Toroidal Dipole BIC-Driven Highly Robust Perfect Absorption with a Graphene-Loaded Metasurface.

Achieving perfect absorption in few-layer two-dimensional (2D) materials plays a crucial role in applications such as optoelectronics and sensing. However, the underlying mechanisms of all reported works imply a strongly inherent dependence of the central wavelength on the structural parameters. Here, we propose a structure-parameter-deviation immune method for achieving perfect absorption at any desired wavelength by harnessing the toroidal dipole-bound state in the continuum (TD BIC). We experimentally demonstrate the versatile design with a monolayer-graphene-loaded compound grating structure. Such a TD BIC built upon the TE31 mode allows for the transition from BIC to quasi-BIC without breaking the structural symmetry, enabling the stable resonance wavelength while tailoring the quality factors via variation of the gap distance. Comparison with traditional literature further reveals the superiority of our method in realizing highly robust perfect absorption, with a wavelength stability ratio of >15. Remarkably, this approach can be straightforwardly applied to other 2D materials.

Polarization-controlled varifocal metalens with a phase change material GSST in mid-infrared.

Detection of aldehyde carbonyl radiation plays an essential role in guaranteeing the safety of fried food. However, the radiation of low-content aldehyde carbonyl is always weak and includes polarized light. Focusing the weak radiation with polarization-sensitive configurations provides an efficient way to improve the signal-to-noise ratio of detection. The advent of dynamic metasurfaces based on phase-change materials (PCMs) have demonstrated superiorities over their traditional counterparts in tunability and miniaturization. In this paper, we propose two reflected varifocal metasurfaces, which combine Ge2Sb2Se4Te1 (GSST) with two materials that have close optical constants with amorphous and crystalline GSST. The first one realizes a four-spot focal system with linearly-polarized incidence based on polarization multiplexing. It adds a new polarization degree of freedom compared with traditional varifocal metasurfaces. Compared with traditional spatial-multiplexing method, our second metasurface enables the independent control of the polarization and phase profiles of circularly-polarized light. Remarkably, it reduces energy loss and crosstalk. We believe the novel scenarios of combing GSST with similar materials provide a new direction for tunable metasurfaces based on PCMs.

Recent Progress in Improving the Performance of Infrared Photodetectors via Optical Field Manipulations.

Benefiting from the inherent capacity for detecting longer wavelengths inaccessible to human eyes, infrared photodetectors have found numerous applications in both military and daily life, such as individual combat weapons, automatic driving sensors and night-vision devices. However, the imperfect material growth and incomplete device manufacturing impose an inevitable restriction on the further improvement of infrared photodetectors. The advent of artificial microstructures, especially metasurfaces, featuring with strong light field enhancement and multifunctional properties in manipulating the light-matter interactions on subwavelength scale, have promised great potential in overcoming the bottlenecks faced by conventional infrared detectors. Additionally, metasurfaces exhibit versatile and flexible integration with existing detection semiconductors. In this paper, we start with a review of conventionally bulky and recently emerging two-dimensional material-based infrared photodetectors, i.e., InGaAs, HgCdTe, graphene, transition metal dichalcogenides and black phosphorus devices. As to the challenges the detectors are facing, we further discuss the recent progress on the metasurfaces integrated on the photodetectors and demonstrate their role in improving device performance. All information provided in this paper aims to open a new way to boost high-performance infrared photodetectors.

Manipulating mode degeneracy for tunable spectral characteristics in multi-microcavity photonic molecules.

Optical microcavities are capable of confining light to a small volume, which could dramatically enhance the light-matter interactions and hence improve the performances of photonic devices. However, in the previous works on the emergent properties with photonic molecules composed of multiple plasmonic microcavities, the underlying physical mechanism is unresolved, thereby imposing an inevitable restriction on manipulating degenerate modes in microcavity with outstanding performance. Here, we demonstrate the mode-mode interaction mechanism in photonic molecules composed of degenerate-mode cavity and single-mode cavity through utilizing the coupled mode theory. Numerical and analytical results further elucidate that the introduction of direct coupling between the degenerate-mode cavity and single-mode cavity can lift the mode degeneracy and give rise to the mode splitting, which contributes to single Fano resonance and dual EIT-like effects in the double-cavity photonic molecule structure. Four times the optical delay time compared to typical double-cavity photonic molecule are achieved after removing the mode degeneracy. Besides, with the preserved mode degeneracy, ultra-wide filtering bandwidth and high peak transmission is obtained in multiple-cavity photonic molecules. Our results provide a broad range of applications for ultra-compact and multifunction photonic devices in highly integrated optical circuits.

