Dielectric Pillars Research Articles

High efficiency independent modulation at dual-wavelength based on Pancharatnam–Berry and propagation phases

Metasurfaces capable of controlling multiple wavelengths independently have attracted broad interests these years due to their significance in multi-channel information processing applications. Previous solving strategies include spatial multiplexing or extensive searching for appropriate structures, both of which have their own disadvantageous, such as low efficiency, large computer resource requirement, or time consumption. In this paper, by combining the Pancharatnam–Berry (PB) phase and propagation phase, we propose a strategy to simplify the design complexity in a dual-wavelength metasurface system, in which two simple rectangular-shaped dielectric pillars (T1 and T2) with different aspect ratios are chosen as basic structures and crossed at the geometric center to achieve manipulation. The larger pillar T2 controls the longer wavelength through the PB phase while the smaller T1 acts as a perturbation to T2. The crossed T1&T2 is studied as a whole to tune the short wavelength. The investigations by the multipole expansion method reveal that the polarization conversion ratio of the meta-atoms is dependent on the interference of the formed multipoles. To validate the proposed strategy, a dual-wavelength achromatic metalens and a wavelength-multiplexed holographic metasurface operating at the infrared thermal imaging band are designed. Our design strategy can find widespread applications in metasurfaces where multiple objectives are required to be realized.

Inverted Transflection Spectroscopy of Live Cells Using Metallic Grating on Elevated Nanopillars.

Water absorption of mid-infrared (MIR) radiation severely limits the options for vibrational spectroscopy of the analytes-including live biological cells-that must be probed in aqueous environments. While internal reflection elements, such as attenuated total reflection prisms and metasurfaces, partially overcome this limitation, such devices have their own limitations: ATR prisms are difficult to integrate with multiwell cell culture workflows, while metasurfaces suffer from a limited spectral range and small penetration depth into analytes. In this work, we introduce an alternative live cell biosensing platform based on metallic nanogratings fabricated on top of elevated dielectric pillars. For the MIR wavelengths that are significantly longer than the grating period, reflection-based spectroscopy enables broadband sensing of the analytes inside the trenches separating the dielectric pillars. Because the depth of the analyte twice-traversed by the MIR light excludes the highly absorbing thick water layer above the grating, we refer to the technique as inverted transflection spectroscopy (ITS). The analytic power of ITS is established by measuring a wide range of protein concentrations in solution, with the limit of detection in the single-digit mg mL-1. The ability of ITS to interrogate live cells that naturally wrap themselves around the grating is used to characterize their adhesion kinetic.

Electromechanically reconfigurable plasmonic nanoslits for efficient nano-opto-electro-mechanical tuning

Plasmonic metamaterials with strong localized surface plasmon resonances (LSPRs) have been improved by incorporating a nano-opto-electro-mechanical system (NOEMS) for tunable optical and electrical responses. However, conventional NOEMS devices suffer from the inefficient free-space coupling due to the limited design of reconfigurable antennas and the low optical tunability due to electrostatic pull-in of moving parts. In this paper, we propose efficient mid-infrared plasmonic nanoslits with variable nanometer-sized gaps giving rise to broad optical tunability. Our finite element method (FEM) analysis shows that by applying a voltage bias to asymmetric nanoslits supported on dielectric pillars, the gap width can be increased by more than 60 nm from an initial 1 nm. The large change in gap width results in a significant spectral shift in reflectance, while maintaining strong free-space coupling and a constant full width at half maximum (FWHM). We also analyze the effect of geometrical parameters on the nanometer-sized gap width and optical responses. Our nanoslits with simple geometry resulting in efficient optical coupling show promise for improving active metasurfaces and electromechanically tunable plasmonic devices.

Enhancing photoelectrochemical CO2 reduction with silicon photonic crystals.

