Subwavelength Structures Research Articles

Plasmonic Cavity Formation by Circular and Spiral Corrugations

Metal surfaces with sub-wavelength structures form a plasmon polariton-like surface mode, i.e., spoof-plasmon. The spoof-plasmon on a corrugated disk propagates radially and is reflected at the edge, resulting in formation of plasmonic cavity. With a concentric circular corrugation, the excited spoof-plasmons form an axisymmetric plasmonic cavity. With a spiral corrugation, the spoof-plasmons have non-zero orbital angular momenta and form a non-axisymmetric plasmonic cavity. Spoof-plasmons consisting of the plasmonic cavity transfer their angular momenta to radiation waves via the corrugated hollow waveguide by conserving their topological charges.

Anomalous reflection with customized high-efficiency bandwidth.

Anomalous reflection from metasurfaces with 100% efficiency at optical frequencies was not achieved until an all-dielectric quasi-three-dimensional subwavelength structure was proposed. The desired nonlocal control of light waves is realized by designing phase responses of multilayer films at a single wavelength. However, a high-efficiency bandwidth is not controllable by designing only the phase response at a single wavelength. Here, we propose the use of a multilayer metasurface to achieve anomalous reflection with a customized high-efficiency bandwidth. The interference of the successive light waves scattered from the structure at multiple wavelengths is controlled by phase dispersion regulation of multilayer films. As a proof of concept, two sets of multilayer films with different phase dispersions were designed to realize broadband (∼110 nm) and narrowband (∼15 nm) anomalous reflections, both with an efficiency of over 80%. The results offer a general strategy to design high-efficiency anomalous reflection with arbitrary bandwidth and might stimulate various potential applications for metadevices.

Measurement of structured tightly focused beams with classical interferometry

We report the first classic interferometric measurement of the full field of propagating tightly focused vector beams (NA up to 1.23) across areas of . This approach outperforms near field measurements of non paraxial propagating beams because it is compatible with large power imbalances across the polarization components and low light intensities. Also, intricate subwavelength structures are resolved due to the exact mapping of the transverse field components by the microscope. Prior to focusing, the structured laser light has a linear or circular polarization state and an arbitrary 2D spatial phase. The transverse polarization components are extracted directly from 12 interferograms using 4 step interferometry, while the longitudinal component is extracted from the measured transverse fields with Gauss law. Different structured propagating vectorial beams are measured to demonstrate that the method works for any field geometry. The measurements are compared to simulations with normalized cross correlations that yield mean values , confirming good agreement.

Subwavelength structure engineered passband filter for the 2-µm wave band.

In this work, we experimentally demonstrate a passband filter for the 2-µm wave band on the silicon-on-insulator platform. The device consists of a strip waveguide and an apodized subwavelength-structured waveguide. Fabricated on a 340-nm-thick silicon membrane, the proposed passband filter shows a 3-dB bandwidth of 16-33 nm, a high sidelobe suppression ratio (SLSR) of 24 dB, and a low insertion loss (IL) of 0.4 dB.

A comprehensive deep learning method for empirical spectral prediction and its quantitative validation of nano-structured dimers

Nanophotonics exploits the best of photonics and nanotechnology which has transformed optics in recent years by allowing subwavelength structures to enhance light-matter interactions. Despite these breakthroughs, design, fabrication, and characterization of such exotic devices have remained through iterative processes which are often computationally costly, memory-intensive, and time-consuming. In contrast, deep learning approaches have recently shown excellent performance as practical computational tools, providing an alternate avenue for speeding up such nanophotonics simulations. This study presents a DNN framework for transmission, reflection, and absorption spectra predictions by grasping the hidden correlation between the independent nanostructure properties and their corresponding optical responses. The proposed DNN framework is shown to require a sufficient amount of training data to achieve an accurate approximation of the optical performance derived from computational models. The fully trained framework can outperform a traditional EM solution using on the COMSOL Multiphysics approach in terms of computational cost by three orders of magnitude. Furthermore, employing deep learning methodologies, the proposed DNN framework makes an effort to optimise design elements that influence the geometrical dimensions of the nanostructure, offering insight into the universal transmission, reflection, and absorption spectra predictions at the nanoscale. This paradigm improves the viability of complicated nanostructure design and analysis, and it has a lot of potential applications involving exotic light-matter interactions between nanostructures and electromagnetic fields. In terms of computational times, the designed algorithm is more than 700 times faster as compared to conventional FEM method (when manual meshing is used). Hence, this approach paves the way for fast yet universal methods for the characterization and analysis of the optical response of nanophotonic systems.

