Metamaterials are artificially structured media with unit cells much smaller than the wavelength of light. They have proved to possess novel electromagnetic properties, such as negative magnetic permeability and negative refractive index. This enables applications such as negative refraction, superlensing and invisibility cloaking. Although the physical properties can already be demonstrated in two-dimensional (2D) metamaterials, the practical applications require 3D bulk-like structures. This prerequisite has been achieved in the gigahertz range for microwave applications owing to the ease of fabrication by simply stacking printed circuit boards. In the optical domain, such an elegant method has been the missing building block towards the realization of 3D metamaterials. Here, we present a general method to manufacture 3D optical (infrared) metamaterials using a layer-by-layer technique. Specifically, we introduce a fabrication process involving planarization, lateral alignment and stacking. We demonstrate stacked metamaterials, investigate the interaction between adjacent stacked layers and analyse the optical properties of stacked metamaterials with respect to an increasing number of layers.

Recently, metamaterials have gained a lot of attention due to their novel electromagnetic properties.[1-4] They consist of artificial structures which are much smaller than the operating wavelength of light. Their main potential is in the area of tailoring permittivity, permeability, and the refractive index of materials. [1][2][3][4] A medium with simultaneous negative permittivity and negative permeability can exhibit negative refraction and unique reversed electromagnetic properties, as discussed by Veselago [5] long before such materials were fabricated. A negative refractive index material, [1,4] which does not exist in nature, can be realized by a specifically designed metamaterial when its negative permeability and negative permittivity are tuned to the same frequency band. In the GHz frequency range, negative permeability was achieved using split-ring resonators (SRRs). [6] In combination with thin continuous wires, which are responsible for negative permittivity, [7] a negative refractive index material was first demonstrated by Smith et al. in 2000. [8] Because metamaterial building block dimensions in the GHz frequency range are on the order of millimeters, it is easy to stack them together to threedimensional (3D) electromagnetic elements for practical applications such as perfect lenses [9,10] and cloaking devices. [11,12] In the optical regime, the most common designs for metamaterials include SRRs, cut-wires, and meshes. [3,[13][14][15][16] These structures are entirely based on two-dimensional (2D) systems, which cannot be used as bulk optical elements. Stacking of 2D metamaterials is assumed to be the pathway towards the third dimension in the optical regime.[17] However, this is a difficult task due to the limitations of current nanofabrication technology. To overcome these difficulties, in this paper we suggest a simple and practical approach to increase the effective number of metamaterial layers, namely introducing a metal mirror. [18] We would like to concentrate on the cut-wire design for 2D metamaterial stacking due to its ease of nanofabrication. In addition, stacking of metamaterials in general confronts another fundamental problem: vertical electromagnetic coupling of neighboring metamaterial layers has to be addressed. In this contribution, we also investigate the interaction of neighboring cut-wire layers and solve it elegantly with the method of plasmon hybridization. [19][20][21][22] We present experimental evidence as well as relevant theoretical simulations to prove that this scheme works well for metamaterial stacking. The general principles, which we present in this paper, might pave the way for new rules of metamaterial designs and for the understanding of the resonances in these systems. Our samples were fabricated using standard electron beam lithography and thermal evaporation of the constitute materials followed by a lift-off procedure. Figure 1A and B schematically illustrate the samples, which consist of gold cut-wires (sample I) and gold cut-wire pairs (sample II) sepa...

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We demonstrate the fabrication of metallic photonic crystals, in the form of a periodic array of gold nanowires on a waveguide, by spin-coating a colloidal gold suspension onto a photoresist mask and subsequent annealing. The photoresist mask with a period below 500 nm is manufactured by interference lithography on an indium tin oxide (ITO) glass substrate, where the ITO layer has a thickness around 210 nm and acts as the waveguide. The width of the nanowires can be controlled from 100 to 300 nm by changing the duty cycle of the mask. During evaporation of solvent, the gold nanoparticles are drawn to the grooves of the grating with apparently complete dewetting off the photoresist for channels less than 2 microm in width, which therefore form nanowires after the annealing process. Strong coupling between the waveguide mode and the plasmon resonance of the nanowires, which is dependent on the polarization and incidence angle of the light wave, is demonstrated by optical extinction measurements. Continuity of the nanowires is confirmed by conductivity properties. Simplicity, high processing speed, and low cost are the main advantages of this method, which may have a plethora of applications in telecommunication, all-optical switching, sensors, and semiconductor devices.

In order to provide a guide for the design and optimization of bowtie slot antennas in the visible and near infrared spectral regime, their optical properties have been investigated with emphasis on geometry and materials. Although primarily theoretical, experimental investigations for reduced thickness cases are also included. As characterized by their field patterns, two types of resonances are discussed: plasmonic and Fabry-Pérot-like resonances. These resonance types show a linear dependence on aperture perimeter and film thickness, respectively, while showing a complementary behavior with near independence of the other respective parameter. Metal properties, as in the Drude model, are also considered. Various metals with respectively different skin depths are studied, showing a nearly linear dependence of the resonance wavelength on skin depth.

We introduce a plasmon hybridization picture to understand the optical properties of double split-ring resonator metamaterials. The analysis is based on the calculated reflectance spectra from a finite-integration time-domain algorithm. Field distributions of the double split-ring resonators at the resonant frequencies confirm the results from the plasmon hybridization analysis. We demonstrate that the plasmon hybridization is a simple and powerful tool for understanding and designing metamaterials in the near infrared and visible regime.

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