Efficient Thermo-Optic Switching and Modulating in Nonlocal Metasurfaces.

High- optical resonances in nonlocal metasurfaces, benefiting from significantly enhanced light/matter interactions, feature strong responses even under a weak external stimulus. In this work, we leverage the high- resonances of quasi-guided modes (QGMs) supported by a photonic crystal slab (PCS) structure to achieve efficient optical switching/modulation. The QGMs with an experimentally measured -factor of ∼2200 are realized by shifting every second column of air holes in a rectangular lattice within a silicon slab.

Ecological effect of microplastics on soil microbe-driven carbon circulation and greenhouse gas emission: A review.

Soil organic carbon (SOC) pool, the largest part of terrestrial ecosystem, controls global terrestrial carbon balance and consequently presented carbon cycle-climate feedback in climate projections. Microplastics, (MPs, <5 mm) as common pollutants in soil ecosystems, have an obvious impact on soil-borne carbon circulation by affecting soil microbial processes, which play a central role in regulating SOC conversion. In this review, we initially presented the sources, properties and ecological risks of MPs in soil ecosystem, and then the differentiated effects of MPs on the component of SOC, including dissolved organic carbon, soil microbial biomass carbon and easily oxidized organic carbon varying with the types and concentrations of MPs, the soil types, etc.

Ultra-narrowband and rainbow-free mid-infrared thermal emitters enabled by a flat band design in distorted photonic lattices.

Most reported thermal emitters to date employing photonic nanostructures to achieve narrow bandwidth feature the rainbow effect due to the steep dispersion of the involved high-Q resonances. In this work, we propose to realize thermal emissions with high temporal coherence but free from rainbow effect, by harnessing a novel flat band design within a large range of wavevectors. This feature is achieved by introducing geometric perturbations into a square lattice of high-index disks to double the period along one direction.

The Strong Coupling Effect between Metallic Split-Ring Resonators and Molecular Vibrations in Polymethyl Methacrylate.

We propose and study a nanoscale strong coupling effect between metamaterials and polymer molecular vibrations using metallic split-ring resonators (SRRs). Specifically, we first provided a numerical investigation of the SRR design, which was followed by an experimental demonstration of strong coupling between mid-infrared magnetic dipole resonance supported by the SRRs fabricated on a calcium fluoride substrate and polymethyl methacrylate molecular vibrations at 1730 cm. Characterized by the anti-crossing feature and spectral splitting behaviors in the transmission spectra, these results demonstrate efficient nanoscale manipulation of light-matter interactions between phonon vibrations and metamaterials.

Efficient and Shape-Sensitive Manipulation of Nanoparticles by Quasi-Bound States in the Continuum Modes in All-Dielectric Metasurfaces.

Current optical tweezering techniques are actively employed in the manipulation of nanoparticles, e.g., biomedical cells.

High-Q optical resonances with robustness based on the quasi-guided modes in waveguide moiré gratings.

High-Q resonances, especially those with high spectral tunability and large robustness of the Q factors, are always sought in photonic research for enhanced light-matter interactions. In this work, by rotating the 1D ridge grating on a slab waveguide in both the clockwise and counterclockwise directions by a certain angle θ, we show that the original subwavelength lattice can be converted into waveguide moiré gratings (WMGs), with the period increased to a larger value determined by the value of θ. These period-increasing perturbations will cause the First Brillouin Zone (FBZ) of the 1D grating to shrink, and thus convert the non-radiating guided modes with the dispersion band below the light line into quasi-guided modes (QGMs) above the light line, which can be accessed by free space radiations.

Exploiting Zone-Folding Induced Quasi-Bound Modes to Achieve Highly Coherent Thermal Emissions.

Thermal emissions with high coherence, although not as high as those of lasers, still play a crucial role in many practical applications. In this work, by exploiting the geometric perturbation-induced optical lattice tripling and the associated Brillion zone folding effect, we propose and investigate thermal emissions in the mid-infrared with simultaneous high temporal and spatial coherence. In contrast with the case of period-doubling perturbation in our previous work, the steeper part of the guided mode dispersion band will be folded to the high-symmetry Γ point in the ternary grating.

Narrowband mid-infrared thermal emitters based on the Fabry-Perot type of bound states in the continuum.

