Periodic Arrays Of Subwavelength Apertures Research Articles

Structural Stability of Optofluidic Nanostructures in Flow-Through Operation.

Optofluidic sensors based on periodic arrays of subwavelength apertures that support surface plasmon resonance can be employed as both optical sensors and nanofluidic structures. In flow-through operation, the nanoapertures experience pressure differences across the substrate in which they are fabricated, which imposes the risk for structural failure. This work presents an investigation of the deflection and structural stability of nanohole array-based optofluidic sensors operating in flow-through mode. The analysis was approached using experiments, simulations via finite element method, and established theoretical models. The results depict that certain areas of the sensor deflect under pressure, with some regions suffering high mechanical stress. The offset in the deflection values between theoretical models and actual experimental values is overturned when only the effective area of the substrate, of 450 µm, is considered. Experimental, theoretical, and simulation results suggest that the periodic nanostructures can safely operate under trans-membrane pressures of up to 20 psi, which induce deflections of up to ~20 μm.

Electromagnetic behavior of dielectric objects on metallic periodically nanostructured substrates.

In this research, we investigate the electromagnetic behavior of a metallic thin-film with a periodic array of subwavelength apertures when dielectric objects are located on it. The influence of size, geometry and optical properties of the objects on the transmission spectra is numerically analyzed. We study the sensitivity of this system to changes in the refractive index of the illuminated volume induced by the presence of objects with sizes from hundreds of nanometers (submicron-sized objects) to a few microns (micron-sized objects). Parameters such as the object volume within the penetration depth of the surface plasmon in the buffer medium or the contact surface between the object and the nanostructured substrate strongly affect the sensitivity. The proposed system models the presence of objects and their detection through the spectral shifts undergone by the transmission spectra. Also, we demonstrate that these can be used for obtaining information about the refractive index of a micron-sized object immersed in a buffer and located on the nanostructured sensitive surface. We believe that results found in this study can help biomedical researchers and experimentalists in the process of detecting and monitoring biological organisms of large sizes (notably, cells).

An Electrically Tunable Terahertz Plasmonic Device Based on Shape Memory Alloys and Liquid Metals

AbstractAn electrically tunable terahertz (THz) plasmonic device is designed and fabricated using liquid metals (eutectic gallium indium) and shape memory alloy wires (Flexinol). The liquid metal is injected into the voids of a poly(dimethyl) siloxane microfluidic mold forming a periodic array of subwavelength apertures, while the wires are inserted into the elastomer below the metal plane. When a DC voltage is applied to the wires, they contract via Joule heating, reducing the aperture periodicity and blueshifting the transmission resonances of the device. When the voltage is removed, the wires cool and elongate back to their original length, allowing the transmission spectrum to return to its original state. The magnitude of this change depends upon the applied voltage. The device is shown to thermally cycle between the relaxed state and the fully contracted state reproducibly over at least 500 thermal cycles. The asymmetric geometry of the device and the contraction process yield transmission properties that are unexpected: two closely spaced resonances, where both resonances correspond to the same scattering indices, and an increase in the transmission amplitude of the lowest order resonance upon contraction. Numerical simulations are used to understand these features.

New approach for extraordinary transmission through an array of subwavelength apertures using thin ENNZ metamaterial liners.

Extraordinary transmission (ET) through a periodic array of subwavelength apertures on a perfect metallic screen has been studied extensively in recent years, and has largely been attributed to diffraction effects, for which the periodicity of the apertures, rather than their dimensions, dominates the response. The transmission properties of the apertures at resonance, on the other hand, are not typically considered 'extraordinary' because they may be explained using more conventional aperture-theoretical mechanisms. This work describes a novel approach for achieving ET in which subwavelength apertures are made to resonate by lining them using thin, epsilon-negative and near-zero (ENNZ) metamaterials. The use of ENNZ metamaterials has recently proven successful in miniaturizing circular waveguides by strongly reducing their natural cutoff frequencies, and the theory is adapted here for the design of subwavelength apertures in a metallic screen. We present simulations and proof-of-concept measurements at microwave frequencies that demonstrate ET for apertures measuring one-quarter of a wavelength in diameter and suggest the potential for even more dramatic miniaturization simply by engineering the ENNZ metamaterial dispersion. The results exhibit a fano-like profile whose frequency varies with the properties of the metamaterial liner, but is independent of period. It is suggested that similar behaviour can be obtained at optical frequencies, where ENNZ metamaterials may be realized using appropriately arranged chains of plasmonic nanoparticles.

