International Journal of High Speed Electronics and Systems

The terahertz frequency absorption spectra of DNA molecules reflect low-frequency internal helical vibrations involving rigidly bound subgroups that are connected by the weakest bonds, including the hydrogen bonds of the DNA base pairs, and/or non-bonded interactions. Although numerous difficulties make the direct identification of terahertz phonon modes in biological materials very challenging, recent studies have shown that such measurements are both possible and useful. Spectra of different DNA samples reveal a large number of modes and a reasonable level of sequence-specific uniqueness. This chapter utilizes computational methods for normal mode analysis and theoretical spectroscopy to predict the low-frequency vibrational absorption spectra of short artificial DNA and RNA. Here the experimental technique is described in detail, including the procedure for sample preparation. Careful attention was paid to the possibility of interference or etalon effects in the samples, and phenomena were clearly differentiated from the actual phonon modes. The results from Fourier-transform infrared spectroscopy of DNA macromolecules and related biological materials in the terahertz frequency range are presented. In addition, a strong anisotropy of terahertz characteristics is demonstrated. Detailed tests of the ability of normal mode analysis to reproduce RNA vibrational spectra are also conducted. A direct comparison demonstrates a correlation between calculated and experimentally observed spectra of the RNA polymers, thus confirming that the fundamental physical nature of the observed resonance structure is caused by the internal vibration modes in the macromolecules. Application of artificial neural network analysis for recognition and discrimination between different DNA molecules is discussed.

By illuminating the sample with a broadband 10-500 GHz stimulus and coherently detecting the response, we obtain reflection and transmission spectra of common powdered substances, and compare them as a starting point for distinguishing concealed threats in envelopes and on personnel. Because these samples are irregular and their dielectric properties cannot be modulated, the spectral information we obtain is largely qualitative. To show how to gain quantitative information on biological species at micro- and millimeter-wave frequencies, we introduce thermal modulation of a globular protein in solution, and show that changes in microwave reflections coincide with accepted visible absorption spectra, pointing the way toward gaining quantitative chemical and biological spectra from broadband terahertz systems.

THz time domain spectroscopy of biomolecules was performed to determine applicability of the technique for chemical and conformational identification of biomolecules. Measurements were performed on samples of DNA, bovine serum albumin, collagen, hen egg white lysozyme, myoglobin, and bacteriorhodopsin as a function of temperature, hydration and photoexcitation. The results are compared to normal mode calculations. We demonstrate a clear resemblance of the observed broad and near featureless THz absorbance spectrum to the calculated density of normal modes. While the magnitude of the absorbance and the center of the broad response depends on biomolecular species, unique chemical identification would appear challenging. The observed dependence on hydration is in agreement with mass and dielectric loading, and the dependence on temperature is in agreement with decreasing conformational flexibility with reduced temperature. Finally THz absorbance dependence on the biomolecular conformation and mutation is demonstrated for bacteriorhodopsin.

Electronic characteristics are difficult to monitor in nanocomposites. Here we describe indirect assessments of these characteristics using THz, Raman and IR spectroscopy. Specifically we seek to gain understanding of the electron mobility in semiconductive and conductive nanostructures for electronic, electrooptic and nonlinear optical purposes.

Having long been the realm of molecular chemistry, astronomy, and plasma diagnostics, the upper millimeter-wave band (~100 to 300 GHz) and the THz region above it have recently become the subject of heightened activity in the engineering community because of exciting new technology (e.g., sub-picosecond optoelectronics) and promising new "terrestrial" applications (e.g., counter-terrorism and medical imaging). The most challenging of these applications are arguably those that demand remote sensing at a stand-off of roughly 10 m or more between the target and the sensor system. As in any other spectral region, remote sensing in the THz region brings up the complex issues of sensor modality and architecture, free-space electromagnetic effects and components, transmit and receive electronics, signal processing, and atmospheric propagation. Unlike other spectral regions, there is not much literature that addresses these issues from a conceptual or system-engineering viewpoint. So a key theme of this chapter is to review or derive the essential engineering concepts in a comprehensive fashion, starting with fundamental principles of electromagnetics, quantum mechanics, and signal processing, and building up to tradc-off formulations using system-level metrics such as noise-equivalent power and receiver operating characteristics. A secondary theme is to elucidate aspects of the THz region and its incumbent technology that are unique, whether advantageous or disadvantageous, relative to other spectral regions. The end goal is to provide a useful tutorial for graduate students or practicing engineers considering the upper mm-wave or THz regions for system research or development.

