Subwavelength Slits Research Articles

Effects of interplay between metal subwavelength slits on extraordinary optical transmission

In this paper we study the extraordinary optical transmission of one-dimensional multi-slits in an ideal metal film. The transmissivity is calculated as a function of various structural parameters. The transmissivity oscillates, with the period being just the light wavelength, as a function of the spacing between slits. As the number of slits increases, the transmissivity varies in one of three ways. It can increase, attenuate, or remain basically unchanged, depending on the spacing between slits. Each way is in an oscillatory manner. The slit interaction responsible for the oscillating transmission strength that depends on slit spacing is the subject of more detailed investigation. The interaction most intuitively manifests as a current distribution in the metal surface between slits. We find that this current is attenuated in an oscillating fashion from the slit corners to the center of the region between two adjacent slits, and we present a mathematical expression for its waveform.

Optical forces on cylinders near subwavelength slits illuminated by a photonic nanojet

We discuss optical forces exerted on particles, either dielectric or metallic, near a subwavelength slit illuminated by a photonic nanojet. We compare those cases in which the Mie resonances are or are not excited. The configurations on study are 2D, hence those particles are infinite cylinders and, in order to obtain extraordinary transmission, the illuminating beam is p-polarized. We show the different effects of these particle resonances on the optical forces: while whispering gallery modes under those illumination conditions weaken the force strength, this latter is enhanced by localized plasmon excitation. Also, illuminating the slit with a nanojet enhances the optical forces on the particle at the exit of the aperture by a factor between 3 and 10 compared with illumination of the slit with a Gaussian beam. In addition, the pulling force that such a small resonant metallic particle suffers on direct illumination by a nanojet can change by the presence of the slit, so that it may become repulsive at certain lateral positions of the particle.

Enhanced optical transmission through metallic slits embedded with rectangular cavities

The non-resonantly enhanced optical transmission phenomenon of sub-wavelength metallic slits on a thin film is significant for broadband light harvesting devices. To improve the efficiency of transmission, in this paper, we propose a compound structure of the tapered metallic slit embedded with rectangular cavity, and its optical properties are investigated by the finite element method. The results show that the compound structure leads to a higher transmission than the tapered metallic slit without rectangular cavity. In addition, the effects of the structure parameters and the position of the rectangular cavity on the transmission property are also studied. These results would be helpful for designing metallic slits with high transmission.

Resonant transmission enhancement at one-dimensional metal gratings

An analysis of phenomena leading to high transmission at a one-dimensional metal grating is presented. It is shown that high resonant transmission can be obtained either for thick or thin gratings for a wide range of filling factors and that the origins of the enhanced transmission are different in each case. We analyse the optical response of structures with subwavelength slits and of various thicknesses. The role of different pure (dielectric cavity modes, surface plasmon polaritons) and coupled resonances on the enhanced transmission is presented.

Polarization Filters for Visible Light Consisting of Subwavelength Slits in an Aluminum Film

We fabricated arrays of subwavelength slits in an Al film and measured their optical properties in the visible light region. Al crossbeams were installed as supports between the Al lines to enable us to precisely fabricate the nanoslits. Visible light polarized parallel to the slits transmits at shorter wavelengths, similar to that of a wire grid array at infrared wavelengths. On the other hand, for light polarized perpendicular to the slits, the transmission is at longer wavelengths and significant absorption takes place at shorter wavelengths due to Fano-type interference. This absorption in the visible wavelength region is due to the precise nanofabrication process and the high plasma frequency of Al. These results show that Al nanoslits can be used as polarization filters in the visible wavelength region.

Tunable, narrow-band, all-metallic microwave absorber

We exploit the metamaterial properties of a thick metallic grating with extreme sub-wavelength slits on a metallic slab to achieve complete absorption of transverse magnetic polarized microwaves. We measure narrow bands of total absorption (up to 99.9999%) from normal to grazing incidence that can be tuned by varying an air gap between the grating and the slab. Unlike typical absorbers, the structure is mostly metallic with a 97% filling factor, and no absorptive material beside the metal itself is employed. We access the absorption properties of metals in the microwave where they are commonly believed to be perfect reflectors.

Optical transmission through double-layer compound metallic gratings with subwavelength slits

Optical transmission through a double-layer compound metallic grating (DCMG) composed of two identical compound metallic gratings (CMGs) with two subwavelength slits filled with different dielectrics inside each period is investigated by using the finite-difference time-domain method. The results show that the transmission properties of the DCMG are dependent on both the separation G between the two metallic layers and the phase configurations of the electromagnetic waves at the exits of adjacent slits of each layer. When a suitable separation (G ∼ 300 nm) is chosen, for the DCMG a notable transmission peak emerges at a certain wavelength, at which phase resonance appears for the corresponding CMG, while the transmission spectra of the corresponding double-layer simple metallic gratings (DSMGs) with the separation (G ∼ 300 nm) exhibit unexpected transmission suppression in a broad spectral region. When G > 340 nm, the intensity of the transmission around the wavelength for the DCMG gradually decreases down to almost zero as G increases, while the high transmission is nearly maintained for the corresponding DSMGs.

