Karsten Siebert

Continuous-wave all-optoelectronic terahertz imaging

We present an all-optoelectronic THz imaging system based on photomixing of two continuous-wave laser beams using photoconductive antennas. For a specific biological sample, we compare continuous-wave THz imaging and pulsed THz imaging at 1 THz with respect to data-acquisition time and signal-to-noise ratio, and discuss image formation from both amplitude and phase data. In addition, we introduce the application of hyperboloidal lenses which allow tighter focusing and a corresponding improvement in spatial resolution compared to off-axis paraboloidal mirrors.

Terahertz dark-field imaging of biomedical tissue.

We investigate dark-field imaging in the terahertz (THz) fre-quency regime with the intention to enhance image contrast through the analysis of scattering and diffraction signatures. A gold-on-TPX test structure and an archived biomedical tissue sample are examined in conventional and dark-field transmission geometry. In particular, the capability of the technique for tumor detection is addressed.

All-optoelectronic continuous wave THz imaging for biomedical applications

We present an all-optoelectronic THz imaging system for ex vivo biomedical applications based on photomixing of two continuous-wave laser beams using photoconductive antennas. The application of hyperboloidal lenses is discussed. They allow for f-numbers less than 1/2 permitting better focusing and higher spatial resolution compared to off-axis paraboloidal mirrors whose f-numbers for practical reasons must be larger than 1/2. For a specific histological sample, an analysis of image noise is discussed.

A fully tunable dual-color CW Ti:Al/sub 2/O/sub 3/ laser

We introduce a CW two-color Ti:sapphire laser for terahertz spectroscopic applications. The laser provides two parallel beams at independently tunable wavelengths allowing one to select a difference frequency in the range from below 20 GHz to 50 THz. The laser is pumped by a single 6-W pump beam in order to keep the setup as simple as possible. Under these conditions, output powers of 150 mW per beam in the gain-maximum region are achieved.

All-optoelectronic continuous-wave terahertz systems.

We discuss the optoelectronic generation and detection of continuous–wave terahertz (THz) radiation by the mixing of visible/near–infrared laser radiation in photoconductive antennas. We review attempts to reach higher THz output–power levels by reverting from mobility–lifetime–limited photomixers to transit–time–limited p–i–n photodiodes. We then describe our implementation of a THz spectroscopy and imaging–measurement system and demonstrate its imaging performance with several examples. Possible application areas of THz imaging in the biomedical field and in surface characterization for industrial purposes are explored.

Field Screening in Low-Temperature-Grown GaAs Photoconductive Antennas

This paper investigates the nature of the signals obtained in double-pulse-excitation THz-emission experiments performed on terahertz antennas with low-temperature-grown GaAs as photoconductive material. Results of such measurements on devices containing photoconductive gaps with an area below 100 µm2 have been interpreted in the past as evidence for screening effects by space charge build-up. We argue that this interpretation leads to discrepancies with what is known about charge-carrier dynamics. In analogy to the much-better-studied response of large-area emitters (photoconductors with an active area on the order of a square centimeter), we argue that radiation-field screening must be considered when interpreting the data of small-area emitters.

All-Optoelectronic Terahertz Imaging Systems and Examples of Their Application

We give an overview over several all-optoelectronic measurement systems which we have developed for transmittive and reflective imaging in the terahertz (THz) frequency range. The systems employ either pulsed or continuous-wave THz radiation. In both cases, they work on the basis of single-pixel scanning. Addressing the potential for imaging in the medical and dental field, and the application of THz radiation for industrial surface and interface characterization, we explore dark-field imaging where the imaging contrast originates from diffraction and scattering effects coming from topography or refractive-index variations.

All-optoelectronic CW THz-imaging

Summary from only given. We demonstrate cw THz imaging with complex sample and compare the results with images taken with a state-of-the-art pulsed system based on an Ti:sapphire amplifier laser operating at 1-kHz repetition rate.

Measurement of the spin-redirection phase using Pancharatnam's theorem

A Michelson interferometer is presented which enables a direct measurement of Berrys geometric phase (spin-redirection phase) resulting from light propagating through a helically wound optical single-mode fibre. The operation of the interferometer is interpreted in terms of Pancharatnams theorem. In addition, we put the spin-redirection phase into the context of Pancharatnams phase.

Investigation of tumor recognition by Terahertz dark-field imaging

Summary form only given. Dark-field imaging deals with contrast enhancement by the detection of that part of the radiation which is deflected out of the beam-propagation direction by either diffraction or scattering in the sample, and requires blocking of the ballistic part of the radiation.