We propose asymmetric D-split resonators as unit cells for high Q metasurfaces. In such resonators, current trapped modes lead to in-phase oscillations of antisymmetric currents. Thus, radiation losses are suppressed, enabling Q-factors beyond the ones obtainable in symmetric designs. We compare the proposed structure against both asymmetric and symmetric split ring metasurfaces and find an improvement in terms of Q by a factor of two and ten, respectively. Transmission measurements in a terahertz spectrometer provide experimental proof of the high Q-factors and agree well with numerical simulations. In the future, asymmetric D-split metasurfaces could be employed as high-performance sensors or filters.

We present a novel measurement setup for monitoring changes in leaf water status using nondestructive terahertz time-domain spectroscopy (THz-TDS). Previous studies on a variety of plants showed the principal applicability of THz-TDS. In such setups, decreasing leaf water content directly correlates with increasing THz transmission. Our new system allows for continuous, nondestructive monitoring of the water status of multiple individual plants each at the same constant leaf position. It overcomes previous drawbacks, which were mainly due to the necessity of relocating the plants. Using needles of silver fir (Abies alba) seedlings as test subjects, we show that the transmission varies along the main axis of a single needle due to a variation in thickness. Therefore, the relocation of plants during the measuring period, which was necessary in the previous THz-TDS setups, should be avoided. Furthermore, we show a highly significant correlation between gravimetric water content and respective THz transmission. By monitoring the relative change in transmission, we were able to narrow down the permanent wilting point of the seedlings. Thus, we established groups of plants with well-defined levels of water stress that could not be detected visually. This opens up the possibility for a broad range of genetic and physiological experiments.

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We propose polarization and angle insensitive metamaterials at terahertz frequencies consisting of two concentric ring resonators with interdigitated fingers placed between the rings. We experimentally demonstrate that the bandstop resonance remains unaffected by changes in both the incident angle and the polarization. Furthermore, high quality-factors of more than 16 are observed as Fano-like modes with small dipole moments are excited. We show that the sharpness of the resonance can be controlled by the number of interdigitated finger pairs. The structures exhibit pronounced normal phase dispersion near the resonance, which renders them attractive candidates for electromagnetic induced transparency and slow light applications.

We report on the excitation of sharp Fano-like resonances in lattices of metamolecules composed of two differing types of metaatoms. The proposed structures exhibit modes originating from the individual metaatoms as well as a very sharp mode from the collective excitation of the metamolecule lattice as a whole. Next-generation thin film sensors (e.g., for bio/chemical hazard detectors) could especially benefit from such artificial materials. Having multiple modes at different spectral positions enables the characterization of dispersive materials, while the high Q-factors of the eigenmodes lead to a very high sensitivity.

In this paper the use of aerosol jet printing as a fabrication and rapid prototyping method for terahertz metamaterials is presented. Since this technique combines the very precise deposition of metallic inks with line widths down to 10 µm and the use of digitally generated tool paths, it is highly suitable for fabricating arbitrary 2D microstructures for terahertz applications. The fabrication of a metamolecule composed of four closed ring resonators with two different radii on conventional polyester foil is shown. Furthermore, the metamaterials are characterized thoroughly using microscopic images, confocal microscopy, and terahertz time‐domain spectroscopy, which demonstrate impressively the suitability of this fabrication technology for terahertz applications. For an additional validation the experimentally gathered data with numerical simulations are compared.

