Numerical study of tunable enhanced chirality in multilayer stack achiral phase-change metamaterials

Tunable and enhanced optical activity in twisted graphene metasurface bilayers

The phenomenon about optical activity has widespread applications in polarization optics, biosensing, and analytical chemistry. The optical activity in twisted graphene metasurface bilayers (TGMBs) is studied theoretically in this paper. It is found that the large circular dichroism (CD) value can be adjusted by various physical parameters of TGMBs such as separation distance, the voltage applied to metasurfaces, and twist angle. By adjusting the twist angle of TGMB, the shapes of the CD spectra, circular birefringence spectra, and ellipticity spectra can be manipulated in the broadband range. When the twisted bilayer metasurfaces are stacked with an ultrathin spacer, it is found that there might exist the strong optical activity responses near the rotated-σ-near-zero regime and topological transition σ-near-zero regime. The corresponding phenomena raise the prospect of tunable, compact, and on-chip terahertz devices with graphene metasurfaces based on optical activity.

Giant 2D-chiroptical response in an achiral metasurface integrated with black phosphorus

In this work, we proposed a black phosphorus (BP) achiral metasurface and theoretically study the chiroptical response arising from extrinsic 2D-chirality in the mid-infrared regime. The achiral metasurface is composed of a monolayer BP sheet sandwiched by a silver ring array and dielectric spacer stacking on a silver substrate. The giant circular conversion dichroism (CCD) of the achiral metasurface is allowed at oblique incidence for the cooperative interaction of BP anisotropic surface plasmon modes and localized surface plasmons in metal rings, and the integrated BP can dynamically modulate the chiroptical response by controlling the doping concentration of BP. Furthermore, we found that a multiband phenomenon for CCD response occurs when tuning the thickness of the spacer. The proposed hybrid achiral metasurface provides more flexible opportunities to realize active polarization modulator, biosensor and chiral detection.

Reconfigurable metamaterial for chirality switching and selective intensity modulation

A reconfigurable metamaterial for chirality switching and selective intensity modulation is demonstrated experimentally. Through simple folding strategy, nonchiral state, single-band chiral states and dual-bands chiral states can be switched. Circular dichroism up to 0.94 is measured with folding angles of 70°. Meanwhile, selective intensity modulation is realized by the combined effect of folding angle and incident angle. The transmission intensity of circularly polarized waves can be modulated by more than 90% at any selected resonating frequency between 8.97 and 10.73 GHz. This work will benefit the researches of foldable metamaterials and have potential applications in the field of reconfigurable devices.

Plasmonic Chirality and Circular Dichroism in Bioassembled and Nonbiological Systems: Theoretical Background and Recent Progress

Nature is chiral, thus chirality is a key concept required to understand a multitude of systems in physics, chemistry, and biology. The field of optics offers valuable tools to probe the chirality of nanosystems, including the measurement of circular dichroism, the differential interaction strength between matter and circularly polarized light with opposite helicity. Simultaneously, the use of plasmonic systems with giant light-interaction cross-sections opens new paths to investigate and manipulate systems on the nanoscale. Consequently, the interest in chiral plasmonic and hybrid systems has continually grown in recent years, due to their potential applications in biosensing, polarization-encoded optical communication, polarization-selective chemical reactions, and materials with polarization-dependent light-matter interaction. Experimentally, chiral properties of nanostructures can be either created artificially using modern fabrication techniques involving inorganic materials, or borrowed from nature using bioassembly or biomolecular templating. Herein, the recent progress in the field of plasmonic chirality is summarized, with a focus on both the theoretical background and the experimental advances in the study of chirality in various systems, including molecular-plasmonic assemblies, chiral plasmonic nanostructures, chiral assemblies of interacting plasmonic nanoparticles, and chiral metal metasurfaces and metamaterials. The growth prospects of this field are also discussed.