The effectiveness of silicon (Si) and silicon-based materials in catalyzing photoelectrochemistry (PEC) CO2 reduction is limited by poor visible light absorption. In this study, we prepared two-dimensional (2D) silicon-based photonic crystals (SiPCs) with circular dielectric pillars arranged in a square array to amplify the absorption of light within the wavelength of approximately 450nm. By investigating five sets of n + p SiPCs with varying dielectric pillar sizes and periodicity while maintaining consistent filling ratios, our findings showed improved photocurrent densities and a notable shift in product selectivity towards CH4 (around 25% Faradaic Efficiency). Additionally, we integrated platinum nanoparticles, which further enhanced the photocurrent without impacting the enhanced light absorption effect of SiPCs. These results not only validate the crucial role of SiPCs in enhancing light absorption and improving PEC performance but also suggest a promising approach towards efficient and selective PEC CO2 reduction.

High-aspect-ratio dielectric pillar with nanocavity backed by metal substrate in the infrared range.

We investigated absorption and field enhancements of shallow nanocavities on top of high-aspect-ratio dielectric pillars in the infrared range. The structure includes a high-aspect-ratio nanopillar array of high refractive index, with nano-cavities on top of the pillars, and a metal plane at the bottom. The enhancement factor of electric field intensity reaches 3180 in the nanocavities and peak absorption reaches 99%. We also investigated the finite-size effect of the presented structure to simulate real experiments. Due to its narrow absorption bandwidth 3.5 nm, it can work as a refractive index sensor with sensitivity 297.5 nm/RIU and figure of merit 85. This paves the way to directly control light field at the nanoscales in the infrared light range. The investigated nanostructure will find applications in multifunctional photonics devices such as chips for culturing cells, refractive index sensors, biosensors of single molecule detection and nonlinear sensors.

Pixelated Photonic Crystals

Photonic crystals can prevent or allow light of certain frequencies to propagate in distinct directions in anomalous and useful ways for use as waveguides, laser cavities, and topological light propagation. However, there exist limited approaches for fundamental reconfiguration of photonic crystals, such as changing the unit cell to various and on‐demand geometries and symmetries. This work introduces the concept of pixelated 2D photonic crystals where the variability of the dielectric profile is achieved by a pixelated matrix of the material. Specifically, the cross sections of dielectric cylindrical pillars distributed in a photonic crystal lattice are replaced with pixelated circles using different resolutions and the corresponding band diagrams are calculated. The comparison to the band diagrams of the original structure shows that the original—and today typically used—cylindrical design can be well approximated by as few as square switchable pixels while retaining less than change in the photonic band structure. Experimental realization of switchable pixelation is proposed based on the liquid crystal display (LCD) technology with high birefringence materials. More generally, the demonstrated approach to reconfigurable 2D photonic crystal based on switchable pixels can enable realization of diverse fundamentally reconfigurable advanced optical materials.

Plasmonic Nanopillars-A Brief Investigation of Fabrication Techniques and Biological Applications.

Nanopillars (NPs) are submicron-sized pillars composed of dielectrics, semiconductors, or metals. They have been employed to develop advanced optical components such as solar cells, light-emitting diodes, and biophotonic devices. To integrate localized surface plasmon resonance (LSPR) with NPs, plasmonic NPs consisting of dielectric nanoscale pillars with metal capping have been developed and used for plasmonic optical sensing and imaging applications. In this study, we studied plasmonic NPs in terms of their fabrication techniques and applications in biophotonics. We briefly described three methods for fabricating NPs, namely etching, nanoimprinting, and growing NPs on a substrate. Furthermore, we explored the role of metal capping in plasmonic enhancement. Then, we presented the biophotonic applications of high-sensitivity LSPR sensors, enhanced Raman spectroscopy, and high-resolution plasmonic optical imaging. After exploring plasmonic NPs, we determined that they had sufficient potential for advanced biophotonic instruments and biomedical applications.