Fractal plasmonic metamaterials: physics and applications

AbstractWe review our recent works on a particular type of metamaterials (MTMs), which are metallic plates drilled with periodic arrays of subwavelength apertures typically in fractal-like complex shapes. We first show that such MTMs can well mimic plasmonic metals in terms of surface plasmon properties, but with plasmon resonances solely dictated by their aperture geometries rather than the constitutional materials. We then develop an effective-medium description for such plasmonic MTMs based on the mode expansion theory. Based on these theoretical understandings, we show that such MTMs exhibit several interesting applications, such as superlensing, hyperlensing, and enhancing light-matter interactions, which are demonstrated by microwave experiments or full-wave simulations.

Terahertz Plasmonic Structures Based on Spatially Varying Conductivities

Terahertz plasmonic structures are demonstrated in which the conductivity of the metallic film is varied spatially in order to further enhance the response. Using a commercially available inkjet printer, in which one cartridge is filled with conductive silver ink and a second cartridge is filled with resistive carbon ink, computer generated drawings of plasmonic structures are printed in which the individual printed dots can have differing amounts of the two inks, thereby creating a spatial variation in the conductivity. The silver ink has a DC conductivity that is only a factor of six lower than bulk silver, while the carbon ink acts as a lossy dielectric at THz frequencies. Both inks sinter at room temperature immediately after contact with the plastic film. Using a periodic array of subwavelength apertures as a test structure, patterns printed with different fractional amounts of the two inks show dramatically different enhanced optical transmission properties. These differences arise from changes in the propagation loss properties as a function of conductivity. This data is used to design and fabricate aperture arrays in which the conductivity varies spatially. The resulting plasmonic effect is found to dramatically alter the spatial beam profile of the transmitted THz radiation, as measured by THz imaging.

Optic-null medium: realization and applications

Optic-null medium (ONM), an electromagnetic (EM) space representing optically nothing, has many interesting applications but is difficult to realize practically due to its extreme EM parameters. Here we demonstrate that a holey metallic plate with periodic array of subwavelength apertures can well mimic an ONM. We develop an effective-medium theory to extract the EM parameters of the designed ONM, and employ full-wave simulations to demonstrate its optical functionalities. Microwave experiments, in excellent agreement with full-wave simulations, are performed to illustrate several applications of the ONM, including the radiation cancellation effect and the hyperlensing effect.

Concentration of broadband terahertz radiation using a periodic array of conically tapered apertures

We describe the optical concentration properties of periodic arrays of conically tapered metallic apertures measured using terahertz (THz) time-domain spectroscopy. As a first step in this process, we optimize the geometrical properties of individual apertures, keeping the output aperture diameter fixed, and find that the optimal taper angle is 30°. A consequence of increasing the taper angle is that the effective cutoff frequency red shifts, which can be readily explained using conventional waveguide theory. We then fabricate and measure the transmission properties of a periodic (hexagonal) array of optimized tapered apertures. In contrast to periodic arrays of subwavelength apertures in thin metal films, which are characterized by narrowband transmission resonances associated with the periodic spacing, here we observe broadband enhanced transmission above the effective cutoff frequency. Further enhancement in the concentration capabilities of the array can be achieved by tilting the apertures towards the array center, although the optical throughput of individual tapered apertures is reduced with increasing tilt angle. Finally, we discuss possible future directions that utilize cascaded structures, as a means for obtaining further enhancement in the amplitude of the transmitted THz radiation.

The collective property of enhanced transmission through compound metal periodic arrays of subwavelength apertures

The transmissions through a two-dimensional compound metal periodic hole arrays comprised of rectangle and cross-shaped holes are calculated by the finite-difference-time-domain (FDTD) method. The results show that the transmissions strongly depend on the collective effect of the compound hole cell. Moreover, the comparisons between transmission characteristics corresponding to the asymmetry of the cross-shaped hole in two different directions (vertical and parallel to the light polarization) are also studied. It is found that the 1168nm peak is split into two peaks when the symmetry of the cross-shaped hole in y-axis direction is broken. However, it is shown that there is no obvious change of transmissions with changing the asymmetry in x-axis direction. Thus, it is concluded that the transmissions are more sensitive to the vertical direction asymmetry. The results may be utilized to tune the electromagnetic wave in subwavelength optics.

