## 2 THz Imaging: First Measurements at the ENEA Center of Frascati  
The first THz imaging measurements, which were carried out at the ENEA center of Frascati in 2005, made use of a Compact Free Electron Laser that operated in the spectral region between 0.1 and 0.15 THz. This source is able to provide coherent radiation with an output power of about 1.5 kW in 4 us pulses at a maximum pulse repetition rate of 10 Hz. The first measurements were carried out on a living plant, in order to see whether it was possible to detect the hydration level of the plant's leaves under different stress conditions, and also on the electronic chip of a smart card [26]. The experiments were successful, showing that it is possible to follow the hydration processes in living plants and to detect the metallic part of the smartcard chip under its plastic surface. More tests were carried out in order to optimize the lateral resolution of the images, thus overcoming the diffraction limit. An upgrade of the system was a 3D mechanical scanner for the sample [27]: an XY translational stage driven by piezomotors with a 50-mm travel range on both axes was coupled to a stepper-motor driven stage for the \(z\)-axis, in order to provide complete control of the sample position under the irradiation head. The imaging system is shown in Figure 1: radiation coming from the FEL is delivered to the control room by means of a copper light-pipe. Radiation is coupled into a focusing cone and injected into a waveguide system consisting of two WR6 10 dB directional couplers. A truncated WR6 waveguide is attached to the second directional coupler for launching the radiation toward the sample under investigation. Two directional couplers, providing a reference signal of the FEL radiation incident on the sample and the signal reflected by the sample, respectively, are necessary in order to normalize the output for the FEL signal fluctuations. Both signals are detected by Schottky diodes at room temperature. Figure 1: Layout of the terahertz (THz) imaging system: (**1**) Light-pipe. (**2**) Focusing cone. (**3**) Schottky diodes. (**4**) 10 dB directional coupler. (**5**) 20 dB directional coupler. (**6**) WR6 imaging probe. (**7**) Sample plane. (**8**) XY axes translational stage. (**9**) \(Z\) axis translational stage. Since the radiation is linearly polarized, when considering a TE\({}_{01}\) mode, the boundary conditions must be imposed on only one of the dimensions of the guide, while the other one can be reduced in order to obtain a better lateral resolution. Utilizing a tapered waveguide, with the shorter dimension tapered up to 0.2 mm, it was possible to overcome the diffraction limit, obtaining a 200-\(\upmu\)m lateral resolution in one dimension. Resolution in the other dimension remains limited by the cutoff frequency, corresponding to half of the radiation wavelength in a rectangular waveguide. In order to obtain a higher resolution in both directions, it is possible to perform a second imaging scan by rotating the sample by 90\({}^{\circ}\) and utilizing software techniques to combine the results of the two scans. Resolution measurements were carried out with the knife-edge method and by means of Heidenhain reference targets. The THz image of a Heidenhain reference target, which was taken by using the probe head in Figure 2(b2), is shown in Figure 2(b1). In Figure 2(c1), the same target was scanned using the tapered head shown in Figure 2(c2). Enhancement of the resolution is evident. A knife-edge measurement was carried out using the tapered probe head shown in c2. The result, which is reported in Figure 2a, confirms the resolution enhancement. This upgraded system was used to determine the reflectivity of soils as a function of their water content. The system was also used, in 2007, to determine the active response of electronic devices to THz radiation [28]: a novel GaN High Electron Mobility Transistor (HEMT) was excited with THz radiation and a spatial map of its response was recorded.