Terahertz radiation

Terahertz (THz) imaging is an emerging technique for non-invasive analysis. Since THz waves can penetrate opaque materials, various imaging systems that use THz waves have been developed to detect, for instance, concealed weapons, illegal drugs, and defects in polymer products. The absorption of THz waves by water is extremely strong, and hence, THz waves can be used to monitor the water content in various objects. THz imaging can be performed either by transmission or by reflection of THz waves. In particular, time domain reflection imaging uses THz pulses that propagate in specimens, and in this technique, pulses reflected from the surface and from the internal boundaries of the specimen are detected. In general, the internal structure is observed in crosssectional images obtained using micro-specimens taken from the work that is being analysed. On the other hand, in THz time-domain imaging, a map of the layer of interest can be easily obtained without collecting any samples. When realtime imaging is required, for example, in the investigation of the effect of a solvent or during the monitoring of water content, a THz camera can be used. The first application of THz time-domain imaging in the analysis of a historical tempera masterpiece was performed on the panel painting Polittico di Badia by Giotto, of the permanent collection of the Uffizi Gallery. The results of that analysis revealed that the work is composed of two layers of gypsum, with a canvas between these layers. In the paint layer, gold foils covered by paint were clearly observed, and the consumption or ageing of gold could be estimated by noting the amount of reflection. These results prove that THz imaging can yield useful information for conservation and restoration purposes.

This paper investigates the radiation characteristics of a Vee dipole antenna operating in the near infra-red and optical frequency regimes. Antenna properties, such as far-field radiation patterns, coupling coefficient, measured by the scattering parameter S11, and directivity are provided. The resonance and directivity behavior of the optical Vee dipole, which is based on Multi-Wall Carbon Nano Tube (MWCNT), are investigated by varying the dipole length in order to exploit the effective operating frequency in the near infra-red range (~120 to 400 THz) and the visible light range (~400- 750 THz). Moreover, a parametric study aimed at optimizing the antenna directivity is performed by varying the angle between the two arms of the dipole using CST Microwave Studio simulation software which is based on the Finite Integration Technique (FIT). The proposed antenna achieved a directivity 3.767 dB higher than the traditional dipole in the visible regime while maintaining the same directivity in the near infrared regime.

Tunable resonant absorption by plasmons in the two-dimensional electron gas (2DEG) of grating-gated HEMTs is known for a variety of semiconductor systems, giving promise of chip-scale frequency-agile THz imaging spectrometers. We present our calculations of transmission spectra and resonant photoresponse due to plasmons in InPand graphene-based HEMTs at millimeter and THz wavelengths. Our experimental approach to measurement of electrical response is also described. Potential applications include man-portable or space-based spectral-sensing.

The observation of THz regime transmission resonances in an InGaAs/InP high electron mobility transistor (HEMT) can be attributed to excitation of plasmons in its two-dimensional electron gas (2DEG). Properties of grating-based, gate-voltage tunable resonances are shown to be adequately modeled using commercial finite element method (FEM) software when the HEMT layer structure, gate geometry and sheet charge concentration are taken into account. The FEM results are shown to produce results consistent with standard analytical theories in the 10-100 cm-1 wavenumber range. An original analytic formula presented here describes how the plasmonic resonance may change in the presence of a virtual gate, or region of relatively high free charge carriers that lies in the HEMT between the physical grating gate and the 2DEG. The virtual gate and corresponding analytic formulation are able to account for the red-shifting experimentally observed in plasmonic resonances. The calculation methods demonstrated here have the potential to greatly aid in the design of future detection devices that require specifically tuned plasmonic modes in the 2DEG of a HEMT, as well as giving new insights to aid in the development of more complete analytic theories.

Gate-voltage tunable plasmon resonances in the two dimensional electron gas of high electron mobility transistors (HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems are reported. Gates were in the form of a grating to couple normally incident THz radiation into 2D plasmons. Narrow-band resonant absorption of THz radiation was observed in transmission for both systems in the frequency range 10 - 100 cm-1. The fundamental and harmonic resonances shift toward lower frequencies with negative gate bias. Calculated spectra based on the theory developed for MOSFETs by Schaich, Zheng, and McDonald (1990) agree well with the GaN results, but significant differences for the InGaAs/InP device suggest that modification of the theory may be required for HEMTs in some circumstances.

A highly sensitive absorption-based sensor based on folded split-ring metamaterial graphene resonators (FSRMGRs) is designed, and its biomedical application in terahertz (THz) spectrum is investigated. The sensor has a nearly perfect absorption, with a spectral absorption coefficient of 99.75% at 4 THz and an average Q-factor of 13.76. The resonance peak frequency is sensitive to the refractive index (RI) of the test medium (analyte), and a fairly high sensitivity of 851 GHz/RIU has been obtained. The specifications of the sensor can be tuned by an external DC-bias voltage applied to the graphene layer. According to the obtained results, the developed absorber appears to be a good candidate bio-sensing applications.

Three-dimensional photonic bandgap (PBG) structures using alumina as the high-permittivity material were designed and then fabricated by using the fused deposition of multimaterials (FDMM) process. The finite-element and real-time electromagnetic modeling ofthe fabricated structure resulted in a 3-D photonic bandgap in the16–24 GHz range, which agreed with experimental measurements. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 30: 305–307, 2001.

While sub-THz/THz solid state multipliers afford outstanding resolution, rapid frequency sweeping has been problematic. In order to sweep via FM modulation the microwave synthesizer phase-lock is turned off and hence the frequency calibration is lost. One way around this, as advanced in the FASSST scan technique, is to sweep the frequency rapidly and use post-scan frequency calibration via cavity mode

Metasurfaces are engineered thin surfaces comprising two-dimensional (2D) arrays of sub-wavelength-spaced and sub-wavelength-sized resonators. Metasurfaces can locally manipulate the amplitude, phase, and polarization of light with high spatial resolution. In this paper, we report numerical and experimental results of a vortex-beam-generating metasurface fabricated specifically for infrared (IR) and terahertz (THz) wavelengths. The designed metasurface consists of a 2D array of dielectric cross-shaped resonators with spatially varying length, thereby providing the desired spatially varying phase shift to the incident light. The metasurface was found to be insensitive to the polarization of the incident light. The dimensions of the cross-resonators were calculated using rigorous finite-difference time-domain analysis. The spectral scalability via physical scaling of the meta-resonators is demonstrated using two vortex-generating optical elements operating at 8.8 μm (IR) and 0.78 THz....

In this work we present the results on combined experimental and computational study of sub-THz spectra of liquid water. The important new result is the detection of hydrogen bonds in liquid water. The experimental study was performed by employing Fourier Transform terahertz (THz) spectroscopy with spectral resolution of 0.25 cm-1. Resonance features in transmission spectra of water layers between thin film substrates are demonstrated in the sub-THz range. The theoretical approach for computer simulation of THz absorption spectra from liquid water is also discussed. The molecular dynamical (MD) simulations of water were performed using Amber 8 and the TIP3P, SPCE (Extended Single Point Charge) and TIP4P water models. Several examples of modeling results are presented. The experimental spectra are compared with the theoretical predictions. The SPCE model better correlates with experimental spectra compare to two other models.