Optical forces on cylinders near subwavelength slits: effects of extraordinary transmission and excitation of Mie resonances

We study the optical forces on particles, either dielectric or metallic, in or out their Mie resonances, near a subwavelength slit in extraordinary transmission regime. Calculations are two-dimensional, so that those particles are infinite cylinders. Illumination is with p-polarization. We show that the presence of the slit enhances by two orders of magnitude the transversal forces of optical tweezers from a beam alone. In addition, a drastically different effect of these particle resonances on the optical forces that they experience; namely, we demonstrate an enhancement of these forces, also of binding nature, at plasmon resonance wavelengths on metallic nanocylinders, whereas dielectric cylinders experience optical forces that decrease at wavelengths exciting their whispering gallery modes. Particles located at the entrance of the slit are easily bound to apertures due to the coincidence in the forward direction of scattering and gradient forces, but those particles at the exit of the slit suffer a competition between forward scattering force components and backward gradient forces which make more complex the bonding or antibonding nature of the resulting mechanical action.

Beam focusing from double subwavelength metallic slits filled with nonlinear material surrounded by dielectric surface gratings

Based on the radiation properties of surface plasmon polaritons (SPPs) can be controlled by adjusting the refractive indexes of dielectric materials in the metallic slits, a novel plasmonic focusing structure formed by two subwavelength metal apertures filled with Kerr nonlinear material surrounded by surface dielectric gratings is proposed and demonstrated numerically. Directions of radiation fields are determined by the phase difference of the surface waves at the exit interface and resonance property of each surface grating. Numerical simulations using two-dimensional (2D) Finite-Difference Time-Domain (FDTD) method verify that the deflection angle and focal length can be controlled easily by changing the intensity of incident light, dynamically tunable on-axis and off-axis focusing effects can be achieved.

Photonic bands from cascaded metallic plates with subwavelength slits

We investigated through experiments and calculations the coupling of the local resonant modes supported by subwavelength H-fractal slits in metallic plates when they are cascaded. The transmission peak of the single plate is split into a multiplet with definite transmission phase values. In addition, we analyzed the transmission passbands resultant from cascading more plates, which shows a similarity between the coupled metallic plates with local resonant modes and electronic systems in solid state physics.

Tunable surface plasmons interference patterns with the same mask in nanolithography

When a transverse magnetic-polarized beam illuminates normally on an Ag grating mask with sub-wavelength slits, the surface plasmon (SP) interference patterns are formed on the Ag surface. According to the dispersion relationship of SPs, the wavelength of SPs (λsp) is tunable by altering the refractive index of the photoresist or the illumination wavelength. Various λsp form interference patterns with different size. The interference patterns are tunable using the same mask with period microns in size, which can save the cost of fabricating a different mask. This method will have potential applications to nanolithography.

Matching and funneling light at the plasmonic Brewster angle

The ultrabroadband impedance matching of metallic gratings at the plasmonic Brewster angle [A. Al\`u et al., Phys. Rev. Lett. 106, 123902 (2011)] is analyzed here in several realistic scenarios and configurations, and in the case of nonmonochromatic excitation. This phenomenon is the analogy of the well-known Brewster transmission for dielectric slabs but, when applied to plasmonic gratings, has the remarkable property of funneling and concentrating light within subwavelength slits. We analyze here how the presence of absorption and of realistic substrates and/or superstrates may influence the phenomenon, its beamwidth and angular selectivity, and its overall performance in the case of broadband, ultrashort incident pulses in the time domain. We prove that broadband signals may be concentrated and transmitted almost unaffected through narrow apertures, even in the presence of absorption, very different from conventional extraordinary optical transmission based on resonant phenomena.

Young's double-slit experiment in photonic crystals

We present an experimental and numerical study of the transmission of a photonic crystal perforated by two subwavelength slits, separated by two wavelengths. The experimental near-field image of the double-slit design of the photonic crystal shows an interference pattern, which is analogous to Young's experiment. This interference arises as a consequence of the excitation of surface states of the photonic crystals and agrees very well with the simulations.

Wideband plasmonic beam steering in metal gratings

We demonstrate controllable light deflection in thick metal gratings with periodic subwavelength slits filled with an active material. Under specific illumination conditions, the grating becomes nearly transparent and acts as a uniform optical phased-array antenna where the phase of the radiating elements is controlled by modifying the index of refraction of the material that fills each slit. The beam-steering operational regime occurs in a wide wavelength band, and it is relatively insensitive to the input angle.

