Chiral metasurfaces exhibit vast application prospects in near-field imaging, polarization conversion, and chiral sensing. In this work, we theoretically propose a temperature-tunable terahertz (THz) chiral absorber based on a metasurface of Ag–VO2 nested split-rings. Perfect absorption of left-handed circularly polarized light (LCP) exceeding 99% and weak absorption of only about 6% of right-handed circularly polarized light (RCP) are achieved at 7.24 THz in frequency. The discrepancy in absorption responses of LCP and RCP results in a large circular dichroism (CD) of 0.94. The CD value in the THz region can be actively modulated between 0.02 and 0.94 by controlling the phase transition of VO2, which is closely related to the ambient temperature. Moreover, near-field imaging and encrypted presentation are realized by leveraging the metasurface and its mirror-image properties. Our work holds significant promise in widespread fields including THz intelligent absorption, imaging, and chiral detection.

Heterogeneous metamaterials containing excitonic materials provide an ideal platform for strong exciton-photon coupling. In this Letter, we theoretically demonstrate four strong couplings in a heterogeneous metamaterial consisting of a TiO2 grating standing on a perovskite-WS2-perovskite waveguide layer by tuning the structural sizes. The quasi-bound state in the continuum (qBIC) and the guided mode resonance (GMR) both strongly coupled with the excitons of both perovskite and WS2 under oblique incident illumination, resulting in four large Rabi splittings of 177.32, 187.53, 406.25, and 435.09 meV via a reasonable combination of oscillator strengths of perovskite and WS2. Double strong coupling behaviors are also achieved when the grating period equals 222 nm with an incident light angle of 19.3°. Moreover, double ultrastrong coupling can even be realized by the GMR and qBIC respectively interacting with the exciton of WS2 when its oscillator strength reaches a certain value. Our work paves an effective avenue to realizing strong coupling and even ultrastrong coupling between multiple excitons and multiple optical modes.

Carefully designed metasurfaces can serve as effective platforms for strong coupling phenomena by the excitation of specific modes and the enhancement of light -matter interactions. However, strong coupling simultaneously involving Fabry-P & eacute;rot (FP) cavity modes, anapole states, and excitons has not been thoroughly explored. In this work, we numerically present the strong coupling of a FP cavity-anapole-exciton supported by a nano -optical metasurface of bulk WS 2 -Si heterogeneous nanodisks inserted into a FP optical cavity. The excited multiorder FP cavity modes enable giant Rabi splittings with tunable population due to all generated odd -order FP cavity modes strongly coupling with the anapole state and exciton of bulk WS 2 -Si heterogeneous nanodisks. Giant Rabi splittings of 643 and 335 meV are, respectively, achieved by the first -order FP and third -order FP cavity-anapole-exciton interactions. Our designed hybrid system provides a robust platform for exploring ultrahigh Rabi splittings and strong coupling of multiple optical responses in light -matter interactions.

Electromagnetically induced transparency (EIT), nonlinearity, and optical chirality hold significant applications in many areas such as optical switches, slow-light devices, chiral harmonic conversion, and optical storage. In this work, we theoretically propose an asymmetric all-dielectric metasurface supporting toroidal dipole-quasi-bound states in the continuum (TD-q-BICs). High quality (Q) EIT, strong third harmonic generation (THG), and giant nonlinear chirality are achieved via the extremely enhanced electric field energy localized in the Si plate by the TD-q-BIC. A huge transition from high Q EIT with transmission of similar to 0.99 to strong chirality with circular dichroism (CD) of similar to 0.9 is realized by tuning the angle and polarization state of incident light. Strong THG with efficiency of 4.5 x 10(-3) under linear polarization light is due to the highly localized electric field supported by the TD-q-BIC and perfect nonlinear CD chirality with theoretically value of similar to 1 originates from the large discrepancy in electric field distributions under different circularly polarized light. Our work provides an innovative paradigm to construct TD-q-BICs-governed EIT analogs, THG, and nonlinear chirality for the development of multifunction nanophotonic meta-devices.

We theoretically demonstrate a thermo-optic and electric-optic dual-channel dynamically switchable terahertz (THz) absorber based on both bandwidth and intensity in a graphene-VO2 complex metamaterial. An ultra-broadband from 2.5 THz to 10.8 THz with near-unity absorption (>= 90 %) is achieved when the ambient temperature (T-a) is equal to 72 degrees C and the Fermi energy (E-F) of graphene is zero and the absorption intensity decreases to less than 5% when T-a = 28degree celsius , accompanied by the sharply reduced bandwidth, verifying the superior thermo-optic modulation function. The absorption intensity is also artificially tuned from similar to 3.9% to similar to 99% in the 2.29-4.86 THz range by tuning the E-F from 0 to 0.9 eV, implying the remarkable electric-optic modulation behavior. Moreover, the designed absorber is independent on the polarization of incident light. The proposed absorber paves an effective avenue for its applications in intelligent absorbers, telecom and THz modulators, etc.

