Circular Nanoholes Research Articles

Buckling of a stretched nanoplate with a nanohole incorporating surface energy

The effect of a nanoplate buckling at a circular nanohole under the remote uniaxial tension is studied incorporating the surface energy in accordance with the Gurtin–Murdoch surface elasticity model. The hole surface within the framework of the plane problem and the faces of the plate within the framework of the Kirchhoff theory of the plate bending are characterized by both the surface elasticity properties and the residual surface tension. The full potential energy of the plate containing the surface tension with non-strain terms of the surface displacement gradient is derived. Based on the principle of virtual displacements, critical values of the load, corresponding symmetrical and asymmetrical forms of the buckling are found by the Ritz method. Numerical investigations reveal that allowing for the non-strain terms of the surface displacement gradient in the Gurtin–Murdoch constitutive relation leads to the essential increasing the rigidity of the plate and the critical Euler load. Two types of the size effect are detected. The nanoplate thickness and the ratio of the hole radius to the thickness influence independently the value of the critical load.

Closed-Form Solution for a Circular Nanohole with Surface Effects Under Uniform Heat Flux

Closed-Form Solution for a Circular Nanohole with Surface Effects Under Uniform Heat Flux

Enhanced Circular Dichroism by F-Type Chiral Metal Nanostructures

Circular dichroism (CD) effects have broad applications in fields including biophysical analysis, analytical chemistry, nanoscale imaging, and nanosensor design. Herein, a novel design of a tilted F-type chiral metal nanostructure composed of circular nanoholes with varying radii has been proposed to achieve remarkable CD effects, and the results demonstrate the generation of a significant current oscillation at the sharp edges where the nanoholes overlap under circularly polarized light, resulting in a strong CD effect. The CD effect can reach up to 7.5%. Furthermore, spectral modulation of the resonant wavelength can be achieved by adjusting the structural parameters, which enhances the tunability of the structure. Overall, these results provide theoretical or practical guidance for enhancing the circular dichroism signal strength of chiral metal nanostructures and designing new types of two-dimensional chiral structures.

Conversion of a Helical Surface Plasmon Polariton into a Spiral Surface Plasmon Polariton at the Outlet of a Metallic Nanohole.

The conversion of a helical surface plasmon polariton (SPP) creeping out of a circular nanohole in a thick metal (Ag or Au) film into a spiral (Hankel type) SPP outward propagating at the film’s interface is studied theoretically. The dispersion relations of SPPs of various modes in a nanohole, calculated from a transcendental equation, show that the propagation length of an SPP of mode 1 is much larger than the other modes in a specific frequency band, which is dependent on the nanohole size. In this band, the streamlines of the Poynting vector (energy flux) of mode-1 SPP in nanohole exhibit helixes; the surface component of the energy flux is perpendicular to the phase front of the SPP. Numerical results show that, after a helical SPP tunnels through a nanohole, most of the energy flux fans out at the outlet as a dipole radiation. The spatial phase distribution of Ez above the interface indicates that the transmission light carries orbital angular momentum with a topological charge of 1. Additionally, a part of the helical SPP creeping along the edge of an outlet naturally converts into a spiral (Hankel type of order 1) SPP outward propagating at the film’s interface; both SPPs have the same handedness. Moreover, the interferences of multi SPPs generating from two nanoholes and even from a two-dimensional nanohole array are also related to the spiral SPP.

Circular nanocavity substrate-assisted plasmonic tip for its enhancement in nanofocusing and optical trapping.

Plasmonic tip nanofocusing has widely been applied in tip-enhanced Raman spectroscopy, optical trapping, nonlinear optics, and super-resolution imaging due to its capability of high local field enhancement. In this work, a substrate with a circular nanocavity is proposed to enhance the nanofocusing and optical trapping characteristics of the plasmonic tip. Under axial illumination of a tightly focused radial polarized beam, the circular nanohole etched on a metallic substrate can form a nanocavity to induce an interference effect and further enhance the electric field intensity. When a plasmonic tip is placed closely above such a substrate, the electric field intensity of the gap-plasmon mode can further be improved, which is 10 folds stronger than that of the conventional gap-plasmon mode. Further analysis reveals that the enhanced gap-plasmon mode can significantly strengthen the optical force exerted on a nanoparticle and stably trap a 4-nm-diameter dielectric nanoparticle. Our proposed method can improve the performance of tip-enhanced spectroscopy, plasmonic tweezers and extend their applications. We anticipate that our methods allow simultaneously manipulating and characterizing single nanoparticles in-situ.

