Norman Born

Terahertz metasurfaces with high Q-factors

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.

Polarization and angle independent terahertz metamaterials with high Q-factors

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.

Monitoring Plant Drought Stress Response Using Terahertz Time-Domain Spectroscopy

A terahertz measurement setup precisely monitors changes in the water status of multiple plants in experiments under controlled environmental conditions. 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.

Excitation of multiple trapped-eigenmodes in terahertz metamolecule lattices

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.

Compression Molded Terahertz Transmission Blaze-Grating

We present a terahertz transmission blaze-grating, which can be fabricated, easily, cost efficiently and in large numbers using compression molding of micro-powder. The diffraction properties of the grating are derived by simulation of electro-magnetic field scattering and are compared with angle-dependent measurements obtained in a terahertz time-domain setup. An excellent match between simulation and measurement is found, demonstrating the ability of the terahertz transmission blaze-grating for spatial dispersion of terahertz waves. Thus, this and similar terahertz transmission blaze-gratings can be used as dispersive elements for applications such as spectrometers or novel THz imaging systems.

Cavity enhanced terahertz modulation

We present a versatile concept for all optical terahertz (THz) amplitude modulators based on a Fabry-Perot semiconductor cavity design. Employing the high reflectivity of two parallel meta-surfaces allows for trapping selected THz photons within the cavity and thus only a weak optical modulation of the semiconductor absorbance is required to significantly damp the field within the cavity. The optical switching yields to modulation depths of more than 90% with insertion efficiencies of 80%.

High-Q terahertz bandpass filters based on coherently interfering metasurface reflections

We present compact and easy-to-realize terahertz bandpass filters with Q values in the order of 500. The filters are based on coherently interfering reflections from two parallel metasurfaces applied to the boundaries of a semiconductor disk. By changing the thickness of the semiconductor disk and the dimensions of the metasurface structures, the filter can be optimized for various terahertz frequencies. Moreover, a precise tuning of the resonance frequency is possible via the temperature of the structure or its angle with respect to the propagation direction.

Channeling the electrical breakdown of air by optically heated plasma filaments

Femtosecond laser pulses with sufficiently high peak power leave tracks of dilute plasma in their wakes. Potential use of this plasma for channeling electrical discharges in the atmosphere has been discussed and demonstrated in laboratory-scale experiments. However, the electron density in femtosecond laser-generated plasma decays rapidly on the nanosecond time scale, due to recombination and electron attachment to air molecules. The finite plasma lifetime limits the maximum extent of the guided electrical breakdown to a few meters. Here, we experimentally demonstrate that the limitation associated with the short plasma lifetime can be overcome though optical heating of the plasma filaments by an auxiliary energetic laser pulse with a duration in the nanosecond range. We show that the breakdown electric field can be reduced by up to a factor of 4 with a heater fluence of about 1  kJ/cm2. This approach could have applications in channeling long-range electrical discharges in the atmosphere and, potentially, in channeling lightning strikes.

Highly sensitive terahertz dielectric sensor for small-volume liquid samples

We present a resonator-based sensor for the measurement of the refractive index of dielectric liquid samples. The proposed sensor operates on the basis of an electromagnetic resonance between a thin metallic grating and a reflecting ground plane. The fluid whose refractive index is to be measured fills the region between the metallic grating and the ground plane and causes a considerable shift in the resonance frequency (>500 GHz/RIU). The sensor has a relatively simple structure; therefore, it can be manufactured economically on industrial scales.

Switchable THz Filter Based on a Vanadium Dioxide Layer Inside a Fabry–Pérot Cavity

A terahertz amplitude switching device is proposed which allows for the efficient manipulation of sharp transmission bands. The switching concept is based on the thermally triggered insulator-to-metal transition of a thin vanadium dioxide layer, placed inside a Fabry-Pérot resonator.