Sub-wavelength Periodic Structures Research Articles

Unveiling the Terahertz Nano-Fingerprint Spectrum of Single Artificial Metallic Resonator.

As artificially engineered subwavelength periodic structures, terahertz (THz) metasurface devices exhibit an equivalent dielectric constant and dispersion relation akin to those of natural materials with specific THz absorption peaks, describable using the Lorentz model. Traditional verification methods typically involve testing structural arrays using reflected and transmitted optical paths. However, directly detecting the dielectric constant of individual units has remained a significant challenge. In this study, we employed a THz time-domain spectrometer-based scattering-type scanning near-field optical microscope (THz-TDS s-SNOM) to investigate the near-field nanoscale spectrum and resonant mode distribution of a single-metal double-gap split-ring resonator (DSRR) and rectangular antenna. The findings reveal that they exhibit a dispersion relation similar to that of natural materials in specific polarization directions, indicating that units of THz metasurface can be analogous to those of molecular structures in materials. This microscopic analysis of the dispersion relation of artificial structures offers new insights into the working mechanisms of THz metasurfaces.

Comprehensive review of the directed design of optomechanical crystal cavities using intelligent algorithms

Optomechanical crystal cavities are devices based on optomechanical interactions to manipulate photons and phonons on periodic subwavelength structures, enabling precise measurement of the force and displacement. The performance of the target structures varies when applied to different applications. Optomechanical crystal cavities now rely on an empirical forward design, which is inefficient. Therefore, a desired shift is toward directed design with a “problem-oriented” strategy. The directed optimization problem’s nonconvex nature and extensive parameter space necessitate substantial computational resources, driving the need for intelligent algorithms in a sub-wavelength structure design. Intelligent algorithms can surpass the constraints of traditional methods and discover novel structures that are effective in different materials, topologies, modes, and wavelengths. This paper provides an extensive overview of intelligent algorithms for guiding the directed design of optomechanical crystal cavities. It presents a systematic classification of 15 algorithmics, including, but not limited to, topology algorithms, particle swarm optimization algorithms, convolutional neural networks, and generative adversarial networks. The article provides a comprehensive review and thorough analysis of the principle and current application state, as well as the advantages and disadvantages of each intelligent algorithm. By using these intelligent algorithms, researchers can enhance the efficiency and accuracy of optimizing optomechanical crystal cavities in a broader design space.

Synthetic Fourier Domain Optical Coherence Tomography

A novel approach for image formation in optical coherence tomography (OCT) and microscopy is presented. The depth resolution of OCT, including recently developed nanosensitive OCT (nsOCT), is limited by the spectral bandwidth of the light source used for illumination. The proposed approach, synthetic OCT (synOCT), permits label‐free, depth‐resolved quantitative visualization of the subwavelength‐sized structures with nanosensitivity. Using synOCT it is possible to estimate the contribution of axial Fourier components of an object's structure in image formation at each small volume within the image. The size of such areas can be smaller than the resolution limit of the imaging system that provides potential for super‐resolution imaging. Visualization of the subwavelength periodic structures and quantitative visualization of the subwavelength internal structures of highly scattering biological samples, within voxels smaller than resolution limit of the imaging system, are demonstrated. In contrast to nsOCT, the trade‐off between spectral and spatial resolution is removed which results in dramatic improvement of both spectral and spatial resolution in synOCT relative to nsOCT.

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.

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.

Lattice-induced strong coupling in symmetric and asymmetric split-ring metamaterial arrays.

The intrinsic link between surface plasmon modes (eigenmodes) and lattice modes in subwavelength periodic structures is investigated based on the split-ring metamaterial structure. The paper shows that the strong coupling between the eigenmodes and the lattice modes can be achieved by appropriately adjusting the period of the metamaterial structure, and the emergence of new, to the best of our knowledge, modes at low frequencies is observed, resulting in a lower spectral loss of a single hybrid resonance and an increase in its Q factor up to 110. In addition, an asymmetric double-split-ring structure is proposed, and the Fano resonance is excited, giving rise to a spectral line with three resonance valleys. The coupled harmonic-oscillator model is used to interpret the underlying coupling mechanism in lattice-induced transparent systems, which agrees well with our simulation results. This strong-coupling scheme between the lattice and the mixed modes of the metamaterial unit provides a new avenue to modulate lattice-induced transparency, high-Q resonance, and strong-field confinement, which may find applications in the design of ultrasensitive sensors, slow-light devices, as well as multiple frequency absorbers and other fields.

