Georgia Theano Papadakis

Applied Physics Watson Laboratories California Institute of Technology

Layered optical metamaterials: effective parameters, magnetic effects and active tunability

Abstract

 Layered optical metamaterials: effective parameters,
magnetic effects and active tunability
Georgia T. Papadakis
Watson Laboratories of Applied Physics, Caltech, Pasadena, 91125
Metamaterials purpose is to expand the range of electromagnetic properties
achievable with materials existing in nature. Due to their complicated geometrical
structure composed of subwavelength elements, wave-propagation in metamaterials is
usually described in terms of effective electromagnetic parameters. In this talk, I will
discuss effective medium theories and mention previous parameter retrieval algorithms
established in the 1990s-2000s. Furthermore, I will present a newly developed retrieval
algorithm revealing that optical magnetism can be achieved in simple one-dimensional
dielectric heterostructures.
We focus our attention in planar metal-dielectric metamaterials that support
hyperbolic (unbound) dispersion diagrams and plasmonic surface waves. Such structures
support exotic electromagnetic properties ranging from negative refraction, diverging
density of optical states and hyper-lensing. However, up until now, their relevance has
been limited to only transverse magnetic polarization fields, due to lack of broadband
magnetic materials at optical frequencies. Utilizing our retrieval approach, we
experimentally demonstrate that such metamaterials exhibit non-trivial magnetic
permeability that ranges variously from μ>1 to μ<0 at optical frequencies. We are, thus,
able to generalize the concepts of plasmonic surface-bound states and hyperbolic media
to both linear polarizations of light, making plasmonic and hyperbolic structures able to
stand on both feet, responding to both electric and magnetic fields.
Moreover, I will discuss mechanisms enabling active tunability of the effective
response of hyperbolic metamaterials in the visible and IR spectral range. In the visible
regime, use of transparent conductive oxides (TCOs) and field-effect gating can give rise
to tuning of the effective optical parameters by as much as 150% in the visible regime
along with the opportunity to opening and closing optical band gaps. In the IR and THz
spectral range, the tunability of graphene monolayers via Fermi level tuning, is sufficient
to induce shifts of the effective permittivity of graphene/polaritonic dielectric
metamaterials in the order of tens of μm. A tunable electromagnetic response via external
DC bias may lead to novel optical devices such as tunable optical polarizers, lasing
applications, smart switches and actively controllable topologically protected photonic
states.
References:
1. G.T. Papadakis, P. Yeh and H. A. Atwater, Phys. Rev. B 91, 155406 (2015)
2. G.T. Papadakis & H. A. Atwater, Phys. Rev. B 92, 184101 (2015)
3. A. Poddybny, I. Iorsh, P. Belov and Y. Kivshar, Nat. Photon. 7, 948 (2013)
4. G. T. Papadakis, D. Fleischman, A. Davoyan, P. Yeh and H. A. Atwater, arXiv:
1608.02909 [physics.optics] (2016)



Date: 15/9/2016
Time:15:00 (coffee & cookies will be served at 14:45)
Place:FORTH Seminar Room 1