Genetic optimization of Brillouin scattering gain in subwavelength-structured silicon membrane waveguides

On-chip Brillouin optomechanics has great potential for applications in communications, sensing, and quantum technologies. Tight confinement of near-infrared photons and gigahertz phonons in integrated waveguides remains a key challenge to achieving strong on-chip Brillouin gain. Here, we propose a new strategy to harness Brillouin gain in silicon waveguides, based on the combination of genetic algorithm optimization and periodic subwavelength structuration to engineer photonic and phononic modes simultaneously. The proposed geometry is composed of a waveguide core and a lattice of anchoring arms with a subwavelength period requiring a single etch step. The waveguide geometry is optimized to maximize the Brillouin gain using a multi-physics genetic algorithm. Our simulation results predict a remarkable Brillouin gain exceeding 3300 W−1m−1, for a mechanical frequency near 15 GHz.

Heat accumulation effects in femtosecond laser-induced subwavelength periodic surface structures on silicon

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for femtosecond laser-induced subwavelength periodic surface structures on silicon. Femtosecond laser micromachining is no longer a nonthermal process, as long as the repetition rate reaches up to 100 kHz due to heat accumulation. Moreover, a higher repetition rate generates much better defined ripple structures on the silicon surface, based on the fact that accumulated heat raises lattice temperature to the melting point of silicon (1687 K), with more intense surface plasmons excited simultaneously. Comparison of the surface morphology on repetition rate and on the overlapping rate confirms that repetition rate and pulse overlapping rate are two competing factors that are responsible for the period of ripple structures. Ripple period drifts longer because of a higher repetition rate due to increasing electron density; however, the period of laser structured surface is significantly reduced with the pulse overlapping rate. The Maxwell–Garnett effect is confirmed to account for the ripple period-decreasing trend with the pulse overlapping rate.

Efficient single-etch surface grating couplers in silicon nitride platforms for telecom and datacom wavebands

We present designs and experiments of single-etched amorphous silicon (α-Si) surface grating couplers on a silicon nitride (SiN) waveguide, operating at the telecom (C-band) and datacom (O-band) wavebands. SiN-only grating couplers demonstrate experimental coupling loss of -3.9 dB at 1.55 μm wavelength with a 1-dB bandwidth of 37 nm. Utilizing the hybrid α-Si/SiN platform with subwavelength grating structure, we obtain improved coupling performances, with the optimized fiber-chip coupling loss of -2.0 dB and a 1-dB spectral bandwidth of 42 nm centered at 1.31 μm.

Experimental Demonstration of Compact S-Band MW-Level Metamaterial-Inspired Klystron

Metamaterials are artificially constructed subwavelength structures exhibiting unprecedented electromagnetic phenomena. In this letter, an S-band MW-level metamaterial-inspired klystron using all-metal complementary electric split ring resonators (CeSRRs) was realized in the experiment. The CeSRR-loaded interaction structure of the klystron developed here features a miniaturized structure, the volume of which is only about 0.44 of the conventional counterparts. The metamaterial-inspired klystron based on the interaction structure loaded with CeSRRs was designed, fabricated and assembled. Using a pencil beam with parameters of 120 kV/80 A in the hot-test, the klystron delivered the biggest output power of 5.51 MW with gain of 55.6 dB and electronic efficiency of 57.4% at 2.852 GHz. This compact metamaterial-inspired klystron has potential applications in proton therapy facilities, tokamaks for the low-hybrid wave heating, and accelerators.

Meta-optics inspired surface plasmon devices

Surface plasmons (SPs) are electromagnetic surface waves that propagate at the interface between a conductor and a dielectric. Due to their unique ability to concentrate light on two-dimensional platforms and produce very high local-field intensity, SPs have rapidly fueled a variety of fundamental advances and practical applications. In parallel, the development of metamaterials and metasurfaces has rapidly revolutionized the design concepts of traditional optical devices, fostering the exciting field of meta-optics. This review focuses on recent progress of meta-optics inspired SP devices, which are implemented by the careful design of subwavelength structures and the arrangement of their spatial distributions. Devices of general interest, including coupling devices, on-chip tailoring devices, and decoupling devices, as well as nascent SP applications empowered by sophisticated usage of meta-optics, are introduced and discussed.

Wide band UV/Vis/NIR blazed-binary reflective gratings for spectro-imagers: two lithographic technologies investigation

We report on subwavelength reflective gratings for hyperspectral applications operating in a very large spectral band (340–1040 nm). Our study concerns a blazed-binary grating having a period of 30 μm and composed of 2D subwavelength structures with size from 120 nm to 350 nm. We demonstrate the manufacturing of the gratings on 3″ wafers by two lithography technologies (e-beam and nanoimprint) followed by classical dry etching process. Optical measurements show that the subwavelength grating approach enables a broadband efficiency, polarization behaviour and wavefront quality improvement with respect to the requirements for the next generation of spectro-imagers for Earth observation missions. An outlook towards spherical substrate based on nanoimprint lithography is also reported with the results of mixed features replication (holes and pillars in the range of 160–330 nm) on a 540 mm concave substrate which demonstrate uniformity and accuracy capabilities over 3″ surface.

