Elliptical Nanohole Research Articles

Circular dichroism in a plasmonic array of elliptical nanoholes with square lattice.

Chiral properties of plasmonic metasurfaces, especially related to different absorption of left and right circularly polarized light leading to circular dichroism (CD), are a research hot topic in nanophotonics. There is often a need to understand the physical origin of CD for different chiral metasurfaces, and to get guidelines for the design of structures with optimized and robust CD. In this work, we numerically study CD at normal incidence in square arrays of elliptic nanoholes etched in thin metallic layers (Ag, Au, Al) on a glass substrate and tilted with respect to the symmetry axes. Strong CD arises in absorption spectra at the same wavelength region of extraordinary optical transmission, indicating highly resonant coupling between light and surface plasmon polaritons at the metal/glass and metal/air interfaces. We elucidate the physical origin of absorption CD by a careful comparison of optical spectra for different polarizations (linear and circular), with the aid of static and dynamic simulations of local enhancement of the electric field. Furthermore, we optimize the CD as a function of the ellipse parameters (diameters and tilt), the thickness of the metallic layer, and the lattice constant. We find that silver and gold metasurfaces are most useful for CD resonances above 600 nm, while aluminum metasurfaces are convenient for achieving strong CD resonances in the short-wavelength range of the visible regime and in the near UV. The results give a full picture of chiral optical effects at normal incidence in this simple nanohole array, and suggest interesting applications for chiral biomolecules sensing in such plasmonic geometries.

Beaming effect of the plasmonic metalens structured with concentric elliptical nanohole arrays

The beaming effect of plasmonic metalens, comprising two concentric elliptical nanohole arrays in a silver film, was explored using experimental methods and numerical simulations. Owing to the birefringent effect of the elliptical nanohole, the rotation of the elliptical major axis and the thickness of the silver film influence the polarization state and magnitude of the light transmitted from the elliptical nanohole. The polarization state of the metalens affected the corresponding diffraction pattern. The birefringent effect of the metallic elliptical nanoholes is related to the ohmic loss of the metal. The ohmic loss influences the beaming effect of plasmonic metals. In this study, we use linearly polarized light in the near-ultraviolet and visible regimes to evaluate the beaming effect through the influence of ohmic loss. Two orthogonal polarizations were generated on the metalens, wherein the major axis of the elliptical nanohole in the inner concentric array was perpendicular to the major axis of the elliptical nanohole in the outer concentric array. The metalens generate transmitted light with a strong focusing ability by generating two orthogonal polarizations. An optical beam with a small focusing spot and long working distance can be obtained from these plasmonic metalens.

Experiments and simulations of chiro-optical response in lowcost nanohole arrays in silver

2D metasurfaces based on periodic nanoholes in metal have been proposed in various plasmonic platforms. Specifically, their resonant features have led to applications spanning in biosensing. Here we investigate additional degree of freedom in elliptical nanohole arrays with hexagonal geometry: chiro-optical effects. Namely, the in-plane asymmetry and a slightly elliptical shape of nanoholes were previously shown to differently extinct light of opposite handedness, even at normal incidence. We now fully characterize nanoholes in Ag, fabricated by low-cost nanosphere lithography. We first measure the dependence of the transmitted intensity for opposite handedness, in a broad spectral and angle of incidence range. We then resolve the circular polarization degree of the transmitted light when the nanohole array is excited with linear polarization. Finally, we numerically investigate the origin of the chiro-optical effect at the nanoscale. We believe that circular polarization resolving of the transmitted degree could be further adapted as a highly sensitive tool in chiral sensing.

Broadband near infrared all-dielectric metasurface absorber

Here we present a broadband all-dielectric metasurface absorber, where elliptical nanohole arrays are entirely embedded into the doped device layer of silicon-on-insulator wafer. From the intraband carrier transitions, the device exhibits enhanced near infrared absorption with a 3-dB bandwidth ∼ 35.3 nm. By the method of detailed multipole decomposition, we find that magnetic dipole resonances predominantly contribute to the absorption enhancement; in addition, the large bandwidth is attributed to the superposition of multiple absorption peaks from the resonances. The absorber designed in this simple structure and compatible with the CMOS techniques is promising for the broadband photodetectors of Si and even Ge. Differently, for the Ge-based photodetectors, the photocarrier excitation originates from interband transitions.

