Towards terahertz microscopy

Terahertz (=1012 Hz) radiation is a form of electromagnetic radiation that is at this moment used rarely for imaging purposes. However, there are indeed reasons to assume that imaging with terahertz radiation could be very useful. First, many materials, such as paper, plastics and clothing are transparent for terahertz radiation, while they block visible light. This opens the opportunity to look through objects. One can, for instance, consider checking the storage life of milk without opening the milk carton, or the security checks on concealed weapons at airports. Secondly, many materials have characteristic properties in the terahertz region that make a clear contrast between these materials possible. For instance, it is possible to specifically measure the concentration of gasses in a gas mixture. Also, different forms of DNA can be distinguished. By the development of stronger sources and more sensitive detection methods, imaging with terahertz radiation becomes ever more attractive. However, for some applications, such as imaging biological cells, the resolutions of many terahertz imaging techniques are not good enough. This is caused by a fundamental physical limit, the diffraction limit, which dictates that the resolution of ordinary imaging techniques is limited to about half the wavelength of the radiation used. For terahertz radiation, the diffraction limit on the resolution is about 0.1 mm. To use terahertz radiation for imaging microscopic objects, such as cells, the diffraction limit will thus have to be circumvented. This thesis explores different aspects of terahertz imaging with the ultimate goal of the development of a terahertz microscopy technique. Two of these aspects are the generation and detection of terahertz pulses. We also describe the different noise sources in our measurements and discuss how the influence of these sources can be minimized. Terahertz waves that propagate over metal surfaces are studied, because of the possibly large influence of these waves in our microscopy setup. The last part of this thesis presents a new terahertz microscopy technique.Applied Science

Electro-optic detection of subwavelength terahertz spot sizes in the near field of a metal tip

We report on a method to obtain a subwavelength resolution in terahertz time-domain imaging. In our method, a sharp copper tip is used to locally distort and concentrate the THz electric field. The distorted electric field, present mainly in the near field of the tip, is electro-optically measured in an (100) oriented GaP crystal. By raster scanning the tip along the surface of the crystal, we find the smallest THz spot size of 18 ?m for frequencies from 0.1 to 2.5 THz. For our peak frequency of 0.15 THz, this corresponds to a resolution of ?/110. Our setup has the potential to reach a resolution down to a few ?m.Imaging Science and TechnologyApplied Science

Effect of a dielectric coating on terahertz surface plasmon polaritons on metal wires

The authors present measurements and calculations on the effect of thin dielectric coatings on the propagation of terahertz pulses along the surface of metal wires. Our measurements show that propagation over only a few centimeters of wire having a thin dielectric coating, strongly distorts the terahertz pulse, which results in a several tens of picoseconds long chirped signal. We demonstrate that the terahertz pulses propagate along the wire as surface waves, and show how a thin coating of a nondispersive material makes this propagation strongly dispersive, giving rise to the chirped signal observed in the measurements. Our results show the potential of terahertz surface plasmon polaritons on metal wires for the sensitive detection of thin dielectric layers.Imaging Science and TechnologyApplied Science

Terahertz polarization imaging

We present a new method to measure the polarization state of a terahertz pulse by using a modified electrooptic sampling setup. To illustrate the power of this method, we show two examples in which the knowledge of the polarization of the terahertz pulse is essential for interpreting the results: spectroscopy measurements on polystyrene foam and terahertz images of a plastic coin. Both measurements show a sampleinduced rotation of the terahertz electric field vector, which is surprisingly large and is a strong function of frequency. A promising aspect of our setup is the possibility of simultaneously measuring both transversal electric field components.Imaging Science and TechnologyApplied Science

Frequency-dependent radiation patterns emitted by THz plasmons on finite length cylindrical metal wires

We report on the emission patterns from THz plasmons propagating towards the end of cylindrical metal waveguides. Such waveguides exhibit low loss and dispersion, but little is known about the dynamics of the terahertz radiation at the end of the waveguide, specifically in the near- and intermediate-field. Our experimental results and numerical simulations show that the near- and intermediate-field terahertz spectra, measured at the end of the waveguide, vary with the position relative to the waveguide. This is explained by the frequency-dependent diffraction occurring at the end of the cylindrical waveguide. Our results show that near-field changes in the frequency content of THz pulses for increasing wire-detector distances must be taken into account when studying surface waves on cylindrical waveguides.Imaging Science and TechnologyApplied Science

Technology evolution of the Tropomi instrument

TROPOMI is the sun backscatter trace gas instrument on ESA's Sentinel-5 precursor satellite. TROPOMI builds upon a rich heritage from similar instruments, the main ones being SCIAMACHY on ESA's ENVISAT and OMI on NASA's AURA satellite. This paper explains how the technology from the heritage instruments evolved, considering high level design aspects such as geometry for Earth and sun viewing, polarization treatment, spectral calibration and whisk- or push-broom concept and leading to the TROPOMI concept. Netherlands parties have played and continue to play an important role in the relevant instruments. TROPOMI is the single payload on the Sentinel-5 precursor mission. The launch in 2015 is meant to bridge the data gap between OMI and SCIAMACHY and the upcoming Sentinel 5 mission. The instrument is funded jointly by the Netherlands Space Office and by ESA1. Dutch Space acts as overall instrument prime