Terahertz radiation detection

The terahertz radiation that has passed through the sample is detected with an optical system having essentially the same structure as the optical system for generating the terahertz radiation and focusing it onto the sample, as depicted in Figure 19.2. The terahertz radiation that has passed through the sample is focused onto the terahertz-radiation detection... 

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. ... 

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

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.

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. 

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

THz-TDS was initiated by the pioneering research on the emission of terahertz radiation by Auston et al. [5] and subsequent studies on the coherent detection of the terahertz radiation by van Exeter and Grischkowski [6], Arjavalingram et al. [7], and Nuss et al. [8], Since then, THz-TDS, enabled by the advent of femtosecond laser technology, has become a new spectrometric method covering the region from the millimeter wavelength to the far-infrared. 

Generation and Detection of the Terahertz Radiation Using a Photoconductive Antenna Element... 

A PCA is effective also as a detector for the terahertz radiation. As shown in Figure 19.2, the terahertz radiation that has passed through the sample is focused onto the PCA in the detection system. The femtosecond laser pulse is synchronously superimposed onto the terahertz radiation at the same PCA. Since the density of the electric current j t) occurring between the electrodes is proportional to the number of carriers formed by photoexcitation A/exc(0. the current formed is also proportional to the complex electric field thz(0 of the terahertz radiation. Therefore, by measuring the electric current between the electrodes as illustrated in Figure 19.2, it is possible to determine the electric field of the terahertz radiation and its phase (t). 

However, it is not possible to determine directly Ej (t) and 4> t) of the terahertz radiation, because the terahertz radiation is emitted as an extremely short pulse. For this reason, as illustrated schematically in Figure 19.4, time delays are instigated between the laser pulses for generating the terahertz radiation and those for detecting the terahertz radiation. The shape of the terahertz radiation on the time axis is determined by scanning the delay time of the laser pulses. 

Figure 19.4 Detection scheme for the terahertz radiation. For details, see text.

If the mechanism for the detection of the terahertz radiation is described in more detail, it is necessary to consider the relationship between the density of current induced in the transmission lines of the PCA in the detection system and the electric field of the terahertz radiation tuzCO as well as the number of photoexcited carriers Agxc(0- The relationship is expressed as a convolution of the latter two quantities as [9]... 

As described in Section 19.3.3.2, the strength of the electric field of the terahertz radiation is detected as a time-series signal thz(0- These signals are actually measured as / (t) signals that are proportional to LLI j(0 thz(0> where LU /0 indicates the Dirac delta comb (see Section D.3.3). thz(0 signals are converted to equivalent complex electric field (< ) values on the frequency axis as... 

Numerical simulations of this model were carried out to test expectations. The results are shown in Figs. 8 and 9. The figure 8 shows the amplitude of the detected signal at the third harmonic of the modulation frequency when the operating point corresponds to the second node, i.e., n = 2, and the Bo is selected for the detection of radiation in the vicinity of 400 GHz, which lies near the lower edge of the Terahertz band, where the device sensitivity to such radiation has been confirmed by our experiments, see Fig. 3, for example. As confirmed by the simulations, the 3 harmonic sensing concept yields indeed a narrow band detector, with sensitivity between roughly 200 and 800 GHz, as... 

Tani, M., Herrmann, M. and Sakai, K. (2002) Generation and detection of terahertz pulsed radiation with photoconductive antennas and its application to imaging. Meas. Sci. Technol, 13, 1739-1745. 

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