A flexible dual-function capacitive sensor enhanced by loop-patterned fibrous electrode and doped dielectric pillars for spatial perception

A flexible dual-function capacitive sensor enhanced by loop-patterned fibrous electrode and doped dielectric pillars for spatial perception

Topological valley crystals in a photonic Su–Schrieffer–Heeger (SSH) variant

Progress on two-dimensional materials has shown that valleys, as energy extrema in a hexagonal first Brillouin zone, provide a new degree of freedom for information manipulation. Then, valley Hall topological insulators supporting such-polarized edge states on boundaries were set up accordingly. In this paper, a two-dimensional valley crystal composed of six tunable dielectric triangular pillars in each unit cell is proposed in the photonic sense of a deformed Su–Schrieffer–Heeger model. We reveal the vortex nature of valley states and establish the selection rules for valley-polarized states. Based on the valley topology, a rhombus-shaped beam splitter waveguide is designed to verify the valley-chirality selection rule above. Our numerical results entail that this topologically protected edge states still maintain robust transmission at sharp corners, thus providing a feasible idea for valley photonic devices in the THz regime.

Optimization and design on multi-layers of dielectric metasurface as broadband terahertz quarter wave plate

Planar metasurface-based quarter wave plates (QWPs) have significant advantages over conventional devices in terms of compactness, flexibility, and simplicity of manufacture; however, they offer a relatively narrow operational bandwidth. A broadband terahertz achromatic QWP is realized in the 0.68–1.48 THz spectral region in this work, which consists of several separate metasurface layers of dielectric elliptic pillars stacked together with various rotation angles. Meanwhile, an improved simulated annealing method is proposed, which introduces the evolutionary strategies to optimize the distinct fundamental microstructural unit cells, and the running speed is greatly increased. Furthermore, the proposed multi-layers metasurface may pave the way for arbitrary polarization control of incident waves and be ideally suited for application by virtue of subwavelength thickness in the other frequency bands as well.

Nanometer-sensitive wideband opto-mechanical switching concept

The present paper is concerned with the mechanically extremely sensitive reflection switching concept of a free-space wave impinging on an array of dielectric or semiconductor pillars. Splitting the pillars of a 2D periodic array in its resonant reflection regime at a prescribed wavelength into two parts with a low-index gap of a few nanometers between parts cancels the reflection of a plane wave under normal incidence. The underlying principle lies in the strong and abrupt discontinuity of the electric field component parallel to the pillar axes caused by the gap. The electromagnetic field distribution is consequently deeply perturbed and no longer corresponds to that of an optical resonance of the array; this suppresses the reflection. The electromagnetic analysis of a silicon pillar array leads to the design of a gapless experimental model fabricated by microsystem technologies that exhibits a broad reflection maximum of a few tens of nm at a prescribed wavelength in the visible and near-IR range, and of a pillar structure with nanometer-thick low-index gap exhibiting no reflection peak over this wide wavelength range. A transmission ratio of 1:30 at a 1080 nm peak wavelength between a gapless and a 1.5 index, 30 nm-thick gap structures was measured.

A High-Temperature and Frequency-Reconfigurable Multilayer Frequency Selective Surface Using Liquid Metal

A high-temperature and frequency-reconfigurable multilayer frequency selective surface (FSS) based on liquid metal is proposed in this work. After construction of a multilayer dielectric cavity with specific dielectric-shaped pillars and the injection of liquid metal (EGaIn) into each layer of the cavity, the liquid metal and dielectric pillars form a slot-type bandpass FSS, and frequency reconfiguration is realized by switching the injection layer. A prototype of the proposed FSS was designed, fabricated and measured. The measured results validated the design well. The good conductivity and fluidity of the liquid metal are utilized to realize the reconfigurable electromagnetic performance of the proposed FSS. This works in the C-band (5 GHz) and Ku band (13 GHz) and has a frequency switching time between 13.1 to 14.2 s. Moreover, due to the good thermal conductivity of liquid metal, the flowing liquid metal can take away heat, and the multilayer FSS can work as cooling plate. The heat transfer performance was investigated by numerical simulations. A 1000 ° plane heat source was set on the bottom surface of the double-layer FSS. The average temperature on the top surface was 62.2 °, when the liquid metal fluid in the FSS had a speed of 1 m/s. The proposed multilayer FSS has strong potential for applications in some special application fields involving stealth and hypersonic aircraft.

