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PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

Plasmonic Metamaterial with Fishnet Superlattice for Enhanced Chemical Sensing

M. Obradov, Z. Jakšić, D. Tanasković

Abstract - In this work we consider a novel type of superla-ttice fishnet metamaterial for chemical sensing. By superimposing two fishnet metamaterial lattices we aim to achieve increased functionality compared to the standard fishnet metamaterials. We investigate spectral response of our structure to the changes in surrounding medium to be utilized in single wavelength readout chemical sensing. Additionally we compare the dispersion relation of plasmon modes in our superlattice fishnet metamaterial to the single lattice metamaterial and analyze the nature of coupling between plasmon modes. We present possibilities for multispectral sensing operation as well as for switching the resonant plasmon modes between sublattices by purely optical means.

I. INTRODUCTION

Metamaterials are artificial structures with effective

electromagnetic properties having values not readily found in nature, like negative, zero or extraordinarily high values of refractive index [1, 2]. Science magazine included metamaterials among the top ten science breakthroughs of the decade, regardless of the science field [3]. A very large number of various applications of metamaterials have been proposed, an important place among them being sensing [4-8]. Especially interesting are the structures combining the propagation of surface plasmons polaritons (SPP) with metamaterials – the plasmonic metamaterials [9-11].

One of the best-known and the most convenient structures for plasmonic metamaterials is the double fishnet, a structure consisting of a metal-dielectric-metal sandwich through which an array of subwavelength holes has been drilled [12-16]. This relatively simple geometry compatible with planar technologies ensures high operating frequencies and large degree of tailorability through variation of geometry and material parameters.

Double fishnets may be regarded as a generalization of the extraordinary optical transmission (EOT) arrays [17], since both of them basically represent 2D matrices of apertures in a noble metal sheet. It is well known that EOT can be used in chemical sensing schemes [18, 19].

All advantages of EOT are also present in double fishnet structures. Owing to the existence of subwavelength openings they can be used in microfluidic schemes and the sensors based on them are sufficiently sensitive to detect adsorption of a single monatomic/monomolecular layer of

analyte [15]. Besides that, double fishnets can be used as sensors in negative refractive index regime, ensuring an additional degree of freedom for their operation.

As far as the authors are informed, all EOT and double fishnet structures considered until now for sensing purposes consisted of regular patterns of apertures with identical shapes and dimensions. However, it is known that introduction of perturbations like defects, superposition of different patterns, the use of quasiperiodicity or aperiodi-city results in the appearance of additional modes and new regimes of operation [20-22]. Thus it is of interest to consider the possibility to increase the designer's freedom by utilizing these new modes.

In this paper we propose a novel type of fishnet metamaterials with a superlattice geometry as a basis for an adsorptive refractometric chemical sensor for visible and near-infrared part of the spectrum. The structure is obtained as the superposition of two fishnets with different dimensions of apertures but with identical unit cell dimensions and shifted by a half of the unit cell length. We present the results of simulation of these structures.

II. THEORY

Double fishnet structures ensure the appearance of

negative values of effective refractive index in a certain wavelength range, as defined by their dimensions and materials. They are a prototypical example of double-negative negative index metamaterials for optical region. Since the value of the refractive index is determined by the values of permittivity and permeability, n , one can separately tailor permittivity and permeability so that they reach negative values in the same wavelength range, and then use their superposition to obtain negative n [23]. Though fishnet metamaterials are realized by etching holes in metal-dielectric-metal layers, they can be also viewed as a mesh of intersecting orthogonal nanorods (whence the name), where each set of parallel layered nanorods is responsible for a single negative optical parameter. Noble metals used in fishnet metamaterials provide negative dielectric permittivity (described by the Drude or Drude-Lorentz resonant electron model) in optical region near plasma frequency for nanorods parallel to the incident light electric field (TE). Negative relative permeability is achieved through the resonant behavior of nanorods parallel to the magnetic field of the incident light (TM polarization). These nanorods behave as inductive

M. Obradov, Z. Jakšić and D. Tanasković are with the Centre of Microelectronic Technologies, Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, Belgrade, Serbia, marko.obradov@nanosys.ihtm.bg.ac.rs

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elements with currents (electron oscillations) in metallic parts closing the circuit through the dielectric displacement field in response to the incident magnetic field. By combining the two sets of nanorods an LC circuit is realized with its magnetic resonance described by negative permeability combined with underlying negative permitti-vity of noble metals. In the spectral ranges of negative refractive index it is possible to impedance match the fishnet to the surrounding medium, resulting in near zero reflection and extraordinary optical transmission. The only limiting factor here is the increased absorption associated with resonance.

