Metallic Nanoslit Research Articles

Numerical and experimental studies on sub-wavelength focusing in nano-slit arrays of metallic stripes with variable widths

We simulated the distributions of light energy transmission through metallic nano-slits for the optimal design of nano-lens structures and observed experimentally the light focusing effect from the nano-lens array. Contrary to reported research where the dielectric slit sizes are controlled, the current design is based on the distributions of metallic claddings of different sizes. The nano-focusing is generated by the surface plasmon polariton (SPP) effect that is known to overcome the diffraction limit and to induce sub-wavelength focusing. SPPs are collective electron charge oscillations in metallic nano-slits that can be excited by electromagnetic waves, amplifying near-field optical waves at resonance. Here, intensity amplifications in near-fields are maximized for a given light source, mainly based on various distributions of metallic claddings of different sizes in an array. For experimentation gold nano-slit arrays designed from the numerical optimization were fabricated through the lift-off process and confirmed by observations in SEM and confocal microscopes. The SPP effect in nano-slit lenses was explored by light focusing on light-sensitive papers macroscopically and by light transmission microscopically using a dispersive Raman spectrometer in scattering mode.

Transmission of TE-polarized light through metallic nanoslit arrays assisted by a quasi surface wave

Optically thick metallic nanoslit arrays are opaque to TE-polarized light, in contrast to enhanced transmission of TM-polarized light. Here, we numerically show that, by introducing an ultrathin high-index dielectric coating on the metal surfaces, a quasi surface wave can be excited at the metasurfaces to enhance the transmission of TE-polarized light. The quasi surface wave is shown to behave like surface plasmon waves, and enhance the transmission in similar mechanisms as surface plasmon waves do for TM-polarized light. In this work, we suggest a way of manipulating TE-polarized light in metallic subwavelength structures.

A Subwavelength Focusing Structure Composite of Nanoscale Metallic Slits Array With Patterned Dielectric Substrate

A novel plasmonic lens consisting of metallic nanoslits with a patterned dielectric substrate is proposed. In the structure, all metallic nanoslits have the same geometrical parameters and interspacing. The phase of incident light is modulated beforehand by the substrate before they bump on the metal slits. The surface plasmon polaritons (SPPs) wave excited by the modulated light will be focused after passing through the slits. Numerical simulation demonstrates that the size of the generated focal spot is very close to half wavelength. The overriding advantage of the proposed structure is that it significantly reduces the difficulty in fabrication while the focusing properties are comparable to the ones with filling materials in the slits.

21309 金属ナノスリット構造を用いた微小光学位相子

21309 金属ナノスリット構造を用いた微小光学位相子

Controllable electromagnetic transmission based on metallic nanoslit with a microcavity

In this paper, we explore the electromagnetic transmission through a vertical metallic nanoslit with a horizontal microcavity. The introduction of microcavity gives rise to the changes of slit transmission spectrum in the number and amplitude of transmission peak and dip, owing to the electromagnetic coupling between slit and microcavity. The transmission through the slit with microcavity strongly depends on whether the isolated slit is in resonance or not, and the microcavity resonance is an additional factor that influences the transmission. The transmission spectrum can be adjusted by changing the opening position of slit, and the slit depth, the width and depth of microcavity. Our results are analysed and discussed from the view of Fano resonance. These results may be of value in tuning electromagnetic wave in subwavelength optics and designing plasmonic devices based on composite metallic nanoslit structures.

Enhanced optical transmission by V-shaped nanoslit in metal film

In this paper, we reveal that the enhanced transmission through a perforated metal film can be further boosted up by a V-shaped nanoslit, which consists of two connected oblique slits. The maximum transmission at resonance can be enhanced significantly by 71.5% in comparison with the corresponding vertical slit with the same exit width. The value and position of transmission resonance peak strongly depend on the apex angle of the V-shaped slit. The optimum apex angle, at which the transmission is maximal, is sensitive to the slit width. Such phenomena can be well explained by a concrete picture in which the incident wave drives free electrons on the slit walls. Moreover, we also simply analyze the splitting of the transmission peak in the symmetry broken V-shaped slit, originating from the resonances of different parts of the V-shaped slit. We expect that our findings will be used to design the nanoscale light sources based on the metal nanoslit structures.

Optimization design of optical waveguide control by nanoslit-enhanced THz field

We discuss design issues of devices which were proposed recently [Opt. Lett. 37 (2012) 3903] for terahertz (THz) control of the propagation of an optical waveguide mode. The mode propagates through a nonlinear dielectric material placed in a metallic nanoslit illuminated by THz radiation. The THz field in the slit is strongly localized and thus significantly enhanced, facilitating nonlinear interactions with the dielectric waveguide material. This enhancement can lead to notable changes in the refractive index of the waveguide. The closer the waveguide is to the slit walls, the higher the nonlinear effects are, but with the cost of increasing propagation losses due to parasitic coupling to surface plasmon polaritons at the metal interfaces. We analyze several optical waveguide configurations and define a figure of merit that allows us to design the optimal configuration. We find that designs with less overlap of the THz and optical fields but also with lower losses are better than designs where both these parameters are higher. The estimated terahertz field incident onto the metallic nanoslit required to manipulate the waveguide mode has reasonable values which can be achieved in practice.

