Session A35: Focus Session: Negative Index Materials I

Discrete Breathers in magnetic metamaterials in one and two dimensions.

We study the formation, stability as well as mobility of discrete breathers (DBs) in magnetic metamaterials in one and two dimensions. Magnetic metamaterials consisted by split ring resonators (SRRs) exhibit large magnetic response at Terahertz and optical frequencies. We use nonlinear arrays of SRRs where DBs arise as a result of the nonlinearity and discreteness. We consider different geometries of SRRs in both dimensions and find several different types of Hamiltonian nonlinear excitations such as dark, single-site and multibreathers. We also consider the dissipative version of the problem where DBs are formed as well. In the latter case, DBs locally alter the paramagnetic character of MMs to a diamagnetic.   

Negative response and intrinsic localization in rf SQUID metamaterials.

A periodic array of rf SQUIDs in an alternating magnetic field acts as an inherently nonlinear magnetic metamaterial, due to the nonlinearity of the Josephson element and the resonant properties of the SQUIDs themselves. Neighboring SQUIDs are weakly coupled due to magnetic dipole-dipole interaction through their mutual inductances, allowing an effective medium description if the wavelength of the applied field is much larger than the array period. We found that SQUID arrays can provide negative magnetic response, and thus negative permeability, above the resonance frequency of the individual SQUIDs. Moreover, that response can be tuned by the applied flux. Dissipative SQUID arrays, modeled as discrete networks, support nonlinear excitations in the form of intrinsic localized modes (discrete breathers). We found that dissipative discrete breathers exist in both one- and two-dimensional SQUID arrays. Furthermore, those breathers alter locally the magnetic response of an array from paramagnetic to diamagnetic or vice versa.   

Hybrid metamaterials for dynamic tuning

Advances in the field of metamaterials have created many new and exciting devices, but the performance and applicability of these devices to date have been hindered by the reliance on a dispersive resonance. In this talk we present a metamaterial device with the ability to dynamically tune the center frequency of its far-infrared resonance in real-time, alleviating many of the limitations of dispersion. Our device combines the familiar Split Ring Resonator (SRR) element with a thin film of rare earth oxide possessing a metal-to-insulator phase transition that occurs just above room temperature. During this phase transition, the electromagnetic responses of the oxide-film and SRR become intertwined, creating a sort of hybrid metamaterial. This interaction allows us to manipulate the resonance of the SRR via the oxide-film material properties. The device exhibits a dynamically tunable resonance -- shifting center frequency of the magnetically active SRR mode by as much as 20{\%} within 2 degrees Kelvin of temperature control.   

Tunable plasmonic nanostructures and nanolenses in optical domain

We have designed and studied various periodic metamaterials with the resonant response in the IR and optical range. In particular, the stacks of metallic films with periodic hole arrays separated by dielectric layers (fishnet, FN) have been demonstrated to have a negative index at IR frequencies. We were able to control the index of refraction of a fishnet with an amorphous semiconductor spacer layer in a pump-probe experiment [1]. This opens up ways for modulating the light at the nanoscale. We have also observed strong second- and third- harmonic generation with this metamaterial. We discuss various uses of a gain material to compensate the losses. Arrays of metallic nanoparticles or holes support individual and collective plasmonic excitations that contribute to surface enhanced Raman scattering (SERS) with thousand-fold field enhancement factor that can be used for single-molecule detection and other applications of the ``light at the nanoscale.'' \newline \newline [1] E. Kim, et al., Appl. Phys. Lett. 91, 173105 (2007) \newline [2] E.V. Ponizovskaya, A.M.Bratkovsky, Appl.Phys. A 87, 161 (2007)   

Negative Refraction Index in Magnetic Semiconductors

A novel principally homogeneous, non-composite magnetic semiconductor, or Chromium doped Indium Oxide, with the Curie temperature well above room temperature with natural negative refraction index in the THz range will be presented. The negative refraction index arises due to the overlapping of the negative permittivity in the plasmon subsystem and the negative permeability in the spin wave (magnon) subsystem within the same frequency domain. Since the losses in the magnetic mode are almost negligible, and the additional scattering losses due to the inhomogeneities are not present in our homogeneous medium, the total losses are exclusively due to the plasmon decay. Consequently, the negative refraction index wave has losses approximately 5 times smaller than losses in any of the currently known inhomogeneous designs. The parameters of both plasmon and magnon subsystems are calculated from the extended Band Theory, and first principles, respectively, and validated with available experimental data. Analytical expressions which describe the negative refraction index band are also presented.   