Enhancement of ablation and ultrafast electron dynamics observation of nickel-based superalloy under double-pulse ultrashort laser irradiation

Temporally shaped femtosecond laser machining is a flexible and effective method to improve the efficiency and quality of cooling film holes. This study investigated the ablation of nickel-based superalloy by double-pulse femtosecond laser with different pulse-separations and fluences. Compared with single-pulse ablation, approximate 1.5 times enhancement of ablation area was obtained in double-pulse ablation with about 2 ps pulse-separation. By varying the pulse-separations, the ablation area can be tuned, and at the same time, the ablation depth can be kept for little fluctuation. An improved two-temperature model and time-resolved transient reflectivity technique were used for analyzing the ablation mechanisms. We found that more energy deposition can happen from electron system to lattice system for double-pulse ablation, which makes ablation area increase. However, mechanical relaxation started at around 2 ps, which could be suppressed by the pressure wave induced by the second sub-pulse, and finally achieved the maximum ablation area at about 2 ps pulse-separation. Besides, laser-induced subwavelength periodic surface structures were observed under irradiation of multiple pulses. The findings may aid in understanding the ablation mechanism between nickel-based superalloy and femtosecond laser, as well as in optimizing the processing of cooling film holes.

Nanostructured back surface amorphization of silicon with picosecond laser pulses

Laser-based amorphization on the back surface of a 525-μm thick crystalline silicon sample is studied. To deposit sufficient energy for a local change from a crystalline to an amorphous state, laser irradiation at 2-μm wavelength with 25-ps pulse duration is combined with Bessel beam shaping. Deterministic single-site modifications and homogeneous continuous lines of amorphous silicon are demonstrated. Optical and electron microscopy together with Raman spectroscopy measurements highlight the material transformations featuring the formation of subwavelength periodic surface structures. The investigations open up possibilities for processing in-built microelectronic devices.

Laser Direct Writing of Silver Clusters‐Based Subwavelength Periodic Structures Embedded in Mid‐Infrared Gallo‐Germanate Glass

The direct laser writing (DLW) using femtosecond lasers allows for the inscription of 3D microstructures embedded inside optical materials. Based solely on the silver ions photochemistry, the DLW in silver‐containing glasses enables to locally induce inside glasses a unique combination of optical properties. The physical modifications of the material encompass not only a refractive index change, but also new physical properties like fluorescence, second‐ and third‐harmonic generations, and surface plasmon resonance. Numerous efforts are deployed to develop the DLW‐assisted silver photochemistry in phosphate glasses. However, this glass family is suffering from its near‐infrared optical cutoff as opposed to the silver‐doped gallo‐germanate glasses. With an extended mid‐infrared (mid‐IR) transmission, these glasses are synthesized via the melt‐quenching technique. Depending on the glass composition, either a glass matrix‐based single track (Type I) or a silver cluster‐based double track (Type A) of refractive index change is produced. By enabling an order of magnitude smaller structures than with Type I, Type A modification is further expanded to embed, for the first time, periodic structures below the inter‐track spacing. Demonstrated with a pitch down to 400 nm, these Type A–based periodic structures bring new insights through the fabrication of 3D diffractive gratings in mid‐IR glasses.

Manipulation of Subwavelength Periodic Structures Formation on 4H-SiC Surface with Three Temporally Delayed Femtosecond Laser Irradiations.