Influence of the Interlayer on Coupling of Surface Plasmons in a Sandwiched Structure with Periodic Array of Nanoapertures

In this work, we investigate the optical properties of a multilayer structure, where a SiO2 film is sandwiched by silver films with periodic array of sub-wavelength apertures. Due to the coupling of surface plasmons (SPs) between different layers, electric and magnetic resonances have been observed. By varying the thickness of the interlayer SiO2, we can modify relative phase of the SPs resonance and control the shifts of transmission peaks. Experimentally the multilayers are fabricated by magnetron sputtering and the array of apertures is milled by focused-ion-beam facility. The measured optical transmission spectra reasonably agree with our numerical calculation, which bases on three-dimensional finite-difference time-domain method. To understand the shifts of the peaks, we present a phenomenological explanation, considering the transmission peaks as energy levels, and the coupling of localized surface plasmons as perturbation. These results may have potential applications in designing plasmonic devices and tuning electromagnetic wave in nanophotonics.

Optical dot antenna and nanohole transmission

In this reported work, the authors optimised a dielectric dot as a radiator, studied its emission properties, verified the far-field pattern multiplication and obtained high directivity from the proposed dot antenna. Also the effect of the dot radiator on the extraordinary optical transmission (EOT) was studied through optically thick metal film with a periodic array of subwavelength apertures as well as a single circular aperture surrounded by finite periodic concentric annular grooves (bull's eye). With dot radiators a high-intensity focused electric field beam and polarisation-independent EOT were obtained through periodic arrays of holes in metal film. Interestingly, the bull's eye structure with an optimised dot radiator has transmission threefold larger and suppressed side lobes of the light beam emerging from it. These features might open a new pathway in the field of nanophotonics for multiple applications such as designing better near-field imaging devices, biosensors and improving the efficiency of solar cells etc.

Nanostructured Metal-Enhanced Raman Spectroscopy for DNA Base Detection

Periodic arrays of subwavelength apertures on Ag film are used as the substrates for nanohole-enhanced Raman spectroscopy. This study presents a rectangular bull's eye structure in an array. We demonstrate the quantitative features of the pattern by the finite-difference time-domain method. This structure is used to enhance the Raman spectroscopy of four DNA bases without labeling. The enhancement factor is up to 106. A 1.5-fold enhancement against the circular pattern experimentally confirms the role of sharp corners. This structure may be used in nucleotide identification and DNA sequencing.

Liquid metal-based plasmonics

We demonstrate that liquid metals support surface plasmon-polaritons (SPPs) at terahertz (THz) frequencies, and can thus serve as an attractive material system for a wide variety of plasmonic and metamaterial applications. We use eutectic gallium indium (EGaIn) as the liquid metal injected into a polydimethylsiloxane (PDMS) mold fabricated by soft lithography techniques. Using this approach, we observe enhanced THz transmission through a periodic array of subwavelength apertures. Despite of the fact that the DC conductivity of EGaIn is an order of magnitude smaller than many conventional metals, we clearly observe well-defined transmission resonances. This represents a first step in developing reconfigurable and tunable plasmonic devices that build upon well-developed microfluidic capabilities.

Gain-assisted extraordinary optical transmission through periodic arrays of subwavelength apertures

We theoretically investigate the amplification of extraordinary optical transmission (EOT) phenomena in periodic arrays of subwavelength apertures incorporating gain media. In particular, we consider a realistic structure consisting of an opaque silver film perforated by a periodic array of slits and clad on each side by an optically pumped dielectric thin film containing rhodamine dye molecules. By solving the semiclassical electronic rate equations coupled to rigorous finite-element simulations of the electromagnetic fields, we show how the resonant electric-field enhancement associated with EOT properties enables complete ohmic loss compensation at moderate pump intensity levels. Furthermore, our calculations show that, as a consequence of the strong spatial hole-burning effects displayed by the considered structures, three separate regimes of operation arise: the system can behave as an absorber, an optical amplifier or a laser, depending on the value of the pump intensity. A discussion on the feasibility of reaching the lasing regime in the considered class of structures is also presented.