Multi-focus plasmonic lens design based on holography

Multi-focus plasmonic lens with metallic nanoslits of variant widths have great potential applications in optical interconnection, integrated optics and nanophotonics. But the design method with simulated annealing algorithm or Yang-Gu algorithm requires complex calculation and multi focuses are limited to be set on the same output plane. In this paper, we propose a design method based on holography. The desired light field distribution and the incident plane wave can be treated as object wave and reference wave, respectively. So the calculation is relative simple and multi focuses can be located in different output plane. Numerical simulation of multi-focus lens design is performed through finite-difference time-domain (FDTD) method and the result confirms the feasibility of our method.

New effects in an ultracompact Young's double nanoslit with plasmon hybridization

We present a theoretical study on the anomalous transmission of an asymmetric Young's double metallic nanoslit with an ultrathin interval. The slit–slit interactions show exotic optical properties, particularly strong suppression of light transmission with narrow bandwidth and red/blue shifting of transmission peaks from individual slits, in contrast to currently well-developed subwavelength optical structures. Dip transmission is also observed to gradually decay with increasing interval distance. Analysis results reveal that such a phenomenon physically lies in the hybridized cross-talking of surface modes within two slits throughout the interval film in between. Depending on the plasmon hybridization type, either symmetric or antisymmetric, the newly induced optical phase retardation between the two slits can decrease and increase, respectively. This investigation suggests a potential way of nonlinearly bonding metallic waveguides and consequently aiding in the design of new plasmonic devices.

Improved extraordinary optical transmission though single nano-slit by nano-defocusing

Abstract In this work, by using the finite difference time domain (FDTD) method, we have numerically studied the transmission properties of a composite structure consisting of a de-focusing tapered slit and nano-strip. For a fixed wavelength, the dependence of transmission efficiency of light on the structure parameters is demonstrated in detail. we have found that the extraordinary optical transmission through a single tapered metallic nano-slit is enhanced by placing a metallic nano-strip over it, in which tapered slit with large taper angle is used for de-focusing of light and be avoid of Fabry–Perot resonance in straight metallic nano-slit. It is shown that a suitable tapered slit assisted by nano-cavity-antenna can enhance the transmission by about 50% (compared with that in straight slit case). Furthermore, the parameters of the tapered slit (the taper angle and the taper length) hardly change the resonance in the horizontal nano-cavity. The transmission spectrum of the tapered slit can also be tuned by adjusting the taper angle. Such striking characteristic tapered slit has promising applications in designing optical nano-scale device.

Angle-Insensitive Structural Colours based on Metallic Nanocavities and Coloured Pixels beyond the Diffraction Limit

To move beyond colorant-based pigmentation display technologies, a variety of photonic and plasmonic crystal based structures have been designed and applied as colour filters. Nanostructure based colour filtering offers increased efficiencies, low power consumption, slim dimensions, and enhanced resolution. However, incident angle tolerance still needs to be improved. In this work, we propose a new scheme through localized resonance in metallic nanoslits by light funneling. Angle insensitive colour filters up to ±80 degrees have been achieved, capable of wide colour tunability across the entire visible band with pixel size beyond the diffraction limit (~λ/2). This work opens the door to angle insensitive manipulation of light with structural filtering.

Beam Focusing by Tapered Metallic Nano-Slits

In this study, we discussed the influence of tapered metallic subwavelength slit array with variant width of slits on beam focus properties. In order to analyze the phenomenon, a finite-difference time-domain numerical algorithm and phase matching theory were adopted for computational numerical simulation of the nano-structures. The structures were flanked with the penetrated slits through a metal film coated on a quartz substrate. We found that the phase delay of each slit will grow with the decrease of slit width. Optical transmission of each slit can be adjusted by using an optimum choice of the taper angle, taper slit width and the wavelength of incident light. So by optimizing the slit width of the input and output apertures, the trapezoidal slit array can show focusing property. The focal width and focal intensity can be controlled effectively by regulating the aperture width of slits. These results are very encouraging for further study on nano-optics and the applied planar lenses.

A Broadband Nanosensor based on Multi-Interference of Surface Plasmon Polaritons

A novel broadband refractive index nanosensor based on multi-interference of surface plasmon polaritons is reported. It is composed of a metallic nanoslit flanked by periodical grooves on its two sides. Extraordinary high-throughput, high-resolution, and high-sensitivity detections can be realized by observing the shift of the resonant wavelength. The sensor covers a large range of the refractive index change due to both the narrow linewidth of the single resonant peak in the broadband spectrum and the sensitive shift of the peak position withthe refractive index change. A theoretical model is developed to well predict the optical response of the sensor. An excellent linearity between the resonant wavelength and the refractive index can be achieved. The sensitivity, which is 620 nm/refractive index unit, can be further increased by tuning the period of the grooves and the high throughput; high resolution can be simultaneously achieved by adding the number of grooves.