Inside Front Cover

AbstractMultispectral fluorescence imaging is performed under the laser illumination of multiple wavelengths simultaneously. A plasmonic metasurface is designed to perform colour filtering, composed of an arrangement of circular and elliptical nanoholes etched into an aluminium thin film. The resultant mosaic colour filter array is flip‐chip bonded onto a single‐photon avalanche diode (SPAD) array. High fidelity, full‐colour fluorescence images are reconstructed from as few as three photons per pixel from a variety of biological samples.Further details can be found in the article by P.W.R. Connolly, J. Valli, Y.D. Shah, Y. Altmann, J. Grant, C. Accarino, C. Rickman, D.R.S. Cumming, and G.S. Buller (e202000505) image

Simultaneous multi-spectral, single-photon fluorescence imaging using a plasmonic colour filter array.

We present the first realisation of simultaneous multi-spectral fluorescence imaging using a single-photon avalanche diode (SPAD) array, where the spectral unmixing is facilitated by a plasmonic metasurface mosaic colour filter array (CFA). A 64 × 64 pixel format silicon SPAD array is used to record widefield fluorescence and brightfield data from four biological samples. A plasmonic metasurface composed of an arrangement of circular and elliptical nanoholes etched into an aluminium thin film deposited on a glass substrate provides the high transmission efficiency CFA, enabling a bespoke spectral unmixing algorithm to reconstruct high fidelity, full colour images from as few as ∼3 photons per pixel. This approach points the way toward real-time, single-photon sensitive multi-spectral fluorescence imaging. Furthermore, this is possible without additional bulky components such as a filter wheel, prism or diffraction grating, nor the need for multiple sample exposures or multiple detectors.

Hierarchical Structures: Tailoring Surface Self‐Organization for Nanoscale Polygonal Morphology on Germanium (Adv. Mater. 21/2021)

Nanoscale polygons are fundamental building blocks toward hierarchical and 3D nanostructures for multifunctional nanodevices. In article number 2008668, Mohammed Aslam, Jing Fu, Rakesh G. Mote, and co-workers demonstrate a single-step process to manipulate the morphology of circular nanoholes via ion-beam irradiation. Through nanoscale control over the self-organization process dynamics on the surface of Ge, they realize highly periodic and protruding polygons.

Tailoring Surface Self-Organization for Nanoscale Polygonal Morphology on Germanium.

The evolution of polygonal-shaped nanoholes on the (100) surface of germanium, aided by focused ion beam induced self-organization, is presented. The energetic beam of ions creates a viscous phase which, at a thermodynamical minimum, leads to surface self-organization. A directed viscous-flow along the predefined nanoholes provides well-ordered polygonal nanostructures, ranging from triangles to hexagons and octagons, as desired. The amorphization exhibiting a confined viscous-flow at the walls of nanoholes is attributed to the localized melting zones induced by site-specific thermal spikes during ion irradiation, as revealed by microscopy and molecular dynamics studies. This leads to a local self-organization in the vicinity of each circular nanohole via a viscous-fingering process at the nanoscale. Such controlled self-organization, with the help of a predefined scanning grid, transforms the circular holes into the desired polygonal shape. The present morphology manipulation promises to surmount the barriers concerning the size reduction efforts in the field of nanofabrication.

Spectral Engineering via Complex Patterns of Circular Nano-object Miniarrays: I. Convex Patterns Tunable by Integrated Lithography Realized by Circularly Polarized Light

