Kai Baaske

Conductivity of single-stranded and double-stranded deoxyribose nucleic acid under ambient conditions: The dominance of water

We investigate the conductivity of single-stranded and double-stranded herring deoxyribose nucleic acid (DNA) in buffer solution spotted and dried on Au nanocontacts. We find an exponential increase of the conductivity with increasing humidity that is identical for single- and double-stranded DNA within the measurement accuracy. While the small conductivity of dry DNA is comparable to that of a large band-gap semiconductor, we attribute the increase at high humidity levels to water molecules accumulated at the phosphate backbone. For high humidities we observe s-shaped current-voltage characteristics that can be well explained by the dissociation of water attached to the DNA molecules.

Multifrequency continuous wave terahertz spectroscopy for absolute thickness determination

We present a tunable multifrequency continuous wave terahertz spectrometer based on two laser diodes, photoconductive antennas, and a coherent detection scheme. The system is employed to determine the absolute thickness of samples utilizing a proposed synthetic difference frequency method to circumvent the 2π uncertainty known from conventional photomixing systems while preserving a high spatial resolution.

Terahertz Radiation at 0.380 THz and 2.520 THz Does Not Lead to DNA Damage in Skin Cells In Vitro

The question whether nonionizing electromagnetic radiation of low intensity can cause functional effects in biological systems has been a subject of debate for a long time. Whereas the majority of the studies have not demonstrated these effects, some aspects still remain unclear, e.g., whether high-frequency radiation in the terahertz range affects biological systems. In particular for frequencies higher than 0.150 THz, investigations of the ability of radiation to cause genomic damage have not been performed. In the present study, human skin cells were exposed in vitro to terahertz radiation at two specific frequencies: 0.380 and 2.520 THz. Power intensities ranged from 0.03–0.9 mW/cm2 and the cells were exposed for 2 and 8 h. Our goal was to investigate whether the irradiation induced genomic damage in the cells. Chromosomal damage was not detected in the different cell types after exposure to radiation of both frequencies. In addition, cell proliferation was quantified and found to be unaffected by the exposure, and there was no increase in DNA damage measured in the comet assay for both frequencies. For all end points, cells treated with chemicals were included as positive controls. These positive control cells clearly showed decreased proliferation and increased genomic damage. The results of the present study are in agreement with findings from other studies investigating DNA damage as a consequence of exposure to the lower frequency range (<0.150 THz) and demonstrate for the first time that at higher frequencies (0.380 and 2.520 THz), nonionizing radiation does not induce genomic damage.

Field Exposure and Dosimetry in the THz Frequency Range

With a growing number of applications utilizing THz radiation appearing on the market the question of health protection against non-ionizing electromagnetic fields arises in this frequency range, as at lower frequencies before. To date, about 50 independent empirical studies on living organisms, model systems and cells have been performed to clarify bio-electromagnetic interaction in the THz frequency range. Many of these studies find behavioral effects or effects on the cellular level, even at non-thermal exposure levels, while others do not report effects other than thermally induced damage. We discuss the general challenges in performing reliable field exposure experiments in the THz frequency range and describe a methodology that was adopted in a large campaign searching for genotoxic effects of THz radiation in vitro.

Folded dipole antenna for increased cw THz output power

We present simulations and measurements which verify the improved continuous wave THz output power of folded dipole antennas compared to full wavelength dipole antennas.

In vitro field exposition of skin cells between 100 GHz and 2.52 THz

Initiated by the German Federal Office for Radiation Protection (BfS) field exposition experiments have been designed to examine genotoxic effects of THz radiation in vitro. Under defined environmental conditions, two different human skin cell types are exposed to continuous-wave radiation at six distinct frequencies between 100 GHz and 2.52 THz originating from different sources of THz radiation. The cell containers are irradiated with free space power densities between 0.1 mW/cm2 and 10 mW/cm2 measured traceable to the SI units.

Z-scan based fiber-coupled coherent cw THz imaging system

We present a fiber-coupled cw THz system for fast and accurate imaging in the field of non-destructive testing. Fibercoupled THz antennas make this system flexible and robust. The time delay is not realized via an optical delay line but by moving the detector antenna along the optical axis (Z-Scan).

Mail inspection using THz imaging : a comparison of three different systems

To evaluate the potential of THz imaging systems for mail and luggage inspection we study a set of letters containing different hazardous items. The samples are investigated with three different THz systems available in our group: A microwave based system working around 100 GHz, a THz time-domain system and a THz gas laser. We provide a comparative discussion on our results and the advantages and disadvantages of each system.

An alternative to the geometric addition method for calculating the rise time of fast oscilloscopes and pulse generators

Abstract In this paper, we propose a new method for calculating the rise time of pulse generators and oscilloscopes using a correction factor. This new method is advantageous over the well known geometric rule, also known as the root-sum-of-squares (RSS) rule, because its corresponding uncertainty contribution can be estimated, whereas the uncertainty assigned to the RSS method due to systematic error is typically unknown in any given measurement scenario. In our method, the correction factor is estimated from a large set of representative classical response functions. Furthermore, the systematic error caused by the time base distortion of sampling oscilloscopes is corrected in order to reduce the uncertainty of the calibration process.