Anomalous far-field polarization around bound states in the continuum in non-Bravais lattices.

It is generally believed that at-Γ bound states in the continuum (BICs) are enclosed by a linearly polarized vortex in momentum space when the structures have mirror (σz) symmetry, in-plane inversion (I) symmetry, and time reversal symmetry (T). Here, we reveal an anomalous situation in which at-Γ BICs can be enclosed by linearly and elliptically polarized far-field even when the σz, I, and T symmetries are all maintained in non-Bravais lattices, which is radically different from previous cognition. Asymmetric, diatomic structures are designed to elaborate this intriguing phenomenon. By controlling the geometric parameters or refractive indexes of the two meta-atoms, the far-field polarization around the at-Γ BICs gradually deviates from linear polarization and approaches circular polarization. Our findings reveal that non-Bravais lattices can provide a novel platform to manipulate the far-field polarization, showing important applications in quantum entanglement, structured light, and radiation modulation.

Key Role of Asymmetric Photothermal Effect in Selectively Chiral Switching of Plasmonic Dimer Driven by Circularly Polarized Light.

Strong interaction between circularly polarized light and chiral plasmonic nanostructures can enable controllable asymmetric photophysical processes, such as selective chiral switching of a plasmonic nanorod-dimer. Here, we uncover the underlying physics that governs this chiral switching by theoretically investigating the interplay between asymmetric photothermal and optomechanical effects. We find that the photothermally induced local temperature rises could play a key role in activating the dynamic chiral configurations of a plasmonic dimer due to the temperature-sensitive molecular linkages located at the gap region. Importantly, different temperature rises caused by the opposite handedness of light could facilitate selective chiral switching of the plasmonic dimer driven by asymmetric optical torques. Our analyses on the wavelength-dependent selectively chiral switching behaviors are in good agreement with the experimental observations. This work contributes to a comprehensive understanding of the physical mechanism involved in the experimentally designed photoresponsive plasmonic nanosystems for practical applications.

Thermal Emission Manipulation Enabled by Nano-Kirigami Structures.

The nano-kirigami metasurfaces have controllable 3D geometric parameters and dynamic transformation functions and therefore provide a strong spectral regulation capability of thermal emission. Here, the authors propose and demonstrate a dynamic and multifunctional thermal emitter based on deformable nano-kirigami structures, which can be actuated by electronic bias or mechanical compression. Selective emittance and the variation of radiation intensity/wavelength are achieved by adjusting the geometric shape and the transformation of the structures. Particularly, a thermal management device based on a composite structure of nano-kirigami and polydimethylsiloxane (PDMS) thin film is developed, which can dynamically switch the state of cooling and heating by simply pressing the device. The proposed thermal emitter designs with strong regulation capability and multiple dynamic adjustment strategies are desirable for energy and sensing applications and inspire further development of infrared emitters.

Broadband and high-efficiency polarization conversion with a nano-kirigami based metasurface.

Nano-kirigami metasurfaces have attracted increasing attention due to their ease of three-dimension (3D) nanofabrication, versatile shape transformations, appealing manipulation capabilities and rich potential applications in nanophotonic devices. Through adding an out-of-plane degree of freedom to the double split-ring resonators (DSRR) by using nano-kirigami method, in this work we demonstrate the broadband and high-efficiency linear polarization conversion in the near-infrared wavelength band. Specifically, when the two-dimensional DSRR precursors are transformed into 3D counterparts, a polarization conversion ratio (PCR) of more than 90% is realized in wide spectral range from 1160 to 2030 nm. Furthermore, we demonstrate that the high-performance and broadband PCR can be readily tailored by deliberately deforming the vertical displacement or adjusting the structural parameters. Finally, as a proof-of-concept demonstration, the proposal is successfully verified by adopting the nano-kirigami fabrication method. The studied nano-kirigami based polymorphic DSRR mimic a sequence of discrete bulk optical components with multifunction, thereby eliminating the need for their mutual alignment and opening new possibilities.