Illumination of colloid sphere monolayers by circularly polarized beams enables the fabrication of concave patterns composed of circular nanohole miniarrays that can be transferred into convex metal nano-object patterns via a lift-off procedure. Unique spectral and near-field properties are achievable by controlling the geometry of the central nanoring and quadrumer of slightly rotated satellite nanocrescents and by selecting those azimuthal orientations that promote localized plasmon resonances. The spectral and near-field effects of hexagonal patterns composed of uniform gold nanorings and nanocrescents, which can be prepared by transferring masks fabricated by a perpendicularly and obliquely incident single homogeneous circularly polarized beam, were studied to uncover the supported localized plasmonic modes. Artificial rectangular patterns composed of a singlet nanoring and singlet nanocrescent as well as quadrumer of four nanocrescents were investigated to analyze the effect of nano-object interactions and lattice type. It was proven that all nanophotonical phenomena are governed by the azimuthal orientation independent localized resonance on the nanorings and by the C2, C1, and U resonances on the nanocrescents in case of bar {E}-field direction perpendicular and parallel to their symmetry axes. The interaction between localized surface plasmon resonances on individual nano-objects is weak, whereas scattered photonic modes have a perturbative role at the Rayleigh anomaly only on the larger periodic rectangular pattern of miniarrays. Considerable fluorescence enhancement of dipolar emitters is achievable at spectral locations promoting the C and U resonances on the constituent nano-object.

Spectral Engineering Via Complex Patterns of Circular Nano-Object Miniarrays: II. Concave Patterns Tunable by Integrated Lithography Realized by Circularly Polarized Light

The use of circularly polarized beams in interferometric illumination of colloid sphere monolayers enables the direct fabrication of rectangular patterns composed of circular nanohole miniarrays in metal films. This paper presents a study on the spectral and near-field effects of complex rectangular patterns consisted of a central nanoring and slightly rotated satellite nanocrescents in azimuthal orientations, which promote coupling between localized and propagating plasmons. To inspect the localized modes separately, we investigate the spectral responses and near-field phenomena of hexagonal patterns composed of uniform nanorings and nanocrescents, which can be fabricated by a single, homogeneous, circularly polarized beam incident perpendicularly and obliquely, respectively. To understand the interaction of localized and propagating modes, we analyze artificial rectangular patterns composed of a singlet nanoring, a singlet horizontal nanocrescent, and a quadrumer of four slightly rotated nanocrescents. The results demonstrate that on the rectangular pattern of a singlet horizontal nanocrescent the interacting C2 and C1 localized resonances in the C orientation (0^{circ } azimuthal angle) and the U localized resonance coupled with propagating surface plasmon polaritons (SPPs) in the U orientation (90^{circ } azimuthal angle) manifest themselves in similar split spectra. Moreover, split spectra appear due to the coupling of the azimuthal orientation independent localized resonance on the nanorings and the SPPs propagating on their rectangular pattern in the U orientation. The spectral response of the complex miniarray pattern can be precisely tuned by varying the geometrical parameters of the moderately interacting nanoholes and the pattern period. In appropriate configurations, the fluorescence of the dipolar emitters is enhanced, which has potential applications in bio-object detection.

Regulation of failure mechanism of a bilayer Gr/h-BN staggered stacked heterostructure via interlayer sp3 bonds, interface connection, and defects

The combination of vertical and in-plane heterostructures will create unprecedented structures that may produce novel physical properties. In this study, the failure mechanism of a bilayer Gr/h-BN staggered stacked heterostructure (BGBN-SS) with different interlayer sp3 bonds, different interface connection, and various defects has been investigated. The results show that interlayer sp3 bonds and various defect affect the failure mechanism of BGBN-SS in two contrary ways. The sp3 bonds raise the primary strain of the BGBN-SS-containing various defects and different interface connection, and can weaken tensile stresses and strain and Young’s modulus. However, the creation of interlayer bonding leads the bilayer heterostructure gradually changed to “quasi three-dimensional” structure. The stronger interlayer interaction induced by sp2–sp3 bonds in “quasi three-dimensional” structure can strengthen the interlayer shell modulus and load transfer rate. In addition, the mechanical properties of interface C–N bonding are greater than that of interface C–B bonding, indicating that C–N bonding at interface could improve the stability and ductility of the composite effectively. The square nanoholes are more likely to accumulate the local stress of the system, compared with circular nanoholes. The changing of sp2 hybridization of interlayer bonds transforms to a weak hybrid sp3 bonds. As a result, the special defects (interlayer bonding) introduce a new stress transfer mode (different from vdW heterostructures and in-plane hybrid nanostructures.