Tunable and polarization-sensitive perfect absorber with a phase-gradient heterojunction metasurface in the mid-infrared.

Inspired by the growing family of Van der Waals materials, hBN supported phonon polaritons have attracted much attention due to their inherent hyperbolic dispersion properties in the mid-infrared. However, the lack of tunability imposes a severe restriction on the diversified, functional and integrated applications. Here, we propose a phase-gradient heterostructure metasurface to realize a dynamically tunable and polarization-sensitive perfect absorber in the mid-infrared through combining hBN and phase change VO2. Narrow-band perfect absorption at 7.2 µm can be switched to broadband around 11.2 µm through controlling the temperature of VO2. The governed physics of the bandwidth and absorption differences are demonstrated. Phonon polaritons in hBN phase-gradient configurations and plasmon polaritons in periodic VO2 blocks are respectively excited. We also investigate the absorption dependence on the polarization states of designed absorber. The method of engineering the absorption through controlling the temperature and polarization states opens up a new avenue for tunable applications such as data storage and integrated optical circuits.

The trend of direct medical costs and associated factors in patients with chronic hepatitis B in Guangzhou, China: an eight-year retrospective cohort study.

BACKGROUND: Although the expenses of liver cirrhosis are covered by a critical illness fund under the current health insurance program in China, the medical costs associated with hepatitis B virus (HBV) related diseases is not well addressed. In order to provide evidence to address the problem, we investigated the trend of direct medical costs and associated factors in patients with chronic HBV infection. METHODS: A retrospective cohort study of 65,175 outpatients and 12,649 inpatients was conducted using a hospital information system database for the period from 2008 to 2015. Generalized estimating equations (GEE) were applied to explore associations between annual direct medical costs and corresponding factors, meanwhile quantile regression models were used to evaluate the effect of treatment modes on different quantiles of annual direct medical costs stratified by medical insurances. RESULTS: The direct medical costs increased with time, but the proportion of antiviral costs decreased with CHB progression. Antiviral costs accounted 54.61% of total direct medical costs for outpatients, but only 6.17% for inpatients. Non-antiviral medicine costs (46.06%) and lab tests costs (23.63%) accounted for the majority of the cost for inpatients. The direct medical costs were positively associated with CHB progression and hospitalization days in inpatients. The direct medical costs were the highest in outpatients with medical insurance and in inpatients with free medical service, and treatment modes had different effects on the direct medical costs in patients with and without medical insurance. CONCLUSIONS: CHB patients had a heavy economic burden in Guangzhou, China, which increased over time, which were influenced by payment mode and treatment mode.

Tunable phase change polaritonic perfect absorber in the mid-infrared region.

Realizing tunable light-polaritons interaction, such as perfect absorption in a controllable and compact manner holds great promise in nanophotonic systems. In this work, we engineer the hyperbolic surface phonon polaritons and surface plasmons polaritons to dynamically tune the perfect absorption in mid-infrared by combing the two van der Waals materials: the natural hyperbolic material hBN and phase change material VO2. Two spectrally separated and physically distinct perfect absorption peaks are alternatively observed and can be tuned through changing the temperature. The absorption in the resonant wavelengths can reach around 100%. We also demonstrate the flexibility of the absorber by investigating the absorption dependence on the polarization state and angle of incidence. The structural parameters sweep also confirms the robustness of our design. Our findings may open new possibilities to many versatile minimized applications such as optical modulators, optical switching, and temperature control system.

Selectively thermal radiation control in long-wavelength infrared with broadband all-dielectric absorber.

Artificial control of the thermal radiation is of growing importance to fundamental science and technological applications, ranging from waste heat recovery to thermophotovoltaics. Nanophotonics has been proven to be an efficient approach to manipulate the radiation. In comparison with structures utilizing planar subwavelength scale lithography, in this paper, we propose a cascaded all-dielectric multilayer structure to selectively manipulate the thermal radiation characteristics in long-wavelength infrared (LWIR). The broadband emissivity in non-atmospheric windows (6.3-7.5 µm) can reach 0.95 and the average absorption rate is below 3% in atmospheric windows (8-14 µm). The multilayer structure is insensitive to the polarization of the incident waves and maintains a good rectangular absorptivity curve even with large oblique incidence angle at 45 degrees. The outstanding properties of the nanostructures promise various applications in infrared sensing and thermal imaging.