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

Plasmonic Elliptical Nanohole Arrays for Chiral Absorption and Emission in the Near-Infrared and Visible Range

Chiral plasmonic nanostructures with tunable handedness-dependent absorption in the visible and infrared offer chiro-optical control at the nanoscale. Moreover, coupling them with emitting layers could lead to chiral nanosources, important for nanophotonic circuits. Here, we propose plasmonic elliptical nanohole arrays (ENHA) for circularly dependent near-infrared and visible emission. We first investigate broadband chiral behavior in an Au-ENHA embedded in glass by exciting it with plane waves. We then study the coupling of ENHA with a thin emitting layer embedded in glass; we focus on the emission wavelengths which provided high chirality in plane-wave simulations. Our novel simulation set-up monitors the chirality of the far-field emission by properly averaging a large set of homogeneously distributed, randomly oriented quantum sources. The intrinsic chirality of ENHA influences the circular polarization degree of the emitting layer. Finally, we study the emission dependence on the field distribution at the excitation wavelength. We demonstrate the chiral absorption and emission properties for Au-ENHA emitting in the near-infrared range, and for Ag-ENHA which is excited in green range and emits in the Lumogen Red range. The simple geometry of ENHA can be fabricated with low-cost nanosphere lithography and be covered with emission gel. We thus believe that this design can be of great importance for tunable chiral nanosources.

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.

Fabrication of metal nanorod arrays using anodic porous alumina mask with elliptical apertures prepared by lattice conversion process

Highly ordered Au nanorod arrays were fabricated using anodic porous alumina with elliptical nanoholes as an evaporation mask and applied as a substrate for measurements of surface-enhanced Raman scattering (SERS) spectra. Shape of nanoholes in anodic porous alumina obtained by self-ordering anodization process is usually circular. In the present study, elliptical nanoholes ordered in rectangular lattice were successfully fabricated by lattice conversion process using naturally ordered anodic porous alumina. The Au nanorod arrays were fabricated by thermal evaporation using the obtained anodic porous alumina. The SERS activity of the Au nanorods were higher than that of a circular Au nanodots.

Plasmonic elliptical nanoholes for chiroptical analysis and enantioselective optical trapping.

A simple yet effective achiral platform using elliptical nanoholes for chiroptical analysis is demonstrated. Under linearly polarized excitation, an elliptical nanohole in a thin gold film can generate a localized chiral optical field for chiroptical analysis and simultaneously serve as a near-field optical trap to capture dielectric and plasmonic nanospheres. In particular, the trapping potential is enantioselective for dielectric nanospheres, i.e., the hole traps or repels the dielectric nanoparticles depending on the sample chirality. For plasmonic nanospheres, the trapping potential well is much deeper than that for dielectric particles, rendering the enantioselectivity less pronounced. This platform is suitable for chiral analysis with nanoparticle-based solid-state extraction and pre-concentration. Compared to plasmonic chiroptical sensing using chiral structures or circularly polarized light, elliptical nanoholes are a simple and effective platform, which is expected to have a relatively low background because chiroptical noise from the structure or chiral species outside the nanohole is minimized. The use of linearly polarized excitation also makes the platform easily compatible with a commercial optical microscope.

Anisotropic silicon nanowire arrays fabricated by colloidal lithography.

The combination of metal-assisted chemical etching (MACE) and colloidal lithography allows for the affordable, large-scale and high-throughput synthesis of silicon nanowire (SiNW) arrays. However, many geometric parameters of these arrays are coupled and cannot be addressed individually using colloidal lithography. Despite recent advancements towards higher flexibility, SiNWs fabricated via colloidal lithography and MACE usually have circular, isotropic cross-sections inherited from the spherical templates. Here we report a facile technique to synthesize anisotropic SiNWs with tunable cross-sections via colloidal lithography and MACE. Metal films with an elliptical nanohole array can form from shadows of colloidal particles during thermal evaporation of the metal at tilted angles. The aspect ratio of these anisotropic holes can be conveniently controlled via the deposition angle. Consecutive MACE using these patterned substrates with or without prior removal of the templating spheres results in arrays of anisotropic SiNWs with either elliptical or crescent-shaped cross-sections, respectively. As a consequence of the anisotropy, long SiNWs with elliptical cross-sections preferentially collapse along their short axis, leading to a controlled bundling process and the creation of anisotropic surface topographies. These results demonstrate that a rich library of SiNW shapes and mesostructures can be prepared using simple spherical colloidal particles as masks.