Dielectric chiral metasurfaces for enhanced circular dichroism spectroscopy at near infrared regime.

Numerous applications of chiro-optical effects can be found in nanophotonics, including imaging and spin-selective absorption, particularly in sensing for separating and detecting chiral enantiomers. Flat single-layer metasurfaces composed of chiral or achiral sub-wavelength structures offer unique properties to manipulate the light due to their extraordinary light-matter interaction. However, at optical wavelengths, the generation of strong chirality is found to be challenging via conventional chiral metasurface approaches. This work intends to design and optimize a dielectric chiral meta-nano-surface based on a diatomic design strategy to comprehend giant chiro-optical effects in the near-infrared (NIR) regime for potential application in circular dichroism (CD) spectroscopy. Instead of using a single chiral structure that limits the CD value at optical wavelengths, the proposed metasurface used a diatomic (two meta-atoms with distinct geometric parameters) chiral structure as a building block to significantly enhance the chiro-optical effect. Combining both meta-atoms in a single periodicity of the building block introduces constructive and destructive interferences to attain the maximum circular dichroism value exceeding 75%. Moreover, using multipolar resonance theory, the physics behind the generation of giant chiro-optical effects have also been investigated. The proposed dielectric chiral meta-platform based on the extra degree of freedom can find application in compact integrated optical setups for CD spectroscopy, enantiomer separation and detection, spin-dependent color filters, and beam splitters.

Broadband acoustic triggers based on multiport waveguide structures

The study of acoustic information processing has attracted great attention owing to its advantages of anti-electromagnetic interference and low energy consumption. Acoustic logic device, as a fundamental component, plays an important role in designing integrated acoustic systems. In the past few years, with the rapid development of sonic crystals, acoustic metamaterials and metasurfaces, researchers have demonstrated a variety of acoustic logic gates based on different mechanisms, and have devoted their efforts to the promotion of the practical applications. The more complex acoustic triggers with broad bandwidth and subwavelength size are very important for developing integrated sound devices, but it is difficult to realize them. In this work, we design two types of acoustic triggers based on the mechanisms of linear interference and phase modulation. The acoustic trigger with a width of 0.32<i>λ</i> and length of 0.82<i>λ</i> is composed of phased unit cells and multi-port waveguide structures, showing a subwavelength structure. Based on the phase modulation of the phased unit cells and the mechanism of linear interferences, the acoustic T-type trigger and D-type trigger with the same threshold are designed and demonstrated experimentally. The corresponding working bands of the T-type and D-type triggers are 3.293–4.069 kHz and 3.400–4.138 kHz, and their fractional bandwidths (the ratio of the bandwidth to the center frequency) can reach about 0.23 and 0.22, respectively, showing a broadband characteristic of both triggers. The mechanism of the T-type trigger is attributed to the linear interference caused by two phased unit cells with a phase difference of π. However, the realization of the D-type trigger is closely related to the incident sound energy and the phase modulation caused by the phased unit cell in the control port. The measured results and simulated results agree well with each other. Compared with other types of acoustic logic devices, the designed acoustic triggers have the advantages of broad bandwidth, subwavelength size, same threshold, and passive structure, as well as being easy to integrate, thus providing great potential applications in acoustic computing, acoustic communication, acoustic information processing and integrated acoustics. Our experimental demonstration of acoustic triggers can further promote the theoretical and experimental investigations of basic acoustic components.

Design and Fabrication of the Antireflection Subwavelength Structure of Convex Germanium Optical Window

Design and Fabrication of the Antireflection Subwavelength Structure of Convex Germanium Optical Window

PtS2 metamaterials: Fabrication and physical properties

Metamaterials are electromagnetic materials with engineered sub-wavelength structures, which can be coupled with the incident electromagnetic wave to produced peculiar physical properties. Currently, many studies are being conducted to the transition metal dichalcogenides (TMDs) due to their unique structures and novel physical properties. Here, we produced honeycomb nanostructure with various sizes on few-layer PtS2 through a well-developed nanosphere lithography technology. In this way, the unique properties of PtS2 metamaterials can be tuned to explore more exotic physical characteristics. The honeycomb surface morphology of PtS2 metamaterials has been demonstrated using the scanning electron microscope and the atomic force microscope. The optical absorption spectra of Pt and PtS2 metamaterials were investigated and the results show that the absorption spectra can be modulated by changing the size of the nanostructure. In addition, the optical anisotropic properties of Pt and PtS2 metamaterials were characterized through the angle-resolved absorption spectra. Raman spectroscopy shows the surface-enhanced Raman spectroscopy (SERS) effect of Pt metamaterials. The simulation exhibits the electric field distribution of PtS2 metamaterials at different wavelengths. The results offer a new approach to develop novel optoelectronic devices based on the meta-structure of TMDs.

