Daniel M. Mittleman

Metal wires for terahertz wave guiding

Sources and systems for far-infrared or terahertz (1 THz = 1012 Hz) radiation have received extensive attention in recent years, with applications in sensing, imaging and spectroscopy. Terahertz radiation bridges the gap between the microwave and optical regimes, and offers significant scientific and technological potential in many fields. However, waveguiding in this intermediate spectral region still remains a challenge. Neither conventional metal waveguides for microwave radiation, nor dielectric fibres for visible and near-infrared radiation can be used to guide terahertz waves over a long distance, owing to the high loss from the finite conductivity of metals or the high absorption coefficient of dielectric materials in this spectral range. Furthermore, the extensive use of broadband pulses in the terahertz regime imposes an additional constraint of low dispersion, which is necessary for compatibility with spectroscopic applications. Here we show how a simple waveguide, namely a bare metal wire, can be used to transport terahertz pulses with virtually no dispersion, low attenuation, and with remarkable structural simplicity. As an example of this new waveguiding structure, we demonstrate an endoscope for terahertz pulses.

Sensing with terahertz radiation

Spectroscopy in the Terahertz Spectral Region.- Terahertz Imaging.- Free-Space Electro-Optic Techniques.- Photomixers for Continuous-Wave Terahertz Radiation.- Applications of Optically Generated Terahertz Pulses to Time Domain Ranging and Scattering.- Bio-medical Applications of THz Imaging.- Electronic Sources and Detectors for Wideband Sensing in the Terahertz Regime.

Imaging with terahertz radiation

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

T-ray tomography

We demonstrate tomographic T-ray imaging, using the timing information present in terahertz (THz) pulses in a reflection geometry. THz pulses are reflected from refractive-index discontinuities inside an object, and the time delays of these pulses are used to determine the positions of the discontinuities along the propagation direction. In this fashion a tomographic image can be constructed.

A single-pixel terahertz imaging system based on compressed sensing

We describe a terahertz imaging system that uses a single pixel detector in combination with a series of random masks to enable high-speed image acquisition. The image formation is based on the theory of compressed sensing, which permits the reconstruction of a N-by-N pixel image using much fewer than N2 measurements. This approach eliminates the need for raster scanning of the object or the terahertz beam, while maintaining the high sensitivity of a single-element detector. We demonstrate the concept using a pulsed terahertz time-domain system and show the reconstruction of both amplitude and phase-contrast images. The idea of compressed sensing is quite general and could also be implemented with a continuous-wave terahertz source.

Short-Range Ultra-Broadband Terahertz Communications: Concepts and Perspectives

We propose the concept of ultra-broadband terahertz communication, based on directed non-line-of-sight (NLOS) transmissions. Potential applications of such a system supporting multi-gigabit data rates are given, and put into the context of currently emerging WLANs/WPANs. The technology and propagation constraints serve as boundary conditions for the determination of the required antenna gain to support ultra-broadband communication. Resulting high-gain antenna requirements will necessitate highly directed transmissions. We propose the use of omni-directional dielectric mirrors to support directed NLOS paths. Their performance is investigated with ray-tracing simulations of a terahertz propagation channel in a dynamic office environment, which is calibrated with measured building-material and mirror parameters. We demonstrate that a directed NLOS path scheme will make a terahertz communication system robust to shadowing. Furthermore, we show that dielectric mirrors covering only parts of the walls will significantly enhance the signal coverage in a typical indoor scenario.

A spatial light modulator for terahertz beams

We design and implement a multipixel spatial modulator for terahertz beams using active terahertz metamaterials. Our first-generation device consists of a 4×4 pixel array, where each pixel is an array of subwavelength-sized split-ring resonator elements fabricated on a semiconductor substrate, and is independently controlled by applying an external voltage. Through terahertz transmission experiments, we show that the spatial modulator has a uniform modulation depth of around 40% across all pixels, and negligible crosstalk, at the resonant frequency. This device can operate under small voltage levels, at room temperature, with low power consumption and reasonably high switching speed.

CHEMICAL RECOGNITION OF GASES AND GAS MIXTURES WITH TERAHERTZ WAVES

A time-domain chemical-recognition system for classifying gases and analyzing gas mixtures is presented. We analyze the free induction decay exhibited by gases excited by far-infrared (terahertz) pulses in the time domain, using digital signal-processing techniques. A simple geometric picture is used for the classif ication of the waveforms measured for unknown gas species. We demonstrate how the recognition system can be used to determine the partial pressures of an ammonia-water gas mixture.

NONCONTACT SEMICONDUCTOR WAFER CHARACTERIZATION WITH THE TERAHERTZ HALL EFFECT

We demonstrate noncontact measurements of the Hall mobility of doped semiconductor wafers with roughly 250 μm spatial resolution, using polarization rotation of focused beams of terahertz (THz) radiation in the presence of a static magnetic field. Quantitative and independent images of both carrier density and mobility of a doped semiconductor wafer have been obtained.

Scattering Analysis for the Modeling of THz Communication Systems

Modeling propagation channels for future pico-cellular indoor THz communication systems requires the knowledge of the reflective properties of building materials. The reflectivity of smooth, optically thick materials can be modeled with Fresnel equations. In case of materials with a rough surface, diffuse scattering reduces the power reflected in the specular direction. Kirchhoff scattering theory can be employed to derive modified Fresnel equations which account for these losses by introducing a Rayleigh roughness factor calculated from the measured surface height distribution of the sample under observation. Using the resulting, analytically derived reflection coefficient based on material parameter and surface measurements in propagation models enables the simulation of arbitrary configurations. We present a set of calculated and measured reflection coefficients for a selection of common indoor building materials which are in good agreement, thus verifying our modeling approach. Furthermore, we illustrate by ray-tracing simulations the effect of wall and ceiling roughness on propagation in future indoor scenarios. Both, absolute power levels and propagation patterns are shown to be strongly influenced by scattering. In some cases, reflected transmissions with longer propagation paths can be more efficient than the shorter ones in terms of incurred losses.