Tuning of Classical Electromagnetically Induced Reflectance in Babinet Chalcogenide Metamaterials

Metamaterials analog of electromagnetically induced reflectance (EIR) has attracted intense attentions since they can provide various applications for novel photonic devices such as optical detectors with a high sensitivity and slow-light devices with a low loss. The development of dynamic photonic devices desires a tunable EIR feature in metamaterials. However, most metamaterials-induced EIR is not spectrally controllable particularly for the near-infrared (NIR) region. Herein, a tuning of EIR is illustrated in Babinet chalcogenide metamaterials in the NIR region. The EIR response is created by weak hybridization of two dipolar (bright) modes of the paired Au slots. Such a mode interference can be engineered through non-volatile phase transition to the refractive index of the Ge₂Sb₂Te₅ (GST), resulting in an active controlling of the reflection window. A 15% spectral tuning of the reflectance peak is observed experimentally in the NIR region as switching the GST state between amorphous and crystalline.

Plasmonic metamaterials for chiral sensing applications

Plasmonic metamaterials are artificially designed materials which exhibit optical properties that cannot be found in nature. They have unique and special abilities related to electromagnetic wave control, including strong field enhancement in the vicinity of the surfaces. Over the years, scientists have succeeded in dramatically improving the detection limit of molecular chirality utilizing a variety of plasmonic metamaterial platforms. In this mini-review, we will discuss the principles of most recent issues in chiral sensing applications of plasmonic metamaterials, including suggested formulas for signal enhancement of chiroptical plasmonic sensors, and studies on various platforms that employ different sensing mechanisms.

Reconfigurable parity-time symmetry transition in phase change metamaterials

Although the conceptually remarkable, experimental illustration of parity-time (PT) symmetric quantum systems stays unexplored, their photonics analogues are explored by taking advantage of the similarity between the Helmholtz and Schrödinger equations. However, the static nature of the constitutive parameters of photonics structure inherently limits an achievement of dynamic PT-symmetry transition. We propose mid-infrared metamaterials (MMs) consisting of two orthogonally orientated metallic meta-atoms with one resonator integrated by a chalcogenide glass, GeTe. The PT-symmetry breaking can be actively transited to PT-symmetry by varying the structural state of GeTe between amorphous and crystalline while fixing the MM geometry. An electric-thermal model is constructed to illustrate that reversible switching between PT-symmetry and PT-symmetry breaking can be obtained in ∼270 ns. Our theoretical work lays the basis for designing high-speed reconfigurable PT-symmetry photonics systems that provide a promising approach for dynamically engineering non-Hermitian quantum symmetry.

Tunable chiroptical response of graphene achiral metamaterials in mid-infrared regime

We numerically investigate a tunable and extrinsic chiroptical response of a graphene achiral metamaterial in mid-infrared regime. The achiral metamaterial is composed of cascaded metallic split ring apertures and complementary graphene rings patterned on a dielectric layer. The strong extrinsic chiroptical responses of the metamaterial are allowed at oblique incidence and the integrated graphene can dynamically modulate extrinsic chirality by changing its Fermi level. The spectra of the chiroptical responses will show a blue shift with increasing the Fermi level of the patterned graphene. The maximal values of circular dichroism in the reflection and transmission modes can reach 80% and 50%, respectively. The maximal values of polarization rotation angle in the reflection and transmission modes can reach 80° and 60°, respectively. This graphene-based metamaterial design paves a way for a myriad of important terahertz (THz) and mid-infrared applications, such as optical modulators, absorbers and polarizers.

Thermally Tunable Ultra-wideband Metamaterial Absorbers based on Three-dimensional Water-substrate construction

Distilled water has frequency dispersive characteristic and high value of imaginary part in permittivity, which can be seen as a good candidate of broadband metamaterial absorbers(MAs) in microwave. Here, an interesting idea based on the combination of water-substrate and metallic metamaterial in the three-dimensional construction is proposed, which can achieve outstanding broadband absorption. As a proof, the distilled water is filled into the dielectric reservoir as ultra-thin water-substrate, and then the water-substrates are arranged on the metal backplane periodically as three-dimensional water-substrate array(TWA). Simulation shows that the TWA achieves broadband absorption with the efficiency more than 90% from 8.3 to 21.0 GHz. Then, the trigonal metallic fishbone structure is introduced here between the water-substrate and the dielectric reservoir periodically as three-dimensional water-substrate metamaterial absorber(TWMA). The proposed TWMA could achieve ultra-broadband absorption from 2.6 to 16.8 GHz, which has increase by 64.8% in relative absorption bandwidth. Meanwhile, due to the participation of distilled water, the thermally tunable property also deserves to be discussed here. In view of the outstanding performance, it is worth to expect a wide range of applications to emerge inspired from the proposed construction.