Controlling laser-induced periodic surface structures on semiconductor materials is of significant importance for micro/nanophotonics. We here demonstrate a new approach to form the unusual structures on 4H-SiC crystal surface under irradiation of three collinear temporally delayed femtosecond laser beams (800 nm wavelength, 50 fs duration, 1 kHz repetition), with orthogonal linear polarizations. Different types of surface structures, two-dimensional arrays of square islands (670 nm periodicity) and one-dimensional ripple structures (678 nm periodicity) are found to uniformly distribute over the laser-exposed areas, both of which are remarkably featured by the low spatial frequency. By altering the time delay among three laser beams, we can flexibly control the transition between the two surface structures. The experimental results are well explained by a physical model of the thermally correlated actions among three laser-material interaction processes. This investigation provides a simple, flexible, and controllable processing approach for the large-scale assembly of complex functional nanostructures on bulk semiconductor materials.

Self-organizing half-wave gratings on the surface of silica glass

The interaction of femtosecond laser pulses with the surface of silica glass has been studied. As a result of interference between the incident radiation and surface plasmon polaritons, the formation of self-organizing subwavelength periodic structures with a period of 250 nm was observed. The minimum pulse energy at which recording occurs without surface ablation has been revealed. Keywords: direct laser recording, femtosecond laser pulses, silica, nanogratings.

Theoretical investigation of two-dimensional sub-wavelength structures fabricated by multi-beam surface plasmon interference lithography

In this study, multi-beam surface plasmon (SP) interference lithography was theoretically proposed and demonstrated to fabricate periodic two-dimensional sub-wavelength structures. The lithography structures were based on an attenuated total reflection prism used to excite the SPs using a 442 nm laser. Circular lattices with periods of 166 and 288 nm were successfully obtained via three- and six-beam SP interference lithography, respectively; these lattices exhibited an identical feature size of 96 nm. Square dot arrays with a 177 nm period and 88.5 nm spot size were obtained by four-beam SP interference lithography. The proposed lithography technology has a simple structure and is economical; further, it may provide an effective method for the fabrication of two-dimensional sub-wavelength structures.

Acoustic Metamaterials: A Review of Theories, Structures, Fabrication Approaches, and Applications

AbstractAcoustic metamaterials (AMs), which are materials composed of subwavelength periodic artificial structures with specific designs, have drawn significant research attention. This is because of the extraordinary abilities of AMs to manipulate acoustic waves compared with those of traditional acoustic materials. In this review, current advances in AM research are comprehensively investigated and summarized. First, major theories regarding AMs, including the acoustic wave equation, crystal lattice and energy band theory, effective medium theory, and general Snell's law, are explained in detail. Existing AM structures are then described and divided into several categories based on their structural characteristics and responses to acoustic waves. Furthermore, to bridge the gap between design and practical application, both conventional and advanced AM manufacturing methods are summarized. The applications of AMs in the fields of acoustic cloaking, acoustic lensing, acoustic absorption, noise reduction, and supernormal sound transmission are particularly delineated. Finally, contemporary challenges, trends, and strategies relevant to AMs are discussed. This review aims to provide a comprehensive collection of AM‐related knowledge, including information regarding physical theories, structures, fabrication approaches, and applications, which will help promote the further development of AMs.

Generation of microstructures and extreme sub-wavelength laser-induced periodic structures on the Si surface using $$\hbox {N}_{2}$$ nanosecond pulsed laser for the reduction of reflectance

Generation of microstructures and extreme sub-wavelength laser-induced periodic structures on the Si surface using $$\hbox {N}_{2}$$ nanosecond pulsed laser for the reduction of reflectance

Surface Superconductivity Changes of Niobium Sheets by Femtosecond Laser-Induced Periodic Nanostructures

Irradiation with ultra-short (femtosecond) laser beams enables the generation of sub-wavelength laser-induced periodic surface structures (LIPSS) over large areas with controlled spatial periodicity, orientation, and depths affecting only a material layer on the sub-micrometer scale. This study reports on how fs-laser irradiation of commercially available Nb foil samples affects their superconducting behavior. DC magnetization and AC susceptibility measurements at cryogenic temperatures and with magnetic fields of different amplitude and orientation are thus analyzed and reported. This study pays special attention to the surface superconducting layer that persists above the upper critical magnetic field strength Hc2, and disappears at a higher nucleation field strength Hc3. Characteristic changes were distinguished between the surface properties of the laser-irradiated samples, as compared to the corresponding reference samples (non-irradiated). Clear correlations have been observed between the surface nanostructures and the nucleation field Hc3, which depends on the relative orientation of the magnetic field and the surface patterns developed by the laser irradiation.