Using multi-polar scattering and near-field plasmonic resonances to achieve optimal emission enhancement from quantum emitters embedded in dielectric pillars

Recently, high refractive index micro-pillars have been widely used for enhancing the fluorescence of quantum emitters (vacancy/defect centers) embedded within the pillar. However, the maximum observed enhancement from these pillars has been limited to about a factor of 10. Within the dielectric pillars, the Purcell enhancement is restricted to around unity, and the fluorescence enhancement is mainly due to the enhancement of the collection efficiency of the dipole emission from inside the pillar if compared to a bulk substrate. Using multi-polar electromagnetic scattering resonances and near-field plasmonic field enhancement/confinement, here we report a simple metal–dielectric pillar resonator scheme to achieve a close to three orders of magnitude fluorescence enhancement from embedded solid state vacancy centers. The scheme comprises a silver (Ag) cylinder fabricated on top of a silicon-carbide (SiC) dielectric pillar, with both the SiC and Ag cylinders having the same diameter. A selective dipole orientation relative to the metal–dielectric interface for emitters close to the SiC pillar’s top surface leads to a large Purcell enhancement of the dipole’s emission. The Ag cylinder was found to function as an efficient resonator as well as an antenna, enhancing as well as directing a significant fraction of the dipole’s emission into far-field free space.

Broadband achromatic metalens and meta-deflector based on integrated metasurface

Metasurfaces have attracted much attention due to their great ability to manipulate the electromagnetic field over an ultra-compact profile, which facilities the realization of the next generation diffractive optical elements. However, the broadband performance of the metasurface-based optical devices is still subject to the chromatic aberration. Although the broadband achromatic metalenses have recently been studied, the diameters and numerical apertures of the metalens are still limited by the requirement of large phase compensation. Such large phase dispersion is usually realized by sophisticated nanostructures of extremely high aspect ratios that are not easy to fabricate. Here we propose a new kind of metalens by integrating the dielectric pillars with a multi-level phase plate, and achromatic focusing and imaging can be achieved in the wavelength range from 1100 to 1500 nm. In addition, a broadband achromatic meta-deflector is also proposed, which could refract broadband incident light with the same refractive angle. Our integrated metasurface can provide large phase compensation while alleviating the aspect ratios of the composing nanopillars, and shows potential applications in designing arbitrary metadevices with dispersion engineering properties.

High-efficiency all-silicon metasurfaces with 2π phase control based on multiple resonators

Flat all-silicon metasurfaces have significant potential as ultrathin optical devices and systems that offer the advantages of mature fabrication technology and complementary metal–oxidesemiconductor compatibility. However, previously reported all-silicon metasurfaces suffered from the drawback of low transmission owing to the large intrinsic reflection loss induced by high-index silicon substrates. To achieve high transmission, silicon-based metasurfaces are usually composed of low-index dielectric substrates and silicon pillars. It is still a substantial challenge for an all-silicon metasurface to realize high transmission. In this paper, we present a design to overcome this drawback by integrating two-dimensional periodic arrays of subwavelength silicon cylinders as perfect antireflection resonators on the bottom and top surfaces of the silicon substrate. Using this design, a uniform array of all-silicon unit cells comprising a metasurface can exhibit high transmission, up to 99.5%, at the target wavelength of 10.6 µm, which indicates a significant improvement over the design without the antireflection scheme. Furthermore, as an example of a metasurface application, an all-silicon polarization-insensitive metalens is built based on a series of all-silicon unit cells with high transmission and 2π phase control, which indicates subwavelength-focusing capability. The transmission of the entire metalens reached 81.9%, and the focusing efficiency was 71%. This study not only recommends a means to develop ultrathin all-silicon optical devices with high efficiency but also proposes a general approach to achieve high-quality metasurfaces based on a single high-index dielectric.

