Spin-Dependent Visible Dyakonov Surface Waves in a Thin Hyperbolic Metamaterial Film.

A Dyakonov surface wave offers a promising way of guiding light in two dimensions with almost no loss but also additional freedom of modulation through the anisotropy at the interface. Here we experimentally demonstrated a spin-dependent Dyakonov surface wave mode propagating in a hyperbolic metamaterial film at a visible frequency. Due to the strong anisotropy of the hyperbolic metamaterial, a Dyakonov type mode shows a large allowed angle band and transverse spin with its direction determined by the orientations of the hyperbolic anisotropy and surface normal, based on which we experimentally observed the photonic spin Hall effect of the surface wave mode. This work enables a new generation of two-dimensional photonic circuits with spin-dependent excitation/detection and modulation devices at the subwavelength level.

Optical Vector Vortex Generation by Spherulites with Cylindrical Anisotropy.

Materials with crystalline structures of circular symmetry are rare in nature; however, they are highly desired in optical applications with structured lights, whose characteristics are of cylindrical symmetry. In this work, using a naturally existing circular anisotropy from a spherulite formed by molecular self-assembly, we obtain a cylindrical vector optical vortex beam generation transformed from the spin angular momentum in the wide visible range. The proposed strategy provides promising and broad opportunities for the applications of spherulites in the generation of structured lights and modulations of both the polarization and the angular momentum.

Broadband second-harmonic generation from artificial optical nonlinearity.

In this Letter, we present a mechanism for effectively broadening the bandwidth of second-harmonic generation (SHG) with the metamaterial-based artificial optical nonlinearity. As the nonlinear response of the artificial nonlinearity arising from the magnetoelectric coupling constructed by the meta-molecule (MM) structure, the broadband second-order nonlinearity can be built by simply combining the MMs with different geometrical sizes together. The physical model and the numerical simulation fully support the artificial generation and modulation of the broadband second harmonics. Our work suggests a new route for realizing the on-chip custom-designed nonlinear optical devices with broadband operation.

Artificial Nonlinearity Generated from Electromagnetic Coupling Metamolecule.

A purely artificial mechanism for optical nonlinearity is proposed based on a metamaterial route. The mechanism is derived from classical electromagnetic interaction in a metamolecule consisting of a cut-wire meta-atom nested within a split-ring meta-atom. Induced by the localized magnetic field in the split-ring meta-atom, the magnetic force drives an anharmonic oscillation of free electrons in the cut-wire meta-atom, generating an intrinsically nonlinear electromagnetic response. An explicit physical process of a second-order nonlinear behavior is adequately described, which is perfectly demonstrated with a series of numerical simulations. Instead of "borrowing" from natural nonlinear materials, this novel mechanism of optical nonlinearity is artificially dominated by the metamolecule geometry and possesses unprecedented design freedom, offering fascinating possibilities to the research and application of nonlinear optics.

Diode-based microbolometer with performance enhanced by broadband metamaterial absorber.

This Letter reports a microbolometer integrated with a broadband metamaterial absorber (MMA) to enhance its performance, which contains series-connected silicon diodes as the temperature sensor. The broadband MMA is readily integrated into the device by introducing an array of different-sized square resonators on the silicon nitride structural layer, while the widened titanium interconnecting wires between individual diodes serve as the ground plane. In a comparative experiment, the broadband MMA was demonstrated to be superior to the ordinary silicon nitride absorber in a broad spectra range, especially in a long-wavelength IR regime, which directly leads to an increase in IR responsivity by 60%. More importantly, this enhancement in responsivity was achieved with no sacrifice of the response time due to the negligible thermal mass of the introduced resonator array.

Planar broadband and high absorption metamaterial using single nested resonator at terahertz frequencies.

A planar broadband metamaterial absorber with high absorptivity working at terahertz frequencies was designed and fabricated in this work. Two nested back-to-back split-ring resonators (BSRRs) constitute a single resonator, which achieves three strong resonances, with two of them merged into a broadband peak. Cobalt silicide and parylene-C were innovatively applied as ground plane and dielectric spacer. The nested BSRR absorber experimentally realizes a bandwidth of 0.66 THz with the absorptivity above 0.8, and the highest absorptivity reaches 0.97. Taking the central frequency at 2.74 THz, the measured FWHM is 47% and the Q factor is 2.13.

Broadband Spin and Orbital Momentum Modulator Using Self-Assembled Nanostructures.

