Realizing multi-function absorptions through arbitrary octagonal meta-atoms.

Metasurface absorbers (MA) typically exhibit a single type of absorption function due to their regular structures. In this study, we propose an irregular MA structure with octagonal meta-atoms. The presence of eight vertices in each meta-atom allows for tunable coordinates and offers a multitude of degrees of freedom in terms of geometry. As a result, the proposed MA exhibits diverse functionalities, including perfect absorption, multi-peaks absorption, and high absorption with a filtering window. To predict the geometric parameters of the MA structure based on a given target absorption spectrum, as well as the inverse design of the structure using the absorption spectrum as input, we employ a deep neural network combined with the particle swarm optimization algorithm. Remarkably, the mean-square error for spectrum prediction and inverse design of the MA structure is found to be as low as 0.0008 and 0.0031, respectively. This study opens up new possibilities for designing irregular electromagnetic structures and holds great potential for applications in multifunctional metasurfaces and metamaterials.

Achieving focal invariance in different background refractive indices through a dual-environment metalens.

A conventional metalens is designed with a fixed working environment, and its focal length depends on the background refractive index. In this study, we propose a dual-environment metalens that can maintain the same focal length in both media of air and water. The metalens consists of 16 types of meta-atoms with different geometries, which can cover the 0-2π phase range in both air and water. We perform finite-difference time-domain simulations to investigate the metalens and demonstrate that its focal length remains unchanged, regardless of whether the background medium is air or water. Furthermore, we investigated the optical forces within the focal field of the metalens in both air and water, indicating its potential trapping capability in these media. Our method provides a new insight into dual-environment metasurfaces and advances the methodology of electromagnetic structures in extensive applications.

Compact, low-loss, and high-polarized-extinction ratio terahertz TM-pass polarizer based on a hybrid plasmonic waveguide with a graphene ridge.

A compact, low-loss, and high-polarized-extinction ratio TM-pass polarizer based on a graphene hybrid plasmonic waveguide (GHPW) has been demonstrated for the terahertz band. A ridge coated by a graphene layer and the hollow HPW with a semiround arch (SRA) Si core is introduced to improve structural compactness and suppress the loss. Based on this, a TM-pass polarizer has been designed that can effectively cut off the unwanted TE mode, and the TM mode passes with negligible loss. By optimizing the angle of the ridge, the height of the ridge, air gap height, and the length of the tapered mode converter, an optimum performance with a high polarization extinction ratio of 30.28 dB and a low insert loss of 0.4 dB is achieved in the 3 THz band. This work provides a scheme for the design and optimization of polarizers in the THz band, which has potential application value in integrated terahertz systems.

Breaking symmetry restriction of chirality through spin-decoupled phase modulation utilizing non-mirror-symmetric meta-atoms.

The geometric phase in metasurfaces follows a symmetry restriction of chirality, which dictates that the phases of two orthogonal circularly polarized waves are identical but have opposite signs. In this study, we propose a general mechanism to disrupt this symmetric restriction on the chirality of orthogonal circular polarizations by introducing mirror-symmetry-breaking meta-atoms. This mechanism introduces a new degree of freedom in spin-decoupled phase modulation without necessitating the rotation of the meta-atom. To demonstrate the feasibility of this concept, we design what we believe is a novel meta-atom with a QR-code structure and successfully showcase circular-polarization multiplexing metasurface holography. Our investigation offers what we believe to be a novel understanding of the chirality in geometric phase within the realm of nanophotonics. Moreover, it paves the way for the development of what we believe will be novel design methodologies for electromagnetic structures, enabling applications in arbitrary wavefront engineering.

Ultracompact parabolic sidewall-profiled hybrid plasmonic waveguide Bragg grating with optimized spectral properties of long-wavelength passband.

An ultracompact hybrid plasmonic waveguide Bragg grating (HPWBG) with improved spectral properties of long-wavelength passband is proposed. A hollow HPW is introduced to suppress the entire loss, and a parabolic profiled sidewall is designed to optimize the spectral properties for specific wave bands. The transfer matrix method and finite element method are combined to ensure the efficiency of numerical research. The results show that the parabolic profile effectively reduces the reflection and strengthens the resonance of the mode in the long-wavelength passband, suppressing the oscillations and realizing significant smoothness and improvement in transmission. The optimized transmittance is greater than 99%, and insertion loss is as low as 0.017 dB. A wide bandgap of 103 nm is also attained. The structure also has a compactness with a length of 3.4 µm and exhibits good tolerance. This work provides a scheme for designing and optimizing wavelength selecting devices and has potential application value in integrated photonic devices.

Predicting the eigenstructures of metamaterials with QR-code meta-atoms by deep learning.