The underlying effect responsible for negative index and EOT behavior is coupling of incident light into plasmon modes supported by the structure, as determined by its geometry and material composition. The interfaces between negative and positive permittivity materials (metal-dielectric) support surface plasmons polaritons (SPP), waves propagating along the interface and evanescent in the perpendicular direction. Orderly perfora-ted metallic films are known to support “designer” plas-mons, designated as such because of the ability to achieve any desired plasma frequency based on the design of the holey metal film. If the structure already supports surface plasmons, like a perforated metal film on dielectric, natural SPP and designer SPP merge to become indistinguishable. Plasmonic modes are characterized by high electric field localization within the air openings in the film, and the metallic layer acts as an impedance matching layer between air and the underlying dielectric (which can also be air – the freestanding metamaterial nanomembrane) for extra-ordinary optical transmission. For fishnet metamaterial, plasmons modes from two metallic-dielectric interfaces can couple to each other if the dielectric layer is thin enough, forming two new states, one with energy higher + and one with energy lower – than the original mode supported by the individual interface. They “squeeze” the incident light through the subwavelength openings, opening two extraordinary optical transmission windows (one for each supported plasmon mode), which in terms of macroscopic parameters are described by negative index of refraction.

III. RESULTS AND DISCUSSION

We calculated the electromagnetic parameters of the

superlattice utilizing finite element (FEM) simulations by the Comsol Multiphysics RF module. We considered a sandwich Au–Al2O3–Au (30nm-40nm-30nm) suspended in air, with rectangular holes (40nm x 80nm) in the corners of the 300 nm x 300 nm unit cell and with a narrow slit (10 nm x 150 nm) in the center of the cell, the long side of the slit parallel to the magnetic field direction (Figs. 1, 2). Perfect electric conductor and perfect magnetic conductor boundary conditions were used to simulate in-plane periodicity. We assumed normal incidence of the incoming light. Thus we determined spectral transmission, reflection and absorption of the structures (Fig. 3).

Fig. 1. Top view of superlattice metamaterial composed of two 2D sublattices. Each set of apertures is given in different color.

Fig. 2. A single unit cell of fishnet superlattice. The cross section is visible, showing metal-dielectric-metal layer sandwich

The parameters were calculated for different wavelengths and for relative permittivities of the environ-ment 1, 1.1 and 1.2. Small variations in the surrounding medium permittivity are used to simulate the presence of adsorbate on the metal surfaces and within metamaterial holes. A redshift of the spectral curves has been observed with increasing permittivity. The adsorbate-induced redshift coupled with the very narrow bandwidth of EOT windows, ensuring a high sensitivity chemical sensor in a single-wavelength operation mode. Signal readout can be obtained either by reflected or by transmitted beam.

Our results show that the principal difference between the spectral curves of the superlattice fishnet and the conventional structure is the appearance of an additional operating band stemming from the introduction of the additional array of apertures. Due to the strong in-plane asymmetry of the unit cell introduced by the narrow central slit the superlattice fishnet metamaterial exhibits strong polarization effects. Electric field distributions for resonant plasmon modes for fishnet superlattice suspended in air are shown in Figs. 4, 5. High field localizations are observed within holes of the fishnet metamaterial. Coupling of plasmonic modes can be seen by comparing the field distributions from Figs. 4 and 5, the first being the in-plane electric field distribution within the metallic layer and the second the in-plane electric field distribution within the dielectric layer (the middle stratum of the structure).

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Fig. 3. Scattering parameters of the superlattice for different permittivities of the environment (1, 1.1 and 1.2). Top: absorption coefficient; middle: spectral reflection; bottom: transmission. A redshift with increasing permittivity is observed.

For the first resonant wavelength the electric field is mainly distributed in the rectangular holes in the corners of the unit cell, to further shift towards high field confinement in the central slit as the resonant wavelength increases. Extremely weak coupling of plasmon modes is observed within the central slit at shorter wavelengths, i.e. the slit modes in the top and bottom metal are practically independent of each other. They only begin to couple at longer wavelengths, though this coupling is still weaker compared to the plasmon modes coupling within the rectangular holes.

This points out to another conclusion: one is able to “switch” between the response of the two lattices by choosing the operating frequency and polarization of incident light, ensuring in this way a field concentration in one targeted sublattice only, or alternatively in both of them simultaneously.

Fig. 4. Spatial distribution of electric field within metal layers across the superlattice unit cell for relative permittivity equal to 1 at three resonant wavelengths (top: 620 nm; middle: 780 nm; bottom: 940 nm.)

IV. CONCLUSION

We used numerical simulation to demonstrate an improvement in double fishnet metamaterial functionality for chemical sensing by superimposing two hole arrays on a single metal-dielectric-metal layered structure without sacrificing any of the metamaterial beneficial features since the resulting structure acts as a hybrid of the constituent structures in more than one way. A narrow spectral response necessary for sensing is enhanced by an intro-duction of an additional passband ensuring a for a wider choice of options in sensing applications. The hybrid nature of the superlattice metamaterial is best shown in plasmon modes dispersion relation where depending on the frequency the metamaterial can have either any of the sublattice resonant plasmon modes or a hybrid plasmon mode associated with the additional pass band. Switching between plasmon modes and consequently between the volumes with high field localizations within metamaterial by purely optical means introduces additional freedom of the tuning of the metamaterial response in addition to the inherent tunability by design as well as possible multispectral operation.

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Fig. 5. Spatial distribution of electric field across the superlattice unit cell for relative permittivity equal to 1 at three resonant wavelengths (top: 620 nm; middle: 780 nm; bottom: 940 nm.) within dielectric layer.

ACKNOWLEDGEMENT

The paper is a part of the research funded by the

Serbian Ministry of Education, Science and Technological Development within the project TR32008.

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