Broadband polarization resolving based on dielectric metalenses in the near-infrared.

We demonstrate a novel polarization-resolved device (PRD) with the ability to accurately resolve the polarization states via a simple measurement process. The PRD is composed of two elaborately designed metalenses, which are capable of focusing the two circularly polarized (CP) lights. Therefore, for an arbitrary polarized light (treated as a combination of the two CP lights), a discrepancy is exhibited on focusing efficiency, which inversely provides a way to calculate the ellipticity. With such a strategy, the generalized form for polarization resolving is derived, with which the ellipticity of the incident polarized light can be calculated (through just measuring the efficiencies of the two spots). This process is accomplished by utilizing the numerical simulations and theoretical analysis. Moreover, resolving the polarization states can be achieved within a wavelength range of 400nm, due to the broadband effect of the designed metalenses. With the merits of compact configuration, broadband and compatibility with the existing semiconductor technology, the designed PRD holds potential applications in characterizing the polarization states.

Annihilating optical angular momentum and realizing a meta-waveplate with anomalous functionalities.

Manipulating the polarization states of electromagnetic waves, a fundamental issue in optics, has attracted intense attention. However, most of the reported devices are either so bulky or with specific functionalities. Here we propose a conceptually new approach to design an ultra-thin meta-waveplate (MWP) with anomalous functionalities. By elaborately designing the structural units of the metasurface, the incident right circular polarized (CP) light carrying spin angular momentum can be coupled into two surface plasmon modes with opposite orbital angular momenta which interaction with each other in the near-field, degenerating to a linear polarized (LP) light in the far-filed. The incoming spin angular momentum is annihilated and the designed MWP can function as a quarter-waveplate. However, compared with the conventional quarter-waveplates, our designed MWP owns the unidirectional function (only converting CP light to LP light) with a certain output polarization angle, which provides an extra degree of freedoms in controlling the polarization. Moreover, the designed MWP can function as a chiral material and exhibiting optical rotation properties within a broad bandwidth.

Polarization-independent metalens constructed of antennas without rotational invariance.

We propose a novel approach to designing an ultrathin polarization-independent metalens (PIM) by utilizing antennas without rotational invariance. Two arrays of nanoblocks are elaborately designed to form the super cell of the PIM, which are capable of focusing right-handed circularly polarized and left-handed circularly polarized lights. With such a strategy, the PIM is able to achieve polarization-independent focusing, since the light with any polarization can be treated as a combination of the two orthogonal ones. A theoretical analysis based on the Jones vector is proposed to detailedly explore the underlying physics. The polarization-independent characteristic of the designed PIM is also demonstrated by utilizing finite difference time domain simulations. Moreover, polarization-independent focusing can be achieved within a wavelength range of 400 nm. These results can deepen our understanding of polarization-independent focusing and provide a new method for designing ultrathin polarization-independent devices.

Room-Temperature Band-Aligned Infrared Heterostructures for Integrable Sensing and Communication.

The demand for miniaturized and integrated multifunctional devices drives the progression of high-performance infrared photodetectors for diverse applications, including remote sensing, air defense, and communications, among others. Nonetheless, infrared photodetectors that rely solely on single low-dimensional materials often face challenges due to the limited absorption cross-section and suboptimal carrier mobility, which can impair sensitivity and prolong response times. Here, through experimental validation is demonstrated, precise control over energy band alignment in a type-II van der Waals heterojunction, comprising vertically stacked 2D Ta2NiSe5 and the topological insulator Bi2Se3, where the configuration enables polarization-sensitive, wide-spectral-range photodetection. Experimental evaluations at room temperature reveal that the device exhibits a self-powered responsivity of 0.48 A·W-1, a specific directivity of 3.8 × 1011 cm·Hz1/2·W-1, a response time of 151 µs, and a polarization ratio of 2.83. The stable and rapid photoresponse of the device underpins the utility in infrared-coded communication and dual-channel imaging, showing the substantial potential of the detector. These findings articulate a systematic approach to developing miniaturized, multifunctional room-temperature infrared detectors with superior performance metrics and enhanced capabilities for multi-information acquisition.

Fractal H-shaped plasmonic nanocavity.