The development of narrow-band thermal emitters operating at mid-infrared (MIR) wavelengths is vital in numerous research fields. However, the previously reported results obtained with metallic metamaterials were not successful in achieving narrow bandwidths in the MIR region, which suggests low temporal coherence of the obtained thermal emissions. In this work, we demonstrate a new design strategy to realize this target by employing the bound state in the continuum (BIC) modes of the Fabry-Perot (FP) type.

Investigations on the optical forces from three mainstream optical resonances in all-dielectric nanostructure arrays.

Light can exert radiation pressure on any object it encounters, and the resulting optical force can be used to manipulate particles at the micro- or nanoscale. In this work, we present a detailed comparison through numerical simulations of the optical forces that can be exerted on polystyrene spheres of the same diameter. The spheres are placed within the confined fields of three optical resonances supported by all-dielectric nanostructure arrays, including toroidal dipole (TD), anapoles, and quasi-bound states in continuum (quasi-BIC) resonances.

Highly sensitive terahertz fingerprint sensing based on the quasi-guided modes in a distorted photonic lattice.

Using photonic structures resonating at the characteristic absorption frequency of the target molecules is a widely-adopted approach to enhance the absorption and improve the sensitivity in many spectral regions. Unfortunately, the requirement of accurate spectral matching poses a big challenge for the structure fabrication, while active tuning of the resonance for a given structure using external means like the electric gating significantly complicates the system. In this work, we propose to circumvent the problem by making use of quasi-guided modes which feature both ultra-high Q factors and wavevector-dependent resonances over a large operating bandwidth.

Quasi-guided modes resulting from the band folding effect in a photonic crystal slab for enhanced interactions of matters with free-space radiations.

We elucidate that guided modes supported by a regular photonic crystal slab structure composed of a square lattice of air holes in a silicon slab will transition into quasi-guided (leaky) modes when the radius of every second column of air holes is changed slightly. This intentional geometric perturbation will lead to a doubling of the period in one direction and the corresponding shrinkage of the first Brillouin zone. Because of the translational symmetry in the -space, leaky waves inheriting the spatial dispersion of the original guided modes, which do not interact with external radiation, will appear with the dispersion curves above the light cone.

Ultra-high-Q substrate-mode coupled resonances in complementary THz metamaterial.

Achieving high-Q resonances in the THz frequency range is significant for applications such as sensors, filters, and emitters. A promising approach for obtaining such resonances is by using metamaterials. However, high-Q resonances in THz metamaterials are usually limited by metallic radiation losses in the meta-atoms.

Relative humidity sensing using THz metasurfaces combined with polyvinyl alcohol.

Relative humidity (RH) plays an important role in almost every industrial field. Thus, the detection of RH is of great significance in these fields. Terahertz (THz) waves are extremely sensitive to the changes in RH because water absorbs strongly in this electromagnetic band.

Strong coupling between quasi-bound states in the continuum and molecular vibrations in the mid-infrared.

Strong light-matter coupling is of much interest for both fundamental research and technological applications. The recently studied bound state in the continuum (BIC) phenomenon in photonics with controlled radiation loss rate significantly facilitates the realization of the strong coupling effect. In this work, we report the experimental observation of room temperature strong coupling between quasi-BIC resonances supported by a zigzag metasurface array of germanium elliptical disks and the vibrational resonance of polymethyl methacrylate (PMMA) molecules in the mid-infrared.

High-directionality spin-selective routing of photons in plasmonic nanocircuits.

Efficient on-chip manipulation of photon spin is of crucial importance in developing future integrated nanophotonics as is electron spin in spintronics. The unidirectionality induced by the interaction between spin and orbital angular momenta suffers low efficiency in classical macroscopic optics, while it can be highly enhanced on subwavelength scales with suitable architectures. Here we propose and demonstrate a spin-sorting achiral split-ring coupler to unidirectionally excite dielectric-loaded plasmonic modes in two independent waveguides.

Spin-orbit-enabled sorting of optical flows in plasmonic nanocircuits.

On-chip controlling of photon spin is essential in developing future integrated nanophotonics with complex functionalities. Here we propose and demonstrate a robust spin-sorting nanocircuit, which consists of a spin-orbit coupler (i.e.

Metasurface circular polarizer based on rotational symmetric nanoholes.

A circular polarizer is proposed based on a single layered metasurface. This metasurface circular polarizer is composed of L-shaped nanoholes etched on the silver film. The L-shaped nanoholes are rotational symmetric, and the special symmetric structure determines the polarization selection transmission of the metasurface.

