David Jahn

Terahertz plasmonic Bessel beamformer

We experimentally demonstrate terahertz Bessel beamforming based on the concept of plasmonics. The proposed planar structure is made of concentric metallic grooves with a subwavelength spacing that couple to a point source to create tightly confined surface waves or spoof surface plasmon polaritons. Concentric scatterers periodically incorporated at a wavelength scale allow for launching the surface waves into free space to define a Bessel beam. The Bessel beam defined at 0.29 THz has been characterized through terahertz time-domain spectroscopy. This approach is capable of generating Bessel beams with planar structures as opposed to bulky axicon lenses and can be readily integrated with solid-state terahertz sources.

Attenuated Total Reflection Terahertz Time-Domain Spectroscopy: Uncertainty Analysis and Reduction Scheme

Terahertz time-domain spectroscopy (THz-TDS) in the attenuated total reflection (ATR) configuration is ideally suited to characterize highly absorptive media. In this paper, we analyze the impact of random phase and amplitude errors and prism misalignment on the optical constants extracted from the ATR THz-TDS measurements. Analytical models together with empirical data suggest that, among those factors, small prism misalignment has a relatively strong impact on the optical constants. We propose an alternative configuration for ATR THz-TDS that significantly reduces the impact of prism misalignment between the reference and sample measurements. As a result, this configuration facilitates repetitive reference and sample scans to mitigate the effect from long-term signal drifts. Based on this proposed method, the measured optical constants of distilled water and pure ethanol have a significantly reduced uncertainty. The presented analysis and method can be used to improve the measurement accuracy in standard ATR THz-TDS setups.

3D printed dielectric rectangular waveguides, splitters and couplers for 120 GHz

We use a 3D printer to fabricate rectangular dielectric single mode waveguides for 120 GHz. The rectangular waveguides consisting of polystyrene showed an attenuation of 6.3 dB/m, which is low enough for short devices. We also characterize 3D printed Y-splitters and a 1x3-splitter based on multimode interference. Further, we construct and measure a variable planar waveguide coupler which can be used as a 3-dB coupler, a cross-coupler and no coupler at all.

Periodic sampling errors in terahertz time-domain measurements

An extensive investigation of the origin and the impact of periodic sampling errors of terahertz time-domain spectroscopy systems is given. We present experimental findings and compare them to a theoretical model which is developed in this work. Special attention is given to the influence on the extraction of the refractive index from measurements. It can be shown that even distortions of the spectrum at frequencies higher than the used bandwidth can have a significant impact on the extracted refractive index.

Fabry-Pérot interferometer for sensing polar liquids at terahertz frequencies

We propose and validate a sensor for polar liquids that operates in conjunction with terahertz time-domain spectroscopy. The sensor is constructed from an optically thick silicon wafer and a ground plane, separated by a gap into which the liquid is injected. This arrangement represents a Fabry-Perot interferometer that causes a sharp minimum in the reflection spectrum. Compared to resonance-based sensors, this sensor design can maintain its sharp spectral response when loaded with highly absorbing polar liquids. This overcomes an issue of damped resonance caused by material losses in resonance-based sensors. We report a reflection minimum shift of 8 GHz per percent ethanol in water. The sensor can be readily integrated with a microfluidic channel for real-time fluid monitoring.