J. D. Ortiz

Self-Complementary Metasurface for Designing Narrow Band Pass/Stop Filters

A self-complementary metasurface is studied in this paper. The metasurface is a 2-D periodical arrangement of unit cells formed by a metallic printed split ring resonator and its complementary counterpart. It is demonstrated that this structure behaves like a very selective band-pass filter for a certain linear polarization while band-stop filtering is achieved for the orthogonal polarization over the same frequency range. This idea opens the door to a new class of frequency selective surfaces made of connected and unconnected elements, whose filtering properties are mechanically tunable from band-pass to band-stop by rotating the surface or the polarization.

Spatial Angular Filtering by FSSs Made of Chains of Interconnected SRRs and CSRRs

Two frequency selective surfaces (FSSs) made of an infinite set of parallel 1-D chains of interconnected split ring resonators (I-SRRs) and interconnected complementary split ring resonators (I-CSRRs) are studied. The main result was that the central frequencies of the stopband and passband can be strongly tuned by controlling the angle of incidence.

A band-pass/stop filter made of SRRs and C-SRRs

Frequency Selective Surfaces (FSS) are usually classified into two big groups depending on wether they are made of unconnected elements or connected elements. Close to their resonance frequency, the first type behaves like a band-stop filter while the second type like a band-pass filter. In this paper we propose a new type of surface made of Split Ring Resonators (SRRs) and, at the same time, Complementary Split Ring Resonators (CSRRs), placed in such a way that the surface is self-complementary. The main result is that this FSS shows band-stop features for one linear polarization state and band-pass features for the orthogonal polarization. Therefore, it is in the middle between the two usual groups of FSS, what could drive us to new designs of band-pass/stop filters which are are easily switchable from band-pass to band-stop, and viceversa, by simply rotating the surface through 90 degrees.

Experimental demonstration of the saturation and weakening of the resonant response of the SRR and the CSRR

Summary form only given. In 1999, J. B. Pendry and co-workers proposed the Split Ring Resonator (SRR) for designing magnetic materials at high frequencies from non magnetic metal (J. B. Pendry et al., IEEE Trans. MTT 47, 2075, 1999). This possibility was experimentally demonstrated in microwaves (D. R. Smith et al., Phys. Rev. Lett. 84, 4184, 2000) and millimeter waves (T. J. Yen et al., Science 303, 1494, 2004). However, when the particle is scaled down enough to resonate in the optical range, the resonance frequency achieves a saturation level and the magnetic response becomes very weak (J. Zhou et al., Phys. Rev. Lett. 95, 223902, 2005). This phenomenon was only demonstrated through numerical simulations, but there has not been yet an experimental probe. Separately, Zentgraf et al. experimentally demonstrated that the Babinets principle is almost valid for the SRR and its complementary screen, the so called Complementary-SRR (CSRR), in spite of bad conductivity of metals and big thickness of the screens (T. Zentgraf et al., Phys. Rev. B 76, 033407, 2007). Now, in this paper, we experimentally study the saturation and weakening phenomenon for both SRR and CSRR. Figure 1 shows the unit cell of a 2D array of square SRRs, being yellow for silver and blue for the substrate. The CSRR looks the same but with yellow and blue colors interchanged. The samples were manufactured by ion milling over silver lying on an Infrasil substrate. The geometry was checked by S EM microscopy. Transmission and reflection coefficients were obtained by FTIR. Table 1 shows the resonance frequencies for geometries gradually scaling from period a = 2000 nm down to a = 500 nm. For instance, the fundamental resonance of the SRR was found at 38.3 THz for a = 2000 and 73.9 THz for a = 500 nm. Thus, the resonance frequency grew up by a factor of 1.9, while the geometry was reduced by a bigger factor of 4. It is clearly evidencing that the beginning of the saturation. Also we observed the weakening of the resonant response for smaller particles. This phenomenon will be critical for application of metamaterials in optics.

