Terahertz radiation

Sinyukov, A. M. Hayden, L. M., Generation and detection of terahertz radiation with multilayered electro optic polymer films, Opt. Lett. 2002, 27, 55 57... 

Terahertz, or far infrared spectroscopy, covers the frequency range from 0.1 to lOTHz (300 to 3cm ) where torsional modes and lattice vibrations of molecules are detected. It is increasing in use in many application areas, including analysis of crystalline materials. Several dedicated conunercial instruments are available which use pulsed terahertz radiation which results in better signal to noise than those using blackbody sources for radiation (and associated with the terminology far infrared spectroscopy). Work using extended optics of FTIR instrumentation as weU as continuous-wave source THz has also been recently reported. ... 

E. Pickwell, B. E. Cole, A. J. Fitzgerald, M. Pepper, and V. P. Wallace, In vivo study of human skin using pulsed terahertz radiation, Physics in Medicine and Biology, vol. 49, pp. 1595-1607, 2004. 

Terahertz radiation poses either no or minimal health risk [2-6] to either a suspect being scanned by a THz system or the system s operator. 

Table 1. Comparison of terahertz radiation with other radiation for explosive detection...

Terahertz radiation lies between the microwave and the infrared regions of the electromagnetic spectrum. Terahertz typically ranges from 0.1 x 1012 to 10 x 1012Hz. One THz is equivalent to 300 microns in wavelength, 1 ps in time, 4.1 meV, and 47.6 K. THz radiation bridges the gap between photonic and electronic devices and offers a large expanse of unused, unexplored bandwidth. Historically, the lack of sources and detectors, as well as the perceived lack of need, had contributed to the dearth of activity in THz. For example, the first commercially available THz spectrometer did not arrive until 2000. 

F. C. De Lucia, Sensing with Terahertz Radiation, D. Mittleman (Ed.), Springer, New York... 

Fig. 5) This figure shows a sketch of the investigated detector concept. An irradiated high mobility two-dimensional electron gas device is subjected to a constant magnetic field Bo, where Bo is chosen to correspond to a fixed point (marked as a dot on the top inset) of the resistance oscillations for incident radiation at a frequency f. The detector device function is realized by superimposing on the static magnetic field, a small time varying component, which has been shown here in blue. Then, a high harmonic, tuned band Terahertz sensor is realized by detecting the device resistance at a odd-harmonic multiple of the field modulation frequency, as the detector is illuminated by Terahertz radiation.

As we explore the interaction of cold-atom systems with microwave and terahertz radiation, we find that they have some unique properties as detectors. A comparison with superconductor-based detectors such as SQUlDs is instractive. Because of the third law of thermodynamics, i.e., a system in a single quantum state has zero entropy, the response of a SQUID is almost free of thermal noise. But an additional properly of SQUIDs is that they exhibit the phenomenon of coherence, i.e., wave interference, which leads to entirely new effects, e.g. the AC and DC Josephson effects. Cold atom clouds share this behavior, as we will discuss below. 

Mapping with Pulsed Terahertz Radiation 59 Beamsplitter... 

THz sensing and imaging technology is relatively new with numerous applications from sectors as diverse as semiconductor, medical, manufacturing, space, and defense industries. In this chapter, a broad survey of terahertz-biodetection technology from its infancy to more recent biomedical use is presented. The focus is directed mainly on terahertz radiations that can be specifically applied to label-free ligand-analyte interaction. The uniqueness, limitations, and potential capabilities of THz biosensor are discussed. 

The Ti-sapphire oscillator is extremely useful as a stand-alone source of femtosecond pulses in the near-IR region of the spectrum. Some ultrafast experiments, especially of the pump-probe variety (see below), can be conducted with pulses obtained directly from the oscillator or after pulse selection at a lower repetition rate. Far-IR (terahertz) radiation is usually generated using a semiconductor (usually GaAs) substrate and focused Ti-sapphire oscillator pulses [7]. If somewhat higher-energy pulses are required for an experiment, the Ti-sapphire oscillator can be cavity dumped by an intracavity acousto-optical device known as a Bragg cell. 

CHROMATIC POLARIZATION CONVERSION OF TERAHERTZ RADIATION BY A DENSITY-MICROSTRUCTURED TWO-DIMENSIONAL ELECTRON SISTEM... 

Simple three level fits to the observed pump-probe signal for the two absorption lines in Fig. 2(b) (at a sample temperature of 4.2 K) indicated decay lifetimes of 350 and 360 ps for the C and D lines respectively. These figures are two orders of magnitude longer than the equivalent intersubband lifetime in quantum well systems. Furthermore, measurements up to 60 K showed that this lifetime was quite insensitive to temperature [20]. Both qualities offer great potential for a population inversion of the 2p level over the Is level and hence show the promise of a solid-state source of Terahertz radiation. 

INTERACTION OF OPTICALLY CREATED ELECTRON ENSEMBLE WITH TERAHERTZ RADIATION IN A SHORT SEMICONDUCTOR SUPERLATTICE... 

We present a theoretical study of interplay between terahertz radiation and an electron ensemble in a GaAs/AlGaAs superlattice after ultra-short optical excitation. The simulation was performed by means of a single-particle Monte Carlo method. We found that the time that needs for the ensemble to reach the state ensuring the terahertz amplification is determined by the electron intraminiband relaxation time. The stationary state may be attained faster for the special initial distribution of electrons in /<-space. 

Electromagnetic methods such as eddy current, capacitance, microwaves, and terahertz radiation are not traditional inspection methods for composites, but they can be used in some circumstances [36]. Microwaves (300 MHz—300 GHz, 1000—1 mm) and terahertz (300 GHz—3 THz, 1—0.1 mm) are applicable to fiberglass composite inspection and have been successful at the detection of damage and internal features [37—45]. However, electromagnetic radiation at these wavelengths does not penetrate conductive materials. For CFRP, which is mildly conductive, they are only useful for sensing very near the surface. Capacitance measurements can be used to measure dielectric property changes in composites such as moisture uptake or cure condition. 

Mittleman D. Sensing with terahertz radiation. New York Springer Verlag 2003. 

E. J. Madras et al., Application of terahertz radiation to the detection of corrosion under the... 

---