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Various physical mechanisms leading to terahertz (THz) emission from semiconductor surfaces illuminated by femtosecond laser pulses are analyzed. Results obtained on different materials are described and relative efficiency of these materials as THz emitters is compared.

We report on the status of symmetric varactor diode multipliers for signal generation in the terahertz frequency range. The progress and basic principles of heterostructure barrier varactor (HBV) diodes are presented. Furthermore, the design methodology and electro-thermal simulation results of high-power HBV multipliers for signal generation in the millimeter and submillimeter wave region are also presented. Finally, a state-of-the-art HBV tripler with an output power of 0.2 Watt at 113 GHz is presented.

We describe our work towards THz sources which employ the “Bloch gain”, a stimulated-emission mechanism which has been predicted as early as 1971 to exist for semiconductor superlattices but which researchers – in spite of much recent work – have not yet been able to take advantage of for the implementation of THz amplifiers and lasers. From a basic-physics point-of-view, the interest in Bloch gain arises from its dynamical, second-order character, involving simultaneous scattering of an electron and emission of a THz photon. This aspect has the practically important implication that the temperature dependence of the gain is determined to a large degree by the optical-phonon energy scale and not that of the photon energy, with the consequence that there is a rather slow roll-off the gain with temperature. This feature together with the rather high-gain values which are calculated to be comparable with those of THz quantum cascade lasers at low temperature, fosters the hope that a Bloch THz laser could be the first semiconductor-based THz laser operating at room temperature.

For THz quantum cascade lasers to prove useful for applications, certain requirements for their spectral performance will have to be met. Here, we focus on the provision of single mode operation. Distributed feedback devices lasing on a single longitudinal mode are reported using both first and second order gratings. We also report the operation of terahertz master oscillator power amplifier structures with the potential to increase the output power which is available in a single mode.

We demonstrate a new waveguiding structure for terahertz (THz) radiation, in which broadband THz pulses are confined and guided along a bare metal wire. This waveguide exhibits close to the lowest attenuation of any waveguide for broadband THz pulses reported so far. It also supports propagation of broadband radiation with negligible group-velocity dispersion, making it especially suitable for use in pulsed terahertz sensing and diagnostic systems. In addition, the structural simplicity lends itself naturally to the facile manipulation of the guided pulses, including coupling, directing, and beam splitting. These results can be described in terms of a model developed by Sommerfeld, for waves propagating along the surface of a cylindrical conductor.

The paper discusses and compares the concepts, performance potential, and most recent experimental results of both classical and novel active two-terminal devices for low-noise RF power generation at submillimeter- wave frequencies up to 1 THz. These devices use transit-time, transferred-electron, and quantum-mechanical effects (or a combination of them) to create a negative differential resistance at the frequency of interest. Examples of state-of-the-art results are output power levels of more than 70 mW at 62 GHz and more than 10 mW at 132 GHz from GaAs/AlGaAs superlattice electronic devices; more than 9 mW at 280 GHz, 3.7 mW at 300 GHz, 1.6 mW at 329 GHz, and more than 40 μW at 422 GHz from InP Gunn devices; and more than 140 μW at 355 GHz from a GaAs tunnel-injection transit-time diode.

We review a selection of recent technological advances in terahertz frequency time-domain spectroscopy. We discuss the coherent generation of ultra-broadband terahertz radiation using a biased and asymmetrically excited low-temperature-grown GaAs photoconductive (PC) emitter. Using a backward collection method, terahertz radiation with frequency components over 30 THz can be collected, the highest observed for PC emitters. We outline two detection schemes, electro-optic (EO) detection using a ZnTe crystal, and PC-detection using a low-temperaturegrown GaAs PC receiver. The use of the PC receiver provides the timedomain spectroscopy system with a smooth spectral distribution between 0.3 and 7.5 THz, ideal for spectroscopic applications. We illustrate the technological developments with examples of transmission spectroscopy of polycrystalline organic materials. Specifically, we review measurements of the vibrational spectra of polycrystalline purine and adenine over the temperature range 4–290K. A number of well-resolved absorption peaks are observed, which are interpreted as originating from intermolecular vibrational modes mediated by hydrogen bonds. As the temperature is reduced, the observed absorption bands resolve into narrower peaks and some shift towards higher frequencies, which can be explained by the anharmonicity of the vibrational potentials. An empirical expression is given to describe this frequency shift.

Terahertz (THz) time-domain spectroscopy (TDS) is used to study two types of amorphous materials: glasses and polymers. The theory of far-infrared (IR) absorption in amorphous materials is used to analyse the results, and to understand the differences in THz absorption among the sample materials. A family of related borosilicate glasses has been examined along with silica glass, and their THz absorption coefficients and refractive indices are compared. Two chalcogenide glasses are also studied. Three types of polymer plates have been examined, and their THz transmission properties are compared with those of glasses. Polymerisation in SU8 films has been studied by exposing samples to UV for different lengths of time and comparing their THz transmission properties.

The vibrational modes corresponding to protein tertiary structural motion lay in the far-infrared or terahertz (THz) frequency range. These collective large-scale motions depend on global structure and thus will necessarily be perturbed by ligand-binding events. We discuss the use of THz dielectric spectroscopy to measure these vibrational modes and the sensitivity of the technique to changes in protein conformation, oxidation state and environment. A challenge of applying this sensitivity as a spectroscopic assay for ligand binding is the sensitivity of the technique to both bulk water and water bound to the protein. This sensitivity can entirely obscure the signal from the protein or protein–ligand complex itself, thus necessitating sophisticated sample preparation making the technique impractical for industrial applications. We discuss methods to overcome this background and demonstrate how THz spectroscopy can be used to quickly assay protein binding for proteomics and pharmaceutical research.

Spectroscopic imaging with terahertz (THz) or submillimeterwave (SMM) sources holds great promise for both defense and dual-use applications, such as for detection of chemical/biological weapons (CBW), concealed explosives, and other weapons (particularly nonmetallic varieties), and even through-the-wall imaging. To perform spectroscopy with active illumination of the target, either multiple or tunable continuous-wave (CW) sources or broadband pulsed sources are needed; passive illumination (e.g. using the cold sky) is limited to outdoor settings. While using incoherent (or intentionally decohered) illumination, either from the sky, a noise source, or a frequency-modulated CW source helps to reduce the interference caused by standing-wave phenomena (analogous to laser speckle), all such approaches have severe limitations in that they cannot perform accurate ranging, they are limited to a narrow range of frequencies, or are relatively weak. They are also all fundamentally limited to incoherent detection, which has limited signal-to-noise ratio (SNR) performance, lacking the advantages of heterodyne downconversion and detection. Using pulsed, broadband, coherent THz or SMM sources, and detectors is ideal for spectroscopic imaging and detection.