The structural symmetry of solids plays an important role in defining their linear and nonlinear optical properties. The quest for versatile, cost-effective, large-scale, and defect-free approaches and materials platforms for tailoring structural and optical properties on demand is underway since decades. A self-assembled spherulite material comprised of synthesized molecules with large dipole moments aligned azimuthally, forming a vortex polarity with spontaneously broken symmetry, is experimentally demonstrated. This unique self-assembled structure enables new linear and nonlinear light-matter interactions, including generating optical vortex beams with complex spin states and on-demand topological charges at the fundamental, doubled, and tripled frequencies. This work will likely enable numerous applications in areas such as high-dimensional quantum information processing with large capacity and high security, spatiotemporal optical vortices, and a novel optical manipulation and trapping platform.

Broadband metamaterial absorber at mid-infrared using multiplexed cross resonators.

In this paper, we theoretically and experimentally demonstrate broadband metamaterial absorbers that work in the mid-infrared regime. In the absorbers, two or four gold cross resonators with different sizes are multiplexed in a unit cell on SiO(2) spacing layer on top of gold ground plane. Compared with the single cross resonator absorbers with a Q factor of 6.39, the developed absorber with two cross resonators multiplexed reduces the Q factor to 3.78. When four different cross resonators are integrated, the Q factor drops to as low as 1.85, and the bandwidth almost covers the full mid-infrared regime from 3 µm to 5 µm with absorbance higher than 50%.

Polarization-independent dual-band terahertz metamaterial absorbers based on gold/parylene-C/silicide structure.

We design, fabricate, and characterize dual-band terahertz (THz) metamaterial absorbers with high absorption based on structures consisting of a cobalt silicide (Co-Si) ground plane, a parylene-C dielectric spacer, and a metal top layer. By combining two periodic metal resonators that couple separately within a single unit cell, a polarization-independent absorber with two distinct absorption peaks was obtained. By varying the thickness of the dielectric layer, we obtain absorptivity of 0.76 at 0.76 THz and 0.97 at 2.30 THz, which indicates the Co-Si ground plane absorbers present good performance.

Perfect absorption based on a ceramic anapole metamaterial.

Metamaterials, from concept to application level, is currently a high-trending topic. Due to the strict requirements of the simultaneous reasonable structural design and stability of materials, the construction of a high-performance metamaterial for extreme environments is still difficult. Here, combining metamaterial design with materials optimization, we propose a completely different strategy and synthesize a type of monomeric ceramic meta-atom to construct metamaterials. Based on a geometric design with multiple degrees of freedom and dielectric properties, hybrid anapole modes with impedance matching can be produced, experimentally inducing nearly perfect absorption with high temperature stability (high tolerable temperature of approximately 1300 °C, with almost zero temperature drift) in microwave/millimeter-wave bands. We surpass the oxidation temperature limitation of 800 °C in conventional plasmonic absorbers, and provide an unprecedented direction for the further development of integrated high-performance metamaterial wireless sensors responding to extreme environmental scenarios, which will also lead to a new direction of specific ceramic research toward device physics.

A universal route to efficient non-linear response via Thomson scattering in linear solids.

Non-linear materials are cornerstones of modern optics and electronics. Strong dependence on the intrinsic properties of particular materials, however, inhibits the at-will extension of demanding non-linear effects, especially those second-order ones, to widely adopted centrosymmetric materials (for example, silicon) and technologically important burgeoning spectral domains (for example, terahertz frequencies). Here we introduce a universal route to efficient non-linear responses enabled by exciting non-linear Thomson scattering, a fundamental process in electrodynamics that was known to occur only in relativistic electrons in metamaterial composed of linear materials. Such a mechanism modulates the trajectory of charges, either intrinsically or extrinsically provided in solids, at twice the driving frequency, allowing second-harmonic generation at terahertz frequencies on crystalline silicon with extremely large non-linear susceptibility in our proof-of-concept experiments. By offering a substantially material- and frequency-independent platform, our approach opens new possibilities in the fields of on-demand non-linear optics, terahertz sources, strong field light-solid interactions and integrated photonic circuits.

Spin homojunction with high interfacial transparency for efficient spin-charge conversion.

