Elliptical Nanohole Research Articles

Numerical study of tunable enhanced chirality in multilayer stack achiral phase-change metamaterials.

We numerically demonstrate a multiband circular dichroism (CD) by tilting achiral metamaterials (MMs) composed of an elliptical nanoholes array (ENA) penetrating through metal/ phase-change material (PCM) /metal multilayer stack, with respect to the incident light. The CD spectrum can be actively tuned across a wide range from the near-infrared (NIR) to mid-infrared (MIR) regime by transiting the state of the PCM (Ge2Sb2Te5) from amorphous to crystalline. Thus, it can switch on/off a multiband chiroptical response in the infrared region. Our simulation also elucidates that the achiral multilayer stack MMs, which have strong magnetic resonances, can enhance the optical chirality inside the elliptical apertures for both amorphous and crystalline states. The switching of the enhanced chirality may pave the way to manipulate electromagnetic waves, such as tunable circular polarizers, chiroptical spectroscopy, and chiral biosensors.

Plasmonic sensor with high figure of merit based on differential polarization spectra of elliptical nanohole array.

Using the difference of the polarization transmission spectra of elliptical nanohole arrays (ENAs), the figure of merit (FOM) of the sensor performance of ENA can be significantly improved, and is inversely proportional to the measurement resolution. By optimizing the aspect ratio of the elliptical holes, Ag thickness, substrate-effect, and adhesive layer, the sensitivity, FOM, and relative sensitivity of the ENA can be improved to be 775 nm RIU-1, 705 RIU-1, and 70.23%, respectively, with an excellent linear dependence on the change of refractive index. Such a high-performance sensor also can be used in monitoring the molecule adsorption and RNA hybridization, revealing a highly localized near-field enhancement. This will benefit the sensing of surface-specific binding events in biologic detection and medical diagnosis.

Generation and detection of orbital angular momentum via metasurface.

Beams carrying orbital angular momentum possess a significant potential for modern optical technologies ranging from classical and quantum communication to optical manipulation. In this paper, we theoretically design and experimentally demonstrate an ultracompact array of elliptical nanoholes, which could convert the circularly polarized light into the cross-polarized vortex beam. To measure the topological charges of orbital angular momentum in a simple manner, another elliptical nanoholes array is designed to generate reference beam as a reference light. This approach may provide a new way for the generation and detection of orbital angular momentum in a compact device.

Efficient broadband light absorption in elliptical nanohole arrays for photovoltaic application.

We propose a perpendicular elliptical silicon nanohole (PE-SiNH) array for light absorption in thin film silicon solar cells. Our analysis shows that this architecture is capable of increasing the absorption of a thin film silicon solar cell by 11.3% in comparison to that of the optimal circular SiNH array. The process of breaking the mirror symmetries is responsible for the increase of the coupled modes. The PE-SiNH structures show additional near-zero spatial Fourier components compared with the circular SiNH structure, which helps to couple more incident light into slow Bloch modes. The mode interaction between adjacent elliptical nanoholes is in favor of the coupling of the incident light into channeling modes and, therefore, enhances light absorption in the short wavelength region.

Tailoring of spectral response and spatial field distribution with corrugated photonic crystal slab.

We report a new physical mechanism for simultaneous tuning of quality factors, spectral responses, and field distributions in photonic crystal slabs through removal of polarization mode degeneracy using a lattice of elliptical nano-holes. The quality factors in these structures can become higher than those obtained with much smaller circular nano-holes. Furthermore, the modes can be superimposed by either rotating or morphing the elliptical nano-holes into a corrugated grating. These findings will enable improved radiation-matter interaction in optical, microwave, and THZ frequencies along with enhanced opto-acoustic coupling.

Enhanced sensitivity of gold elliptic nanohole array biosensor with the surface plasmon polaritons coupling

Surface plasmon resonance (SPR) spectroscopy of the metal nanostructure (MN) is widely used in chemical and biological sensing. The biosensor characteristics of the MN are affected by the size, shapes, substrate, and so on. Especially, some studies about the mode of the MN show that the surface plasmon polaritons (SPPs) coupled mode excitation has a significant impact on the sensitivity of the MN biosensor. To reach the aim of obtaining the high sensitivity MN biosensor, the coupled mode of the gold elliptic nanohole array (GENA) was simulated and analyzed. It shows that the coupled mode was influenced by the refractive indexes of medium and substrate. The coupled mode resonance peak shows a higher sensitivity than that of other SPPs mode peaks. The GENA biosensor chips were fabricated and integrated with polydimethylsiloxane (PDMS) microfluidics. The sensitivity of the GENA is characterized by transmission spectra of GENA in deionized water and aqueous NaCl solution. The coupled mode peak sensitivity of the biosensor reaches 549 nm/RIU. An antigen-antibody interaction experiment was also adopted to verify the sensitivity of the surface binding reaction. The sensitivity of detected concentration of the AFP (α-fetoprotein) in our experiment has been reached 25 ng/ml closed to the clinic concentration. The GENA biosensor chip has potential application in label-free chemical and biomedical fields, especially, cancer biomarker testing.