Large-area 1D selective emitter for thermophotovoltaic applications in the mid-infrared

Two- or three-dimensionally patterned subwavelength structures, also known as metamaterials, have the advantage of arbitrarily engineerable optical properties. In thermophotovoltaic (TPV) applications, metamaterials are commonly used to optimize the emitter’s radiation spectrum for various source temperatures. The output power of a TPV device is proportional to the photon flux, which is proportional to the emitter size. However, using 2D or 3D metamaterials imposes challenges to realizing large emitters since fabricating their subwavelength features typically involves complicated fabrication processes and is highly time-consuming. In this work, we demonstrate a large-area (78 cm2) thermal emitter. This emitter is simply fabricated with one-dimensional layers of silicon (Si) and chromium (Cr), and therefore, it can be easily scaled up to even larger sizes. The emissivity spectrum of the emitter is measured at 802 K, targeting an emission peak in the mid-infrared. The emissivity peak is ∼0.84 at the wavelength of 3.75 μm with a 1.2 μm bandwidth. Moreover, the emission spectrum of our emitter can be tailored for various source temperatures by changing the Si thickness. Therefore, the results of this work can lead to enabling TPV applications with higher output power and lower fabrication cost.

Performance of a 200 mm Diameter Achromatic HWP with Laser-Ablated Sub-Wavelength Structures

Performance of a 200 mm Diameter Achromatic HWP with Laser-Ablated Sub-Wavelength Structures

On-Chip Reconfigurable Focusing through Low-Loss Phase Change Materials Based Metasurfaces.

Metasurfaces are useful subwavelength structures that can be engineered to achieve useful functionality. While most metasurfaces are passive devices, Phase Change Materials can be utilized to make active metasurfaces that can have numerous applications. One such application is on-chip beam steering which is of vital utility for numerous applications that can potentially lead to analog computations and non-Von Neumann computational architectures. This paper presents through numerical simulations, a novel metasurface that can realize beam steering through active phase switching of in-planted arrays of phase change material, Sb2S3. For the purpose of numerical demonstration of the principle, beam focusing has been realized, on-chip, through active switching of the Sb2S3 unit cell between the amorphous and crystalline phases. The presented architecture can realize on-chip transformation optics, mathematical operations, and information processing, thus opening the gates for future technologies.

Dispersion-engineered broadband diffractive optical elements with multilayer subwavelength structures.

Diffractive optical elements (DOEs) play an important role in modern optical applications such as spectral and imaging systems, but it is challenging to balance the diffraction efficiency with the working bandwidth. The core issue is controlling the broadband dispersion of all phase units to achieve achromatic 2π-phase modulation in the broadband domain. Here, we demonstrate broadband DOEs utilizing multilayer subwavelength structures with different materials, making it possible to freely control the phase and phase dispersion of the structural units on a much larger scale than monolayer structures. The desired dispersion-control abilities arose due to a dispersion-cooperation mechanism and vertical mode-coupling effects between the top and bottom layers. An infrared design comprised of two vertically concatenated T i O 2 and Si nanoantennas separated by a S i O 2 dielectric spacer layer was demonstrated. It showed an average efficiency of over 70% in the three-octave bandwidth. This work shows enormous value for broadband optical systems with DOEs such as spectral imaging and augmented reality.

Theoretical study on fabrication of sub-wavelength structures via combining low-order guided mode interference lithography with sample rotation

Abstract Guided mode interference lithography based on a symmetric metal-cladding dielectric waveguide structure (a resist between two metal layers), is studied. The low-order guided modes in the waveguide structure excited by 325 nm laser, and then sub-wavelength gratings with half-periods of 47 nm ( λ /6.91) and 60 nm ( λ /5.42) can be obtained by excited TM0 and TE0 guided modes, respectively. Among them, the period of the sub-wavelength grating fabricated by TM0 guided mode interference is smaller, but it is limited by the thickness of resist. TE0 guided mode interference is more suitable for thick resist. On this basis, taking TM0 guided mode as an example, periodic sub-wavelength structures, such as square lattice and quasi-hexagonally close-packed structure, can be obtained by rotating and exposing the sample in different ways. The proposed method of combining guided mode interference lithography with sample rotation has simple operation and low cost, and provides a theoretical reference for fabricating two-dimensional sub-wavelength structures.