Based on complementary fractal geometry structures, we design a novel infrared quasi-three-dimensional (3D) nanocavity with a localized enhanced field with multiband resonant frequencies. The fractals offer the nanostructure two important characteristics, multiband functionality and a subwavelength effect. The electric field, power flow, and the field intensity distributions are given to indicate the internal mechanism of the localized enhanced field in the nanocavity. Additionally, the effective medium method is established to retrieve the permittivity and impedance of the structure. It is shown that a strongly enhanced localized field is achieved in the nanocavity at two different resonant frequencies by using the finite difference time domain method. The field intensity in the nanocavity is enhanced by a factor of up to 60 times over that of the incident light because of the important contribution of the loss factor in the permittivity. The surface plasmon hybridization is thought to play an important role in the strong localized field enhancement. The multiband property and high localized intensity offer the nanocavity great potential for applications in surface enhanced Raman scattering and other nanoscale novel devices.

Interface Analysis between Inconel 625 and Cobalt-Chromium Alloy Fabricated by Powder Bed Fusion Using Pulsed Wave Laser.

A few components used in the aerospace and petrochemical industries serve in corrosive environments at high temperatures. Corrosion-resistant metals or unique processes, such as coating and fusion welding, are required to improve the performance of the parts. We have used laser powder bed fusion (LPBF) technology to deposit a 5 mm thick corrosion-resistant CoCrMo layer on a high-strength IN625 substrate to improve the corrosion resistance of the core parts of a valve. This study found that when the laser volumetric energy density (EV) ≤ 20, the tensile strength increases linearly with the increase in EV, and the slope of the curve is approximately 85°. The larger the slope, the greater the impact of EV on the intensity. When EV > 20, the sample strength reaches the maximum tensile strength. When the EV increases from 0 to 20, the fracture position of the sample shifts from CoCrMo to IN625. When EV ≤ 38, the strain increases linearly with the increase in EV, and the slope of the curve is approximately 67.5°. The sample strain rate reaches the maximum when EV > 38. Therefore, for an optimal sample strength and strain, EV should be greater than 38. This study provides theoretical and technical support for the manufacturing of corrosion-resistant dissimilar metal parts using LPBF technology.

Continuous-Spectrum-Polarization Recombinant Optical Encryption with a Dielectric Metasurface.

The Jones matrix, with eight degrees of freedom (DoFs), provides a general mathematical framework for the multifunctional design of metasurfaces. Theoretically, the maximum eight DoFs can be further extended in the spectrum dimension to endow unique encryption capabilities. However, the topology and intrinsic spectral responses of meta-atoms constrains the continuous engineering of polarization evolution over wavelength dimension. In this work, a forward evolution strategy to quickly establish the mapping relationships between the solutions of the dispersion Jones matrix and the spectral responses of meta-atoms is reported. Based on the eigenvector transformation method, arbitrary conjugate polarization channels over the continuous-spectrum dimension are successfully reconstructed. As a proof-of-concept, a silicon metadevice is demonstrated for optical information encryption transmission. Remarkably, the arbitrary combination forms of polarization and wavelength dimension increase the information capacity (210 ), and the measured polarization contrasts of the conjugate polarization conversion are >94% in the entire wavelength range (3-4 µm). It is believed that the proposed approach will benefit secure optical and quantum information technologies.

Ultrahigh-Q guided mode resonances in an All-dielectric metasurface.

High quality(Q) factor optical resonators are indispensable for many photonic devices. While very large Q-factors can be obtained theoretically in guided-mode settings, free-space implementations suffer from various limitations on the narrowest linewidth in real experiments. Here, we propose a simple strategy to enable ultrahigh-Q guided-mode resonances by introducing a patterned perturbation layer on top of a multilayer-waveguide system. We demonstrate that the associated Q-factors are inversely proportional to the perturbation squared while the resonant wavelength can be tuned through material or structural parameters. We experimentally demonstrate such high-Q resonances at telecom wavelengths by patterning a low-index layer on top of a 220 nm silicon on insulator substrate. The measurements show Q-factors up to 2.39 × 105, comparable to the largest Q-factor obtained by topological engineering, while the resonant wavelength is tuned by varying the lattice constant of the top perturbation layer. Our results hold great promise for exciting applications like sensors and filters.

Selective Enhancement of Photoresponse with Ferroelectric-Controlled BP/In2 Se3 vdW Heterojunction.