Ultrafast tunable chirped phase-change metamaterial with a low power.

We numerically demonstrate an all-optical tunable dual-band double negative (DNG) index chirped metamaterial (MM) in the mid-infrared (M-IR) region. This MM possesses an ultrafast and significant tunability under low pump light power, realized by combining phase change material (PCM). It has a configuration of elliptical nanohole array (ENA) penetrating through metal/PCM/metal (Au-Ge(2)Sb(2)Te(5)-Au) films. Here, we consider the case when the chirp is introduced by displacing the positions of the ENA along the short axis of the elliptical apertures inside the primitive cell, which can achieve multiple internal surface-plasmon polariton (SPP) modes at the inner metal-dielectric interfaces of the structure and thus providing a dual-band negative index with simultaneous negative permittivity and permeability. The influence of amorphous and crystalline states of Ge(2)Sb(2)Te(5) on the effective optical parameters of the structure is analyzed. Switching between these states provides a large wavelength shift of the structure's effective optical parameters. A photothermal model is used to study the temporal variation of the temperature of the Ge(2)Sb(2)Te(5) layer to show a potential to switch the phase of Ge(2)Sb(2)Te(5) by optical heating. Generation of the tunable dual-band DNG index presents clear advantages as it possesses a fast tuning time of 0.4 ns, a low pump light intensity of 7.3μW/μm(2), and a large tunable wavelength range of 978 nm. We expect that our design may have potential applications in actively tunable multi-band nanodevices.

Low-loss dual-band double-negative chirped metamaterial

We present numerical modelling of a dual-band double-negative chirped metamaterial (MM) in the optical region. The MM has an elliptical nanohole array (ENA) penetrating through a metal/dielectric material/metal (Au-Al2O3-Au) film. We consider the case when the chirp is introduced by displacing the positions of the ENA along the short axis of the elliptical apertures inside the primitive cell. We demonstrate that these chirped MMs can provide a dual-band negative refractive index (NRI) with simultaneous negative permittivity and permeability in both near infrared (N-IR) region and mid-infrared (M-IR) region, respectively. As the neighboring elliptical holes are moved closer to each other the NRI in the N-IR region blue shifts to the shorter wavelength with a decreasing magnetic resonance, whereas the NRI in the M-IR region red shifts to the longer wavelength with an increasing magnetic resonance. This structure possesses a high value of figure-of-merit owing to the large size of the elliptical apertures.

Fast tuning of Fano resonance in metal/phase-change materials/metal metamaterials

We propose fast tuning of a Fano resonance (FR) in a three dimensional metamaterial (MM). The MM consists of an elliptical nanohole array (ENA) embedded through a metal/ phase-change material (PCM)/metal multilayer. The results show that the interference between the electric and magnetic resonances can be significantly enhanced when the elliptical nanoholes occupy the sites of a rectangular lattice, thus providing a FR with a higher value quality factor (Q) compared to the ENA with a square lattice. By switching the PCM (Ge2Sb2Te5) between its amorphous and crystalline states, the FR peak can be red-shifted by up to 42%. The FR can be tuned with a ultra low energy mid-infrared laser pulse of 0.38 ns duration and an intensity of 3.2μW/μm2.

Modeling of multi-band circular dichroism using metal/dielectric/metal achiral metamaterials

We show that an elliptical nanohole array (ENA) penetrating through a metal/dielectric/metal (MDM) film produces multi-band circular dichroism (CD) in the near infrared (N-IR) region when the incident light is off-normal incidence. This extrinsic CD is a result of the elliptical symmetry of the nanohole, which gives rise to different polarization modes along its short and long axes. These two polarization modes introduce a net polar vector that forms a chiral triad with the off-normal incidence and the vector normal to the plane of the ENA. The proposed structure possesses four resonance peaks arising from the excitation of both internal- and external- surface plasmon polariton (SPP) modes with different diffraction orders to those wavelengths at which they couple to the incoming light. The formation of these resonance peaks is responsible for the multi-band CD.

