Remigius Zengerle

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.

Determination of single-mode fiber coupler design parameters from loss measurements

A method is presented for determining the minimum spacing between the fiber core centers in a single-mode fiber coupler which consists of two substrate blocks each provided with a fiber fixed in a curved groove. Immersion liquids with known refractive indices are placed on the top surface of one coupler block and the introduced power loss of the light guided within the fiber is measured. By fitting these results to calculated curves the minimum spacing may be determined with an error of less than 0.2 μm.

Wavelength-selective directional coupler made of nonidentical single-mode fibers

Low-loss asymmetrical single-mode fiber directional couplers applicable to WDM between transmission channels at 1300 and 1500 nm have been constructed. The crossover wavelength of these couplers can be tuned by a suitable intermediate layer. Theoretical calculations show that fiber multiplexers with channel spacings less than 20 nm Can be fabricated.

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.

Narrow-band wavelength-selective directional couplers made of dissimilar single-mode fibers

Asymmetrical wavelength-selective fiber couplers with a bandwidth of 22.5 nm have been constructed and investigated. The special shapes of the measured sidelobes in the power transmission characteristics are explained by the bending profiles of the fibers.

Effects of nonlinear dispersion in EDFA's on optical communication systems

The signal-induced change of the refractive index in an erbium-doped fiber amplifier (EDFA) causes a phase modulation imposed on a signal when passing the EDFA. In this paper, we apply our extended EDFA model on an optical communication system. The model includes this phase modulation, by including the nonlinear dispersion in an EDFA, and the spontaneous emission noise. The influence of these effects on an optical communication system is examined by means of Q-factor and eye diagram. We assume an intensity modulated-direct detection (IM-DD) system operating at 193 THz (1552.5 nm) with a bit rate of 10 Gb/s in the anomalous dispersion regime and a total fiber length of 500 km. The fibers are assumed to be dispersion shifted ones, EDFAs are used to compensate for the fiber loss. By numerical simulation we obtain results for the influence of the phase modulation (nonlinear dispersion) due to the signal induced change of the refractive index in an EDFA and the spontaneous emission noise at different input peak powers. Neglecting the signal-induced change of the refractive index strongly underestimates the Q-factor in the anomalous dispersion regime. Therefore it should be included for reliable system simulations. This can be done with the numerical model presented here.

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.

Wide-angle beam refocusing using negative refraction in non-uniform photonic crystal waveguides

The phenomena of beam-steering and negative ray refraction in a graded 2D photonic crystal structure interface with plane tapered transitions to homogeneous waveguide regions is used for efficient far-field refocusing of highly divergent Gaussian beams. It is shown by numerical simulation that a 14microm long photonic crystal structure is able to refocus a small-spot Gaussian beam with an efficiency of about 90%.

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.