High interfacial transparency is vital to achieve efficient spin-charge conversion for ideal spintronic devices with low energy consumption. However, in traditional ferromagnetic/nonmagnetic heterojunctions, the interfacial Rashba spin-orbit coupling brings about spin memory loss (SML) and two-magnon scattering (TMS), quenching spin current crossing the heterointerfaces. To address the intrinsic deficiency of heterointerface, we design a ferromagnetic FeRh/antiferromagnetic FeRh spin homojunction for efficient spin-charge conversion, verified by a high interfacial transparency of 0.75 and a high spin torque efficiency of 0.34 from spin pumping measurements. First-principles calculations demonstrate that the interfacial electric field of homojunction is two orders of magnitude smaller than that of traditional heterojunction, producing negligible interfacial spin-orbit coupling to drastically reduce SML and TMS. Our spin homojunction exhibits potential and enlightenment for future energy-efficient spintronic devices.

Artificial Generation of High Harmonics via Nonrelativistic Thomson Scattering in Metamaterial.

High harmonic generation allows one to extend the frequency of laser to a much broader regime and to study the electron dynamics of matters. However, severely limited by the vague high-order process in natural material and the unfriendly state of the commonly applied gas and plasma media, the ambitious goal of custom-design high harmonics remains exceptionally challenging. Here, we demonstrate that high harmonics can be artificially designed and tailored based on a metamaterial route. With the localized reconstruction of magnetic field in a metamaterial, the nonlinear Thomson scattering, a ubiquitous electromagnetic process which people used to believe that it only occurs with the relativistic velocity, can be stimulated in a nonrelativistic limit, which drives anharmonic oscillation of free electrons and generates high harmonics. An explicit physical model and the numerical simulations perfectly demonstrate the artificial generation and tailoring of the high harmonics. This novel mechanism is entirely dominated by the artificial structure instead of the natural nonlinear compositions. It not only provides unprecedented design freedom to the high harmonic generation but breaks the rigorous prerequisite of the relativistic velocity of the nonlinear Thomson scattering process, which offers fascinating possibilities to the development of new light source and ultrafast optics, and opens up exciting opportunities for the advanced understanding of electrodynamics in condensed matters.

Tailoring Nanohole Plasmonic Resonance with Light-Responsive Azobenzene Compound.

Metal-based nanohole structures, featuring a continuous matrix and discrete voids, have seen a wide spectrum of practical applications, ranging from plasmonic sensing to extraordinary optical transmission. It would not be uncommon to pursue further enhancement of their optical tunability, and incorporation with other functional materials offers an intriguing lead. In this study, the first step involves colloidal lithography fabrication of gold-based, short-range ordered nanohole structures on a glass substrate with varying geometrical parameters. Plasmonic resonance in optical waveband is readily achieved from the coupling between bonding surface plasmons and nanohole lattices. Resonant features observed in transmission measurements could also be well reproduced both from numerical simulations as well as theoretical calculations based on the grating coupling mechanism. With the introduction of a thin layer of azobenzene compound by spin-coating comes the critical transformation that not only alters optical performances by impacting the surface environment but also bestows the structures with light responsiveness. After 488 nm of laser irradiation, it is observed that the structures underwent cross polarization conversion, which could be attributed to the photoalignment behavior from trans-cis isomerization within the azobenzene layer, yielding further optical tunability with the linearly polarized probe light compared to that in the preirradiated state. The tuning of plasmonic resonances through light stimuli paves a noncontacting path for achieving desired optical responses with potentially high spatial and temporal resolution. This work may serve as a fountainhead for future efforts on optically tailorable photonic devices associated with nanohole plasmonics.

Photomechanical meta-molecule array for real-time terahertz imaging.

Real-time terahertz (THz) imaging offers remarkable application possibilities, especially in the security and medical fields. However, most THz detectors work with scanners, and a long image acquisition time is required. Some thermal detectors can achieve real-time imaging by using a focal plane array but have the drawbacks of low sensitivity due to a lack of suitable absorbing materials. In this study, we propose a novel photomechanical meta-molecule array by conveniently assembling THz meta-atom absorbers and bi-material cantilevers together, which can couple THz radiation to a mechanical deflection of the meta-molecules with high efficiency. By optically reading out the mechanical deflections of all of the meta-molecules simultaneously, real-time THz imaging can be achieved. A polyimide sacrificial layer technique was developed to fabricate the device on a glass wafer, which facilitates the transmission of a readout light while the THz wave radiates onto the meta-molecule array directly from the front side. THz images and video of various objects as well as infrared images of the human body were captured successfully with the fabricated meta-molecule array. The proposed photomechanical device holds promise in applications in single and broadband THz as well as infrared imaging.