Stefan F. Busch

Optically controlled terahertz beam steering and imaging

We propose a spatial modulator for terahertz waves based on light induced electron plasma in photo-active semiconductors. A two-dimensional array of computer controlled light is used to create free carries in bulk silicon, which results in a spatial modulation of the transmission at terahertz frequencies. This method not only exhibits a remarkable modulation depth over a broad frequency range but also allows for an optically controlled beam steering of terahertz waves by inducing virtual grating structures. In addition, we analyze the possibility of all-optically controlled terahertz imaging.

Scaled Bistatic Radar Cross Section Measurements of Aircraft With a Fiber-Coupled THz Time-Domain Spectrometer

The knowledge of the radar cross section (RCS) of aircraft and other objects is of great interest both for civil and military applications. Scaled setups are often used in order to facilitate RCS measurements in a well-defined laboratory environment. As radar frequencies steadily increase, for high scaling factors these measurements have to be carried out in the THz regime. In this paper, we propose an experimental setup consisting of a fiber-coupled THz time-domain spectrometer integrated with a two circle goniometer, which enables bistatic scaled RCS measurements. To assess the accuracy of the setup, measurements on reference objects as well as on scale model aircraft are performed. The measured data of the reference objects is compared to the theoretical predictions. As for the aircraft, the comparison between a Panavia 200 Tornado and a Lockheed F117 Nighthawk is made and the influence of individual components like bombs on the overall RCS is evaluated.

Optically gated tunable terahertz filters

We present a fast and flexible terahertz filter based on diffractive gratings combined with an optically gated modulator. The terahertz radiation is diffracted by blazed gratings, and the individual frequency components are focused onto an optically excited semiconductor. Different light patterns are used to create free carriers in the bulk semiconductor material, leading to a spatial modulation of its transmission. This approach enables us to damp arbitrary frequencies and allows us to design and implement filters with almost any frequency response. By using the digital light processing technique, the switching time between different filter settings is as short as 16 ms.

Fabrication of gradient-refractive-index lenses for terahertz applications by three-dimensional printing

In this article we present the design, fabrication, and characterization of gradient-refractive-index lenses for terahertz (THz) applications. The fabrication was performed by three-dimensional printing. These new low-cost lenses were tested using THz time-domain spectroscopy and imaging in order to measure their optical properties. The results show a focusing capacity within the diffraction limit for frequencies below 700 GHz.

3D printed dielectric rectangular waveguides, splitters and couplers for 120 GHz

We use a 3D printer to fabricate rectangular dielectric single mode waveguides for 120 GHz. The rectangular waveguides consisting of polystyrene showed an attenuation of 6.3 dB/m, which is low enough for short devices. We also characterize 3D printed Y-splitters and a 1x3-splitter based on multimode interference. Further, we construct and measure a variable planar waveguide coupler which can be used as a 3-dB coupler, a cross-coupler and no coupler at all.

Cost-efficient delay generator for fast terahertz imaging

We present a fast and low-cost delay generator for terahertz (THz) waves that transfers a rotational motion of a transparent dielectric cube into an effective THz delay. The device is easily implemented in the THz beam path and allows for coherent sampling over 40 ps with a scan rate of hundreds of hertz. Furthermore, we show that our approach is particularly suitable for fast THz imaging.

Terahertz transceiver concept

We present a photoconductive terahertz transceiver based on a modulation of the optical pulses used for generation and detection at different rates. External modulation of the THz pulses is not required as opposed to previously reported approaches. Devices from fiber-optic technology are used, providing flexibility and stability to the system. Imaging and thickness measurement experiments are carried out to demonstrate the performance of the transceiver.

Spectrum to space transformed fast terahertz imaging.

We present an imaging technique in which the broadband frequency information of terahertz (THz) pulses is transformed into spatial resolution. Efficient blazed diffractive gratings spread the individual frequency components over a wide and defined spatial range and f-theta optics are employed to focus the individual components onto a one-dimensional image-line. Measuring the time domain waveform of the THz waves allows therefore for a direct reconstruction of spatial sample characteristics as the spatial domain information is encoded in the terahertz spectrum. We will demonstrate terahertz imaging on selected samples with an improvement in acquisition speed up to two orders of magnitude.

Terahertz brewster lenses

Typical lenses suffer from Fresnel reflections at their surfaces, reducing the transmitted power and leading to interference phenomena. While antireflection coatings can efficiently suppress these reflections for a small frequency window, broadband antireflection coatings remain challenging. In this paper, we report on the simulation and experimental investigation of Brewster lenses in the THz-range. These lenses can be operated under the Brewster angle, ensuring reflection-free transmission of p-polarized light in an extremely broad spectral range. Experimental proof of the excellent focusing capabilities of the Brewster lenses is given by frequency and spatially resolved focus plane measurements using a fiber-coupled THz-TDS system.

Quality Control of Sugar Beet Seeds With THz Time-Domain Spectroscopy

Sugar beet seeds have a comparably high rate of defective seeds of about 35%-40% which are discarded before dissemination in the field. The separation of proper and defective seeds is a complex and labor-intensive process. We show how THz time-domain spectroscopy can be used for the examination of the seeds including an algorithm for automated classification of the seeds.