Fabrication of three-dimensional high-aspect-ratio structures by oblique-incidence Talbot lithography.

Developing a suitable production method for three-dimensional periodic nanostructures with high aspect ratios is a subject of growing interest. For mass production, Talbot lithography offers many advantages. However, one disadvantage is that the minimum period of the light intensity distribution is limited by the period of the diffraction grating used. To enhance the aspect ratio of fabricated nanostructures, in the present study we focus on multiple wave interference between diffracted waves created using the Talbot effect. We propose a unique exposure method to generate multiple wave interference between adjacent diffraction orders by controlling the angle of incidence of an ultraviolet (UV) light source. Using finite-difference time-domain simulations, we obtain fringe patterns with a sub-wavelength period using a one-dimensional periodic grating mask. Moreover, we demonstrate the practical application of this approach by using UV lithography to fabricate sub-wavelength periodic photopolymer-based structures with an aspect ratio of 30 in millimeter-scale areas, indicating its suitability for mass production.

Subwavelength Periodic Structures Design and its Role in Photon Control of Thin Film Solar Cells

A subtle modification of the device surface is able to reduce optical loss and to further achieve high photoelectric conversion efficiency for thin film solar cells. This work shows the manipulation properties of subwavelength periodic structures on incident light at air/glass surface. In order to explore the mechanisms of optical loss, the spectral response and energy distribution of light are investigated by using rigorous coupled wave analysis and finite difference time domain methods. Calculation results show that the diffraction scattering and gradient refraction index play a significant role for better photon harvesting. With an optimized design of [Formula: see text][Formula: see text]nm, [Formula: see text], and hemispherical shape structure, obvious improvement in transmittance, external quantum efficiency and photo-generated current is achieved. The photoelectric conversion efficiencies of amorphous silicon thin film cells with an absorbing layer thickness of 400[Formula: see text]nm is 8.04%, improved by 5.9% compared with the flat cell of equivalent size.

Capture of femtosecond plasmon excitation on transient nonequilibrium states of the metal surface

This paper utilizes the time dependent orientation change of the laser-induced periodic subwavelength structures to capture the nonequilibrium states on copper metal surface.

Goos–Hanchen shift in a metasurface of core–shell nanoparticles

Metasurfaces are periodic subwavelength structures that have found applications in functional devices such as image sensors, cameras, and microscopes. They are able to modulate the amplitude, phase, and polarization of incident light. Reflected light from a surface exhibits a deviation from the rules governed by geometric optics and displaces in directions parallel and perpendicular to plane of incidence. The sharp phase changes at reflection from a metasurface would be useful in enhancing the longitudinal reflection shifts, known as Goos–Hanchen​ (GH) shift. In this article, using generalized sheet transition conditions, we study GH shift in a metasurface composed of spherical metal-oxide core with plasmonic shell nanoparticles in a square array. A detailed analysis reveals that reflection shifts of this type of nanostructures can be tuned delicately by varying the size, core-to-shell ratio and periodicity of particles as well as light wavelength and angle of radiation. Large reflection shifts are achievable in both visible and ultraviolet ranges due to plasmonic resonance of metallic shell. Such sensitivity could be useful in very sensitive beam deflection measurements like in AFM or in reflective UV sensors.

Lattice-induced plasmon hybridization in metamaterials.

We explore an inherent connection between two fundamental concepts of physics-resonance (eigen mode) hybridization and lattice effect in sub-wavelength periodic structures. Our study reveals that coupling with lattice mode is the prime deciding factor to determine the nature, position, and line shape of the hybridized states. Modulating lattice modes can effectively control mode hybridization and tune the relative position of hybridized modes [symmetric (electric), anti-symmetric (magnetic)] without changing any other structural dimensions in subwavelength plasmonic metamaterials. Outcomes of this study can be exploited in designing linear and nonlinear photonic structures toward futuristic meta devices.