Christian Imhof

Negative index bulk metamaterial at terahertz frequencies

We present a bulk metamaterial with negative refractive index in the terahertz frequency range. The structure is composed of pairs of metallic crosses embedded in Benzocyclobutene (BCB). The design is specifically chosen to provide a low-loss, free-standing material which operates under normal incidence and independently of the polarization of the incident radiation. These qualities allow the fabrication of 3D structures by mechanical stacking of multiple thin films.

Polarization-independent active metamaterial for high-frequency terahertz modulation

We present a polarization-independent metamaterial design for the construction of electrically tunable terahertz (THz) devices. The implemented structure consists of an array of gold crosses fabricated on top of an n-doped gallium arsenide (GaAs) layer. Utilizing THz time-domain spectroscopy, we show that the electric resonance and thus the transmission properties of the cross structure can be tuned by an externally applied bias voltage. We further demonstrate the fast amplitude modulation of a propagating THz wave for modulation frequencies up to 100 kHz.

Strongly birefringent metamaterials as negative index terahertz wave plates

We report an alternative approach for the design and fabrication of thin wave plates with high transmission in the terahertz regime. The wave plates are based on strongly birefringent cut-wire-pair metamaterials that exhibit refractive indices of opposite signs for orthogonal polarization components of an incident wave. As examples, we fabricated and investigated a quarter- and a half-wave plate with high intensity transmittance. The wave plates displayed a maximum figure of merit (FOM) of 23 at a frequency around 1.3 THz where the refractive index was n′=−1.7. This corresponds to one of the highest FOMs reported at terahertz frequencies.We report a new approach for the design and fabrication of thin wave plates with high transmission in the terahertz (THz) regime. The wave plates are based on strongly birefringent cut-wire pair metamaterials that exhibit refractive indices of opposite signs for two orthogonal polarization components of an incident wave. As specific examples, we fabricated and investigated a quarterand a half-wave plate that revealed a peak intensity transmittance of 74% and 58% at 1.34 THz and 1.3 THz, respectively. Furthermore, the half wave plate displayed a maximum figure of merit (FOM) of 23 at 1.3 THz where the refractive index was −1.7. This corresponds to one of the highest FOMs reported at THz frequencies so far. The presented results evidence that negative index materials enter an application stage in terms of optical components for the THz technology.

Pairs of metallic crosses as a left-handed metamaterial with improved polarization properties.

We designed and analysed a new structure for the realization of left-handed metamaterials in the GHz region. The material is composed of pairs of metallic crosses and reveals improved polarization behaviour. Left-handed properties can be observed as long as the electrical field vector is located in the plane of the crosses. Negative refraction as indication for simultaneous negative effective epsilon and mu is numerically verified by direct comparison of the wavefronts inside and outside the metamaterial at nonzero angles of incidence.

Low-loss compact high-Q 3D THz grating resonator based on a hybrid silicon metallic slit waveguide

We present a high-Q 3D waveguide transmission filter for the THz-domain, based on an inhomogeneous Bragg grating, incorporated into the walls of a metallic slit waveguide. The reasons for the occurring loss mechanisms in the compact component are presented and the losses are minimized by selective mode adaptation and by tapering the transitions to the corrugated regions. The performance of the device and the influence of parameter variations are analyzed by detailed numerical simulations. These 3D simulations clearly show the drastic drawback of 2D calculations in designing narrowband 3D metal-dielectric waveguide filters and could even lead to a better performance than known designs in 2D technology.

Compact three-dimensional terahertz resonators based on periodically corrugated metallic slit waveguides

We present a compact terahertz filter design realized as a short inhomogeneous Bragg grating formed by variation in the slot width of a three-dimensional (3D) metallic slit waveguide. By introducing a phase-shift into the grating, a narrowband 3D waveguide transmission resonator is created. The functionality of the filter as well as the influence of parameter variations on the performance of the device are analyzed by detailed numerical 3D simulations. By tapering the depth of the grooves in the modulated region the radiation loss is minimized. The simulation results are verified by measurements of the transmission characteristic of a realized structure and show excellent agreement.

Pump beam diameter dependent terahertz generation from surface emitters - experiment and simulation

We report on terahertz wave generation using transient photoconductivity induced by femtosecond laser pulses in semiconductor surfaces. In particular the dependence of terahertz emission characteristics on pump pulse parameters like diameter and input angle is investigated in detail both experimentally and theoretically. It is shown that the emission in forward (quasi-optical reflection) as well as in backward direction (back to the laser) is strongly depending on the diameter of the pump beam on the semiconductor. This is explained in terms of Huygenspsila principle of fundamental waves together with a Hertzian dipole model. The emission properties are simulated using CST MICROWAVE STUDIOreg and compared with experimental results.

Bulk negative index metamaterial operating at THz frequencies

We present a freestanding bulk metamaterial operating in the THz frequency range. The implemented structure consists of pairs of metallic crosses for which the resulting composite medium exhibits a negative index of refraction at about 1 THz.

A three-dimensional lefthanded metamaterial operating at THz frequencies

We present a bulk metamaterial operating in the THz frequency range. The implemented structure consists of pairs of metallic crosses for which the resulting composite medium exhibits a lefthanded transmission band at about 1.1 THz.