An Ultrathin, Fast-Response, Large-Scale Liquid-Crystal-Facilitated Multi-Functional Reconfigurable Metasurface for Comprehensive Wavefront Modulation.

The rapid advancement of prevailing communication/sensing technologies necessitates cost-effective millimeter-wave arrays equipped with a massive number of phase-shifting cells to perform complicated beamforming tasks. Conventional approaches employing semiconductor switch/varactor components or tunable materials encounter obstacles such as quantization loss, high cost, high complexity, and limited adaptability for realizing large-scale arrays. Here, a low-cost, ultrathin, fast-response, and large-scale solution relying on metasurface concepts combined together with liquid crystal (LC) materials requiring a layer thickness of only 5 µm is reported. Rather than immersing resonant structures in LCs, a joint material-circuit-based strategy is devised, via integrating deep-subwavelength-thick LCs into slow-wave structures, to achieve constitutive metacells with continuous phase shifting and stable reflectivity. An LC-facilitated reconfigurable metasurface sub-system containing more than 2300 metacells is realized with its unprecedented comprehensive wavefront manipulation capacity validated through various beamforming functions, including beam focusing/steering, reconfigurable vortex beams, and tunable holograms, demonstrating a milli-second-level function-switching speed. The proposed methodology offers a paradigm shift for modulating electromagnetic waves in a non-resonating broadband fashion with fast-response and low-cost properties by exploiting functionalized LC-enabled metasurfaces. Moreover, this extremely agile metasurface-enabled antenna technology will facilitate a transformative impact on communication/sensing systems and empower new possibilities for wavefront engineering and diffractive wave calculation/inference.

Chiral optical solitons in an electrically active multiferroic guiding structure.

A stack of a dielectric planar waveguide with a Kerr-type nonlinearity, sandwiched between two oxide-based helical multiferroic layers is shown to support electrically-controlled chiral solitons. These findings follow from analytical and full numerical simulations. The analytical scheme delivers explicit material parameters for the guided mode soliton and unveils how the soliton propagation characteristics are controlled by tuning the multiferroic helicity and amplitude of the injected electromagnetic wave. Silicon and CS2 are considered as the optical media in the guiding region enclosed by the multiferroic slabs. CS2 has very similar nonlinearity characteristics to silicon but in the linear regime it exhibits a smaller refractive index in the THz frequency range. The scattering simulations are performed using our developed numerical code based on the rigorous coupled wave method and the results for the dispersion curve for the guided mode agree very well with the analytical formula that we derive in this work. The results demonstrate a case of nonlinear pulse generation with field-controlled, nontrivial topological properties.

Tunable chiral photonic cavity based on multiferroic layers.

Realization of externally tunable chiral photonic sources and resonators is essential for studying and functionalizing chiral matter. Here, oxide-based stacks of helical multiferroic layers are shown to provide a suitable, electrically-controllable medium to efficiently trap and filter purely chiral photonic fields. Using analytical and rigorous coupled wave numerical methods we simulate the dispersion and scattering characteristics of electromagnetic waves in multiferroic heterostructures. The results evidence that due to scattering from the spin helix texture, only the modes with a particular transverse wavenumber form standing chiral waves in the cavity, whereas all other modes leak out from the resonator. An external static electric field enables a nonvolatile and energy-efficient control of the vector spin chirality associated with the oxide multilayers, which tunes the photonic chirality density in the resonator.

Broadband plasmonic chiral meta-mirrors.

Chiral meta-mirrors provide a unique opportunity for achieving handedness-selective strong light-matter interaction at the nanometer scale. Importantly, the chiral resonances observed in chiral meta-mirrors arise from the spin-dependent resonant cavity which, however, is generally narrowband. In this paper, by exploiting a genetic algorithm (GA) based optimization method, we numerically validate a chiral meta-mirror with octave bandwidth. In particular, in the wavelength range from 1000 to 2000 nm, the proposed chiral meta-mirror strongly absorbs circularly polarized light of one handedness while highly reflecting the other. A field analysis indicates that the observed broadband chiroptical response can be attributed to the multiple chiral resonances supported by the optimized meta-mirror across the band of interest. The observed broadband chiral response confirms the potential of advanced inverse-design approaches for the creation of chiral metadevices with sophisticated functionalities. Based on the Lorentz reciprocity theorem, we show that the proposed meta-mirror can enable chiral-selective broadband second harmonic generation (SHG). Our study indicates that the application of advanced inverse-design approaches can greatly facilitate the development of metadevices with strong chiral response in both the linear and nonlinear regimes.

Tailored compliant mechanisms for reconfigurable electromagnetic devices.

