James C. Ginn

Realizing optical magnetism from dielectric metamaterials.

We demonstrate, for the first time, an all-dielectric metamaterial resonator in the mid-wave infrared based on high-index tellurium cubic inclusions. Dielectric resonators are desirable compared to conventional metallo-dielectric metamaterials at optical frequencies as they are largely angular invariant, free of ohmic loss, and easily integrated into three-dimensional volumes. With these low-loss, isotropic elements, disruptive optical metamaterial designs, such as wide-angle lenses and cloaks, can be more easily realized.

Optical magnetic mirrors without metals

The reflection of an optical wave from metal, arising from strong interactions between the optical electric field and the free carriers of the metal, is accompanied by a phase reversal of the reflected electric field. A far less common route to achieving high reflectivity exploits strong interactions between the material and the optical magnetic field to produce a “magnetic mirror” that does not reverse the phase of the reflected electric field. At optical frequencies, the magnetic properties required for strong interaction can be achieved only by using artificially tailored materials. Here, we experimentally demonstrate, for the first time to the best of our knowledge, the magnetic mirror behavior of a low-loss all-dielectric metasurface at infrared optical frequencies through direct measurements of the phase and amplitude of the reflected optical wave. The enhanced absorption and emission of transverse-electric dipoles placed close to magnetic mirrors can lead to exciting new advances in sensors, photodetectors, and light sources.

Infrared plasmons on heavily-doped silicon

We examine the long-wave infrared (LWIR) optical characteristics of heavily-doped silicon and explore engineering of surface plasmons polaritons (SPP) in this spectral region. Both phosphorus (n-type Si) and boron (p-type Si) implants are evaluated and various cap layers and thermal annealing steps are examined. The optical properties are measured using ellipsometry and fit to a Drude model for the infrared (IR) permittivity. The predicted metallic behavior for Si in the thermal IR and its impact on the spatial confinement and dispersion for surface plasmons is studied. We find that the transverse spatial confinement for a surface plasmon on highly doped Si is strongly sub-wavelength near the plasma edge, and the confinement to the surface is enhanced to greater than 10× that of the metal confined SPP over the entire LWIR spectrum.

Micrometer‐Scale Cubic Unit Cell 3D Metamaterial Layers

www.MaterialsViews.com C O M Micrometer-Scale Cubic Unit Cell 3D Metamaterial Layers M U N I By D. Bruce Burckel , * Joel R. Wendt , Gregory A. Ten Eyck , James C. Ginn , A. Robert Ellis , Igal Brener , and Michael B. Sinclair C A IO N The electromagnetic (EM) behavior of most bulk materials can be summarized by two frequency dependent tensors, the dielectric permittivity ( ε ) and the magnetic permeability ( μ ). The fi eld of metamaterials is predicated upon the possibility that man-made materials can exhibit absolute control over the magnitude and sign of both ε and μ within a specifi ed spectral band through the use of designed inclusions with specifi c engineered EM properties to create a new class of devices with enhanced functionality. For non-chiral, artifi cially structured materials, exotic EM behaviors such as perfect lensing [ 1 ] and negative refraction [ 2 ] are possible where ε and μ are simultaneously negative while cloaking requires the ability to spatially vary the permittivity and/or permeability over a wide range of values according to prescriptions from transformation optics. [ 3 ] In practice, achieving simultaneous negative ε and μ is diffi cult. The permittivity of metals below the bulk plasma frequency is inherently negative, and can be large; [ 4 ] however, naturally occurring negative permeability is quite rare, being presently limited to ferromagnetic materials at RF frequencies and lower. Furthermore, at high frequencies (optical and infrared) the response of materials to incident electromagnetic fi elds is dominated by the permittivity of the material, since the magnetic susceptibility is typically 4 orders of magnitude smaller in bulk materials than the electric susceptibility, [ 5 ] making μ r ≈ 1. In 1999 Pendry proposed an isotropic cubic unit cell with split ring resonators (SRR) located on its faces as a method for creating an artifi cially magnetic material in the microwave frequency range. [ 6 ] Shortly after Pendry proposed using SRRs to achieve magnetic permeability tuning of the resulting material, a negative index material was demonstrated with design wavelength λ = 30 mm (microwave) using a combination of SRRs and wires. [ 7 ] The individual SRRs measured 2.6 mm/side, were printed on circuit board material and then assembled into a macroscopic 2D array. One might expect that straightforward extension of the design, fabrication and characterization aspects of microwave metamaterials would lead to analog IR and visible metamaterials merely scaled to the wavelength of interest. In practice, however, translation of metamaterials

Strong coupling between nanoscale metamaterials and phonons.

