Markus Koeberle

Terahertz time-domain spectroscopy of nematic liquid crystals

Liquid crystals are candidate materials for optical devices operating in the Terahertz (THz) frequency region of the electromagnetic spectrum. Proposed devices include THz phase shifters and THz quarter wave plates. To assist in designing for these applications, the fundamental properties of the materials should be determined. Fundamental optical properties to be determined over the frequency range of interest are the refractive index n and the absorption coefficient α. According to the orientation of the liquid crystals relative to the polarisation of the light field, ordinary and extraordinary values for the refractive index may be distinguished. In early work, employing time-domain spectroscopy, a rise in both no and ne with optical frequency in the THz region was reported. Later work, employing two-colour generation of THz radiation, indicated the values of no and ne were both relatively constant in the THz region. We have now made measurements of the two common nematic liquid crystals K15 and E7 using time-domain THz spectroscopy and confirm that no and ne show little change over the spectral region 0.15 to 1 THz.

W-Band Characterization of Anisotropic Liquid Crystals at Room Temperature

This paper describes the characterization of Liquid Crystals (LCs) at W-Band using the standard WR-10 rectangular waveguide. The reflection parameter of a 10 mm long waveguide - terminated by a short-circuit - is measured. The LC is oriented with a homogeneous magnetostatic field strength of 400 A/mm, generated with an air-cooled coil. The extracted complex permittivity of the LC K15 from Merck KgaA (also known as 5CB) is presented, showing permittivities well below 3 with a dielectric anisotropy of Deltaepsiv<0.19 (equivalent to tau=Deltaepsiv/epsivmax<8.5%) and dielectric losses less than tandelta<0.023, all at W-Band.

Liquid crystal based electronically steerable 4×4 antenna array with single horn feed at Ka-Band

This paper presents the design setup of an 4times4 array, each element consists of two Vivaldi antennas connected by an overlapped antipodal finline phase shifter. Simulations of an infinite expanded array showed a good matching in the whole investigated frequency range and fairly low insertion losses of less than 7.7 dB at 35 GHz. The phase can be continuously tuned up to 465deg. Measurement results of the realized array will be presented in the full paper. The presented results depict the first investigations of electronically steerable arrays fed from the back, based on liquid crystals. The results are very promising, encouraging further research: Rising the number of element cells and adding orthogonal arranged Vivaldi antennas for dual polarized signals, [8]. The setup can easily scaled down to rise the frequency. It is aimed to process the structure on fused silica, which improves the mechanical strength as well as excellent microwave properties.

Reconfigurable Vivaldi antenna array with integrated antipodal finline phase shifter with liquid crystal for W-Band applications

This paper presents a new finline phase shifter with an antipodal Vivaldi antenna. The structure offers the potential to be used in a wideband array configuration for electronic beam steering. The individual phase shift is realized by applying a DC voltage to liquid crystal inside the finline.

CW THz spectroscopy of polymers using plasmonic surface-wave sensors

Lamellar metal gratings with sub-wavelength slits have been fabricated and characterized. They feature a transmission resonance which is highly sensitive to material layers attached to the grating surface. At first, gratings with resonant features at 85GHz are investigated. The transmission spectrum recorded with a commercial Vector Network Analyzer is in good agreement with the data obtained from a photoconductive continuous-wave THz system. Further, the grating dimensions are scaled down for an operation frequency of 430GHz. Sandwiched between polymer foils, the transmission resonance of the grating is shifted in frequency and the permittivity of the layers is extracted from the induced shift with an analytical model. The received results coincide with literature data very well.