Reconfigurable electromagnetic devices, specifically reconfigurable antennas, have shown to be integral to the future of communication systems. However, mechanically robust designs that can survive real-world, harsh environment applications and high-power conditions remain rare to this day. In this paper, the general framework for a field of both discrete and continuously mechanically reconfigurable devices is established by combining compliant mechanisms with electromagnetics. To exemplify this new concept, a reconfigurable compliant mechanism antenna is demonstrated which exhibits continuously tunable performance across a broad band of frequencies. Moreover, three additional examples are also introduced that further showcase the versatility and advanced capabilities of compliant mechanism enabled electromagnetic devices. Unlike previous approaches, this is achieved with minimal part counts, additive manufacturing techniques, and high reliability, which mechanical compliant mechanism devices are known for. The results presented exemplify how compliant mechanisms have the capacity to transform the broader field of reconfigurable electromagnetic devices.

Generalized PT Symmetry in Non-Hermitian Wireless Power Transfer Systems.

We show that, by using a saturable gain g_{sat}, generalized PT (GPT) symmetry can be achieved in the intrinsically unbalanced (non-PT-symmetric) high-order wireless power transfer systems. A topology decomposition approach is implemented to analyze the parity of the high-order wireless power transfer systems. In the coupling parametric space, a global GPT-symmetric eigenstate is observed along with the spontaneous phase transition of the local GPT-symmetric eigenstates on the exceptional contour. GPT symmetry guarantees a highly efficient and stable power transfer across the distinct coupling regions, which introduces a new paradigm for a broad range of application scenarios involving asymmetric coupling.

Thickness-dependent slow light gap solitons in three-dimensional coupled photonic crystal waveguides.

The thickness-dependent multimodal nature of three-dimensional (3D) coupled photonic crystal waveguides is investigated with the aim of realizing a medium for controlled optical gap soliton formation in the slow light regime. In the linear case, spectral properties of the modes (dispersion diagrams), location of the gap regions versus the thickness of the 3D photonic crystal, and the near-field distributions at frequencies in the slow light region are analyzed using a full-wave electromagnetic solver. In the nonlinear regime (Kerr-type nonlinearity), we infer an existence of crystal-thickness-dependent temporal solitons with stable pulse envelope and use the solitonic pulses for driving quantum transitions in localized quantum systems within the photonic crystal waveguide. The results may be useful for applications in optical communications, multiplexing systems, nonlinear physics, and ultrafast spectroscopy.

Temporal multi-stage energy pumping.

As an important theoretical concept, temporal boundaries provide researchers with new insights for tailoring electromagnetic waves in the time domain. Because a temporal boundary breaks the time translation symmetry, a source is necessary to satisfy energy conservation. In this Letter, we quantify the relationship between refractive index contrast and the required energy exchange. More specifically, to realize a temporal boundary with a large refractive index contrast, a correspondingly large and abrupt energy exchange is required. Considering this practical difficulty, we propose to mimic a large-contrast temporal boundary by staggering a series of small-contrast temporal boundaries separated by carefully designed durations. In this way, the process of energy input/output is distributed over an elongated duration, but their effect can still be cumulative. This process is analogous to a multi-resonant system with a periodic energy input. Based on this design principle, we discuss several scenarios for different temporal profiles of refractive index and their corresponding energy requirements.

Terahertz Chiral Metamaterials Enabled by Textile Manufacturing.

Easy-to-fabricate, large-area, and inexpensive microstructures that realize control of the polarization of terahertz (THz) radiation are of fundamental importance to the development of the field of THz wave photonics. However, due to the lack of natural materials that can facilitate strong THz radiation-matter interactions, THz polarization components remain an undeveloped technology. Strong resonance-based responses offered by THz metamaterials have led to the recent development of THz metadevices, whereas, for polarization control devices, micrometer-scale fabrication techniques including aligned photolithography are generally required to create multilayer microstructures. In this work, leveraging a two-step textile manufacturing approach, a chiral metamaterial capable of exhibiting strong chiroptical responses at THz frequencies is demonstrated. Chiral-selective transmission and pronounced optical activity are experimentally observed. In sharp contrast to smart-clothing-related devices (e.g., textile antennas), the investigated chiral metamaterials gain their THz properties directly from the yarn-twisting enabled microhelical strings. It is envisioned that the interplay between meta-atom designs and textile manufacturing technology will lead to a new family of metadevices for complete control over the phase, amplitude, and polarization of THz radiation.

Plasmon-Mediated Chiroptical Second Harmonic Generation from Seemingly Achiral Gold Nanorods.

Throughout nature, simple rules explain complex phenomena, such as the selective interaction of chiral objects with circularly polarized light. Here, we demonstrate chiroptical signals from gold nanorods, which are seemingly achiral structures. Shape anisotropy due to atomic-level faceting and rounding at the tips of nanorods, which are free of chiral surface ligands, induces linear-to-circular polarization modulation during second harmonic generation. The intrinsic nanorod chiroptical response is increased by plasmon-resonant excitation, which preferentially amplifies circularly polarized harmonic signals. This structure-plasmon interplay is uniquely resolved by polarization-resolved second harmonic generation measurements. The material's second-order polarizability is the product of the structure-dependent lattice-normal susceptibility and local surface plasmon field vectors. Synthetically scalable plasmon-supporting nanorods that amplify small circular dichroism signals provide a simple, assembly-free platform for chiroptical transduction.

