Session D32: Focus Session: Optical Properties of Nanostructures and Metamaterials III

Spatial gradient tuning in metamaterials

Gradient Index (GRIN) metamaterials have been used to create devices inspired by, but often surpassing the potential of, conventional GRIN optics. The unit-cell nature of metamaterials presents the opportunity to exert much greater control over spatial gradients than is possible in natural materials. This is true not only during the design phase but also offers the potential for real-time reconfiguration of the metamaterial gradient. This ability fits nicely into the picture of transformation-optics, in which spatial gradients can enable an impressive suite of innovative devices. We discuss methods to exert control over metamaterial response, focusing on our recent demonstrations using Vanadium Dioxide. We give special attention to role of memristance and mem-capacitance observed in Vanadium Dioxide, which simplify the demands of stimuli and addressing, as well as intersecting metamaterials with the field of memory-materials.   

Carrier concentration dependence of the tunability of the dipole resonance peak in optically excited metamaterials

Currently, there is strong interest to explore the dynamic control of the electromagnetic properties of metamaterials, which have important implications on their optoelectronic applications. While the design, fabrication and photo-doping of metamaterial/semiconductor structures have been actively pursued, some fundamental issues related to highly photo-excited states, their dynamic tuning and temporal evolution remain open. Using optical-pump terahertz probe spectroscopy, we report on the pump fluence dependence of the electric dipole resonance tunability in metamaterials. We find a previously undiscovered large non-monotonic variation on the strength of the dipole resonance peak with the photo-injected carrier concentration.   

Optically-Nonactive Assorted Helices Array with Interchangeable Magnetic/Electric Resonance

We report here the designing of optically-nonactive metamaterial by assembling metallic helices with different chirality. With linearly polarized incident light, pure electric or magnetic resonance can be selectively realized, which leads to negative permittivity or negative permeability accordingly. Further, we show that pure electric or magnetic resonance can be interchanged at the same frequency band by merely changing the polarization of incident light for 90 degrees. This design demonstrates a unique approach to construct metamaterial.   

Three-dimensional optical metamaterials and nanoantennas: Chirality, Coupling, and Sensing

This abstract not available.   

Temperature-Tunable Transparency Window in Metamaterials Utilizing Superconducting Dark Resonators

We have developed a high quality-factor microwave frequency metamaterial to demonstrate a coherent optical phenomena analogous to electrically induced transparency (EIT). The two-dimensional design employs double planar Nb split rings acting as dark resonators symmetrically placed around a thick Au strip which is a bright resonator [1]. When Nb is in the superconducting state, the significant loss gradient between Nb and Au opens a transparency window along with a strongly enhanced group delay. The data show a systematic evolution with increasing temperature in the superconducting state of Nb, and the features disappear in the resistive state when the loss gradient between the two types of resonators closes. We have observed no RF power dependence of the magnetic response coming out of the EIT configuration, which indicates the process is linear. Laser scanning microscopy images of the RF current distributions in the dark resonators and the other microwave measurements are in good agreement with the simulations run on the same structure. \\[0pt] [1] L. Zhang, \textit{et al}. arXiv:1010.2976   

Light propagation and Anderson localization in superlattices containing metamaterials: effects of correlated disorder

We discuss the effect of correlated disorder on light propagation and Anderson localization in a one-dimensional superlattice made up of air (A) and a dispersive metamaterial (M). Disorder is incorporated by assuming the layer widths to be random variables; however, here we consider the cases of correlated (i.e., the A and M layers with the same width) and completely anti-correlated (the total width of the A and M layers is fixed). We use transfer matrix techniques to obtain the localization length, and compare with the uncorrelated case. We have found that the photonic gaps of the corresponding periodic structure are not completely destroyed in the presence of disorder, giving rise to minima in the localization length. Near a gap, the behavior the localization length depends crucially on the physical origin of the gap (Bragg or non-Bragg gaps). We have found that the asymptotic behavior for the localization length $\xi \propto \lambda^{6}$ for disordered metamaterials is not affected by correlations, and the Brewster anomalies, at which light is delocalized, are also present.   