Rechargeable Metasurfaces for Dynamic Color Display Based on a Compositional and Mechanical Dual-Altered Mechanism.

Dynamic color display can be realized by tunable optical metasurfaces based on the compositional or structural control. However, it is still a challenge to realize the efficient modulation by a single-field method. Here, we report a novel compositional and mechanical dual-altered rechargeable metasurface for reversible and broadband optical reconfiguration in both visible and near-infrared wavelength regions. By employing a simple fabrication and integration strategy, the continuous optical reconfiguration is manipulated through an electro-chemo-mechanical coupled process in a lithium ion battery, where lithiation and delithiation processes occur dynamically under a low electric voltage (≤1.5 V). By controlling the phase transformation from Si to Li xSi, both structural morphology and optical scattering could be rapidly and dramatically tailored within 30 s, exhibiting high-contrast colorization and decolorization in a large-area nanofilm and showing long cyclic stability. Significant wide-angle reconfiguration of high-resolution structural colors in bowtie metasurfaces is demonstrated from anomalous reflection. The results provide a multifield mechanism for reconfigurable photonic devices, and the new platform can be introduced to the multidimensional information encryption and storage.

Ultra-sensitive amplitude engineering and sign reversal of circular dichroism in quasi-3D chiral nanostructures.

Circular dichroism (CD), as one of the most representative chiroptical effects, provides a simple strategy for the detection and characterization of the molecular chirality. The enhancement and sign reversal of CD are of great importance for its practical applications in chiral bio-sensing, chirality switching and optical filtering, etc. Here, we realize considerable adjustments and the sign reversal of CD in quasi-three-dimensional (quasi-3D) combined Archimedean spiral nanostructures. With special local and lattice configurations, the nanostructures have both right-handed and left-handed geometric chirality, which are designed based on the proximity effect of stencil lithography. We find that the CD response of the nanostructures becomes obvious once its height exceeds 200 nm and can be adjusted by the further increase of the height or the change of the blade spacing of the nanostructures. The CD reversal is achieved by utilizing the competition of two chiral centers when the height or blade spacing exceeds a critical value. Further analysis of the scattering power of multipole moments reveals that the CD modulation is determined by both magnetic dipole moment and electric quadrupole moment. Benefiting from the highly sensitive CD response to the height, the extreme sign reversal of CD is achieved when a sub-10-nm ultrathin medium layer is anchored on the surface of the nanostructures, which provides a promising strategy for ultra-sensitive chiral bio-sensing.

Plasmonic Nanosensors with Extraordinary Sensitivity to Molecularly Enantioselective Recognition at Nanoscale Interfaces.

Molecular chirality recognition plays a pivotal role in chiral generation and transfer in living systems and makes important contribution to the development of diverse applications spanning from chiral separation to soft nanorobots. To detect chirality recognition, most of the molecular sensors described to date are based on the design and preparation of the host-guest complexation with chromophore or fluorophore at the reporter unit. Nevertheless, the involved tedious procedures and complicated chemical syntheses hamper their practical applications. Here, we report the plasmonically chiroptical detection of molecular chirality recognition without the need for a chromophore or fluorophore unit. This facile methodology is based on plasmonic nanotransducers that can convert molecular chirality recognitions occurring at nanoscale interfaces into asymmetrically amplified plasmonic circular dichroism readouts, enabling enantiospecific recognition and quantitative determination of the enantiomeric excess of small amino acids. Importantly, such a plasmon-based chirality sensing shows 102-103 amplification in the plasmonic circular dichroism signals from the detections of racemate and near-racemate of molecular analysts, demonstrating an extraordinary sensitivity to the host-guest enantioselective interactions. Furthermore, with advantages of easy-processing, cost-effective, and specific to interfacial molecular chirality, our chiroptical sensing scheme could hold considerable promise toward applications of enantioselective high-throughput screening in biology, stereochemistry, and pharmaceutics.