Analytical transient analysis of temporal boundary value problems using the d'Alembert formula.

Temporal boundary value problems (TBVPs) provide the foundation for analyzing electromagnetic wave propagation in time-varying media. In this paper, we point out that TBVPs fall into the category of unbounded initial value problems, which have traveling wave solutions. By dividing the entire time frame into several subdomains and applying the d'Alembert formula, the transient expressions for waves propagating through temporal boundaries can be evaluated analytically. Moreover, unlike their spatial analogs, TBVPs are subject to causality. Therefore, the resulting analytical transient solutions resulting from the d'Alembert formula are unique to temporal systems.

Photonic Signatures of Spin-Driven Ferroelectricity in Multiferroic Dielectric Oxides.

We study the dispersion and scattering properties of electromagnetic modes coupled to a helically ordered spin lattice hosted by a dielectric oxide with a ferroelectric polarization driven by vector spin chirality. Quasianalytical approaches and full-fledged numerics evidence the formation of a chiral magnonic photonic band gap and the presence of gate-voltage dependent circular dichroism in the scattering of electromagnetic waves from the lattice. Gating couples to the emergent ferroelectric polarization and hence, to the underlying vector-spin chirality. The theory relies on solving simultaneously Maxwell's equations coupled to the driven localized spins taking into account their spatial topology and spatial anisotropic interactions. The developed approach is applicable to various settings involving noncollinear spins and multiferroic systems with potential applications in noncollinear magnetophotonics.

Conductive mixed-order generalized dispersion model for noble metals in the optical regime.

Various dispersion models can be expressed as special cases of the Generalized Dispersion Model (GDM), which is composed of a series of Padé polynomials. While important for its broad applicability, we found that some materials with Drude dispersive terms can be accurately modeled by mixing a 1st order Padé polynomial with an extra conductivity term. This conductivity term can be separated from the auxiliary differential equation (ADE). Therefore, the proposed mixed-order model can achieve the same accuracy with fewer unknowns, thus realizing higher computational efficiency and lower memory consumption. For examples, we derive the model parameters and corresponding numerical errors for noble metals including Au, Ag, and Al in the optical regime. Finally, the proposed model's efficiency improvements are validated through implementation within a Discontinuous Galerkin Time Domain (DGTD) framework. The proposed model can achieve up to 12.5% efficiency improvement in theory compared to the conventional GDM with the same accuracy. A numerical example validates that, in practice, 9% memory reduction and 11% acceleration can be realized.

Active terahertz spin Hall effect in vanadium dioxide metasurfaces.

Insulator-to-metal transition induces large material property variations in vanadium dioxide (VO2) over a broad frequency band. VO2, therefore, has been introduced into metallic resonating structures to realize reconfigurable metadevices from microwave to optical wavelengths. Beyond enabling metal/VO2 hybrid meta-atoms, in the THz regime metallic-phase VO2 micro-structures can support strong electromagnetic resonances, offering great potential in active manipulation of THz radiation. In this paper, we show that VO2 dipole antennas can be used to realize geometric phase coded metasurfaces for wave-front shaping and polarization rotation of THz waves. Moreover, we demonstrate that the corresponding efficiency of the THz spin Hall effect is closely related to VO2's THz electrical conductivity. In light of the dispersionless nature of the geometric phase, our study indicates that metasurfaces constructed by VO2 subwavelength resonators are good candidates for active control of broadband THz radiation.

Complete polarization conversion using anisotropic temporal slabs.

It is well known that control over the polarization of electromagnetic waves can be achieved by utilizing artificial anisotropic media such as metamaterials. However, most of the related research has been focused on time-invariant systems. Inspired by the concept of temporal boundaries, we propose a method to realize polarization conversion in real time by employing time-variant materials, whose permittivity or permeability switches between isotropic and anisotropic values. The criteria for complete polarization conversion are studied for several polarization angles, both analytically and numerically.

Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces.

Lithium niobate (LN) has been widely used for second-harmonic generation (SHG) from bulk crystals. Recent studies have reported improved SHG efficiency in LN micro-ring resonators and hybrid waveguiding structures, as well as in LN nanostructures supporting anapole modes and plasmon-assisted dipole resonances. Here we numerically demonstrate that high Q-factor resonances associated with symmetry-protected bound states in the continuum can lead to highly efficient frequency doubling in LN metasurfaces. Simulations show that the radiative Q-factor and on-resonance field enhancement factor observed in the metasurface are closely dependent on the asymmetric parameter α of the system. Furthermore, high Q-factor resonances boost the SH conversion process in the LN nanostructures. In particular, for a LN metasurface with a Q-factor of ∼8×104, a 0.49% peak SH conversion efficiency is achieved at a pump intensity of 3.3kW/cm2. This suggests that such high Q-factor LN metasurfaces may be good candidates for practical blue-ultraviolet light sources. Our work provides insight into the possible implementation of metadevices based on nanoengineering of conventional nonlinear crystals.