Negative refraction and an optical analogue of a directional spin valve in multiferroic materials

We present a new mechanism for negative refraction caused by a magnetoelectric (ME) effect. The ME effect appears in multiferroic materials in which spatial inversion and time-reversal symmetries are simultaneously broken. Such symmetry breakings allow us to control the spontaneous electric polarization by a magnetic field. We study polaritonic states in multiferroics and show that an asymmetric dispersion relation due to the ME effect gives rise to an optical analogue of a directional spin valve and a one-way waveguide as well as the negative refraction. We also estimate a realistic size of the effect.   

Infrared Properties of NiO-SrTiO$_{3}$ Composites

Magnetic-dielectric composites with overlapping magnetic- and electric-dipole resonances are promising candidates in the search for artificial systems with negative refractive index [1]. Here we report on the fabrication and infrared characterization of NiO-SrTiO$_{3 }$~ceramics. Transmission and reflection data were obtained in the 10-700 cm$^{-1}$ range using both a THz time-domain and a FTIR spectrometer. The spectra show features associated with the bulk antiferromagnetic resonance of NiO and the soft mode of SrTiO$_{3}$, as well as new collective modes of the aggregate. The results are in qualitative agreement with effective medium theories. \newline [1] S. D. Kirby, M. Lee, R. B. van Dover, J. Phys. D \textbf{40,} 1161 (2007)   

Measurements of the electric susceptibilities of Au nanorods at optical frequencies

Accurate knowledge of the electric susceptibilities of nanoparticles is of key importance in the design of optical metamaterials. We have determined the principal values of the susceptibility tensor of Au nanorods by measuring the real and imaginary phase shift of light transmitted by Au nanorod suspensions in organic solvents. The nanorods were aligned by an externally applied low frequency electric field. The real and imaginary parts of the phase shift were determined using a conoscopic Mach-Zehnder interferometer with a dye laser and a spectrophotometer, respectively. We discuss our procedure of extracting the principal values of the susceptibility tensor as function of wavelength from the experimental data. We consider the implications of our results for the construction of optical negative index metamaterials.   

Tuned permeability in terahertz split-ring resonators for devices and sensors

A process is demonstrated for tuning the magnetic resonance frequency of a fixed split-ring resonator array, by way of adding material near the split-ring elements. Applying drops of a silicon-nanospheres/ethanol solution to the surface of the sample decreases the magnetic resonance frequency of the split-ring array in incremental steps of 0.03 THz. This fine tuning is done post fabrication and is demonstrated to be reversible. The exhibited sensitivity of the split-ring resonance frequency to the presence of silicon nanospheres also suggests further application possibilities as a sensor device.   

Double Negative Index of Refraction Observed in a Single Layer of Closed Ring Resonators

We report the results of a spectroscopic study of a single layer of nanoscale metallic single closed ring resonators on a free- standing thin membrane at near-normal and grazing angles of incidence$^{[1]}$. When the magnetic component of the light is perpendicular to the ring plane, we observe a so-called ``double'' negative index of refraction at near-infrared frequencies attributed to a strong magnetic dipolar resonance and a broad electric resonance in this metamaterial. We experimentally identify the different resonance modes and the spectral region of negative refractive index on a series of samples with different feature and lattice sizes, using multi- oscillator fits and comparing to electromagnetic simulations. \newline [1] Z. Hao, M. C. Martin, B. Harteneck, S. Cabrini, E. H. Anderson, Appl. Phys. Lett., in press (2008).   

Z-scan measurement of oriented Au nanoparticle suspensions

The Z-scan technique, developed by the CREOL group$^{1}$, is a simple and effective method for measuring intensity dependent optical nonlinearities of materials. We have carried out Z-scan measurements of gold nanorods suspended in organic solvents using a CW laser. A low frequency external electric field was used to orient the nanoparticles$^{2}$. We present our experimental results for the real and imaginary parts of the nonlinear phase shift as function of the applied aligning electric field. We consider a variety of possible contributing physical mechanisms, and compare their expected contributions with experimental observations. [1] M. Sheik, A.A. Said, and E.W. Van Stryland, \textit{Opt. Lett.} \textbf{14}, 955 (1989). [2] J. Fontana, and P. Palffy-Muhoray, APS March meeting 2008, New Orleans, LA (2008).   

Numerical simulation of the non-local optical response of nanoparticles

The interaction of nanoparticles with light is a primary focus of research in negative index materials. When the wavelength of light is comparable to the particle size, significant non-local effects are expected in the electric and magnetic response of the nanoparticles. It has been suggested that the spatially non-local response may be taken into account via the bianisotropic formalism for the constitutive equations. We have carried out computer simulations of the optical response of nanoparticles using both the discrete dipole approximation and the finite integration technique to determine the effectiveness of these bianisotropic constitutive equations. We present our results, which indicate that the approach of Agranovich et al.[1] provides a better description of the non-local optical response than the bianisotropic formalism. \newline [1] V.M. Agranovich and V.L. Ginzburg, ``Spatial dispersion in crystal optics and the theory of excitons'', (Interscience, London, 1966).