Time-of-Flight Model for the Extraordinary Transmission Through Periodic Arrays of Subwavelength Apertures at THz Frequencies

A qualitative model explaining the extraordinary optical transmission of terahertz (THz) radiation through two-dimensional periodic arrays of subwavelength apertures is presented. Systematic terahertz time-domain spectroscopy studies have been undertaken to investigate the combined effects of the lattice arrangement, aperture shape, area and aspect ratio on the transmission properties of electroformed copper arrays. The extensive results presented provide a unified example of how aperture geometry dictates SPP activity. The novel fabrication method creates exemplary peak resonances, allowing the onset of surface plasmon polariton (SPP) decoupling to be distinguished from direct transmission. Furthermore, we provide the first evidence as to how the temporal properties of SPPs are governed by the single-cycle THz pulse. The time-of-flight model presented can not only be used to explain the results observed in both the presented and previously published experiments but serves as a method to engineer specific resonances for sensor applications.

Super imaging with a plasmonic metamaterial: Role of aperture shape

Through analyzing the transmission properties of metallic plates with periodic arrays of subwavelength apertures in different shapes, we found two conditions for such structures to work as super lenses. The working wavelength dictated by the aperture's shape resonance should be much larger than the array's periodicity, meanwhile the coupling between the aperture's waveguide modes and external radiations should be as small as possible. These two conditions contradict with each other for a square-shape aperture so that high index materials should be inserted into the apertures, while a fractal-shape aperture satisfies these two conditions simultaneously without using high-index insertions. Numerical simulations were performed to illustrate the imaging properties of these plasmonic lenses.

Enhanced transmission of transverse electric waves through periodic arrays of structured subwavelength apertures

Transmission through sub-wavelength apertures in perfect metals is expected to be strongly suppressed. However, by structural engineering of the apertures, we numerically demonstrate that the transmission of transverse electric waves through periodic arrays of subwavelength apertures in a thin metallic film can be significantly enhanced. Based on equivalent circuit theory analysis, periodic arrays of square structured subwavelength apertures are obtained with a 1900-fold transmission enhancement factor when the side length a of the apertures is 10 times smaller than the wavelength (a/lambda =0.1). By examining the induced surface currents and investigating the influence of the lattice constant and the incident angle to the resonant frequency, we show that the enhancement is due to the excitation of the strong localized resonant modes of the structured apertures.

Spectral and spatial investigation of midinfrared surface waves on a plasmonic grating

A patterned metal film with a periodic array of subwavelength apertures, fabricated upon a semiconductor substrate and designed to possess transmission resonances in the midinfrared is interrogated with a wavelength-tunable external cavity quantum cascade laser. The interaction of the coherent light with this plasmonic structure is studied using a spatially resolved transmission experiment, allowing for the far-field imaging of propagating waves on the surface of the metal film. Spatial and spectral transmission is investigated for a range of near-normal incidence angles. For nonzero angles of incidence, coupling of laser light, at distinct frequencies, to surface waves propagating in opposite directions is demonstrated.

Active control and spatial mapping of mid-infrared propagating surface plasmons

Periodic arrays of subwavelength apertures in metal films have been shown to exhibit strongly enhanced transmission at wavelengths determined by the periodicity of the film as well as the optical properties of the metal and surrounding dielectric material. Here we investigate the coupling between such a grating and a Quantum Cascade Laser. By actively tuning the optical properties of our grating, we control the coupling of laser light to the plasmonic structure, switching our grating from a predominantly transmitting state to a state that allows coupling to propagating surface waves, which can then be imaged on the metallic surface.

Loss mechanisms in mid-infrared extraordinary optical transmission gratings

The optical properties of periodic arrays of subwavelength apertures in metal films on GaAs substrates are studied. Specifically, geometric and material losses for these plasmonic structures are characterized using angular dependent transmission, normal incidence reflection, and angular dependent diffraction experiments, in addition to a crossed-polarizer transmission experiment. The optical properties of the samples as a function of engineered material losses are studied. Using this comprehensive approach to the characterization of the plasmonic structures, we are able to identify and isolate specific loss mechanisms, as well as identify the effect of free carriers on the optical properties of the structures.