Narrow Results

THz technology applications, sensors and sources are briefly reviewed. Emphasis is placed on the less familiar components, instruments or subsystems. Science drivers, some historic background and future trends are discussed.

Terahertz (THz) radiation occupies part of the electromagnetic spectrum between the infrared and microwave bands. Until recently, technology at THz frequencies was under-developed compared to the rest of the electromagnetic spectrum, leaving a gap between millimeter waves and the far-infrared (FIR). In the past decade, interest in the THz gap has been increased by the development of ultrafast laser-based T-ray systems and their demonstration of diffraction-limited spatial resolution, picosecond temporal resolution, DC-THz spectral bandwidth and signal-to-noise ratios above 10⁴.

This chapter reviews the development, the state of the art and the applications of T-ray spectrometers. Continuous-wave (CW) THz-frequency sources and detectors are briefly introduced in comparison to ultrafast pulsed THz systems. An emphasis is placed on experimental applications of T-rays to sensing and imaging, with a view to the continuing advance of technologies and applications in the THz band.

The development of an effective biological (bio) agent detection capability based upon terahertz (THz) frequency absorption spectra will require insight into how the constituent cellular components contribute to the overall THz signature. In this work, the specific contribution of ribonucleic acid (RNA) to THz spectra is analyzed in detail. Previously, it has only been possible to simulate partial fragments of the RNA (or DNA) structures due to the excessive computational demands. For the first time, the molecular structure of the entire transfer RNA (tRNA) molecule of E. coli was simulated and the associated THz signature was derived theoretically. The tRNA that binds amino acid tyrosine (tRNA_tyr) was studied. Here, the molecular structure was optimized using the potential energy minimization and molecular dynamical (MD) simulations. Solvation effects (water molecules) were also included explicitly in the MD simulations. To verify that realistic molecular signatures were simulated, a parallel experimental study of tRNAs of E. coli was also conducted. Two very similar molecules, valine and tyrosine tRNA were investigated experimentally. Samples were prepared in the form of water solutions with the concentrations in the range 0.01-1 mg/ml. A strong correlation of the measured THz signatures associated with valine tRNA and tyrosine tRNA was observed. These findings are consistent with the structural similarity of the two tRNAs. The calculated THz signature of the tyrosine tRNA of E. coli reproduces many features of our measured spectra, and, therefore, provides valuable new insights into bio-agent detection.

Motivated by the possibility of identifying and detecting certain biochemical species using Fourier transform infrared spectroscopy (FTIR), we have investigated porcine, bovine, lispro, and human insulins. We have successfully observed and identified all the transition peaks for the four types of insulins in the frequency domains from mid-IR to THz. In the mid-IR region, ten transition peaks have been observed for all four insulins. Although these four insulins are made from amino acids which have either slightly different sequences or slight variations, the ten transition frequencies are virtually indistinguishable. However, for protamine sulfate some of the transition frequencies in the same mid-IR region are either red-shifted or blue-shifted relative to the corresponding ones for the four insulins. Furthermore, the strengths for several peaks of protamine sulfate are significantly reduced whereas the strength for only one transition peak is significantly, enhanced, compared with the corresponding ones for the insulins. In the far-IR to THz transition region, the shapes and locations of the two transitions are quite different between the insulin species and protamine sulfate. In the THz region we have observed a linear dependence of the absorption coefficients on frequency for each of the four insulins and protamine sulfate.

The experimental and computational study of bacterial thioredoxin, an E. coli protein, at THz frequencies is presented. The absorption spectrum of the entire protein in water was studied numerically in the terahertz range (0.1 – 2 THz). In our work, the initial X-ray molecular structure of thioredoxin was optimized using the molecular dynamical (MD) simulations at room temperature and atmospheric pressure. The effect of a liquid content of a bacterial cell was taken into account explicitly via the simulation of water molecules using the TIP3P water model. Using atomic trajectories from the room-temperature MD simulations, thioredoxin's THz vibrational spectrum and the absorption coefficient were calculated in a quasi harmonic approximation.

For our terahertz transmission measurements, we used solutions of thioredoxin in distilled water obtained from Sigma. The experimental and simulated signatures are correlated and dominant peaks are close in frequencies. The results of this study demonstrate that terahertz spectroscopy is a promising tool in generating spectral data for cellular components of bio agents such as bacterial cells and spores.