Based on the generalized Huygens-Fresnel principle, an analytical expression for the spectral intensity of the radial array Gaussian Schell mode beam (RAGSM) in inhomogeneous atmospheric turbulence is derived using the theory of partially coherent light. The variation of the normalized spectral intensity and the on-axis relative spectral shift are investigated. The effects of beam parameters and turbulent parameters on the spectra are numerically calculated. The results show that there is a critical position where a rapid transition of the spectrum takes place. The critical position becomes greater with an increase in the number of sub-beams, the radial distance, initial coherence length. At the longer propagation distance, the effect of structural constants of refractive index is weaker. The spectral transition distances are proportional to the turbulence parameters.

AbstractAltered rocks widely exist in rock mass engineering. Alteration has an obvious deterioration effect on the physical and mechanical properties of rocks, which may cause engineering geological prob-lems or geological hazards. The main method to obtain the uniaxial compressive strength of altered rocks is indoor uniaxial compression test, which has the reality of long period, high cost, difficult sampling, and limited transportation. In this paper, the uniaxial compression test and short wavelength infrared spectral test analysis are performed on the altered rocks of a hydropower station in Southeast China to study the relationship between the strength and the short wavelength infrared spectral curve of altered rocks. The results show that, there is a correlation between the number of absorption peaks of short wavelength infrared spectral curve and the uniaxial compressive strength of altered rocks. In the wavelength ranges of 1880–1950 nm and 2170–2230 nm, the uniaxial compressive strength of altered rocks with two absorption peaks in the short wavelength infrared spectral curves are generally higher than 150 MPa. In the wavelength ranges of 1380–1430 nm, 1880–1950 nm, and 2170–2230 nm, the uniaxial compressive strength of altered rocks with three absorption peaks in the short wavelength infrared spectral curves are generally lower than 100 MPa. In the wavelength ranges of 1380–1430 nm, 1630–1660 nm, 1880–1950 nm, 2170–2230 nm, 2230–2380 nm, the uniaxial compressive strength of altered rocks with four absorption peaks in the short wavelength infrared spectral curves are generally between 100–150 MPa. This research will provide a new method for rapid evaluation of strength of altered rocks in the field.

Light absorbers are desirable for wide applications in optoelectronic devices. In this work, we propose and demonstrate a new plasmonic absorber platform, which can support two sharp absorption bands with differential features via tuning the incident angle. The absorber is formed by an asymmetrical metal-semiconductor-metal (MSM) stack intercalated by metal micro-cavities. The maximal absorption for the two peaks respectively reaches 99.8% (lambda = 0.983 mu m) and 98.9% (lambda = 1.187 mu m). Importantly, the dual-band absorber shows remarkable differential properties when the incident angle is tuned. For instance, the wavelength position is only with a 2 nm fluctuation for the first band in the shorter wavelength range, suggesting a nearly stable state during the angle changes from 0 degrees to 50 degrees. Nevertheless, the wavelength position shifts up to 37.5 nm for the second band in the longer wavelength region, indicating an angle-sensitive absorption. These properties are mainly related to the relaxed resonant fields from the MSM cavities and their different coupling modes to the adjacent metal cavities in this asymmetric structure. These new findings could pave applications for differential optoelectronic manipulations, filters and photo-detection, etc.

The propagation characteristics of rectangular array beams through a bifocal lens system have been studied, including irradiance distribution and focal shift. By using Collins formula and diffraction integral, the analytical expressions for irradiance distributions of the array beams have been derived. The results show that the irradiance distributions exhibit diversified variations due to the different focal length in two directions, and the changes of the irradiance distributions for the correlated and uncorrelated superposition array beam are different. There are usually two maxima of the axial irradiance on propagation. The focal shifts demonstrate that the axial maximum irradiance near the larger focal distance is displaced toward the lens system when the Fresnel number is smaller. The focal shift results from the competition of the two irradiance maxima, which can lead to a permutation of the focal points. This work would be useful for the practical design and use of the bifocal lens in focusing array beams.

Metamaterial absorbers have attracted great attention over the past few years and exhibited a promising prospect in solar energy harvesting and solar thermophotovoltaics (STPVs). In this work, we introduce a solar absorber scheme, which enables efficient solar irradiance harvesting, superb thermal robustness and high solar thermal energy conversion for STPV systems. The optimum structure demonstrates an average absorbance of 97.85% at the spectral region from 200 nm to 2980 nm, indicating the near-unity absorption in the main energy range of the solar radiance. The solar-thermal conversion efficiencies surpassing 90% are achieved over an ultra-wide temperature range (100-800 °C). Meanwhile, the analysis indicates that this metamaterial has strong tolerance for fabrication errors. By utilizing the simple two-dimensional (2D) titanium (Ti) gratings, this design is able to get beyond the limit of costly and sophisticated nanomanufacturing techniques. These impressive features can hold the system with wide applications in metamaterial and other optoelectronic devices.