Fast Tuning of Double Fano Resonance Using A Phase-Change Metamaterial Under Low Power Intensity

In this work, we numerically demonstrate an all-optical tunable Fano resonance in a fishnet metamaterial(MM) based on a metal/phase-change material(PCM)/metal multilayer. We show that the displacement of the elliptical nanoholes from their centers can split the single Fano resonance (FR) into a double FR, exhibiting higher quality factors. The tri-layer fishnet MMs with broken symmetry accomplishes a wide tuning range in the mid-infrared(M-IR) regime by switching between the amorphous and crystalline states of the PCM (Ge2Sb2Te5). A photothermal model is used to study the temporal variation of the temperature of the Ge2Sb2Te5 film to show the potential for switching the phase of Ge2Sb2Te5 by optical heating. Generation of the tunable double FR in this asymmetric structure presents clear advantages as it possesses a fast tuning time of 0.36 ns, a low pump light intensity of 9.6 μW/μm2, and a large tunable wavelength range between 2124 nm and 3028 nm. The optically fast tuning of double FRs using phase change metamaterials(PCMMs) may have potential applications in active multiple-wavelength nanodevices in the M-IR region.

The effect of surface elasticity and residual surface stress in an elastic body with an elliptic nanohole

The deformation of an elastic plane with an elliptic hole in a uniform stress field is considered, taking into account the surface elasticity and the residual surface tension. The solution of the problem, based on the use of the linearized Gurtin–Murdoch surface elasticity relations and the complex Goursat–Kolosov potentials, is reduced to a singular integrodifferential equation. Using the example of a circular hole, for which an exact solution of the equation is obtained in closed form, the effect of the residual surface tension and the surface elasticity on the stress state close to and on the boundary of a nanohole is analysed for uniaxial tension. It is shown that the effect of the residual surface stress and the surface tension, due to deformation of the body, depends on the elastic properties of the surface, the value of the stretching load and the dimensions of the hole.

Topological insulator metamaterials with tunable negative refractive index in the optical region.

A blueshift tunable metamaterial (MM) exhibiting a double-negative refractive index based on a topological insulator (bismuth selenide, Bi2Se3) has been demonstrated in the near-infrared (NIR) spectral region. The potential of Bi2Se3 as a dielectric interlayer of the multilayer MM is explored. The optical response of elliptical nanohole arrays penetrating through Au/Bi2Se3/Au films is numerically investigated using the finite difference time domain (FDTD) method. The blueshift tuning range of the MM is as high as 370 nm (from 2,140 to 1,770 nm) after switching the Bi2Se3 between its trigonal and orthorhombic states.

Nanoarchitectures with Controllable Anisotropic Features in Structures and Properties from Simple and Robust Holographic Lithography

Anisotropic nanostructures with precise orientations or sharp corners display unique properties that may be useful in a variety of applications; however, precise control over the anisotropy of geometric features, using a simple and reproducible large-area fabrication technique, remains a challenge. Here, we report the fabrication of highly uniform polymeric and metallic nanostructure arrays prepared using prism holographic lithography (HL) in such a way that the isotropy that can be readily and continuously tuned. The prism position on the sample stage was laterally translated to vary the relative intensities of the four split beams, thereby tuning the isotropy of the resulting polymer nanostructures through the following shapes: circular nanoholes, elliptical nanoholes, and zigzag-shaped nanoarrays. Corresponding large-area, defect-free anisotropic metallic nanostructures could then be fabricated using an HL-featured porous polymer structure as a milling mask. Removal of the polymer mask left zigzag-shaped metallic nanostructure arrays in which nanogaps separated adjacent sharp edges. These structures displayed two distinct optical properties, depending on the direction along which the excitation beam was polarized (longitudinal and transverse modes) incident on the array. Furthermore, bidirectional anisotropic wetting was observed on the anisotropic polymer nanowall array surface.

Enhancement and tunability of Fano resonance in symmetric multilayer metamaterials at optical regime

Fano resonance (FR) is routinely observed in three-dimensional symmetric metamaterials (MMs) consisting of elliptical nanoholes array (ENA) embedding through metal–dielectric–metal (MDM) multilayers. It is shown theoretically that a square periodic ENA perforating through MDM layers produces an FR response in the near infrared regime. This FR response is attributed to the interplay between the bright modes and dark modes, where the bright modes originate from the electric resonance (localized surface plasmon resonance) caused by the ENA and the dark modes are due to the magnetic resonance (inductive–capacitive resonance) induced by the MDM multilayers. Notably, one can achieve a narrower FR when the elliptical nanoholes occupy the sites of a rectangular lattice, owing to the interaction of the magnetic resonances with the enhanced electric resonances. Moreover, a higher varying degree of the lattice constant along the horizontal direction allows for an FR with a higher value of the quality factor and the tuning of the amplitude and the resonant frequency of the transparency window. Such an FR created by the interference among the magnetic and electric dipolar resonances opens up an alternative way of forming a sharp FR in the symmetric multilayer MMs, and could be exploited for sensing.