Artificial Propeller Chirality and Counterintuitive Reversal of Circular Dichroism in Twisted Meta-molecules.

Here we demonstrate an optical propeller chirality in artificially twisted meta-molecules, which is remarkably different from conventional optical helical chirality. Giant circular dichroism (CD) is realized in a single layer of meta-molecule array by utilizing the surface lattice resonances that are formed by the coupling of chiral electric quadrupole modes to the diffractive lattice mode. Due to the special twist of the propeller blades, the periodic meta-molecule array is hybridized by unit cells with two different chiral centers. As a result, the CD response is readily reversed by tailoring the interference phase through engineering the structural blades without inverting the geometric chirality. Importantly, the enhanced CD and its sign reversal are demonstrated in experiments by using a nano-kirigami fabrication technique.

Topological rainbow based on graded topological photonic crystals.

Topological photonic crystal provides a robust platform for nanophotonic devices. However, few reports have been found to realize multiple frequency routing based on topological photonic states, which have restricted further applications in the field of nanophotonic devices. Here, for the first time, to the best of our knowledge, we propose an efficient method to realize a topological rainbow based on graded dielectric topological photonic crystals, which are constructed by changing the degree of lattice contraction and expansion. The topological edge states of different frequencies are separated and trapped at different positions. The all-dielectric planar nanostructures of graded topological photonic crystals are low-loss, robust, and easy for integration. This Letter plays a key role in the use of robust nanophotonic wavelength routers, optical storage, and optical buffers.

Electromechanically reconfigurable optical nano-kirigami.

Kirigami, with facile and automated fashion of three-dimensional (3D) transformations, offers an unconventional approach for realizing cutting-edge optical nano-electromechanical systems. Here, we demonstrate an on-chip and electromechanically reconfigurable nano-kirigami with optical functionalities. The nano-electromechanical system is built on an Au/SiO2/Si substrate and operated via attractive electrostatic forces between the top gold nanostructure and bottom silicon substrate. Large-range nano-kirigami like 3D deformations are clearly observed and reversibly engineered, with scalable pitch size down to 0.975 µm. Broadband nonresonant and narrowband resonant optical reconfigurations are achieved at visible and near-infrared wavelengths, respectively, with a high modulation contrast up to 494%. On-chip modulation of optical helicity is further demonstrated in submicron nano-kirigami at near-infrared wavelengths. Such small-size and high-contrast reconfigurable optical nano-kirigami provides advanced methodologies and platforms for versatile on-chip manipulation of light at nanoscale.

Fano-Enhanced Circular Dichroism in Deformable Stereo Metasurfaces.

2D metasurfaces have emerged as a paradigm-shifting platform for light management with considerable miniaturization and alleviated fabrication challenges than their 3D counterparts. However, the appearance of in-plane mirror symmetry and reduced dimensions impose fundamental restraints to advanced chiroptical responses and reconfiguration capabilities. Here, a new concept of Fano-enhanced circular dichroism by introducing a reconfigurable stereo metasurface, which possesses deformable out-of-plane twists that are readily achieved by a simple nano-kirigami fabrication method, is demonstrated. The stereo height and twisting geometries can be reproducibly controlled, providing a facile and automated fashion to tailor the distinct profiles of Fano resonances under circularly polarized incidence. As a result, a recorded high efficiency of circular dichroism generation per unit sample thickness is achieved with Fano resonances in opposite lineshapes. Leveraging this feature, large-range reconfiguration of circular dichroism at optical wavelengths is demonstrated through reversible compression of the stereo metasurfaces with a fiber tip. The studied stereo metasurface unfolds a new degree of freedom for advanced photonic applications in a quasi-flat optical platform, and the proposed concept of Fano-enhanced circular dichroism opens new venues to explore interesting fundamental phenomena of chiral optics.