We propose and demonstrate a novel perfect absorber platform by combining the colloidal crystal and refractory metal layer. Dual-band absorption with the efficiency up to 99.8% (0.336 mu m) and 99.1% (1.382 mu m) is achieved. Moreover, the absorption bandwidths are 81 nm and 1237 nm for the two bands in the UV and near-infrared ranges, suggesting the simultaneous achievement of narrowband and ultra-broadband absorption. Furthermore, the resonant wavelengths in the two bands are with the geometric multiples up to 3 similar to 6, indicating the large differential frequency responses. In addition, the absorption properties can be artificially adjusted via the structural parameters. These features can hold potential applications in the nonlinear optics such as frequency multiplication, difference and mixing, and the selective spectral filtering and manipulation, etc.

Perfect light absorption is desirable for applications in the devices related to photo-electrical effects, photo thermal process and photo-chemical/catalysis solutions, etc. In this work, we propose and demonstrate a feasible way to achieve broadband perfect absorption in the full visible range based on the refractory material resonators. This structure shows excellent absorption efficiency with the average absorption up to 94% in the wavelength range from 380 nm to 760 nm (the visible region), suggesting the impressive capability of energy harvesting. The photonic cavity mode and the dipolar plasmon resonance by the half-cylinder cavity structure are the main contributions for the broadband absorption. The full-spectrum visible light absorption is observed to be maintained in a wide angle range under the oblique illumination. The high absorption is also retained under different polarization states. Moreover, the resonant materials such as titanium dioxide (TiO2) and titanium nitride (TiN) have impressive thermal stability due to their high melting points. These findings pave new insights on the refractory perfect visible light absorber for nano-scale photo-thermal process, micro-fluidic thermal disinfect and local high-temperature process, etc.

Light trapping is an important performance of ultra-thin solar cells because it cannot only increase the optical absorption in the photoactive region but it also allows for the efficient absorption with very little materials. Semiconductor-nanoantenna has the ability to enhance light trapping and raise the transfer efficiency of solar energy. In this work, we present a solar absorber based on the gallium arsenide (GaAs) nanoantennas. Near-perfect light absorption (above 90%) is achieved in the wavelength which ranges from 468 to 2870 nm, showing an ultra-broadband and near-unity light trapping for the sun's radiation. A high short-circuit current density up to 61.947 mA/cm2 is obtained. Moreover, the solar absorber is with good structural stability and high temperature tolerance. These offer new perspectives for achieving ultra-compact efficient photovoltaic cells and thermal emitters.

We theoretically propose a simple ultra-narrow multi-band perfect absorber for sensing applications. The perfect absorber consists of periodically arranged metallic nanodisks etched with regular prismatic holes standing on the dielectric-metal bi-layer films. Multiple ultra-narrow perfect absorption bands are obtained in the near-infrared region with the maximum bandwidth less than 21 nm and the intensity as high as 99.86%. The ultra-narrow multi-band perfect absorption originates from the synergy of localized surface plasmons, propagating surface plasmons and lattice resonances. The perfect absorber also presents other significant advantages, e.g. polarization insensitivity and high sensitivity of surrounding environments. Moreover, the prominent sensing performance for detecting the trace amounts of glucose in water is demonstrated. These features make it a promising candidate with great potential in the fields of perfect absorbers, plasmonic sensors, filters and multiplexing binding bio-molecular detection.

Semiconductors have been widely utilized to fabricate optoelectronic devices. Nevertheless, it is still a challenging task to achieve high-quality (Q) resonant light absorption using the high refractive index semiconductors. In this work, we propose a facile scheme for multi-band perfect absorption in the near-infrared range using an array of core-shell cylinder-shaped resonators which are composed of gold nanowires and thin silicon shells. Based on the cooperative effects between the photonic modes of the semiconductor cavity and the plasmonic resonances of the metal resonator, five sharp absorption peaks are observed with the maximal absorption close to 100% (99.98%) and a high Q factor up to 208. The multi-band sharp absorption is observed to be angle-insensitive and polarization-adjustable. Absorption efficiency can be quantitatively tuned via the polarization states following the classical Malus law. Moreover, different semiconductors such as gallium arsenide, indium arsenide, indium phosphide have been exploited to reproduce the sharp perfect absorption in this core-shell resonators platform. The remarkable features make the proposed system potential for multiple applications such as multispectral filtering, photo-detection and hot electron generation.