Beam Manipulation by Metallic Nanoslit Arrays with Perpendicular Cuts inside Slits

Beam manipulation by metallic nanoslit arrays with perpendicular cuts inside the slits was investigated numerically. The simulated results performed by the finite element method (FEM) show that perpendicular cuts with different heights can modulate phase retardation of the transmitted light through the slits. With the proper distribution of cut height, a focused beam is achieved in our metallic nanostructure with four-time amplitude at the focus point and half focal length compared to a slit array without cuts inside. By using asymmetric distribution of height amplitude, a beam deflection around 6° can also be realized in our design.

Narrow-band optical transmission of metallic nanoslit arrays

Metallic nanoslit arrays usually demonstrate wide transmission bands for transverse-magnetic-polarized incidence light. Here, we show that by introducing multi-dielectric layers underneath the metallic structure layer on the substrate, a narrow peak is formed, whose bandwidth can be down to a few nanometers. Three types of resonance modes in the region under the metal layer are identified responsible for the formation of the peak, i.e., a two-dimensional cavity resonance mode, which supports optical transmission, and two in-plane hybrid surface plasmon resonance modes locating on both sides of the peak that suppresses the transmission. Such structures can be applied in advanced photonic devices.

Optical waveguide mode control by nanoslit-enhanced terahertz field

In this Letter we propose a scheme providing control over an optical waveguide mode by a terahertz (THz) wave. The scheme is based on an optimization of the overlap between the optical waveguide mode and the THz field, with the THz field strength enhanced by the presence of a metallic nanoslit surrounding the waveguide. We find an optimum balance between the optical mode attenuation and Kerr-induced change in the propagation constant. The criterion for a π/2-cumulative phase shift, for instance for application in a Mach-Zehnder interferometer configuration, requires 10 kV/cm THz field, which in turn is estimated to result in a nonlinear change of the refractive index in the waveguide of 0.001. Our simulations prove that it is quite reasonable to observe the effect experimentally.

Analytic Theory of the Resonance Properties of Metallic Nanoslit Arrays

By applying formulation based on time reversibility, we provide the analytic theory of resonance properties of metallic nanoslit arrays. We model lossy resonant systems in which a resonance is induced by a single quasi-bound mode (QBM). It is consistent with the Fano resonance theory of quantum interference of auto-ionizing atoms and captures the essential characteristics of dissipative resonant systems. We show that time-reversibility requirements lead to analytical solutions for the resonant transmission and the associated nonreciprocal absorption in terms of a minimal number of independent basic parameters that include partial decay probabilities of resonant pathways and the non-resonant transmission amplitude. With a clear view of interfering electromagnetic field configurations and the associated absorbing processes, the theory reveals the essential physics of resonant optical transmission. In particular, the enhanced transmission peak is given by the product of partial decay probabilities and is independent of the non-resonant light wave amplitude. In a highly asymmetric coupling regime, the excitation of the QBM leads to anti-resonant extinction of the transmission, indicating a negative role of the QBM. The parity of the QBM determines occurrence of red or blue tails in the spectral profile. Absorbance measurements yield direct determination of the partial decay probabilities by which the main features of the resonant transmission are quantitatively explained. Thus, these basic parameters can be directly established experimentally. Full numerical calculations of the transmission spectra are in complete quantitative agreement with our analytical formulation for optical transmission mediated by both slit cavity modes and plasmonic modes.

Investigation of surface plasmon resonance in super-period gold nanoslit arrays

Surface plasmon resonance in super-period metal nanoslits can be observed in the first order diffraction as well in the zeroth order transmission. In this paper, surface plasmon resonance modes in various super-period gold nanoslit arrays are investigated. It is found that the surface plasmon resonance frequencies are determined by the small period of the nanoslits in super-period nanoslits. The number of nanoslits in the unit cell super-period and the nanoslit width do not control the surface plasmon resonance frequencies. It is also found that the resonance wavelength observed in the first order diffraction reveals more accurate the real surface plasmon resonance wavelength in the metal super-period nanoslit array device.

Metallic Planar Lens With Binary Nanoscale Slits

We propose a binary plasmonic lens based on metallic nanoslit array with different fillings. The phase range of π is achieved by changing the refractive indices of the filling materials in the slits. These index-modulated slits are demonstrated to have unique advantages in phase delay compared with the width-modulated ones. The proposed binary structures can keep the advantage in phase modulation and reduce the difficulty in fabrication. Lenses with different functions, such as focusing light to one or two points or correcting oblique incident light, are designed by a simulated annealing algorithm and investigated by the finite-difference time-domain method.

Plasmonic Nanoslit Array Enhanced Metal–Semiconductor–Metal Optical Detectors

Metallic nanoslit arrays integrated on germanium metal-semiconductor-metal photodetectors show many folds of absorption enhancement for transverse-magnetic polarization in the telecommunication C-band. Such high enhancement is attributed to resonant interference of surface plasmon modes at the metal-semiconductor interface. Horizontal surface plasmon modes were reported earlier to inhibit photodetector performance. We computationally show, however, that horizontal modes enhance the efficiency of surface devices despite reducing transmitted light in the far field.