Deep neural networks (DNNs) facilitate the reverse design of metamaterial perfect absorbers (MPAs), usually by predicting the MPA structure from the input absorptivity. However, this suffers from the difficulty that the spectrum that actually exists is unknown before the structure is known. We propose an MPA structure with quick response (QR)-code meta-atoms and construct a novel DNN to predict and reverse design the eigenstructures by inputting designated eigenfrequencies. In addition, the meta-atom has a tremendous number of degrees of freedom, providing rich properties such as multiple absorption peaks. This work paves the way for the study of eigenproblems of complicated metamaterials and metasurfaces.

Focusing characteristics of cylindrical vector beams through a multi-focal all-dielectric grating lens.

A novel, to the best of our knowledge, type of multi-focal all-dielectric grating lens is proposed in this work, and focusing characteristics of cylindrical vector beams through the lens are investigated in detail. Based on the negative refraction mechanism of negative-first-order diffraction and Fermat's principle, a multi-focal lens is designed. By analyzing the diffraction effect of the grating, the essential factor that affects the focus quality is found. Through a two-step optimization process, secondary foci and the focal displacement of primary foci caused by high-order diffractions are overcome, and the quality of the focal field is significantly improved. This work provides a reference for micro-lens design for focus modulation, and the research results also have potential applications in the fields of light-field manipulation and optical tweezers.

Metamaterial perfect absorber with morphology-engineered meta-atoms using deep learning.

Metamaterial perfect absorbers (MPAs) typically have regularly-shaped unit structures owing to constraints on conventional analysis methods, limiting their absorption properties. We propose an MPA structure with a general polygon-shaped meta-atom. Its irregular unit structure provides multiple degrees-of-freedom, enabling flexible properties, such as dual-band absorption. We constructed a deep neural network to predict the parameters of the corresponding MPA structure with a given absorptivity as input, and vice versa. The mean-square error was as low as 0.0017 on the validation set. This study provides a basis for the design of complicated artificial electromagnetic structures for application in metamaterials and metasurfaces.

Polarization-multiplexed wavefront-engineering by all-dielectric metasurface with asymmetric polarization-decoupled meta-atoms.

Polarization multiplexing of metasurfaces conventionally requires the synthesis of both geometric and dynamic phases of meta-atoms. We propose a dynamic-phase-only polarization-multiplexing metasurface that consists of three types of polarization-decoupled meta-atoms and covers the 0-2π phase range. As illustrative examples, we designed and investigated a polarized beam splitter that can independently deflect x- and y-polarized incident lights at arbitrary angles. Furthermore, we designed and studied polarization-multiplexing metasurface-holography embracing double channels of orthogonal polarizations. Both metadevices demonstrate the effectiveness of our approach. This study paves the way for the design of polarization-multiplexing electromagnetic structures for application in metamaterials and metasurfaces.

Optical spanner for nanoparticle rotation with focused optical vortex generated through a Pancharatnam-Berry phase metalens.

Based on the focused optical vortex (OV) generated by a metalens, we studied the physical mechanism for optical manipulation of metal (Ag) nanoparticles in the orbital angular momentum (OAM) field. We found that metal nanoparticles can be stably trapped inside the OV ring and rotated by the azimuthal driving force originating from OAM transfer. The azimuthal force and rotation speed are directly and inversely proportional to the particle size, respectively. The torque for the same particle at the OV ring increases with the increase of the topological charge of the metalens. Considering the same topological charge, the radius of the OV ring or the range of the optical spanner has a positive correlation with the focal length. These kinds of optical tweezers by vortex metalenses can be used as an optical spanner or micro-rotor for lab-on-chip applications.

Generating a plasmonic vortex field with arbitrary topological charges and positions by meta-nanoslits.

A novel plasmonic vortex lens (PVL) consisting of an array of gold film nanoslits for vortex-field generation with arbitrary topological charges and positions is proposed. The performance of the PVL is analyzed theoretically and demonstrated numerically by the finite-element method. By utilizing a symmetric and antisymmetric phase, the plasmonic vortex generated at the center of the PVL can carry arbitrary topological charges (integer or fraction). Two circularly polarized illuminations with opposite spins can excite a composite plasmonic vortex field with symmetry-breaking distribution, in which the breaking point rotates with the phase difference between two spins. In addition, we can shift the center of the plasmonic vortex to any desired position such that a flexible location manipulation of the plasmonic vortex is achieved. The designed PVL can also work as a miniaturized polarimeter for characterizing the state of polarization of the incident light.

Optical manipulation of Rayleigh particles by metalenses-a numerical study.

Based on the focusing feature of a metalens, we numerically studied its application in optical manipulation of Rayleigh particles. Three types of metalenses-point focusing, line focusing, and line focusing with phase gradient-were designed. Simulation results using the finite-difference time-domain method showed that the incident optical beams could be focused into a spot or a line for stable particle trapping. Through engineering a gradient phase in the direction of the focal line, the proposed metalens can push the particles along the line. This provides a unique capability to move particles along a line without the need of any mechanical movement. Given its thin sheet structure and compactness, the proposed metalens can be easily integrated into microfluidic and optical tweezers systems, and it can find potential applications in optical sorting of biological cells.

