems with gain and loss that respect parity-time (PT) symmetry can have real eigenvalues despite their non-Hermitian character. Chiral systems impose circularly polarized waves which do not preserve their handedness under the combined space- and time-reversal operations and, as a result, seem to be incompatible with systems possessing PT symmetry. Nevertheless, in this work we show that in certain configurations, PT symmetric permittivity, permeability, and chirality is possible; in addition, real eigenvalues are maintained even if the chirality goes well beyond PT symmetry. By obtaining all three constitutive parameters in realistic chiral metamaterials through simulations and retrieval, we show that the chirality can be tailored independently of permittivity and permeability; thus, in such systems, a wide control of new optical properties including advanced polarization control is achieved.

Perfect optical absorption with nanostructured metal films: design and experimental demonstration.

Structuring metal surfaces on the nanoscale has been shown to alter their fundamental processes like reflection or absorption by supporting surface plasmon resonances. Here, we propose metal films with subwavelength rectangular nanostructuring that perfectly absorb the incident radiation in the optical regime. The structures are fabricated with low-cost nanoimprint lithography and thus constitute an appealing alternative to elaborate absorber designs with complex meta-atoms or multilayer structuring. We conduct a thorough numerical analysis to gain physical insight on how the key structural parameters affect the optical response and identify the designs leading to broad spectral and angular bandwidths, both of which are highly desirable in practical absorber applications. Subsequently, we fabricate and measure the structures with an FT-IR spectrometer demonstrating very good agreement with theory. Finally, we assess the performance of the proposed structures as sensing devices by quantifying the dependence of the absorption peak frequency position on the superstrate material.

Experimental Demonstration of Ultrafast THz Modulation in a Graphene-Based Thin Film Absorber through Negative Photoinduced Conductivity.

We present an experimental demonstration and interpretation of an ultrafast optically tunable, graphene-based thin film absorption modulator for operation in the THz regime. The graphene-based component consists of a uniform CVD-grown graphene sheet stacked on an SU-8 dielectric substrate that is grounded by a metallic ground plate. The structure shows enhanced absorption originating from constructive interference of the impinging and reflected waves at the absorbing graphene sheet. The modulation of this absorption, which is demonstrated via a THz time-domain spectroscopy setup, is achieved by applying an optical pump signal, which modifies the conductivity of the graphene sheet. We report an ultrafast (on the order of few ps) absorption modulation on the order of 40% upon photoexcitation. Our results provide evidence that the optical pump excitation results in the degradation of the graphene THz conductivity, which is connected with the generation of hot carriers, the increase of the electronic temperature, and the dominant increase of the scattering rate over the carrier concentration as found in highly doped samples.

Nonlinearity in the Dark: Broadband Terahertz Generation with Extremely High Efficiency.

Plasmonic metamaterials and metasurfaces offer new opportunities in developing high performance terahertz emitters and detectors beyond the limitations of conventional nonlinear materials. However, simple meta-atoms for second-order nonlinear applications encounter fundamental trade-offs in the necessary symmetry breaking and local-field enhancement due to radiation damping that is inherent to the operating resonant mode and cannot be controlled separately. Here we present a novel concept that eliminates this restriction obstructing the improvement of terahertz generation efficiency in nonlinear metasurfaces based on metallic nanoresonators. This is achieved by combining a resonant dark-state metasurface, which locally drives nonlinear nanoresonators in the near field, with a specific spatial symmetry that enables destructive interference of the radiating linear moments of the nanoresonators, and perfect absorption via simultaneous electric and magnetic critical coupling of the pump radiation to the dark mode. Our proposal allows eliminating linear radiation damping, while maintaining constructive interference and effective radiation of the nonlinear components. We numerically demonstrate a giant second-order nonlinear susceptibility ∼10^{-11} m/V, a one order improvement compared with the previously reported split-ring-resonator metasurface, and correspondingly, a 2 orders of magnitude enhanced terahertz energy extraction should be expected with our configuration under the same conditions. Our study offers a paradigm of high efficiency tunable nonlinear metadevices and paves the way to revolutionary terahertz technologies and optoelectronic nanocircuitry.

The role of Anderson's rule in determining electronic, optical and transport properties of transition metal dichalcogenide heterostructures.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se, Te) possess unique properties and novel applications in optoelectronics, valleytronics and quantum computation. In this work, we performed first-principles calculations to investigate the electronic, optical and transport properties of the van der Waals (vdW) stacked MX2 heterostructures formed by two individual MX2 monolayers. We found that the so-called Anderson's rule can effectively classify the band structures of heterostructures into three types: straddling, staggered and broken gap. The broken gap is gapless, while the other two types possess direct (straddling, staggered) or indirect (staggered) band gaps. The indirect band gaps are formed by the relatively higher energy level of Te-d orbitals or the interlayer couplings of M or X atoms. For a large part of the formed MX2 heterostructures, the conduction band maximum (CBM) and valence band minimum (VBM) reside in two separate monolayers, thus the electron-hole pairs are spatially separated, which may lead to bound excitons with extended lifetimes. The carrier mobilities, which depend on three competitive factors, i.e. elastic modulus, effective mass and deformation potential constant, show larger values for electrons of MX2 heterostructures compared to their constituent monolayers. Finally, the calculated optical properties reveal strong absorption in the ultraviolet region.

