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.
We experimentally demonstrate intensity and phase modulation of terahertz radiation using actively controlled large-area planar metamaterial (metafilm) hybridized with a 12 m thick layer of a liquid crystal. Active control was introduced through in-plane electrical switching of the liquid crystal, which enabled us to achieve a reversible singlepass absolute transmission change of 20 % and a phase change of 40 deg at only 20 V.Efficient active control of terahertz radiation is one of the main challenges of the terahertz technology. The mainstream solutions that have been demonstrated during the past decade include the use of semiconducting structures and liquid crystals (LC) [1,2,3,4,5]. Despite the proven advantage of being broadband these solutions have suffered from a number of drawbacks. In particular, superconductor-based THz modulators have a very small active area and are able to modulate transmission by only a few percent [2] and normally require cryogenic temperatures [1]. Whereas LC optical cells are unable to control the intensity of terahertz radiation, cannot be thinner than several hundred microns (due to the relatively low THz birefringence of liquid crystals), and also require bulky magnets [3] or a driving voltage in excess of 100 V [4,5].
Using terahertz near-field imaging we experimentally investigate the interaction between split-ring resonators (SRRs) in metamaterial arrays. Depending on the inter-SRR spacing two regimes can be distinguished for which strong coupling between SRRs occurs. For dense arrays SRRs couple via their electric and magnetic near-fields. In this case distinct deformations of the SRRs' characteristic near-field patterns are observed as a signature of their strong interaction. For larger separations with a periodicity matching the resonance wavelength, the SRRs become diffractively coupled via their radiated fields. In this regime hybridization between plasmonic and lattice modes can be clearly identified in the experimentally obtained near-field maps.
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.
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 frequency 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 vision filters. Practical implementations of the new concept include applications in wavefront manipulation, observational instruments, vision and free-space communication in light-scattering environments. Keywords: flat optics; metafilms; metasurfaces; planar metamaterials; wavefront manipulation INTRODUCTION Metasurfaces (also known as planar metamaterials or metafilms) are a special low-dimensional class of artificially structured media. This class is represented by thin metal films and surfaces periodically patterned on a sub-wavelength scale, which can be readily fabricated using the existing planar technologies. Apart from their spectral selectivity 1 metasurfaces have demonstrated intriguing electromagnetic effects such as asymmetric transmission 2,3 and optical activity without structural chirality 4 . They can exhibit resonant dispersion mimicking electromagnetically induced transparency and the slow-light phenomenon 5-7 , be invisible 8 , efficiently convert polarization 9-11 , or perfectly absorb radiation 12,13 . Metasurfaces with gradient structuring anomalously reflect and refract light [10][11][12][13][14][15] and can act as lenses, wave-plates, and diffraction gratings [16][17][18][19] . Planar metamaterials are also able to enhance the light-matter interaction facilitating sensing 20 , energy harvesting 21 , and generation of coherent radiation 22,23 . The functionality of the most common types of planar metamaterials is determined by the individual resonant response of their basic structural elements -metamolecules, which are only weakly coupled to each other. When electromagnetic coupling between the metamolecules is strong 24 the relative phase of their excitation becomes important and the resulting spectral response is no longer determined by the individual resonances of the metamolecules. The metamaterial spectrum is then shaped by the collective, spatially coherent modes of metamolecular excitations that engage a large ensemble of metamolecules 25 . The introduction of structural disorder in such an ensemble reduces the degree of coherency and leads to the weakening and broadening of its collective resonant response, which might be seen to vanish
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