In the last few years, a number of researchers including our collaboration have assembled databases of terahertz (THz) time-domain spectroscopy (TDS) absorption spectra from bulk explosives. While this was a necessary and important step in demonstrating the feasibility of THz TDS for explosives detection, the goal of our research is to demonstrate selectivity of THz spectra from the clutter of background spectra coming from the substrate such as soil or sand. We have investigated THz TDS reflection spectra from sand with different grain sizes as well as from metallic powders in order to distinguish between the signals reflected from the rough surfaces compared to distributed reflections at finite depths in the granular material. With marker materials such as tartaric acid, which have absorption features in the 1-2 THz range, we have investigated the reflection spectra of granular substrates with marker chemicals, and compared this to reflection and transmission spectra of solid materials prepared in polyethylene sample pellets. In principle, the same experiments can then be performed using TNT, RDX, HMX and PETN, which all have characteristic features in the 0.5-8 THz frequency range. Absolute molecular absorption coefficients can be measured as well, and we include here preliminary values for RDX. A full analysis will be reported elsewhere.

Pivotal in the design of circuits is the ability to efficiently translate available transistor gain to high gain per stage. Remarkably, for 35-nm InP HEMT transistors, the efficiency of this translation remains high even up to ~0.5 THz. The ever shrinking wavelength correlated with higher frequencies necessitates a scaling of not only the device layout, but also of the passive elements and wafer thickness. Furthermore, to avoid distributed effects, the length of transistor gate fingers must be reduced.

A CMOS cascode amplifier, biased near the threshold voltage of a MOSFET, for terahertz direct detection is proposed. A CMOS terahertz imaging circuit (size: 250 × 180 ìm) is designed and fabricated on the basis of low-cost 180-nm CMOS process technology. The imaging circuit consists of a microstrip patch antenna, an impedance-matching circuit, and a direct detector. It achieves a responsivity of 51.9 kV/W at 0.915 THz and a noise equivalent power (NEP) of 358 pW/Hz^1/2 at a modulation frequency of 31 Hz. NEP is estimated to be reduced to 42 pW/Hz^1/2 at 100 kHz. These results suggest that cost-efficient terahertz imaging is possible in the near future.

Field effect transistors (FETs) in plasmonic regimes of operation could detect terahertz (THz) radiation and operate as THz interferometers, spectrometers, frequency-to-digital converters and THz modulators and sources. We report on the development of compact models for Si MOS (Metal-Oxide-semiconductor) and heterostructure-based plasmonic FETs (or TeraFETs) suitable for circuit design in the THz range and based on the multi-segment unified charge control model. This model accounts for the electron inertia effect (by incorporating segmented Drude inductances), for the ballistic field effect mobility, which is proportional to the channel length, for parasitic resistances and capacitances and for the leakage current. It is validated by comparison with experimental data and TCAD simulation results. The model can be used for simulation and optimization of sub-THz and THz detectors. Our simulations use up to 200 segments in the device channel. The results are also in good qualitative agreement with the hydrodynamic simulations. Applications of our model could dramatically reduce astronomical design costs of nanoscale VLSI reaching US$1.5 billion for the 3 nm technological node.

TeraFET arrays operating in plasmonic regimes could support the transition from 5G to 6G communication if the constituent TeraFETs operate in synchrony. Such arrays are plasmonic crystals supporting Bloch-like waves of electron density oscillations. The key issues are breaking symmetry and maintaining appropriate boundary conditions between the unit cells. The symmetry must be broken to choose the response polarity to detect the direction of the plasmonic instability growth for generating THz oscillations. The coherence of plasma waves propagating in individual cells of the plasmonic crystal results in continuous waves in the entire structure. Using the narrow stripes at the unit cell edges (called plasmonic stubs) could maintain such coherence. Another advantage of TeraFET arrays is the reduced effects of parasitic contact resistance. This advantage is even more pronounced in ring plasmonic structures used for converting THz radiation into a magnetic field (giant inverse Faraday effect).

Zn-doped p-type InP has been examined by far-infrared absorption magnetospectroscopy to fields much higher than previously. The impurity-related transitions observed confirm and greatly extend previous data and reveal new phenomena.

We demonstrate that lightly-doped Si(P) displays extremely sharp absorption lines – the narrowest yet reported for any impurity in natural Si. The Zeeman splitting of many of these lines in magnetic fields <10 T has been studied previously by a number of groups. In this paper we focus on the behavior of metastable states associated with conduction-band Landau levels. The use of a rather heavily doped sample and strong magnetic fields, up to 18 T, assists in the observation of these.

