Subwavelength Annular Aperture Research Articles

Embedded arrays of annular apertures with multiband near-zero-index behavior and demultiplexing capability at near-infrared

In this paper, we study transmission through the embedded arrays of subwavelength annular apertures at near-infrared. Single, i.e. non-embedded arrays of annular holes are known for their capability for high-efficiency transmission even through rather thick apertures in a wide frequency range, extending from microwaves to the visible. In the suggested structures, which contain up to four embedded arrays, multiband operation can be obtained, so that each array is mainly responsible for one of four transmission bands. In such a way, a demultiplexing-like functionality can be realized, i.e. the desired parts of the incident-wave spectrum are distributed between several transmission channels. In the studied structures, we obtain (nearly) zero phase advancement and that indicates near-zero-index behavior at the expected propagation thresholds of plasmonic modes in the frequency domain. Therefore, the earlier developed concept of supercoupling and squeezing into very narrow waveguide channels is applicable to the studied structures. The number of near-zero-index bands is determined by the number of the embedded arrays. The effects of thickness, width of the slits, and permittivity of the filling material are numerically studied and discussed in detail. It is shown that multiband transmission may exist in the near-zero-index regime in a very wide range of parameter variations.

Resonant optical transmission through sub-wavelength annular apertures caused by a plasmonic transverse electromagnetic (TEM) mode

We demonstrate enhanced transmission through annular aperture arrays (AAA) by the excitation of the transverse electromagnetic () guided mode. A complete numerical study is performed to correctly design the structure before it is experimentally characterized. Actually, the challenge was to get efficient -based transmission in the visible range. It turned out to be a hard task because of the strong absorption associated with this guided mode. Nevertheless, we have succeeded to experimentally prove its excitation thanks to the enhanced transmission measured in the far-field. This is the first time we demonstrate experimental evidence of this phenomenon with such AAA structure illuminated at oblique incidence in the visible range. This increases the potential applications of such structures as well, single molecule spectroscopy, photovoltaic, spectral filtering, optical trapping, etc...

Tunable enhanced optical transmission in a metal–dielectric multilayer structured with annular aperture arrays

Recently, it has been demonstrated that nano-metric subwavelength annular aperture arrays (AAAs) in silver films can exhibit greatly enhanced transmission in visible and near infrared (NIR) range resulted from a waveguide mode described as TE11-like plasmonic mode. Here we propose a structure of metal–dielectric cascading multilayer perforated with AAAs, in which the plasmonic guided modes located in each cavity and the horizontal Bloch SPP modes (HSPs) excited on the in-plane metal–dielectric interfaces compete in the optical transmission and the coupling of HSPs between the upper and lower interfaces of one single layer and even among interfaces of different layers also play an important role to the transmitted energy redistribution, thus to result in tunable enhanced transmission in the NIR range. These properties can provide insight into the design of tunable metal–dielectric optical devices and are of significant potential applicable value to the nanotechnologies.

Generating a sub-wavelength Bessel-like light beam using a tapered hollow tube

We present a series of sub-wavelength annular aperture (SAA) structures with annular width equal to the tip of a tapered hollow tube, which was fabricated using a heat-pulled method. The light beams emitted from the SAA-like structures created by the tapered hollow tube produced light beams characteristic of Bessel beams. We obtained a sub-micrometer focal spot with a depth-of-focus larger than 7 μm and identified the proper structure parameters needed to generate Bessel-like light beams. Our new design has potential application to areas such as optical lithography, optical trapping, and the fabrication of high aspect ratio structures.

Laser ablation of silicon using a Bessel-like beam generated by a subwavelength annular aperture structure

Using a femtosecond laser incident to an oxide-metal-oxide film engraved with a subwavelength annular aperture (SAA) structure, we generated a Bessel-like beam to ablate silicon. Experimental results show that the silicon can be ablated with a 0.05 J/cm(2) input ablation threshold at 120 fs pulse duration. We obtained a surface hole possessing a diameter less than 1 μm. Optical performance, including depth-of-focus and focal spot of the SAA structure, were simulated using finite-different time-domain calculations. We found that a far-field laser beam propagating through a SAA structure possesses a submicrometer focal spot and high focus intensity. Our method can be easily adopted for surface machining in microfabrication applications.

