Markus Wächter

Frequency-dependent characterization of THz Sommerfeld wave propagation on single-wires

In this work, measurements and numerical field simulations highlighting the characteristic propagation behavior of THz surface-wave pulses along bare and dielectrically coated metal wires are presented. An optoelectronic time-domain measurement setup with a freely-positionable probe-tip is used for detection of electrical field transients after different propagation lengths along the wires. Frequency-dependent attenuation and dispersion parameters are determined in the range of 0.02 THz to 0.4 THz. Our results are in good agreement with numerical field simulations considering the propagation of an axial Sommerfeld surface-wave with metallic and dielectric losses. We discuss the influence of wire radius on wave propagation behavior and the application of THz single-wires for sensing.

Metallic slit waveguide for dispersion-free low-loss terahertz signal transmission

Transmission characteristics of a metallic waveguide are presented combining negligible dispersion, very low attenuation, and two-dimensional mode confinement. The waveguide is fabricated by sawing a 270μm wide slit through a 40mm wide and 300μm thick silicon slab and subsequent metalization of all faces. Single-mode terahertz pulses propagating through the slit are excited at one end of the waveguide using a photoconductive antenna array and detected with a freely positionable photoconductive probe tip. Low-loss propagation with negligible group velocity dispersion is observed in the 0.1–1.0THz frequency range. Compared to former approaches considerably stronger mode confinement is achieved.

Tapered photoconductive terahertz field probe tip with subwavelength spatial resolution

In this work a photoconductive probe tip applicable for near- and far-field measurements in the terahertz frequency regime is demonstrated as a powerful alternative to existing terahertz scanning near-field optical microscopy approaches. The probe tip is based on a triangular-shaped patch of freestanding low-temperature-grown GaAs of only 1.3 μm thickness with a pair of tapered metallic wires on top. Using nonresonant electric field enhancement at the tip of the probing device, 10 μm wide metallic structures are spatially resolved and a bandwidth of 2 THz is demonstrated.

Optical generation of terahertz and second-harmonic light in plasma-activated silicon nanophotonic structures

Plasma-activated silicon structures exhibit symmetry broken surfaces through chemical surface modification leading to a considerable second-order nonlinear optical response. This nonlinear response is demonstrated in second-harmonic and difference frequency generation measurements including the generation of terahertz radiation in silicon photonic nanowires using telecom wavelength excitation pulses.

Low-loss terahertz transmission through curved metallic slit waveguides fabricated by spark erosion

Propagation characteristics of broadband terahertz pulses in straight and curved metallic slit (MS) waveguides fabricated using spark erosion technique are investigated in the 0.1–1.0THz frequency range. As the straight MS waveguides, the curved MS waveguides, having radii of curvature as small as 5mm, exhibit comparably undistorted terahertz pulse transmission. The observed bending loss is unexpectedly low, giving rise to the assumption that surface oxidization and roughness generated by the fabrication process are responsible for the improved signal routing behavior. Experimental results are compared with theoretical data from numerical field simulations incorporating the influence of a thin dielectric cover layer.

Fluidic channel waveguide coupler for nl-volume sensing applications

A waveguide coupler configuration enabling the investigation of streaming liquids at terahertz frequencies is presented. The device is based on two electromagnetically coupled hollow core dielectric waveguides. One waveguide is air-filled and is used for terahertz signal transmission while the second waveguide is filled with the sample liquid. Experimental data supported by numerical field simulations are presented for an air-, water- and isopropanol-filled waveguide in the 0.2–0.8 THz frequency range.

Dispersion-free and low-loss propagation of THz signals in a metallic slit waveguide

In this paper, a metallic waveguide concept that combines dispersion-free and low-loss propagation characteristics with two-dimensional mode confinement over a broad frequency range. Waveguide consists of two planar arranged metalized slabs of 300 mum thickness separated by a 270 mum wide slit. The width a of each slab in x-direction is 20 mm and is considered as quasi-infinite in terms of the propagating mode. The considered transmission signal is the fundamental TEM mode propagating along the slit in z-direction with the main field component in x-direction. For the evaluation of propagation characteristics and field distribution we apply a commercially available Maxwell solver software package based on the finite element method with a non-uniform adaptive mesh optimization routine. We plot the electric field amplitude at the waveguides cross-section at 0.5 THz. At this frequency more than 60% of the guided power is confined within an area not exceeding one wavelength around the slit.

Sommerfeld Wires at Terahertz Frequencies

In this work, we investigate the propagation behavior of THz surface-wave pulses along bare and dielectrically coated metal wires. Frequency-dependent attenuation and dispersion parameters are determined in the range of 0.02 THz to 0.4 THz with an optoelectronic time-domain measurement setup. Our results are in good agreement with numerical field simulations considering the propagation of an axial Sommerfeld surface-wave with metallic and dielectric losses. We discuss the influence of wire radius on wave propagation behavior and the application of THz Sommerfeld wires for sensing

Terahertz generation in locally plasma-activated silicon nanophotonic waveguides

We present a novel approach for efficient terahertz signal generation at silicon nanophotonic waveguides via electro-optic difference frequency generation using telecom wavelengths excitation pulses. Second-order nonlinearity of silicon is achieved by plasma-activated surface modification.

Free-standing tapered probe-tip for high resolution photoconductive terahertz field sampling

We present a photoconductive probe-tip applicable for near- and far-field measurements in the terahertz frequency range. The probe-tip is based on a triangular patch of freestanding LT-GaAs of only 1.3 μm thickness with a pair of tapered metallic wires on top. Using non-resonant electric field enhancement at the tip of the probing device a spatial resolution of 5 μm and a bandwidth of 2 THz is demonstrated.