Metasurfaces made of transmission lines: A way to spatial filtering

Since decades ago, many researchers have been involved in the design of Frequency Selective Surfaces (FSS). A comprehensive review of the theory was collected by the well-known author B. A. Munk (B. A. Munk, Frequency Selective Surfaces: Theory and Design, 2000.). In more recent years, FSS based on particles previously used in metamaterials are being investigated. These are commonly called metasurfaces and share the good property of having a unit cell much smaller than the wavelength. One example of is given in Fig. 1(a). This metasurface is based on the use of the Split Ring Resonator (SRR). It was demonstrated by F. Falcone and co-workers that it behaves as a band-pass filter (F. Falcone et al., Phys. Rev. Lett., vol. 93, p. 197401, 2004). It has been also reported that this behaviour remains stable under oblique incidence (M. Beruete et al., Electromagnetics, vol. 26, p. 247, 2006.).The aim of this paper is to investigate the effects of connecting these resonators forming 1D chains as shown in Fig. 1(b), and compare its behaviour with that of the sample with unconnected particles. We found that the new structure (Fig. 1(b)) provide angle-dependent tunability, being possible to significantly shift the central frequency while keeping the bandwidth constant. 1n other words, this kind of metasurfaces has the capability of spatial filtering. Figure 2 shows the simulated transmission coefficients for metasurfaces made with unconnected (dotted lines) and interconnected SRRs (solid lines). Geometrical and physical parameters for these two samples were similar to those used by M. Beruete in the paper cited above. The difference between these two samples is very clear: when the incidence angle is modified, the central frequency moves for the sample with interconnected SRRs while it stays invariant for the case of unconnected SRRs. A model of surface waves based on a transmission line model has been proposed to study these structures. The expected tuning was finally checked by numerical simulations and measurements. Model and measurements will be shown during the conference days.

Extremely thin infrared absorbers made of metallo-dielectric core-shell nanospheres

During the first years of Metamaterials, losses were usually considered undesirable for the most of envisioned applications. Oppositely, in 2008 a paper titled “Perfect Metamaterial Absorbers” just took advantage of the losses (N. I. Landy et al., Phys. Rev. Lett., vol. 100, p. 207402, 2008). A perfect absorber was built as a 2D periodical array of resonant particles responding to both the electric and magnetic field. Just at the resonance frequency a narrow band of quasi-perfect absorption appears. An important drawback is that this absorber is polarization dependent and very sensible to the incidence angle. Subsequent papers (X. Liu et al., Phys. Rv. Lett., vol. 104, p. 207403, 2010; J. Hao et al., High performance optical absorber based on a plasmonic metamaterial, Appl. Phys. Lett., vol. 96, p. 251104, 2010) addressed this problem by using as unit cell a metallic cross or a metallic square over a metallic plane. The same plausible explanation was given in all these papers: in order to avoid any reflected power, the effective medium impedance must match to that of vacuum and, in order to reduce the transmitted power, the imaginary parts of ε and μ must be high enough, so that most of the power is dissipated inside the slab.

On the Lorentz's homogenization method applied to metamaterials presenting strong spatial dispersion

Metamaterial concept is being attracting the attention of many electrical engineers and physicists since some time ago, because it opens the door to a new branch of exotic phenomena and devices which would not be possible by means of natural media [1–4]. Most of times in the literature metamaterials are periodic arrangements of metallic resonant particles. The unit element is designed to resonate at a wavelength much bigger than the periodicity, so that a local or mean field theory may be used. However, close to the resonance frequency the constitutive parameters of the medium reach very high values which implies a shortening of the medium wavelength. Then, there is a sub-band inside the metamaterial frequency band that requires a non-local theory including spatial dispersion into the constitutive parameters. Several attempts has been done in this direction [5, 6]. In this paper we critically review and modify the key steps of the classical Lorentzs local field. We apply the modified Lorentzs method to an infinite cubic arrangement of split ring resonators (SRRs).