By employing the characteristics matrix method, we have investigated the transmission properties of one-dimensional dielectric–semiconductor metamaterial photonic crystals (PC) at Terahertz (THz) range theoretically. The numerical results show the appearance of cutoff frequency within THz range. Furthermore, the thicknesses of the constituents materials and the filling factor have a significant effect on the cutoff frequency. The proposed structure may be useful in many applications, particularly in THz frequency regions.

In the present work, we discuss the transmittance properties of one-dimensional (1D) superconductor nanocomposite photonic crystals (PCs) in THz frequency regions. Our modeling is essentially based on the two-fluid model, Maxwell–Garnett model and the characteristic matrix method. The numerical results investigate the appearance of the so-called cutoff frequency. We have obtained the significant effect of some parameters such as the volume fraction, the permittivity of the host material, the size of the nanoparticles and the permittivity of the superconductor material on the properties of the cutoff frequency. The present results may be useful in the optical communications and photonic applications to act as tunable antenna in THz, reflectors and high-pass filter.

We present a theoretical mechanism for electric field enhancement with SERS of InAs particles of subwavelength apertures under THz excitation. The distribution of electric field confirms that there is a strong enhancement in the InAs particles at THz frequencies. The InAs with a Drude-like behavior in THz range, which is similar to metals at optical frequencies, leads to different SERS when the parameters of these two particles change. The SERS enhancement factor can reach $10^{11}$ under the certain conditions.

Transient characteristics of SiGe-QW laser structures were studied. The excitation of stimulated THz emission is shown to be the result of carrier injection through contacts. The mechanism of intra-center population inversion caused by carrier injection is suggested.

Zn-doped p-type InP has been examined by far-infrared absorption magnetospectroscopy to fields much higher than previously. The impurity-related transitions observed confirm and greatly extend previous data and reveal new phenomena.

The development of an effective biological (bio) agent detection capability based upon terahertz (THz) frequency absorption spectra will require insight into how the constituent cellular components contribute to the overall THz signature. In this work, the specific contribution of ribonucleic acid (RNA) to THz spectra is analyzed in detail. Previously, it has only been possible to simulate partial fragments of the RNA (or DNA) structures due to the excessive computational demands. For the first time, the molecular structure of the entire transfer RNA (tRNA) molecule of E. coli was simulated and the associated THz signature was derived theoretically. The tRNA that binds amino acid tyrosine (tRNA_tyr) was studied. Here, the molecular structure was optimized using the potential energy minimization and molecular dynamical (MD) simulations. Solvation effects (water molecules) were also included explicitly in the MD simulations. To verify that realistic molecular signatures were simulated, a parallel experimental study of tRNAs of E. Coli was also conducted. Two very similar molecules, valine and tyrosine tRNA were investigated experimentally. Samples were prepared in the form of water solutions with the concentrations in the range 0.01-1 mg/ml. A strong correlation of the measured THz signatures associated with valine tRNA and tyrosine tRNA was observed. These findings are consistent with the structural similarity of the two tRNAs. The calculated THz signature of the tyrosine tRNA of E.coli reproduces many features of our measured spectra, and, therefore, provides valuable new insights into bio-agent detection.

Motivated by the possibility of identifying and detecting certain biochemical species using Fourier transform infrared spectroscopy (FTIR), we have investigated porcine, bovine, lispro, and human insulins. We have successfully observed and identified all the transition peaks for the four types of insulins in the frequency domains from mid-IR to THz. In the mid-IR region, ten transition peaks have been observed for all four insulins. Although these four insulins are made from amino acids which have either slightly different sequences or slight variations, the ten transition frequencies are virtually indistinguishable. However, for protamine sulfate some of the transition frequencies in the same mid-IR region are either red-shifted or blue-shifted relative to the corresponding ones for the four insulins. Furthermore, the strengths for several peaks of protamine sulfate are significantly reduced whereas the strength for only one transition peak is significantly enhanced, compared with the corresponding ones for the insulins. In the far-IR to THz transition region, the shapes and locations of the two transitions are quite different between the insulin species and protamine sulfate. In the THz region we have observed a linear dependence of the absorption coefficients on frequency for each of the four insulins and protamine sulfate.