Bernhard Hils

THz Active Imaging Systems With Real-Time Capabilities

This paper presents a survey of the status of five active THz imaging modalities which we have developed and investigated during the last few years with the goal to explore their potential for real-time imaging. We start out by introducing a novel waveguide-based all-electronic imaging system which operates at 812 GHz. Its salient feature is a 32-pixel linear detector array heterodyne-operated at the eighth subharmonic. This array in combination with a telescope optics for object distances of 2-6 m reaches a data acquisition speed suited for real-time imaging. The second system described then is again an all-electronic scanner (now for around 300 GHz ), designed for object distances of ≥ 8 m , which combines mechanical scanning in vertical direction, synthetic-aperture image generation in horizontal direction, and frequency-modulated continuous-wave sweeping for the depth information. The third and fourth systems follow an optoelectronic approach by relying on several- to multi-pixel parallel electrooptic detection. One imager is based on a pulsed THz-OPO and homodyne detection with a CCD camera, the other on either continuous-wave electronic or femtosecond optoelectronic THz sources and a photonic-mixing device (PMD) camera. The article concludes with a description of the state of the art of imaging with focal-plane arrays based on CMOS field-effect transistors.

Terahertz profilometry at 600 GHz with 0.5 μm depth resolution

Characterization of the topography of materials by interferometry in the visible or near-IR wavelength regime becomes difficult or impossible if the surface is rough on the length scale of a tenth of the wavelength and more. In this case, THz radiation can provide an interesting alternative. We demonstrate heterodyne profilometry at 600 GHz as a method for the accurate determination of surface topography with an achievable expanded standard uncertainty of 0.5 μm.

Continuous-wave terahertz imaging with a hybrid system

A hybrid system for THz reflectometric imaging at 0.6 THz synchronizes a multiplied Gunn source with a femtosecond lasers for electro-optic detection and reaches 60 db dynamic range and 25 ms measurement time per pixel.

Towards an active real-time THz camera: first realization of a hybrid system

We report the realization of a hybrid system for stand-off THz reflectrometry measurements. The design combines the best of two worlds: the high radiation power of sub-THz micro-electronic emitters and the high sensitivity of coherent opto-electronic detection. Our system is based on a commercially available multiplied Gunn source with a cw output power of 0.6 mW at 0.65 THz. We combine it with electro-optic mixing with femtosecond light pulses in a ZnTe crystal. This scheme can be described as heterodyne detection with a Ti:sapphire fs-laser acting as local oscillator and therefore allows for phase-sensitive measurements. Example images of test objects are obtained with mechanical scanning optics and with measurement times per pixel as short as 10 ms. The test objects are placed at a distance of 1 m from the detector and also from the source. The results indicate diffraction-limited resolution. Different contrast mechanisms, based on absorption, scattering, and difference in optical thickness are employed. Our evaluation shows that it should be possible to realize a real-time multi-pixel detector with several hundreds of pixels and a dynamic range of at least two orders of magnitude in power.

Fast active THz camera with range detection by frequency modulation

We report on the realization of an active fully electronic THz camera operating at 645 GHz and room temperature. It currently acquires images with about 9000 pixels in 10 seconds. Each pixel contains the amplitude as well as the phase information of the reflected THz signal. Frequency modulation allows to select a specific working distance and to suppress spurious reflections. The typical image size is in the order of hundreds of cm2.

In-vivo, non-invasive detection of hyperglycemic states in animal models using mm-wave spectroscopy

Chronic or sustained hyperglycemia associated to diabetes mellitus leads to many medical complications, thus, it is necessary to track the evolution of patients for providing the adequate management of the disease that is required for the restoration of the carbohydrate metabolism to a normal state. In this paper, a novel monitoring approach based on mm-wave spectroscopy is comprehensively described and experimentally validated using living animal models as target. The measurement method has proved the possibility of non-invasive, in-vivo, detection of hyperglycemia-associated conditions in different mouse models, making possible to clearly differentiate between several hyperglycemic states.

On the way to an active terahertz camera: Optic design and its experimental verification

We report on the simulation of a terahertz optics setup with the commercial, optic-design software package Zemax. We show that the analysis of both diffraction and abberation effects are well corroborated by the experimental results.

High-accuracy topography measurement of optically rough surfaces with THz radiation

Characterization of the surface figure of technical materials by interferometry in the visible or near-IR wavelength regime becomes difficult or impossible if the surface is rough on the length scale of a tenth of the wavelength used. In this case, THz radiation can provide an interesting alternative. We demonstrate two methods for the accurate determination of the surface topography: THz heterodyne profilometry and THz ESAD (extended shear angle difference) deflectometry.

Classification of skin phenotypes caused by diabetes mellitus using complex scattering parameters in the millimeter-wave frequency range

The pathological skin phenotype caused by hyperglycemia is an important indicator for the progress of diabetes mellitus. An early detection of diabetes assures an early intervention to regulate the carbohydrate metabolism. In this publication a non-invasive detection principle based on the measurement of complex scattering parameters in the millimeter-wave frequency range is presented. The measurement principle provides evidence of the applicability for the identification of different glycemic states in animal models. The method proposed here can be used to predict diabetes status in animal models and is interesting for application on humans in view of safeness of millimeter-wave radiation. Furthermore the complex scattering parameters give important information about the anatomic varieties between the analyzed skin samples of the different mice strains. In contrast to other methods, our approach is less sensitive to skin variations between animals.

Holograms: made from scratch

Scratch holograms can be described as the limiting case of geometrical optics, where k→ ∞ , in which photons behave like grains of sand. We will show that scratch holograms can in principle be produced and read out by white light interference.