Metamaterial Spacers
Abstract: In this project, we investigate metaspacers as replacement to conventional materials to extend the capabilities of the devices produced by microfabrication. We seek answer to the question, "Can we make new materials (i.e., meta-metamaterials) with novel physical properties using metamaterials made from natural materials?"
Keywords: Metamaterials
Following the studies on backward wave propagation [1-4], in 1968 Veselago systematically showed that media with simultaneously negative permittivity and permeability, referred later as negative index metamaterials, exhibit unusual and interesting properties such as negative refraction, reverse Doppler's effect, and reverse Cherenkov effect [5]. Research on metamaterials did not gain much attention until Pendry et al. proposed structures to artificially achieve negative permittivity [6, 7] and negative permeability [8]. This was followed by actual fabrication of negative index metamaterials by Smith et al. [9, 10]. Since then, numerous metamaterials have been proposed for different frequency regimes [11-14] to demonstrate many interesting properties and applications such as high precision lithography [15], perfect lens [16], high resolution imaging [16, 17], invisibility cloaks [18], small antennas [19], optical analog simulators [20, 21], and quantum levitation [22].
On the other hand, conventional materials that are used as spacers in microfabrication provide inherently limited optical and electronic properties. For example, dielectric spacers have permittivity higher than unity and are generally nonmagnetic. However, metamaterials can be designed to provide almost any value of permittivity and permeability. Utilizing this feature, metamaterials can replace conventional spacers in microfabrication. Such 'metaspacers' may be used in applications requiring very low index materials [23] or high permeability ferrites [24]. Furthermore, metaspacers can lead to devices/applications requiring spacers having index less than unity (even negative). Despite this great potential, metaspacers have not drawn the deserved attention. Here we define a metaspacer as a metamaterial structure that can be used as a spacer, integrated with other materials to fabricate novel devices that are not possible by using conventional materials.
Metaspacers can be designed to support different types of surface plasmon polariton (SPP) modes. A naturally available dielectric and a metal can form an interface with opposite signs of permittivity and support p-polarized SPPs. However, using a metaspacer, an interface with different signs of permittivity and/or permeability can be realized to support p- (or TM-) and/or s- (or TE-) polarized SPPs, respectively [25, 26]. For example, an interface between a metal and a negative index material can support s-polarized SPPs.
As part of this project, we have theoretically investigated metaspacers as replacement to conventional materials to extend the capabilities of the devices produced by microfabrication. We have sought answer to the question, "Can we make new materials (i.e., meta-metamaterials) with novel physical properties using metamaterials made from natural materials?"
In particular, we have studied negative index metaspacer(most unusual spacer) embedded fishnet metamaterial structure (most studied and convenient optical metamaterial structure) [27]. The negative index metaspacer is placed between two metal layers. An inverted optical response in is observed in the retrieved effective permeability contrary to conventional magnetic metamaterials (see Fig. 1). This inverse optical response results in two interesting consequences: (i) permeability similar to ferrites at microwave frequencies [28], (ii) simultaneously negative group and phase velocity at the low-loss region. The latter was observed recently in an experiment in the high-loss region (i.e., single negative region) [29, 30].
Figure 1 Retrieved effective parameters for the (a) conventional fishnet structure and the (b) unconventional fishnet structure. Insets show the corresponding transmission-reflection spectra.
Acknowledgments
This work was supported in part by the National Science Foundation under grant ECCS-1202443 and by the Oak Ridge Associated Universities.
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