Tuning plasmonic resonances of an annular aperture in metal plate

We present theory to describe the plasmonic resonances of a subwavelength annular aperture in a real metal plate. The theory provides the reflection, including the amplitude and phase, of radially polarized surface plasmon waves from the end faces of the aperture with a significant departure from the perfect electric conductor case due to plasmonic effects. Oscillations in the reflection amplitude and phase are observed. These oscillations arise from transverse resonances and depend on the geometry of the annulus. The theory is applied to the design of various aperture structures operating at the same resonance wavelength, and it is confirmed by comprehensive electromagnetic simulations. The results are contrasted to the perfect electric conductor case and they will be of significant interest to emerging applications in metamaterials, plasmonic sensors, and near-field optics.

Enhanced spontaneous emission rate in annular plasmonic nanocavities

The spontaneous emission rate of erbium ions is enhanced by coupling to localized plasmonic resonances in subwavelength annular apertures in a Au film. The Er3+ ions, embedded in SiO2, are selectively located inside the apertures. The annular apertures act as nanocavities, enhancing the local density of optical states at the Er emission wavelength of 1.54 μm when the cavities are tuned to that wavelength. We show that this leads to an eightfold increase of the photoluminescence intensity, in conjunction with a 2.4-fold enhancement of the spontaneous emission rate.

Continuous numerical aperture properties of a cylindrically polarized light illuminated sub-wavelength annular aperture

We investigated the process of focusing a radially polarized (RP) light beam through a sub-wavelength annular aperture (SAA). We found that the result was a non-diffraction doughnut-shaped light beam which propagates in free space. After analyzing the electric field component of the focus generated by the SAA structure, we identified the relationship between the focal field generated by the SAA. We then compared it to a case with a traditional objective lens. From our findings, we propose that a SAA structure can be viewed as a continuous numerical aperture optical element.

Propagation characteristics of silver and tungsten subwavelength annular aperture generated sub-micron non-diffraction beams

We examined the optical properties such as propagation modes, focal length, side lobes, etc. of metallic subwavelength annular apertures (SAA) and used finite-difference time-domain (FDTD) simulation to compare our experimental findings. Using two different metals, silver and tungsten, we examined the different optical transmission properties of the two metallic SAA structures. The far-field propagation of the silver SAA structure was found to be a type of quasi-Bessel beam when compared with a quasi-Bessel beam generated by a perfect axicon. The propagation characteristics of these two beams were found to match qualitatively. The far-field transmitted light generated by the silver SAA structure was found to possess a 390 nm sub-micron focal spot with a 24 microm depth of focus, which was much smaller than the focal spot generated by a perfect axicon. We also found that a silver SAA structure can generate a sub-micron quasi- Bessel beam that has a much lower far-field side-lobe when compared to that of non-diffraction beams generated by a tungsten SAA structure.

Effect of subwavelength annular aperture diameter on the nondiffracting region of generated Bessel beams

A subwavelength annular aperture (SAA) made on metallic film and deposited on a glass substrate was fabricated by electron-beam lithography (EBL) and which was followed by a metal lift-off process to generate a long propagation range Bessel beam. We propose tuning the focal length and depth of focus (DOF) by changing the diameter of the SAA. We used finite-difference time domain (FDTD) simulations to verify our experimental data. We found that the position of the Bessel Beam focus spot (i.e. focal length) will be farther away from the SAA plane as the diameter of the SAA increases. In addition, the depth of focus (DOF) which is the length of the Bessel beam non-diffracting area, also increases as the diameter of the SAA expands.

Subwavelength nondiffraction beam generated by a plasmonic lens

We experimentally examined the near-field and far-field optical properties of a subwavelength annular aperture (SAA) made on silver film and undertook finite difference time domain simulations. In our near-field measurements, an interference pattern with a period very close to half of the surface plasmon (SP) wavelength (λSP∕2) was found on the surface of the silver film. Moreover, we observed that the transmitted light of the 442nm incident laser was focused at several micrometers behind the silver SAA structure at a tiny spot (354nm) and with a remarkable 31μm depth of focus. This implies that the silver SAA structure can generate a nondiffraction beam and can be used to fabricate high aspect ratio subwavelength structures.