Active Terahertz Metamaterials

In recent years terahertz technology has become an optimistic candidate for numerous sensing, imaging, and diagnostic applications. Nevertheless, THz technology still suffers from a deficiency in high-power sources, efficient detectors, and other functional devices ubiquitous in neighboring microwave and infrared frequency bands, such as amplifiers, modulators, and switches. One of the greatest obstacles in this progress is the lack of materials that naturally respond well to THz radiation. The potential of metamaterials for THz applications originates from their resonant electromagnetic response, which significantly enhances their interaction with THz radiation. Thus, metamaterials offer a route towards helping to fill the so-called ``THz gap''. Here, we present a series of novel THz metamaterials. Importantly, the critical dependence of the resonant response on the supporting substrate and/or the fabricated structure enables the creation of active THz metamaterial devices. We show that the resonant response can be controlled using optical or electrical excitation and thermal tuning, enabling efficient THz devices which will be of importance for advancing numerous real world THz applications.   

Optical properties of chiral metal nanoparticle complexes: Plasmonic chirality and circular dichroism

Plasmonic nanocrystals with chiral geometries are able to create strong circular dichroism (CD) signals in the visible wavelength range. This offers an interesting possibility to design colloidal and other nanostructures with strong optical chirality for applications in biosensors and optoelectronic devices. We present a theoretical study of circular dichroism from chiral metal nanoparticle (NP) complexes. Dipolar Coulomb interactions between NPs are involved as the main mechanism of interaction between spherical NPs in a chiral complex. In our analysis, the CD signal shows strong dependence on geometry and composition of chiral pyramids, tetramers, and helices. The CD spectra have both positive and negative bands. Strongest CD signals were found in helical chains of metal NPs, which resemble helical structures of many biomolecules.   

Plasmonic Behavior of Deep Sub-Wavelength Superconducting RF Metamaterials

We have designed and built ultra-small RF metamaterials with magnetically active spiral elements made of superconducting Nb films [1]. RF transmission measurements on single, 1-D and 2-D arrays of spirals show robust magnetic response when Nb is in the superconducting state [2] at frequencies as low as 14 MHz (corresponding to wavelength $\sim $ 3000 * 'atom' size). Numerical simulations capture the main features of the experimental spectra. The resonant features are tunable via variations in temperature and RF magnetic field [3]. As temperature approaches T$_{c}$, the superconducting kinetic inductance contribution to the total inductance increases, placing this RF metamaterial in the plasmonic limit. We study this approach to the plasmonic limit and compare to the analogous situation of frequency approaching the plasma edge in normal metal metamaterials. \\[4pt] [1] S. M. Anlage, J. Opt. \underline {13}, 024001 (2011). \\[0pt] [2] C. Kurter, \textit{et al}., Appl. Phys. Lett. \underline {96}, 253504 (2010). \\[0pt] [3] C. Kurter, \textit{et al}., IEEE Trans. Appl. Supercond., in press. arXiv:1008.2020.   

Invisibility Using Perfect Absorption CNT Carpet at Visible Frequency and Beyond

The concept of invisibility cloak based on transformation optics and metamaterials has tantalized the scientific community. Cloaking of wavelength-size objects were realized at microwave and NIR frequencies. However, the complexity of metamaterials based on the previous principles limits the object to several wavelengths in size. Moreover, cloaking of 3-D objects at visible band demands challenging inhomogeneous 3D nanostructured metamaterials and still unattainable. We propose a perfect absorption ground plane cloak that works at visible range and for large area arbitrarily shaped 3D objects. Such homogeneous perfect absorption carpet is demonstrated by low density carbon nanotube (CNT) forest, which can visually compress arbitrary 3D objects to appear as a 2D perfect absorption sheet. Invisibility was observed by naked eyes for unpolarized light at entire visible band with cloaking area of 10$^{5}$ larger than a wavelength. Such a cloaking approach based on perfect absorption is not restricted to CNT carpet, and can be applied to a broader frequency range from UV to THz and acts as a universal cloak for arbitrarily large objects. In this scheme the deep space is a natural and perfect ``ground plane''. It would only take a ``cloak'' consisting of low density and broadband absorbing particles to render matters and objects totally ``dark'' to our current instruments.   

Collective dynamics in optomechanical arrays

Photonic crystals can support both localized optical and vibrational modes that couple to each other, leading to a very strong optomechanical interaction. These so-called ``optomechanical crystals'' have been demonstrated experimentally recently. Here we explore the dynamics that results in an array of many coupled optomechanical cells, when these are driven into a regime of self-sustained oscillations. We find synchronization of these oscillations beyond a certain coupling strength. We show that the slow phase dynamics can be efficiently described by an effective Kuramoto model. Other dynamical regimes like chaos will also be accessible in these novel systems.