Spin-resolved near-field scanning optical microscopy for mapping of the spin angular momentum distribution of focused beams

We proposed and built a near-field scanning optical microscope (NSOM) to enable the characterization of the spin angular momentum (SAM) distribution of electromagnetic fields with nanoscale resolution. The NSOM probe was composed of a circular nanohole formed in a thick gold film that was deposited on a tapered cone fiber. The near-field signal, when coupled through the nanohole to the fiber, was split and analyzed using a combination of a quarter-wave plate and a polarizer to extract the two circular polarization components of the signal. This allowed us to characterize the out-of-plane SAM component, which was determined using the relationship Sz ∝ IRCP − ILCP. Using the developed system, we mapped the SAM distributions of a variety of tightly focused cylindrical vector vortex beams and thus validated the system's effectiveness. The proposed spin-resolved NSOM could be a valuable tool for studies of both near-field spin optics and topological photonics.

Ultralow-light-level color image reconstruction using high-efficiency plasmonic metasurface mosaic filters

As single-photon imaging becomes progressively more commonplace in sensing applications such as low-light-level imaging, three-dimensional profiling, and fluorescence imaging, there exist a number of fields where multispectral information can also be exploited, e.g., in environmental monitoring and target identification. We have fabricated a high-transmittance mosaic filter array, where each optical filter was composed of a plasmonic metasurface fabricated in a single lithographic step. This plasmonic metasurface design utilized an array of elliptical and circular nanoholes, which produced enhanced optical coupling between multiple plasmonic interactions. The resulting metasurfaces produced narrow bandpass filters for blue, green, and red light with peak transmission efficiencies of 79%, 75%, and 68%, respectively. After the three metasurface filter designs were arranged in a 64 × 64 format random mosaic pattern, this mosaic filter was directly integrated onto a CMOS single-photon avalanche diode detector array. Color images were then reconstructed at light levels as low as approximately 5 photons per pixel, on average, via the simultaneous acquisition of low-photon multispectral data using both three-color active laser illumination and a broadband white-light illumination source.

Fabrication of Si Micropore and Graphene Nanohole Structures by Focused Ion Beam.

A biosensor formed by a combination of silicon (Si) micropore and graphene nanohole technology is expected to act as a promising device structure to interrogate single molecule biopolymers, such as deoxyribonucleic acid (DNA). This paper reports a novel technique of using a focused ion beam (FIB) as a tool for direct fabrication of both conical-shaped micropore in Si3N4/Si and a nanohole in graphene to act as a fluidic channel and sensing membrane, respectively. The thinning of thick Si substrate down to 50 µm has been performed prior to a multi-step milling of the conical-shaped micropore with final pore size of 3 µm. A transfer of graphene onto the fabricated conical-shaped micropore with little or no defect was successfully achieved using a newly developed all-dry transfer method. A circular shape graphene nanohole with diameter of about 30 nm was successfully obtained at beam exposure time of 0.1 s. This study opens a breakthrough in fabricating an integrated graphene nanohole and conical-shaped Si micropore structure for biosensor applications.

Chiral effects in low-cost plasmonic arrays of elliptic nanoholes

In this work we present an approach to induce chiral effects in well-known plasmonic nanohole arrays with triangular unit cell. Arrays with circular nanoholes in metals can exhibit interesting light-matter interaction mechanisms such as surface plasmon polaritons and extraordinary optical transmission. Moving from circular to elliptical shape and tilting the ellipse away from the lattice symmetry lines, a symmetry breaking induces a different absorption of the circularly polarized light of opposite handedness, i.e. circular dichroism. We numerically investigate circular dichroism at normal incidence in elliptic nanoholes in Au in the spectral range 400–1000 nm. We focus on its dependence on the elliptic nanohole tilt, and further proceed with the ellipse radii optimization. The optimized circular dichroism reaches 84%, and we report on the near field absorption distribution at the wavelengths of this maximum value.

Thermally neutral nano-hole and its effect on the elastic field

We use complex variable methods to rigorously design a circular nano-hole which, when introduced into a host material, does not disturb the original thermal field present in the uncut body. We refer to such a hole as ‘thermally neutral’ or ‘neutral with respect to thermal conductivity’. Our design is based on the idea that the boundary of the hole is considered to have a temperature jump attributed to surface phonon scattering. By adjusting the size of the hole, the surface phonon scattering counteracts the effect of the hole in such a way that the heat flux in the surrounding matrix remains undisturbed. Results obtained through rigorous investigation show that a uniform heat flux generates a harmonic elastic field around the neutral circular nano-hole and that the magnitude of the thermal stress is much lower than the corresponding yield stresses of most materials even in the presence of a higher temperature gradient.