wileyonlinelibrary.com COMMUNICATIONfor angle of arrival (AoA) measurements [ 34 ] or as directional fi lters to shield detectors or detector arrays from scattered or refl ected light.The remainder of this work is structured as follows: To illustrate the proposed design principle, we will start with optimizing the angular sensitivity of a conventional asymmetric double split ring resonator (aDSR) metasurface as it is depicted in Figure 1 . [ 35,36 ] Next, the concept is applied to a multiband metamolecule. [37][38][39] We will demonstrate that the proposed design methodology allows for multiband high-Q fi lter structures, while the high angular sensitivity appears only for one of the resonances. Finally, we will present a detailed characterization of the multiband metasurface including a discussion on potential applications.As mentioned above, we will start with the aDSR geometry. In the context of our study, the size of the primitive cell ( p ), and the angle of incidence ( Θ ) for a given THz souvrce are the most important parameters. The relevant dimensions of the aDSR are given in the caption of Figure 1 and remain fi xed in the course of this study. In Figure 2 a, we simulated the transmission of the aDSR shown in Figure 1 between 0.44 and 0.52 THz and varied the size of the primitive cell between 280 and 340 µm.For small sizes of the primitive cell, the metasurface exhibits a sharp and distinct resonance. This mode is characterized by the collective out-of-phase oscillations of the opposing currents on each part of the resonator. They evoke very low radiation losses leading to a high Q-factor of the resonance, due to a strong interaction between neighboring unit cells. Because of the low radiation losses, such eigenmodes are also known as trapped-current modes or dark-modes. [ 35,40 ] In order to engineer the characteristics of this resonance, the asymmetry angle of the unit cell and the lattice constant are of relevance. Both parameters allow for the adjustment of the resonance depth, the Q-factor and the proximity to the dipole resonance. [ 35,41,42 ] An optimum value of the Q-factor can be observed at a period of p opt. = c/nf 0 , where c is the speed of light, n is the refractive index of the substrate, and f 0 is the resonance frequency. These fi ndings are in line with the results demonstrated by Singh et al., [ 41 ] who reported that metasurfaces consisting of split ring resonators exhibit maximal Q-factors under normal incidence when the size of the primitive cell p equals the resonance wavelength normalized with the refractive index of the substrate. At this condition, the 1 st diffracted order of the metamaterial grating is scattered into the resonator plane. [ 42 ] If the periodicity is increased to values larger than p opt. (crossing of the red line), the mode becomes evanescent, resulting in a much broader and less pronounced resonance.Yet besides this fi nding, we observe another interesting effect when the primitive unit cell approaches this threshold: As shown in Figure 2 c, the resonance be...

In this letter we present a switchable THz notch filter. The filter contains a liquid crystal layer that acts as a half wave retarder in one state and as an isotropic layer in the other state. The device combines three unique properties: it can be switched electrically, it provides a filter depth of 35 dB at 350 GHz and it can be tuned over wide frequency range from 350 GHz to 700 GHz. Keywords Liquid crystals . Switchable terahertz devices . BirefringenceIn recent years terahertz technology has experienced a tremendous development [1, 2]. Many applications for THz systems are presently discussed. They range from nondestructive testing [3][4][5] and process monitoring in the industry [5, 6] to free-space communications with THz carrier waves [7][8][9].Besides sources and detectors, a mature THz technology also requires devices to guide and manipulate THz waves. Among them are reflectors [10,11], waveguides [12, 13], splitters [14], lenses [15], filters [16] and modulators [17, 18]. Modulators and switches could be based on liquid crystals (LCs) the dielectric THz properties of which have recently been characterized with high precision [19-25]. First devices have been demonstrated by Pan et al., including THz phase shifters [26, 27] and polarizers [28]. Wilk et al. reported on a LC based switchable THz Bragg structure [29] and Ghattan et al. presented an on-off switch for sub-THz-waves, which was based on a two-dimensional photonic crystal infiltrated by LCs [30]. A THz device, which combines liquid crystals with an artificial medium, was reported by Khoo et al. [31] and Chen et al. presented a magnetically tunable Lyot filter [32].In this paper we report on the fabrication of an electrically tunable liquid crystal filter which uses the THz birefringence of LCs. It provides a high filter depth and is electrically tunable over a broad frequency range. The filter consists of a 2.4 mm thick liquid crystal layer, which is sandwiched between two 710 μm thick fused silica windows (Fig. 1). As a