Refractory materials have wide applications due to their high thermal and chemical stability. In this work, we numerically design the plasmonic structure for perfect light absorption using the resonant behaviours for these materials. The results of the simulation are presented using the three-dimensional finite-difference time-domain method. The maximum absorption reaches 99.9% and the near-unity absorption window spans a bandwidth of 1182 nm from 413 nm in the visible to 1595 nm in the near-infrared range. Strong plasmonic near-field coupling between the adjacent parts in the split circular Ti resonator and their coupling to the plasmon resonances of the TiN layer are the main reasons for the ultra-broadband perfect absorption. Moreover, the absorption is observed to be incident angle insensitive and polarization independent. Absorption properties can be manipulated via tuning the structural parameters. These impressive absorption features and the high thermal stability can have wide applications in the high-intensity or extreme condition operation such as laser pumping and nonlinear optics, and solar related technologies including thermal steam generation and desalination of seawater.

Surface-enhanced Raman scattering (SERS) detection technique has gained much attention as a powerful analytical tool in recent years. Nevertheless, the attention was mainly focused on the efficient scattering platform by structuring metals themselves, leading to more complex platforms and higher costs. Herein, a new and simple strategy to prepare large-area, low-cost, high-performance SERS substrate is introduced. Ultra-thin semiconductor silicon (Si) film is used as the functional layer for the metallic nano-particles based meta-surface. During the SERS sensing process, the emergence of a Si layer is observed to provide three key contributions: (1) to produce a maximal enhancement factor (EF) ∼470% compared to that of the bare meta-surface, (2) to keep a higher spectral stability for the Raman signal, and (3) to physically interdict the contact between the metal and the molecule. Moreover, the Si film's thickness is down to the scale of an electron's Bohr radius, indicating efficient electronic oscillations for the semiconductor material under electromagnetic excitation. The charge transfer behaviors between the molecules and the Si layer and metal nano-particles can also emerge. These findings could pave new insights on the surface-enhanced spectroscopy and lead to applications for the high-performance, large-area, low-cost SERS sensing process.

An ultrawideband perfect absorber (UPA) is designed via a four-layer dielectric/refractory metal structure, which can produce near-unity absorption in the midinfrared region. The maximal absorption is up to 99.6%. Moreover, the absorber can maintain excellent absorption in a wide angle range, which indicates the angle-insensitive absorption and holds potential applications in complex electromagnetic situations. The ultrawideband spectral absorption mainly results from the intrinsic broadband plasmonic resonances by the refractory metals and the combination of the different plasmonic resonances by the resonators, and the cavity resonances by the layered nanostructures. Furthermore, due to the high melting point of titanium and chromium materials, the UPA is with highly thermal stability. The proposed absorber platform is therefore with both advantageous on the absorption properties and material features, which could pave ways for a wide application prospect in solar harvesting, infrared detection, and others. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)

Dual-band light absorption with the maximal absorptivity up to 99.7% and the minimal spectral bandwidth down to 3 nm is obtained in the plasmonic absorbers consisting of triple-layer plasmonic crystal-nonlinear medium cavity-metal substrate structure, where the intercalated dielectric material is chosen to be a Kerr medium cavity. Efficient all-optical controlling with high spectral intensity change ratios and detecting signal-to-noise is achieved for the system after a slight increase of pumping intensity. These impressive results mainly result from the strong plasmonic resonant field confinement in the middle nonlinear Kerr medium cavity and the near-perfect relative intensity change response by the ultra-sharp anti-reflection spectrum. This work can lay a foundation for advanced all-optical devices by exploiting light perfect absorption behavior and resonant optical field enhancement.

Metal films and semiconductor materials have been utilized for numerous optoelectronic devices due to the impressive electrical features. Nevertheless, the feasible way for achieving light anti-reflection or perfect absorption from opaque metal films and high-index semiconductors has not been established yet. In this work, we numerically propose and demonstrate a new functional metal–semiconductor slab, which can produce perfect light absorption in the optical range. High-index semiconductor resonators have been used to provide strong optical field coupling with the incident light. For the Ag–Si resonant slab consisting of a silicon rings array in the Ag slab, dual-band light absorption with the absorption efficiency above 99% is achieved due to the hybridized coupling of photonic and plasmonic modes for the Si and Ag resonators. Additionally, tri-band light absorption can be obtained when a paired silicon rings array is used. Moreover, these absorption properties can be spectrally manipulated via tuning the structural parameters. Furthermore, the absorber scheme is observed to be realizable by other metals and semiconductors. These optical and structural features can not only provide alternative ways for multi-band light absorption but also suggest new insight on the functional optoelectronics such as the infrared photo-detectors, hot-electron excitation, and nonlinear optics.