All-optical self-switching with ultralow incident laser intensity assisted by a bound state in the continuum.

We present a new, to the best of our knowledge, scheme of realizing all-optical switching by exploiting the phenomenon of the bound state in the continuum (BIC) supported by an array of slotted silicon disks. The air slot in an individual silicon disk works as a void antenna and can then help to excite the quasi-BIC (QBIC) mode by a linearly polarized plane wave. Thanks to the high quality factor of the QBIC resonance and the associated large local electric field enhancement, the self-switching effect is quite pronounced, and a laser power intensity level less than 1/ is required to achieve a change of transmittance from 0 to above 65%.

Bragg-Mirror-Assisted High-Contrast Plasmonic Interferometers: Concept and Potential in Terahertz Sensing.

A Bragg-mirror-assisted terahertz (THz) high-contrast and broadband plasmonic interferometer is proposed and theoretically investigated for potential sensing applications. The central microslit couples the incident THz wave into unidirectional surface plasmon polaritons (SPPs) waves travelling to the bilateral Bragg gratings, where they are totally reflected over a wide wavelength range back towards the microslit. The properties of interference between the SPPs waves and transmitted THz wave are highly dependent on the surrounding material, offering a flexible approach for the realization of refractive index (RI) detection.

Ultrasensitive detection of saccharides using terahertz sensor based on metallic nano-slits.

Unambiguous identification of trace amounts of biochemical molecules in a complex background using terahertz spectroscopy is extremely challenging owing to the extremely small absorption cross sections of these molecules in the terahertz regime. Herein, we numerically propose a terahertz nonresonant nano-slits structure that serves as a powerful sensor. The structure exhibits strongly enhanced electric field in the slits (five orders of magnitude), as well as high transmittance over an extra-wide frequency range that covers the characteristic frequencies of most molecules.

Highly sensitive terahertz fingerprint sensing with high-Q guided resonance in photonic crystal cavity.

In order to solve the problem of low sensitivity and poor selectivity in biochemical sensing using terahertz technology, a new sensing scheme based on photonic crystal cavity structure is proposed. It is composed of two identical photonic crystal slabs, each of which consists of a square lattice of silicon-based cylindrical pillars on a silicon substrate. The geometric parameters of the cavity are optimized to obtain a guided resonance peak at 529.

Tunable hybridization induced transparency for efficient terahertz sensing.

Hybridization induced transparency (HIT) resulting from the coupling between the material absorption resonance and the artificial structure (metamaterial) resonance provides an effective means of enhancing the sensitivity in the terahertz spectroscopic technique-based sensing applications. However, the application of this method is limited by the versatility to the samples with different volumes, because the samples usually have a refractive index larger than unity and their presence with different thicknesses will lead to a shift of the structure resonance, mismatching the material absorption. In this work, we demonstrate that by using InSb coupled rod structures, whose electromagnetic response in the terahertz band can be easily controlled by using ambient parameters like the temperature or magnetic field, the HIT effect can be easily tuned so that without the needs to change the rod geometry, one can realize efficient terahertz sensing with different sample thickness.

Planar antenna array as a highly sensitive terahertz sensor.

In this paper, a planar comb-shaped antenna array for terahertz sensing based on the excitation of spoof surface plasmon modes is proposed. The structure is constructed by an array of three periodic rectangular grooves perforated through metal stripes on top of a silicon substrate. The effective detection of lactose is given as an example to demonstrate the ability of this structure to enhance detection sensitivity.

A sensitive and selective terahertz sensor for the fingerprint detection of lactose.

Special recognition to molecules is essential for many biochemical processes, thus highly sensitive sensing methods for molecule recognition are strongly demanded. Recently, metamaterials present a unique platform for sensing applications owing to their exotic properties. In the current work, a metamaterial sensor for enhanced fingerprint detection of lactose based on resonant coupling of plasmonic modes of split-ring resonators (SRRs) and terahertz characteristic modes of lactose was theoretically and experimentally demonstrated.

Bessel-like beam generated by an axicon based on parallel-plate waveguides.

The axicon is the simplest and most effective optical element for generating the zero-order Bessel-like beam. The zero-order Bessel-like beam, which has the characteristics of small spot size, high brightness, good direction, and large collimation distance, can be applied to optical micromanipulation and power transmission. In this paper, we proposed and designed a structure for phase manipulation based on parallel-plate waveguides that can be used to realize the functionality of the axicon in the terahertz (THz) region.