Field-enhanced nanofocusing of radially polarized light by a tapered hybrid plasmonic waveguide with periodic grooves.

This study reports the field-enhanced nanofocusing of radially polarized light by tapered hybrid plasmonic waveguide (THPW) with periodic grooves. The THPW consists of a conical high-index dielectric cone, a sandwiched low-index dielectric thin layer, and a metal cladding. The axially symmetric 3D finite element method is used to investigate the nanofocusing effect. Under radially polarized illumination at 632.8 nm, strongly enhanced nanofocusing occurs. The hybrid plasmonic structure effectively reduces the energy loss and improves the field enhancement nearly 554 times. Furthermore, periodic grooves are constructed on the metallic surface of the THPW, satisfying the phase-matching condition, and they couple the light energy from the inside to the outside. Finally, an optimized nanofocusing performance with field enhancement of approximately 1810 times is obtained. The results offer an important reference for designing related photonic devices, and the proposed scheme could be potentially exploited in the application of light-matter interactions.

Ultralong photonic nanojet formed by dielectric microtoroid structure.

A photonic nanojet (PNJ) is a highly confined light beam formed by a transparent particle under light wave illumination. Here, we propose and numerically investigate the PNJ formed by a dielectric circular toroid with micro dimensions and a homogenous refractive index. Three-dimensional finite-difference time-domain (FDTD) simulations are conducted and demonstrate that ultralong PNJs can be formed by the doughnut-like structure. Besides, microtoroid structures can allow high-index materials (n=3.5) for PNJ generation. Various PNJ properties, including the focal distance, PNJ length, full width at half-maximum, and maximum intensity, can be flexibly tuned by modifying the geometry of the proposed structure.

TM01 mode in a cylindrical hybrid plasmonic waveguide with large propagation length.

This study reports on a cylindrical hybrid plasmonic waveguide (CHPW) consisting of a high-index dielectric core, a sandwiched low-index dielectric layer, and a metal cladding. The CHPW supports the TM01 mode with a radially polarized transverse component of the electric field. Optical fields can be significantly enhanced in the sandwiched low-index dielectric region with nanoscale thickness down to 5 nm, and tight mode confinement with the same order of the normalized mode area compared with that of the plasmonic waveguide is achieved. Moreover, the mode propagation loss is well compensated by adjusting dimensions of the waveguide to obtain a longer propagation distance. The calculated figure of merit reaches a value several times greater than that in the similarly reported structure. The results indicate that this novel type of hybrid structure can support the limited propagation of a radially polarized mode with good confinement and low loss, consummate the whole system of manipulating the cylindrical vector beams, and show great potential of applications for various integrated nano-photonic devices.

Deep survival analysis using pseudo values and its application to predict the recurrence of stage IV colorectal cancer after tumor resection.

An improved DeepSurv model is proposed for predicting the prognosis of colorectal cancer patients at stage IV. Our model, called as PseudoDeepSurv, is optimized by a novel loss function, which is the combination of the average negative log partial likelihood and the mean-squared error derived from the pseudo-observations approach. The public BioStudies dataset including 999 patients was utilized for performance evaluation. Our PseudoDeepSurv model produced a C-index of 0.684 and 0.633 on the training and testing dataset, respectively. While for the original DeepSurv model, the corresponding values are 0.671 and 0.618, respectively.

Generation of vector beam with space-variant distribution of both polarization and phase.

We present an idea to generate an arbitrary space-variant vector beam with structured polarization and phase distributions. The vector beams are synthesized from the left- and right-hand polarized light, each carrying different phase distributions. Both the phase and the state of polarization of vector beams can be tailored independently and dynamically by a spatial light modulator.

Optical trapping with focused Airy beams.

Airy beams are attractive owing to their two intriguing properties--self-bending and nondiffraction--that are particularly helpful for optical manipulation of particles. We perform theoretical and experimental investigations into the focusing property of Airy beams and provide insight into the trapping ability of tightly focused Airy beams. Experiment on optical tweezers demonstrates that the focused Airy beams can create multiple traps for two-dimensional confining particles, and the stable traps exist in the vicinity of the main intensity lobes in the focused beams. The trapping pattern can be varied with changes in the cross section of the focused beam. The focused Airy beam offers a novel way of optically manipulating microparticles.

Polarization structuring of focused field through polarization-only modulation of incident beam.

We proposed a method of polarization shaping in the focal region with the polarization modulation of incident light. By using an iterative optimization based on a vectorial diffraction calculation with the help of the fast Fourier transform, we can tailor the polarization structure in the focal plane. This provides a novel way to control the vectorial feature of the focal volume with the help of polarization tailoring, which is different from the method using wavefront shaping. The capability of polarization-only modulation on the incident light is demonstrated by optical experiments.