Phenomenological study of gold elliptical nanohole array as a plasmonic rotation sensor

Plasmonic engineering has opened up new opportunities in the field of subwavelength photonics for solving real world problems. The analysis of rotation is a key concept across aerospace, medical and industrial fields. Here we present a novel multiwavelength optical rotation sensor that makes use of the rich plasmonic properties of plasmonic nanohole arrays. A quantitative evaluation is provided on the extraordinary optical transmission as well as quality factor of the key wavelengths as a function of rotation. In order to investigate the origin of the peak wavelengths, we have studied the electrical field intensity of the elliptical nanohole array. Three surface plasmon resonance (SPR) peaks corresponding to wavelengths of 631.5 nm, 752 nm and 1090.5 nm are identified which are independent of rotation angle. The third peak at 1090.5 nm has the highest transmission angular dependency whereas the first peak at 631.5 nm has the greatest Q-factor. It is shown that the shape of the electric field profile can be deduced from the transmission. The present results show that elliptical nanohole arrays are suitable as a method for electromagnetic interference (EMI) resistant remote rotation sensing as well as for use on photonic integrated circuits as part of post-Moore optoelectronic devices. Furthermore, the results presented here have a large implication on photolithography as well as biosening applications, in particular ELISA.

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.

High-throughput fabrication of polymethylmethacrylate nano-hole arrays for structural coloration using proton beam writing made diamond stamp

We demonstrate the fabrication of a diamond stamp through proton beam writing (PBW) coupled with reactive ion etching (RIE), and the process of corresponding thermal nano-imprinting to free-standing polymethylmethacrylate (PMMA) film. Strong structural coloration from the imprinted nano-hole based PMMA patterns was observed. Additionally, we see that 8 nm Cr coating can enhance the structural coloration intensity of PMMA nano-hole. Furthermore, a series of Maxwell solver based finite-difference time-domain (FDTD) simulations were performed to find out theoretical explanations to what we observed in the experiments. The simulation results are in a good agreement with the experimental observations in terms of structural color, color intensity contrast, and Cr coating enhancement. The dependence between the scattering cross section of elliptic nano-hole and polarization angle is discussed as well. This study provides an insight into rapid and high-throughput fabrication of polymer based structural coloration, thus paving the way to bring structural color printings into commercialization driven by large-scale industrial production.

Rayleigh-Wood anomaly approximation with FDTD simulation of plasmonic gold nanohole array for determination of optimum extraordinary optical transmission characteristics

Abstract Light confinement in small volumes remains one of the old and important challenges in the realm of optics. One of the widely studied phenomena of light confinement is extraordinary optical transmission (EOT) through holes with a radius much smaller than the wavelength of light. However little work has been done to provide a phenomenological study detailing underpinnings of the numerical evaluation. To this end, we investigate the Rayleigh-Wood anomalies arising from a plasmonic gold thin film nanohole array on glass through theoretical calculations and FDTD simulations. Theoretical investigation of Rayleigh-Wood anomaly provides an approximation of optical transmission features independent of hole geometrical shape. A discussion is presented on the relation between EOT and Q -factor at various orders. A rational is provided as to why elliptical (i.e round and anisotropic) nanohole array leads to optimum transmission characteristics. It is expected that our results will have an important role in the characterization and optimization of EOT based nanophotonic devices such as plasmonic biosensors, photovoltaics, IR detectors and rotation sensors.

Near-field imaging of surface-plasmon vortex-modes around a single elliptical nanohole in a gold film

We present scanning near-field images of surface plasmon modes around a single elliptical nanohole in 88 nm thick Au film. We find that rotating surface plasmon vortex modes carrying extrinsic orbital angular momentum can be induced under linearly polarized illumination. The vortex modes are obtained only when the incident polarization direction differs from one of the ellipse axes. Such a direct observation of the vortex modes is possible thanks to the ability of the SNOM technique to obtain information on both the amplitude and the phase of the near-field. The presence of the vortex mode is determined by the rotational symmetry breaking of the system. Finite element method calculations show that such a vorticity originates from the presence of nodal points where the phase of the field is undefined, leading to a circulation of the energy flow. The configuration producing vortex modes corresponds to a nonzero total topological charge (+1).

Broadband light absorption enhancement in randomly rotated elliptical nanohole arrays for photovoltaic application.