Morphological characteristics of nanoholes induced by single-shot femtosecond laser ablation of borates and aluminate silicates

Single-shot femtosecond laser ablation experiments with linearly and circularly polarized light were conducted in order to investigate the morphological characteristics of surface nanostructures in lithium borate crystals and glasses, a strontium borate crystal, lanthanide containing borate crystals, and aluminate silicate crystals: Li2B4O7 (LTB) and LiB3O5 (LBO) crystals and Li2O⋅2B2O3 (LTB) and Li2O⋅3B2O3 (LBO) glasses; SrB4O7 (SBO) crystal; Li6Gd(BO3)3 (LGB) and LaSc3(BO3)4 (LSB) crystals; and Ca2Al2SiO7 (CAS) and CaSrAl2SiO7 (CSAS) crystals. In the present study, the material and laser polarization dependance of the morphology of nanoholes was examined in these crystals and glasses. A single nanohole or two holes (a primary hole and a secondary hole) were observed in the borate and aluminate silicate crystals. The size of the nanohole is not restricted by the diffraction limit but instead is dependent on the laser fluence and the materials. It is suggested that the formation of these secondary nanoholes in the studied crystals is attributed to a spontaneous reshaping of the incoming Gaussian pulse into a Gaussian–Bessel pulse. In the LTB and LBO crystals, nanoholes (both primary and secondary holes) with subwavelength sides exhibit a quadrilateral (approximately square or rectangular) morphology, regardless of linear or circular polarization. The sides of the quadrilateral nanoholes lie approximately in the {h h 0} planes on the LTB crystal and in the ({h 0 0} and {0 0 l}) planes on the LBO crystal. We found that the nanohole morphology did not reflect the spatial distribution of the laser intensity. These phenomena were the first observations on the anisotropic morphology of nanoholes. These morphologies do not correspond to the circular symmetric pattern of the Gaussian intensity distribution of the incoming laser beam. This is contrary to the expectations based on the generally accepted laser ablation mechanism. The quadrilateral nanoholes could be an inherent morphology in the LTB and LBO crystals. The morphology of the quadrilateral holes in the LTB and LBO crystals is considered to reflect the continuous BO33− and/or BO45− network structure in their respective tetragonal or orthorhombic unit cells, in which self-tapped excitons are formed in an initial process under multiphoton excitation. In contrast, the SBO, LGB, LSB, CAS, and CSAS crystals and the LTB and LBO glasses exhibit circular nanoholes with subwavelength diameters independent of the laser polarization, the structure, or the composition. The isotropic morphology of nanoholes in these samples reflects the circular pattern of the Gaussian intensity profile of the focused laser beam.

Third-Harmonic Generation From Double Nanohole Aperture in Gold Film

We investigate the third-harmonic generation response from a single aperture structure. Two models of circular and triangular double nanohole aperture are compared by using the finite difference time-domain simulations, and we find the double circular hole has a stronger third-harmonic generation response. The influence of gap sizes of the aperture on third-harmonic generation response has also been analyzed. The results show that the aperture with the narrower gap could provide a higher third-harmonic generation signal peak. Further experimental measurements agree with the simulation results.

Effect of defects on mechanical properties of novel hybrid graphene-h-BN/copper layered nanostructures

The geometric imperfections (circular and square nanohole) were created in different regions of graphene/h-BN heterostructure. Defective heterostructures were used as a reinforcing material for copper (defective G/h-BN@Cu). The mechanical properties of defective G/h-BN@Cu were studied, and the effects of defect size and position on mechanical properties were studied by molecular dynamics. The results showed that mechanical properties of the defective G/h-BN@Cu nanostructure decrease with the increase of the diameter of the nanohole. The effects of nanohole on the stress and strain of the G/h-BN@Cu nanostructures are much higher than Young's modulus. The three mechanical values of G/h-BN@Cu nanostructures in the presence of nanoholes change considerably and do not follow the general trends. When the nanohole is created at h-BN region, a greater reduction of mechanical properties is at this location, relative to other positions. The adverse effect of circular defects on mechanical properties of G/h-BN@Cu nanostructures is lower than that of square defects.