Near‐field optical images of subwavelength annular aperture arrays exhibiting an extraordinary transmission

In this paper, we present near-field optical images of nanostructures exhibiting an extraordinary transmission. These structures consist of annular aperture arrays engraved in a metallic film: they are quite promising structures for nanophotonics because of their high transmission directly linked to a guided mode mediated by each annular aperture. We first briefly explain our fabrication process (focused ion beam milling), then we expose the experimental setup of the near-field optical microscope working both in reflection and transmission modes. For the reflection mode, the 'coffee-bean' structure of the electromagnetic field predicted by the theory has been quite well reproduced. For the transmission mode, we present preliminary experimental results concerning the influence of the wavelength and the polarization of the incident beam on the obtained near-field images.

Beaming of electromagnetic waves emitted through a subwavelength annular aperture

We study the diffraction of electromagnetic waves from subwavelength metallic circular apertures in the microwave spectrum. The theoretical and experimental demonstration of the near- and far-field electromagnetic distributions for subwavelength circular annular apertures and circular annular apertures surrounded by concentric periodic grooves are reported here. The metallic samples had a subwavelength hole with a diameter of 8 mm and had concentric grooves with a periodicity of 16 mm. We present the angular transmission distributions from circular annular apertures, and circular annular apertures surrounded by concentric periodic grooves. At the surface-mode resonance frequency the transmitted electromagnetic waves from the subwavelength circular annular aperture surrounded by concentric periodic grooves have a strong angular confinement with an angular divergence of ±3°. This represents a fourfold reduction when compared with the angular divergence of the beam transmitted from the subwavelength circular aperture.

Super-transmission of light through subwavelength annular aperture arrays in metallic films: Spectral analysis and near-field optical images in the visible range

This paper presents experimental studies of the enhanced light transmission through metallic films pierced by subwavelength annular apertures. Two different methods (e-beam lithography and focused ion beam) have been used to build the nano-structures. We have experimentally recorded their far-field spectral response in the visible range and the optical near-field above the nano-structures when they are excited at 633 nm. The spectral response exhibits a transmission peak at 700 nm with maximum efficiency around 16%. The near-field exhibits a characteristic two-lobe structure just above the aperture. Finite difference time domain (FDTD) simulations reproduce quite well the experimental results.

Microwave Transmission through a Single Subwavelength Annular Aperture in a Metal Plate

The resonant transmission of a small annular aperture, with a diameter much smaller than the radiation wavelength, in a thin metal plate is studied at microwave frequencies. It transpires that such an annular aperture supports several resonant guided modes, including those that are not quantized in the azimuthal direction. Such modes have resonant frequencies that are largely independent of the diameter of the annular aperture, thus being supported by annular apertures that tend to zero radius. The transmittance of such a structure at microwave frequencies is detailed and compared with the predictions of a finite element method model.

Extraordinary grating-coupled microwave transmission through a subwavelength annular aperture

We studied coupling phenomena between surface plasmons and electromagnetic waves in the microwave spectrum using circular apertures surrounded by array of grooves.We first present experimental and theoretical results of enhanced microwave transmission though a subwavelength circular aperture with concentric periodic grooves around the surface plasmon resonance frequency. This is followed by transmission studies through circular annular apertures and circular annular apertures surrounded by concentric periodic grooves. We demonstrated that 145 fold enhancement factor could be obtained with a subwavelength circular annular aperture surrounded by concentric periodic grooves. Our results show that, high transmission from a circular annular aperture with grooves is assisted by the guided mode of the coaxial waveguide and coupling to the surface plasmons.

Light transmission by subwavelength annular aperture arrays in metallic films

We study the spectral response of a metallic film with an engraved 2D periodic structure of annular apertures. We show that an enhanced transmission can be obtained and can reach 90%. The variation of the spectral response as a function of the film thickness from 150 nm to a value of 1.8 μm is also studied. We show that a guided mode in the subwavelength coaxial structure is responsible for this large transmission. A wavelength cut-off exists. We study the transmission characteristics on the optical and geometrical parameters of the array (period, coaxial radii, incident polarization and dielectric constants) using a 3D-FDTD algorithm.