Jan Wallauer

Lattice modes mediate radiative coupling in metamaterial arrays

We show that a resonant response with very high quality factors can be achieved in periodic metamaterials by radiatively coupling their structural elements. The coupling is mediated by lattice modes and can be efficiently controlled by tuning the lattice periodicity. Using a recently developed terahertz (THz) near-field imaging technique and conventional far-field spectroscopy together with numerical simulations we pinpoint the underlying mechanisms. In the strong coupling regimes we identify avoided crossings between the plasmonic eigenmodes and the diffractive lattice modes.

Controlling intensity and phase of terahertz radiation with an optically thin liquid crystal-loaded metamaterial.

We experimentally demonstrate intensity and phase modulation of terahertz radiation using an actively controlled large-area planar metamaterial (metafilm) hybridized with a 12 μm thick layer of a liquid crystal. Active control was introduced through in-plane electrical switching of the liquid crystal, which enabled to achieve a reversible single-pass absolute transmission change of 20% and a phase change of 40° at only 20 V.

Wavevector Selective Metasurfaces and Tunnel Vision Filters

Metasurfaces offer unprecedented flexibility in the design and control of light propagation, replacing bulk optical components and exhibiting exotic optical effects. One of the basic properties of the metasurfaces, which renders them as frequency selective surfaces, is the ability to transmit or reflect radiation within a narrow spectral band that can be engineered on demand. Here we introduce and demonstrate experimentally in the THz domain the concept of wavevector selective surfaces – metasurfaces transparent only within a narrow range of light propagation directions operating effectively as tunnel visionfilters. Practical implementations of the new concept include applications in wavefront manipulation, observational instruments, vision and free-space communication in light-scattering environments.

Terahertz imaging modalities of ancient Egyptian mummified objects and of a naturally mummified rat.

During the last few years, terahertz (THz) imaging has been used to investigate artwork and historic artifacts. The application of THz imaging to mummy investigations is very attractive since it provides spectroscopic information over a broad frequency range and its radiation has proven to be harmless to human cells. However, compared with the current standard imaging methods in mummy imaging—X‐ray and computed tomography (CT)—it remains a novel, emerging technique whose potential still needs to be fully evaluated. Here, ancient Egyptian mummified objects as well as a naturally mummified rat have been investigated by two different THz imaging systems: a broadband THz time domain imaging system and an electronic THz scanner. The obtained THz images are compared with conventional CT, X‐ray, and magnetic resonance images. While the broadband THz time domain setup permits analyses of smaller samples, the electronic THz scanner allows the recording of data of thicker and larger samples at the expense of a limited spectral bandwidth. Terahertz imaging shows clear potential for mummy investigations, although currently CT imaging offers much higher spatial resolution. Furthermore, as commercial mobile THz scanners become available, THz imaging could be applied directly in museums or at excavation sites. Anat Rec, 298:1135–1143, 2015.

Optimal plasmonic focusing on a metal disc under radially polarized terahertz illumination

Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated by radially polarized terahertz pulses is demonstrated. By matching the cylindrical symmetry of the metal structure with the radially polarized terahertz field, surface plasmons are excited along its entire circumference. Constructive interference in the disc center produces a sharp frequency-dependent focal spot well described by a zero-order Bessel function. We map the field distributions on the disc by terahertz (THz) near-field microscopy and compare our results with numerical simulations. For comparison, the behavior of the plasmonic lens under linearly polarized THz illumination is also characterized. The remarkable focusing capabilities of such a plasmonic lens together with its simple structure offer considerable potential for THz sensing and imaging applications.

Large area InN terahertz emitters based on the lateral photo-Dember effect

Large area terahertz emitters based on the lateral photo-Dember effect in InN (indium nitride) are presented. The formation of lateral photo-Dember currents is induced by laser-illumination through a microstructured metal cover processed onto the InN substrate, causing an asymmetry in the lateral photogenerated charge carrier distribution. Our design uses simple metal structures, which are produced by conventional two-dimensional micro-structuring techniques. Having favoring properties as a photo-Dember material InN is particularly well-suited as a substrate for our emitters. We demonstrate that the emission intensity of the emitters can be significantly influenced by the structure of the metal cover leaving room for improvement by optimizing the masking structures.

Mapping the coupling between a photo-induced local dipole and the eigenmodes of a terahertz metamaterial

We demonstrate that eigenmodes of a metamaterial structure at terahertz (THz) frequencies can be excited by photo-generation of localized transient dipoles in the semiconductor substrate. We apply this technique to map the coupling of these dipoles to the resonators near-field. The characteristic metamaterial resonances appear as peaks in the spectrum of the THz radiation emitted from the resonant structures into the far-field. Recording two-dimensional THz emission maps allows us to reproduce the frequency-dependent spatial profiles of the metamaterials eigenmodes.

Adjusting the functionality of terahertz split-ring resonators through geometry

We examine planar double split-ring resonators (SRRs) consisting of two concentric rings with either opposite, similar, or asymmetric gap orientation. Depending on the geometry we observe resonance hybridization, metamaterial induced transparency, or the excitation of dark resonances. These properties can be used for SRR based sensing applications, to realize strongly dispersive behavior, or for determining the optical properties of metals. We further find that THz SRRs featuring very narrow gaps on the micro- or nanoscale can provide in-gap enhancement factors of several 10,000, a property particularly useful for the realization of nonlinear THz experiments.

Terahertz Near-Field Imaging of Electric and Magnetic Resonances in Plasmonic High Frequency Devices

We report a terahertz near-field imaging approach providing spatially resolved measurements of amplitude, phase, and polarization of the electric field. Using this approach we extract the microscopic near-field signatures in plasmonic devices and planar metamaterials.