Enhancement of Fano resonance in metal/dielectric/metal metamaterials at optical regime

Fano resonance (FR) within the transmission spectrum is demonstrated in the near infrared (NIR) region using elliptical nanoholes array (ENA) embedding through metal-dielectric-metal (MDM) layers. For the symmetric MDM-ENA, it has been shown that a FR can be excited by the normally incident light. This FR response is attributed to the interplay between the bright modes and dark modes, where the bright modes originate from the electric resonance (localized surface plasmon resonance) caused by the ENA and the dark modes are due to the magnetic resonance (inductive-capacitive resonance) induced by the MDM multilayers. Displacement of the elliptical nanoholes from their centers breaks the structural symmetry to excite a double FR as a result of the coherent interaction of the electric resonance with two splitting sub-magnetic resonances at different wavelengths. Moreover,the degree of the asymmetry allows for the tuning of the amplitude and bandwidth of the double FR window. The sensitivity to the slight variations of the dielectric environment has been calculated and yields a figure-of-merit of 0.8RIU(-1) for the symmetric MDM-ENA and 3.0RIU(-1) for the asymmetric MDM-ENA.

Study of tunable negative index metamaterials based on phase-change materials

The tunable optical response of elliptical nanohole arrays penetrating through metal–phase change material–metal (Au−Ge2Sb1Te4−Au) films is numerically investigated using the finite difference time domain method in the mid-infrared spectral region. The influence of amorphous and crystalline structural states of Ge2Sb1Te4 on the effective optical parameters of the structure is analyzed. Switching between these states provides a large wavelength shift of the structure’s effective optical parameters.

Atomically flat symmetric elliptical nanohole arrays in a gold film for ultrasensitive refractive index sensing

Past works on refractive index sensing using nanohole arrays in metal films typically achieved a resolution of around 10(-4) to 10(-5) refractive index units (RIU), up to 10(-6) with complicated detection setups. This is an order of magnitude worse than commercial Kretschmann-based surface-plasmon resonance (SPR) sensors. Here, we demonstrate intensity-based bulk refractive index sensing in an aqueous environment with a resolution of 9.38 × 10(-8) refractive index units (RIU), showing for the first time comparable performance for nanohole SPR with Kretschmann-based SPR. This is achieved by the combination of three advances in the materials properties: (a) template stripping to achieve ultra-flat Au surfaces of ~0.2 nm roughness, (b) elliptical nanoholes to enhance transmission, and (c) a Cytop substrate to symmetrize the refractive index with the aqueous environment above the metal film. The simple optical microscope geometry and microfluidic integration used in this work is promising for multiplexed lab-on-chip analysis.

Tunable metamaterials using a topological insulator at near-infrared regime

We present an effective method for actively controlling the intrinsic resonances of optical metamaterials in the near-infrared (NIR) spectral region using topological insulator (bismuth selenide, Bi2Se3) materials. The potential of Bi2Se3 as a dielectric interlayer of multilayer metamaterials (MMs) is explored. The optical response of elliptical nanohole arrays penetrating through metal/TI/metal (Au/Bi2Se3/Au) films is numerically investigated using the Finite Difference Time Domain (FDTD) method in the NIR region. The influence of increasing the immersion time of Bi2Se3 in a AgNO3 solution on the effective optical parameters of the structure is analyzed. This result suggests that a continuous wavelength shift of a structure's effective optical parameters can be obtained by modifying the immersion time.

Enhancement of circular dichroism by a planar non-chiral magnetic metamaterial

We investigate the polarization properties of an elliptical nanohole array (ENA) penetrating through a metal/insulator/metal (MIM) film in the visible wavelength region. The calculated transmission spectra reveal that such a non-chiral magnetic planar metamaterial exhibits a strong transmission circular dichroism (CD). The proposed structure exhibits two surface plasmon polariton (SPP) modes polarized along the short and long diameters of the elliptical nanoholes at oblique incidence. When the elliptical nanoholes form a chiral arrangement with off-normal incidence, the two different polarization modes introduce a net polar vector that forms a chiral triad resulting in transmission CD, even though the MIM perforated with an ENA is a non-chiral structure. We also show that the transmission CD can be enhanced in an MIM-ENA compared with a single metal layer-ENA. This effect is due to the MIM-ENA allowing local magnetic dipole moments, hence increasing the polarization effects compared with a single metal layer-ENA where only electric dipole moments occur. The magnitude of the transmission CD can be tuned by modifying the geometrical parameters of the ENA.