Finite-Size Effects in Metasurface Lasers Based on Resonant Dark States.

The quest for subwavelength coherent light sources has recently led to the exploration of dark-mode based surface lasers, which allow for independent adjustment of the lasing state and its coherent radiation output. To understand how this unique design performs in real experiments, we need to consider systems of finite size and quantify finite-size effects not present in the infinite dark-mode surface laser model. Here we find that, depending on the size of the system, distinct and even counterintuitive behavior of the lasing state is possible, determined by a balanced competition between multiple loss channels, including dissipation, intentional out-coupling of coherent radiation, and leakage from the edges of the finite system. The conclusions are crucial for the design of future experiments that will enable the realization of ultrathin coherent light sources.

Investigation of broadband terahertz generation from metasurface.

The nonlinear metamaterials have been shown to provide nonlinear properties with high nonlinear conversion efficiency and in a myriad of light manipulation. Here we study terahertz generation from nonlinear metasurface consisting of single layer nanoscale split-ring resonator array. The terahertz generation due to optical rectification by the second-order nonlinearity of the split-ring resonator is investigated by a time-domain implementation of the hydrodynamic model for electron dynamics in metal. The results show that the nonlinear metasurface enables us to generate broadband terahertz radiation and free from quasi-phase-matching conditions. The proposed scheme provides a new concept of broadband THz source and designing nonlinear plasmonic metamaterials.

Antimatched Electromagnetic Metasurfaces for Broadband Arbitrary Phase Manipulation in Reflection.

Metasurfaces impart phase discontinuities on impinging electromagnetic waves that are typically limited to 0-2π. Here, we demonstrate that multiresonant metasurfaces can break free from this limitation and supply arbitrarily large, tunable time delays over ultrawide bandwidths. As such, ultrathin metasurfaces can act as the equivalent of thick bulk structures by emulating the multiple geometric resonances of three-dimensional systems that originate from phase accumulation with effective material resonances implemented on the surface itself via suitable subwavelength meta-atoms. We describe a constructive procedure for defining the required sheet admittivities of such metasurfaces. Importantly, the proposed approach provides an exactly linear phase response so that broadband pulses can experience the desired group delay without any distortion of the pulse shape. We focus on operation in reflection by exploiting an antimatching condition, satisfied by interleaved electric and magnetic Lorentzian resonances in the surface admittivities, which completely zeroes out transmission through the metasurface. As a result, the proposed metasurfaces can perfectly reflect a broadband pulse imparting a prescribed group delay. The group delay can be tuned by modifying the implemented resonances, thus opening up diverse possibilities in the temporal applications of metasurfaces.

Pairing Toroidal and Magnetic Dipole Resonances in Elliptic Dielectric Rod Metasurfaces for Reconfigurable Wavefront Manipulation in Reflection.

A novel approach for reconfigurable wavefront manipulation with gradient metasurfaces based on permittivity-modulated elliptic dielectric rods is proposed. It is shown that the required 2π phase span in the local electromagnetic response of the metasurface can be achieved by pairing the lowest magnetic dipole Mie resonance with a toroidal dipole Mie resonance, instead of using the lowest two Mie resonances corresponding to fundamental electric and magnetic dipole resonances as customarily exercised. This approach allows for the precise matching of both the resonance frequencies and quality factors. Moreover, the accurate matching is preserved if the rod permittivity is varied, allowing for constructing reconfigurable gradient metasurfaces by locally modulating the permittivity in each rod. Highly efficient tunable beam steering and beam focusing with ultrashort focal lengths are numerically demonstrated, highlighting the advantage of the low-profile metasurfaces over bulky conventional lenses. Notably, despite using a matched pair of Mie resonances, the presence of an electric polarizability background allows to perform the wavefront shaping operations in reflection, rather than transmission. This has the advantage that any control circuitry necessary in an experimental realization can be accommodated behind the metasurface without affecting the electromagnetic response.

Broadband mixing of [Formula: see text]-symmetric and [Formula: see text]-broken phases in photonic heterostructures with a one-dimensional loss/gain bilayer.

Combining loss and gain components in one photonic heterostructure opens a new route to efficient manipulation by radiation, transmission, absorption, and scattering of electromagnetic waves. Therefore, loss/gain structures enabling [Formula: see text]-symmetric and [Formula: see text]-broken phases for eigenvalues have extensively been studied in the last decade. In particular, translation from one phase to another, which occurs at the critical point in the two-channel structures with one-dimensional loss/gain components, is often associated with one-way transmission. In this report, broadband mixing of the [Formula: see text]-symmetric and [Formula: see text]-broken phases for eigenvalues is theoretically demonstrated in heterostructures with four channels obtained by combining a one-dimensional loss/gain bilayer and one or two thin polarization-converting components (PCCs). The broadband phase mixing in the four-channel case is expected to yield advanced transmission and absorption regimes. Various configurations are analyzed, which are distinguished in symmetry properties and polarization conversion regime of PCCs. The conditions necessary for phase mixing are discussed. The simplest two-component configurations with broadband mixing are found, as well as the more complex three-component configurations wherein symmetric and broken sets are not yet mixed and appear in the neighbouring frequency ranges. Peculiarities of eigenvalue behaviour are considered for different permittivity ranges of loss/gain medium, i.e., from epsilon-near-zero to high-epsilon regime.