Narrowband mid-infrared absorber based on a mirror-backed low-index dielectric lattice

This paper reports the design of a narrowband mid-infrared absorber using a structure consisting of low-index dielectric pillar arrays on a highly reflective metal plane. Such a hybrid system can support a high-quality lattice resonance with strongly confined electromagnetic fields at the tops of dielectric structures. For mid-infrared wavelengths around 4 µm, the low-index dielectric can be silica by plasma-enhanced chemical vapor deposition, and the metal can be copper. The lattice resonance can lead to perfect absorption with a quality factor around 1000 when considering realistic dissipative losses for the low-index dielectric pillars. According to Kirchhoff’s radiation law, the narrowband absorber can work as a high- Q surface-emitting thermal emitter in the mid-infrared by heating its substrate.

Scalable and Deterministic Fabrication of Quantum Emitter Arrays from Hexagonal Boron Nitride.

We demonstrate the fabrication of large-scale arrays of single-photon emitters (SPEs) in hexagonal boron nitride (hBN). Bottom-up growth of hBN onto nanoscale arrays of dielectric pillars yields corresponding arrays of hBN emitters at the pillar sites. Statistical analysis shows that the pillar diameter is critical for isolating single defects, and diameters of ∼250 nm produce a near-unity yield of a single emitter at each pillar site. Our results constitute a promising route toward spatially controlled generation of hBN SPEs and provide an effective and efficient method to create large-scale SPE arrays. The results pave the way to scalability and high throughput fabrication of SPEs for advanced quantum photonic applications.

A sound demonstration of Klein tunneling

Phononic Crystals The ability of particles to tunnel through barriers is an important property of quantum mechanical systems, and the extent of the effect is strongly dependent on the properties of the barrier. By contrast, Klein tunneling can exhibit unity transmission that is independent of the width and height of the energy barrier, but direct evidence for this effect remains elusive. Using a phononic system comprising a periodic array of dielectric pillars, Jiang et al. provide evidence for the direct observation of Klein tunneling. Near-unity transmission of acoustic excitations was observed across the barrier independently of its width and height. This ability to tune the effect by controlling the size of the junctions and the frequency of incident waves provides a promising platform to explore complex nontrivial physics and applications in signal processing and information communications. Science , this issue p. [1447][1] [1]: /lookup/doi/10.1126/science.abe2011

Direct observation of Klein tunneling in phononic crystals.

Tunneling plays an essential role in many branches of physics and has found important applications. It is theoretically proposed that Klein tunneling occurs when, under normal incidence, quasiparticles exhibit unimpeded penetration through potential barriers independent of their height and width. We created a phononic heterojunction by sandwiching two types of artificial phononic crystals with different Dirac point energies. The direct observation of Klein tunneling as shown by the key feature of unity transmission is demonstrated. Our experiment reveals that Klein tunneling occurs over a broad band of acoustic frequency. The direct observation of Klein tunneling in phononic crystals could find applications in signal processing, supercollimated beams, and communications.

Design of 3D Printed Multi-Wavelength DRA

This paper presents the design and analysis of a portable multi-wavelength antenna for UWB, wireless data-link, electromagnetic sensor, and multi-beam applications. The antenna structure consists of an air suspended rectangular dielectric resonator antenna (DRA) loaded with a printed horn. The DRA is suspended in air using four supporting dielectric pillars. The overall antenna structure is fabricated using 3-D printing technology. Poly-Lactic Acid (PLA) is used for the implementation of the antenna system (i.e. DRA, horn and supporting pillars integrated with the substrate). It is a biodegradable thermoplastic composite polymer. The antenna shows multibeam characteristics at different frequencies. The radiation patterns are explained using the concept of multi-wavelength long dipole or monopole. The computed efficiency of the projected DRA is more than 70% and maximum peak gain is 12.15 dBi. The measured impedance bandwidth of the antenna is 122.6% (7.5 GHz to 31.3 GHz). The measured group delay is less than 1.5 ns in the band of operation.