Benedikt Scherger

Paper terahertz wave plates

We present a low-cost terahertz wave plate based on form birefringence fabricated using ordinary paper. Measurements of the transfer function of the wave plate between polarizers closely agree with predictions based on the measured complex indices of refraction of the effective medium. For the design frequency, the dependence on wave plate angle also agrees with theory.

Variable-focus terahertz lens

We present a variable focus lens for the THz range. The focal length can be changed by pumping a medical white oil in and out of the lens body. Due to the optical transparency of the liquid and a similar refractive index in the visible frequency range, the THz beam path can be aligned using conventional optical light sources. This type of lens might find applications in terahertz based quality control, stand-off detection and wireless communication systems.

Terahertz lenses made by compression molding of micropowders

We present a simple and versatile approach for fabricating terahertz lenses based on compression molding of micropowder polymer materials in a tabletop hydraulic press. To demonstrate the feasibility of this approach, a biconvex lens shape is calculated using a ray-tracing algorithm and lenses based on two different micropowders are fabricated. As the powder materials have different refractive indices, the resulting lenses share the same geometric shape but differ in their respective focal length. The focusing properties of the lenses are evaluated by transversal and sagittal beam profile measurements in a fiber-coupled terahertz time-domain spectroscopy system, confirming the excellent imaging qualities of the compression molded lenses.

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.

Polarization sensitive terahertz imaging: Detection of birefringence and optical axis

We present a practicable way to take advantage of the spectral information contained in a broadband terahertz pulse for the determination of birefringence and orientation of the optical axis in a glass fiber reinforced polymer with a single measurement. Our setup employs circularly polarized terahertz waves and a polarization-sensitive detector to measure both components of the electromagnetic field simultaneously. The anisotropic optical parameters are obtained from an analysis of the phase and frequency resolved components of the terahertz field. This method shows a high tolerance against the skew of the detection axes and is also independent of a reference measurement.

Compression Molded Terahertz Transmission Blaze-Grating

We present a terahertz transmission blaze-grating, which can be fabricated, easily, cost efficiently and in large numbers using compression molding of micro-powder. The diffraction properties of the grating are derived by simulation of electro-magnetic field scattering and are compared with angle-dependent measurements obtained in a terahertz time-domain setup. An excellent match between simulation and measurement is found, demonstrating the ability of the terahertz transmission blaze-grating for spatial dispersion of terahertz waves. Thus, this and similar terahertz transmission blaze-gratings can be used as dispersive elements for applications such as spectrometers or novel THz imaging systems.

Efficient terahertz en-face imaging

In this work, we develop a pulsed terahertz imaging system in reflection geometry, where due to scanning of the terahertz beam neither the sample nor the emitter and detector have to be moved. We use a two mirror galvanoscanner for deflecting the beam, in combination with a single rotationally symmetric focusing lens. In order to efficiently image planar structures, we develop an advanced scanning routine that resolves all bending effects of the imaging plane already during measurement. Thus, the measurement time is reduced, and efficient imaging of surfaces and interfaces becomes possible. We demonstrate the potential of this method in particular for a plastic-metal composite sample, for which non-destructive evaluation of an interface is performed.

Terahertz Parallel-Plate Ladder Waveguide With Highly Confined Guided Modes

In this paper, we introduce a parallel-plate ladder waveguide (PPLWG) for THz waves. The characteristics of the guided modes of this structure which belong to the category of spoof surface plasmons are studied using the generalized multipole technique and its potential applications are discussed. Moreover, the proposed structure is experimentally investigated in a THz time-domain spectroscopy system and a comparison between the calculated and measured amplitude and phase characteristics is given. It is shown that the dominant mode of the PPLWG can be excited by an incident focused beam and a high degree of field confinement in the THz regime can be achieved.

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.

A fiberstretcher operating as an optical delay line in a fiber-coupled THz spectrometer

We demonstrate the use of a fiber stretcher as an optical delay line in a fiber-coupled THz spectrometer. No measurable dispersion effects occur when extending the fiber. Measurements of the terahertz pulses at a stretching frequency of 20 Hz show the great potential of such a device for future all-fiber terahertz systems.