Direct-Write Spray Coating of a Full-Duplex Antenna for E-Textile Applications.

Recent advancements in printing technologies have greatly improved the fabrication efficiency of flexible and wearable electronics. Electronic textiles (E-textiles) garner particular interest because of their innate and desirable properties (i.e., conformability, breathability, fabric hand), which make them the ideal platform for creating wireless body area networks (WBANs) for wearable healthcare applications. However, current WBANs are limited in use due to a lack of flexible antennas that can provide effective wireless communication and data transfer. In this work, we detail a novel fabrication process for flexible textile-based multifunctional antennas with enhanced dielectric properties. Our fabrication process relies on direct-write printing of a dielectric ink consisting of ultraviolet (UV)-curable acrylates and urethane as well as 4 wt.% 200 nm barium titanate (BT) nanoparticles to enhance the dielectric properties of the naturally porous textile architecture. By controlling the spray-coating process parameters of BT dielectric ink on knit fabrics, the dielectric constant is enhanced from 1.43 to 1.61, while preserving the flexibility and air permeability of the fabric. The novel combination textile substrate shows great flexibility, as only 2 N is required for a 30 mm deformation. The final textile antenna is multifunctional in the sense that it is capable of operating in a full-duplex mode while presenting a relatively high gain of 9.12 dB at 2.3 GHz and a bandwidth of 79 MHz (2.260-2.339 GHz) for each port. Our proposed manufacturing process shows the potential to simplify the assembly of traditionally complex E-textile systems.

Monolithic Full-Stokes Near-Infrared Polarimetry with Chiral Plasmonic Metasurface Integrated Graphene-Silicon Photodetector.

The ability to detect the full-Stokes polarization of light is vital for a variety of applications that often require complex and bulky optical systems. Here, we report an on-chip polarimeter comprising four metasurface-integrated graphene-silicon photodetectors. The geometric chirality and anisotropy of the metasurfaces result in circular and linear polarization-resolved photoresponses, from which the full-Stokes parameters, including the intensity, orientation, and ellipticity of arbitrarily polarized incident infrared light (1550 nm), can be obtained. The design presents an ultracompact architecture while excluding the standard bulky optical components and structural redundancy. Computational extraction of full-Stokes parameters from mutual information among four detectors eliminates the need for a large absorption contrast between different polarization states. Our monolithic plasmonic metasurface integrated polarimeter is ideal for a variety of polarization-based applications including biological sensing, quantum information processing, and polarization photography.

Linear and nonlinear chiroptical response from individual 3D printed plasmonic and dielectric micro-helices.

Sub-wavelength chiral resonators formed from artificial structures exhibit exceedingly large chiroptical responses compared to those observed in natural media. Owing to resonant excitation, chiral near fields can be significantly enhanced for these resonators, holding great promise for developing enantioselective photonic components such as biochemical sensors based on circular dichroism (CD) and spin-dependent nonlinear imaging. In the present work, strong linear and nonlinear chiroptical responses (scattering CD > 0.15 and nonlinear differential CDs > 0.4) at visible and near infrared frequencies are reported for the first time for individual micrometer-scale plasmonic and dielectric helical structures. By leveraging dark-field spectroscopy and nonlinear optical microscopy, the circular-polarization-selective scattering behavior and nonlinear optical responses (e.g., second harmonic generation and two-photon photoluminescence) of 3D printed micro-helices with feature sizes comparable to the wavelength (total length is ∼5λ) are demonstrated. These micro-helices provide potential for readily accessible photonic platforms, facilitating an enantiomeric analysis of chiral materials. One such example is the opportunity to explore ultracompact photonic devices based on single, complex meta-atoms enabled by state-of-the-art 3D fabrication techniques.

Meta-atom library generation via an efficient multi-objective shape optimization method.

Optimizing the shape of metasurface unit cells can lead to tremendous performance gains in several critically important areas. This paper presents a method of generating and optimizing freeform shapes to improve efficiency and achieve multiple metasurface functionalities (e.g., different polarization responses). The designs are generated using a three-dimensional surface contour method, which can produce an extensive range of nearly arbitrary shapes using only a few variables. Unlike gradient-based topology optimization, the proposed method is compatible with existing global optimization techniques that have been shown to significantly outperform local optimization algorithms, especially in complex and multimodal design spaces.