Semiconducting nanohole arrays have been considered as a promising candidate for high-efficiency solar cells. In this paper, the optical absorption property of randomly rotated elliptical nanohole arrays consisting of 1×1, 2×2, and 4×4 cells has been investigated. It is found that the average ultimate efficiency of the arrays increases with the increase of the size of the supercell. The 4×4 array has the highest ultimate efficiency, and the efficiency of the 4×4 array is less sensitive to the parameters of the random rotation angle than that of the 1×1 and 2×2 arrays. The comparison of absorption spectra of the three arrays shows that the number of absorption peaks of the 1×1 array is the least, but the peak is the highest and narrowest, and that of the 4×4 array is the most, but the peak is the lowest and broadest. The spatial Fourier spectroscopy analysis of the array shows that the 4×4 arrays have the most Fourier spectrum components and the largest absorption mode density. The enhancement of optical absorption of the 4×4 array is attributed to the folding of energy bands, the increase of the energy density of states caused by large supercells, and the overcoupling between incident light and Bloch resonant modes in the structure. The improved stability of the ultimate efficiency of the 4×4 arrays is because the distribution of the energy density of states of the 4×4 arrays with different random parameters tends to be the same, which leads to almost the same absorption efficiency of the different 4×4 arrays.

Surface Effects on the Scatter of SH-Wave by a Shallow Buried the Elliptical Hole

The methods of complex function, multi-polar coordinate system, and conformal mapping are used to solve dynamic stress concentration factor. The surface elasticity theory is applied to obtain the stress boundary conditions on the surface. The effects of frequency and the ration of the major and minor axis of the ellipse on the dynamic stress concentration factor around the elliptical nano-hole are discussed in detail. When the size of elliptical hole shrinks to nanometers, the numerical results show that the surface effect has a significant effect on the scattering of SH-wave.

Nanoimprint Lithography–Based Fabrication of Plasmonic Array of Elliptical Nanoholes for Dual-Wavelength, Dual-Polarisation Refractive Index Sensing

We report on the novel fabrication and characterisation of plasmonic arrays of elliptical nanohole, and their use for refractive index based sensing. The substrates were fabricated using nanoimprint lithography into a chromium hard mask followed by transfer of the patterns into the underlying gold layer by dry etching—a combination of processes amendable to mass manufacturing. 3D-FDTD simulations were undertaken and showed the transmission spectrum was dependant upon the polarisation of the incident light, with a series of minima that can be attributed to plasmonic effects on the gold/water or gold/substrate interfaces. Each polarisation showed two peaks on the gold/water interface, one in the visible and one in the near-infrared part of the spectrum. Simulated electric field profiles showed that the electric field in the infrared propagates deep in the bulk while the one in the visible was more tightly bound to the surface. Experimental transmission spectra of the fabricated samples showed good agreement with the simulated ones. Bulk refractive index experiments were carried out and sensitivities of 293 nm/RIU and 414 nm/RIU were obtained for the two spectral features of interest when the polarisation was along the long axis of the elliptical nanohole for the visible and infrared features, respectively, and 293 nm/RIU and 323 nm/RIU measured when the polarisation was along the short axis of the nanohole.

Anodic porous alumina with elliptical apertures

The fabrication of anodic porous alumina with anisotropic structures, i.e., an anisotropic hole arrangement and anisotropic elliptical hole shape, was demonstrated. Through the optimization of the pattern used for the pretexturing and the anodizing conditions, anodic porous alumina with holes having an elliptical cross section could be satisfactory obtained. The diameter of the elliptical nanoholes could be controlled by an additional wet-etching process. The obtained anodic porous alumina with elliptical nanoholes is expected to be applied to various functional optical devices, such as localized plasmonic devices, requiring control of the anisotropy of the incident light.

Surface effects on an electrically permeable elliptical nano-hole or nano-crack in piezoelectric materials under anti-plane shear

An electrically permeable elliptical nano-hole or nano-crack embedded in an infinite piezoelectric material with surface effect is investigated based on the Gurtin–Murdoch surface/interface model, which is subjected to far-field anti-plane mechanical and in-plane electrical loads. The electric field inside the elliptical nano-hole has been taken into consideration, and exact electroelastic fields near the elliptical nano-hole are obtained by using the technique of conformal mapping. The size-dependent stress and electric displacement intensity factors at the crack tip are derived exactly for both electrically permeable and impermeable boundary conditions when the elliptical nano-hole reduces to the nano-crack. Numerical examples are illustrated to show the surface effects on the stress and electric displacement intensity factors of electrically permeable and impermeable cracks, and on the stress and electric field concentrations around the electrically permeable and impermeable holes. The results indicate that the size dependence of an electrically permeable nano-hole or nano-crack is entirely different from that of the electrically impermeable case.