Genetic optimization of double subwavelength metal slits surrounded by surface dielectric gratings for directional beaming manipulation

In this work, a model of double subwavelength metal slits surrounded by surface dielectric gratings is proposed for the directional beaming based on surface plasmon polariton (SPP). A genetic algorithm (GA) in conjunction with the finite-difference time-domain (FDTD) method is employed to optimize this structure, which leads to the high-efficiency directional beaming at any angle in a broad range (from 0° to 70°). The results show that the furthest propagation distance of the beaming can reach more than 25μm, the maximum deflection efficiency can reach 97.37%, and the enhancement factor of the beaming can reach 7.58×102. The acceptable perturbation range of optimum structures is also obtained, which indicates the feasibility and realization of this model.

TE Polarization Extraordinary Transmission through Metallic Grating with Subwavelength Slits

Based on finite-difference-time-domain method, we have shown that the transmission of TE polarization is affected with the geometrical parameters of one-dimensional metallic grating with subwavelength slits at the wavelength range from visible to near-infrared light. The increase of the dielectric refractive index of slits and the slit width leads to the redshift of the position of the peaks in transmission spectra, but the change of the grating period does not affect that. By tuning the structural geometry, the polarization independent element can be achieved at Communication wavelength.

Determination of Surface Plasmon Modes and Guided Modes Supported by Periodic Subwavelength Slits on Metals Using a Finite-Difference Frequency-Domain Method Based Eigenvalue Algorithm

An eigenvalue solution algorithm is formulated based on the finite-difference frequency-domain (FDFD) method for determining guided modes, including the surface plasmon modes, supported by periodic metallic structures. The Yee-mesh grids which have been popularly adopted in the finite-difference time-domain (FDTD) method are used in the FDFD method and standard eigenvalue matrix equations are obtained for easily searching for the guided eigenmodes. Both two-dimensional (2-D) and three-dimensional (3-D) structures are considered and the periodicity is along the propagation direction. The metals are assumed to be perfect ones or real ones without loss. For 2-D structures, an array of grooves drilled in a perfect conductor and a real-metal structure with a periodic arrangement of subwavelength slits in air are analyzed and the dispersion diagrams and mode-field profiles are obtained. For the latter structure, surface plasmon modes and dielectric slab modes are identified to be in agreement with published results based on a different numerical scheme. This subwavelength-slit structure is then extended to a 3-D one having an additional depth and it is demonstrated that the formulated algorithm can solve the same two kinds of modes for the more complicated 3-D problem. The modes guided along drilled periodic rectangle holes on a perfect conductor surface are also calculated.

Reduction of wide-band crosstalk for guiding microwave in corrugated metal strip lines with subwavelength periodic hairpin slits

A new type of microstrip line on which the spoof surface plasmon polaritons (SPPs) can propagate in microwave band is developed and a scheme for reducing the wide-band crosstalk between transmission lines is proposed. The microstrip line structure is designed by introducing periodic subwavelength hairpin structure on the edge of conventional microstrip lines. Numerical methods are used to analyse the dispersion relation and guiding bandwidth in microwave regime. Besides, the authors experimentally verify that such periodically structured microstrip lines support spoof SPPs in the frequency range between 200 and 8 GHz. Compared with the quasi-transmission electron microscopy mode in conventional microstrip line, the spoof SPPs mode can be highly localised on the surface of the structured microstrip lines, and so the crosstalk between different structured microstrip lines is very weak, for example, the crosstalk between one conventional microstrip line and one structured microstrip line ranges from -19.89 to -62.39 dB (which is much lower than the crosstalk between two conventional microstrip lines) when the distance between the two microstrip lines is the same as the width of the microstrip line. Therefore this new kind of periodically structured microstrip line would be of great use in high-density microwave circuits and high-speed systems to guarantee signal integrity.

多层金属介质镀膜光栅的聚焦特性

多层金属介质镀膜光栅的聚焦特性

Microcavity enhanced optical absorption in subwavelength slits

We introduce a compact submicron structure consisting of multiple optical microcavities at both the entrance and exit sides of a subwavelength plasmonic slit filled with an absorbing material. We show that such microcavity structures at the entrance side of the slit can greatly enhance the coupling of the incident light into the slit, by improving the impedance matching between the incident plane wave and the slit mode. In addition, the microcavity structures can also increase the reflectivities at both sides of the slit, and therefore the resonant field enhancement. Thus, such structures can greatly enhance the absorption cross section of the slit. An optimized submicron structure consisting of two microcavities at each of the entrance and exit sides of the slit leads to ~9.3 times absorption enhancement at the optical communication wavelength compared to an optimized slit without microcavities.