Owing to the large built-in field for efficient charge separation, heterostructures facilitate the simultaneous realization of a low dark current and high photocurrent. The lack of an efficient approach to engineer the depletion region formed across the interfaces of heterojunctions owing to doping differences hinders the realization of high-performance van der Waals (vdW) photodetectors. This study proposes a ferroelectric-controlling van der Waals photodetector with vertically stacked two-dimensional (2D) black phosphorus (BP)/indium selenide (In2 Se3 ) to realize high-sensitivity photodetection. The depletion region can be reconstructed by tuning the polarization states generated from the ferroelectric In2 Se3 layers. Further, the energy bands at the heterojunction interfaces can be aligned and flexibly engineered using ferroelectric field control. Fast response, self-driven photodetection, and three-orders-of-magnitude detection improvements are achieved in the switchable visible or near-infrared operation bands. The results of the study are expected to aid in improving the photodetection performance of vdW optoelectronic devices.

Polychromatic full-polarization control in mid-infrared light.

Objects with different shapes, materials and temperatures can emit distinct polarizations and spectral information in mid-infrared band, which provides a unique signature in the transparent window for object identification. However, the crosstalk among various polarization and wavelength channels prevents from accurate mid-infrared detections at high signal-to-noise ratio. Here, we report full-polarization metasurfaces to break the inherent eigen-polarization constraint over the wavelengths in mid-infrared. This recipe enables to select arbitrary orthogonal polarization basis at individual wavelength independently, therefore alleviating the crosstalk and efficiency degradation. A six-channel all-silicon metasurface is specifically presented to project focused mid-infrared light to distinct positions at three wavelengths, each with a pair of arbitrarily chosen orthogonal polarizations. An isolation ratio of 117 between neighboring polarization channels is experimentally recorded, exhibiting detection sensitivity one order of magnitude higher than existing infrared detectors. Remarkably, the high aspect ratio ~30 of our meta-structures manufactured by deep silicon etching technology at temperature -150 °C guarantees the large and precise phase dispersion control over a broadband from 3 to 4.5 µm. We believe our results would benefit the noise-immune mid-infrared detections in remote sensing and space-to-ground communications.

The Impact of Manufacturing Imperfections on the Performance of Metalenses and a Manufacturing-Tolerant Design Method.

Metalenses play an important role in optoelectronic integrated devices, given their advantages in miniaturization and integration. Due to its high aspect ratio subwavelength structure, fabricating metalenses requires a high-level dry etching technology. Consequently, structure deformation of the metalens will exist if the etching process of the material is not mature enough, which will impair the metalens' performance. In this paper, a polarization-independent InP dielectric metalens is designed to focus the incident light from air into the substrate, which is used for monolithically integrating with the InGaAs/InP photodetector in the future. Subsequently, with the simulation method, we investigated the impact of the structure deformation on the metalens' performance, which was found in our InP dry etching process development. We have found that the sidewall slope and aspect ratio-dependent etching effect greatly impaired the focusing efficiency because of the phase modulation deviation. To solve this problem, we proposed a manufacturing-tolerant design method, which effectively improved the performance of the device with structural deformation. Our work is instructive for developing metalenses and can accelerate their integration application.

Photonic slide rule with metasurfaces.

As an elementary particle, a photon that carries information in frequency, polarization, phase, and amplitude, plays a crucial role in modern science and technology. However, how to retrieve the full information of unknown photons in an ultracompact manner over broad bandwidth remains a challenging task with growing importance. Here, we demonstrate a versatile photonic slide rule based on an all-silicon metasurface that enables us to reconstruct incident photons' frequency and polarization state. The underlying mechanism relies on the coherent interactions of frequency-driven phase diagrams which rotate at various angular velocities within broad bandwidth. The rotation direction and speed are determined by the topological charge and phase dispersion. Specifically, our metasurface leverages both achromatically focusing and azimuthally evolving phases with topological charges +1 and -1 to ensure the confocal annular intensity distributions. The combination of geometric phase and interference holography allows the joint manipulations of two distinct group delay coverages to realize angle-resolved in-pair spots in a transverse manner- a behavior that would disperse along longitudinal direction in conventional implementations. The spin-orbital coupling between the incident photons and vortex phases provides routing for the simultaneous identification of the photons' frequency and circular polarization state through recognizing the spots' locations. Our work provides an analog of the conventional slide rule to flexibly characterize the photons in an ultracompact and multifunctional way and may find applications in integrated optical circuits or pocketable devices.