Origami-Based Reconfigurable Metamaterials for Tunable Chirality.

Origami is the art of folding two-dimensional (2D) materials, such as a flat sheet of paper, into complex and elaborate three-dimensional (3D) objects. This study reports origami-based metamaterials whose electromagnetic responses are dynamically controllable via switching the folding state of Miura-ori split-ring resonators. The deformation of the Miura-ori unit along the third dimension induces net electric and magnetic dipoles of split-ring resonators parallel or anti-parallel to each other, leading to the strong chiral responses. Circular dichroism as high as 0.6 is experimentally observed while the chirality switching is realized by controlling the deformation direction and kinematics. In addition, the relative density of the origami metamaterials can be dramatically reduced to only 2% of that of the unfolded structure. These results open a new avenue toward lightweight, reconfigurable, and deployable metadevices with simultaneously customized electromagnetic and mechanical properties.

Novel Lasers Based on Resonant Dark States.

The route to miniaturization of laser systems has so far led to the utilization of diverse materials and techniques for reaching the desired laser oscillation at small scales. Unfortunately, at some point all approaches encounter a trade-off between the system dimensions and the Q factor, especially when going subwavelength, mostly because the radiation damping is inherent to the oscillating mode and can thus not be controlled separately. Here, we propose a metamaterial laser system that overcomes this trade-off and offers radiation damping tunability, along with many other features, such as directionality, subwavelength integration, and simple layer-by-layer fabrication.

The plasma bioavailability of nitrate and betanin from Beta vulgaris rubra in humans.

PURPOSE: To evaluate the plasma bioavailability of betanin and nitric oxide (NOx) after consuming beetroot juice (BTJ) and whole beetroot (BF). BTJ and BF were also analysed for antioxidant capacity, polyphenol content (TPC) and betalain content. METHODS: Ten healthy males consumed either 250 ml of BTJ, 300 g of BF or a placebo drink, in a randomised, crossover design. Venous plasma samples were collected pre (baseline), 1, 2, 3, 5 and 8 h post-ingestion. Betanin content in BTJ, BF and plasma was analysed with reverse-phase high-performance liquid chromatography (HPLC) and mass spectrometry detection (LCMS). Antioxidant capacity was estimated using the Trolox equivalent antioxidant capacity (TEAC) and polyphenol content using Folin-Ciocalteu colorimetric methods [gallic acid equivalents (GAE)] and betalain content spectrophotometrically. RESULTS: TEAC was 11.4 ± 0.2 mmol/L for BTJ and 3.4 ± 0.4 µmol/g for BF. Both BTJ and BF contained a number of polyphenols (1606.9 ± 151 mg/GAE/L and 1.67 ± 0.1 mg/GAE/g, respectively), betacyanins (68.2 ± 0.4 mg/betanin equivalents/L and 19.6 ± 0.6 mg/betanin equivalents/100 g, respectively) and betaxanthins (41.7 ± 0.7 mg/indicaxanthin equivalents/L and 7.5 ± 0.2 mg/indicaxanthin equivalents/100 g, respectively). Despite high betanin contents in both BTJ (~194 mg) and BF (~66 mg), betanin could not be detected in the plasma at any time point post-ingestion. Plasma NOx was elevated above baseline for 8 h after consuming BTJ and 5 h after BF (P < 0.05). CONCLUSIONS: These data reveal that BTJ and BF are rich in phytonutrients and may provide a useful means of increasing plasma NOx bioavailability. However, betanin, the major betalain in beetroot, showed poor bioavailability in plasma.

Near-Infrared and Optical Beam Steering and Frequency Splitting in Air-Holes-in-Silicon Inverse Photonic Crystals.

We present the design of a dielectric inverse photonic crystal structure that couples line-defect waveguide propagating modes into highly directional beams of controllable directionality. The structure utilizes a triangular lattice made of air holes drilled in an infinitely thick Si slab, and it is designed for operation in the near-infrared and optical regime. The structure operation is based on the excitation and manipulation of dark dielectric surface states, in particular on the tailoring of the dark states' coupling to outgoing radiation. This coupling is achieved with the use of properly designed external corrugations. The structure adapts and matches modes that travel through the photonic crystal and the free space. Moreover it facilitates the steering of the outgoing waves, is found to generate well-defined, spatially and spectrally isolated beams, and may serve as a frequency splitting component designed for operation in the near-infrared regime and in particular the telecom optical wavelength band. The design complies with the state-of-the-art Si nanofabrication technology and can be directly scaled for operation in the optical regime.