Semiconductor solid-state lasers based on conduction-valence band recombination are now commonplace for low-power red emission, and are available commercially at nearly continuous wavelengths throughout the near-UV and near-IR communications bands. Furthermore, new lasers and broadband spontaneous emission sources are available through a wide wavelength range, including 3-8μm based on both conduction-valence band and intersubband transitions, and up to 70μm using cascaded intersubband transitions. Competing processes make the design of these semiconductor lasers extremely difficult when extended to the very long, 300μm wavelength regime corresponding to low terahertz frequencies. We discuss material and device-design considerations for extending semiconductor lasers to this regime. We suggest a new set of device structures based on a semiconductor quantum dot (QD) gain medium, where the lasing occurs through discrete conduction states. In one implementation, two QDs are coupled to make a coupled-asymmetric quantum dot (CAD) laser. In another implementation, an ensemble of non-coupled QDs is selectively placed in a high quality cavity, called a microdisk, which is resonant with an intersublevel QD transition. We demonstrate the initial fabrication of these structures.

Photon assisted transport, dynamic localization and absolute negative conductance appear in the terahertz photoconductivity in semiconductor quantum structures and are close analogs of quasi-particle transport in microwave irradiated superconducting junctions. By embedding superlattice devices in quasi-optical arrays and integrating them into terahertz cavities, the dynamical conductance of electrically biased superlattices can be measured. Models including the complications of electric field domains can account for the results in a semi quantitative manner. Uniform electrically biased superlattices appear to be potentially important as a terahertz gain medium.

This work is concerned with the quantum structure of resonant tunneling diodes, which exhibits intrinsic instability that can be exploited for the development of high-speed, high-frequency devices. The article examines in detail the physics underlying the non-liner instability, in both a one-band model and a multiple-band model. The theoretical basis of the description of electronic processes in such structures are described in some detail in terms of nonequilibrium Green's functions. Also presented here is a semi-phenomenological model of the resonant tunneling diode based on nonlinear circuit theory. Recent works and progresses in this and related areas are summarized here as well.

Issues associated with modeling of quantum devices within the framework of the transient Wigner equation are addressed. Of particular importance is the structure of the Wigner function, hysteresis, and device switching time, whose value is determined by the large signal device properties.

Continuous-wave linear-absorption spectroscopy based on THz radiation generated by solid-state photomixers has been applied to the investigation of the dynamics of biomolecules in polyethylene matrices and to line shape studies of HF for diagnostics of semiconductor etching plasmas. The THz spectra of biotin and myoglobin have been obtained using a variable-temperature, cryogenic sampling system. The spectrum of biotin displays a small number of discrete absorptions over the temperature range from 4.2 K to room temperature while the spectrum of myoglobin has no obvious resonance structure at the >10% fractional absorption level. Spectral predictions from the lowest energy ab initio conformations of biotin are in poor agreement with experiment, suggesting the need to include condensed-phase environmental interactions for qualitative predictions of the THz spectrum. Vibrational anharmonicity is used to model the line shapes that result from drastic changes in vibrational sequence level populations of biotin over this temperature range. Anharmonicity factors (χ_eω_e/ω_e) at the levels of 0.1% to 0.8% are obtained from non-linear least squares fits of the observed resonances and illustrate their important for refining model predictions. Application of the photomixer system to line shape studies in etching plasmas has been used to study the formation efficiency and translational temperature of HF at 1.2 THz under different operating conditions. These results will aid in understanding the chemistry of industry-standard fluorocarbon and oxygenated fluorocarbon etching plasmas.