We use split ring resonators (SRRs) at optical frequencies to study strong coupling between planar metamaterials and phonon vibrations in nanometer-scale dielectric layers. A series of SRR metamaterials were fabricated on a semiconductor wafer with a thin intervening SiO(2) dielectric layer. The dimensions of the SRRs were varied to tune the fundamental metamaterial resonance across the infrared (IR) active phonon band of SiO(2) at 130 meV (31 THz). Strong anticrossing of these resonances was observed, indicative of strong coupling between metamaterial and phonon excitations. This coupling is very general and can occur with any electrically polarizable resonance including phonon vibrations in other thin film materials and semiconductor band-to-band transitions in the near to far IR. These effects may be exploited to reduce loss and to create unique spectral features that are not possible with metamaterials alone.

Planar infrared binary phase reflectarray

A reflective, binary phase reflectarray is demonstrated in the infrared, at a wavelength of 10.6 microm. The unique aspect of this work, at this frequency band, is that the specific desired phase shift is achieved using an array of subwavelength metallic patches on top of a ground-plane-backed dielectric stand-off layer. This is an alternative to the usual method of constructing a reflective Fresnel zone plate by means of a given thickness of dielectric. This initial demonstration of the reflectarray approach at infrared is significant in that there is inherent flexibility to create a range of phase shifts by varying the dimensions of the patches. This will allow for a multilevel phase distribution, or even a continuous variation of phase, across an optical surface with only two-dimensional lithography, avoiding the need for dielectric height variations.

Phase Characterization of Reflectarray Elements at Infrared

The feasibility of a square-patch reflectarray element design is demonstrated at a frequency of 28.3 THz in the infrared (10.6 micrometer free-space wavelength) for the first time. Fabrication of arrays of various patch sizes was performed using electron-beam lithography, and the reflected phase as a function of patch size was characterized using an infrared interferometer. A numerical model for the design of these reflectarray elements was developed incorporating measured values of frequency-dependent material properties, and a comparison of computed and measured phase shows close agreement.

Polarized infrared emission using frequency selective surfaces

An emission frequency selective surface, or eFSS, is made up of a periodic arrangement of resonant antenna structures above a ground plane. By exploiting the coupling and symmetry properties of an eFSS, it is possible to introduce polarization sensitive thermal emission and, subsequently, coherent emission. Two surfaces are considered: a linearly polarized emission surface and a circularly polarized emission surface. The linearly polarized surface consisted of an array of dipole elements and measurements demonstrate these surfaces can be fabricated into high polarization contrast patterns. The circularly polarized surface required the use of an asymmetrical tripole element to maintain coherence between orthogonal current modes and introduce the necessary phase delay to realize circularly polarized radiation.

Demonstration of a single-layer meanderline phase retarder at infrared

Meanderline wave plates are in common use at radio frequencies as polarization retarders. We present initial results of a gold meanderline structure on a silicon substrate that functions at a wavelength of 10.6 microm in the IR. The measured results show a distinct change in the polarization state of the incident beam after passing through the device, inducing a 74 degrees phase retardance between horizontal and vertical components. A high degree of polarization (88%) is maintained in the transmitted beam with an overall power transmittance of 38% and a beam profile that remains essentially unchanged.

Reflectarray Design at Infrared Frequencies: Effects and Models of Material Loss

Reflectarray designs at infrared (IR) frequencies are investigated in this paper. At the short-wavelength region, material loss becomes an important consideration in reflectarray designs. Based on the measured properties of conductors and dielectrics at infrared frequency, this paper investigates the loss effects on the reflection magnitude and phase of reflectarray elements. It is revealed that when the material loss exceeds a certain limit, the element reflection phase will vary within a narrow phase range instead of a full 360° phase range. An equivalent circuit model is used to understand this phenomenon. Based on the investigation, alternative design methods for infrared reflectarrays are suggested to lower the loss effect. The low loss reflectarrays have great potential for infrared and visible range applications, such as a low profile planar concentrator for solar energy systems.