A series of ferroelectric poly(vinylidene fluoride-trifluoroethylenechlorotrifluoroethylene), P(VDF-TrFE-CTFE), have been synthesized by a two-step approach. The first step is copolymerization of VDF and CTFE via solution or suspension methods to produce P(VDF-CTFE) copolymers with different molecular weights. The second step is partial de-chlorination to convert copolymers into P(VDF-TrFE-CTFE) terpolymers with precisely controlled compositions. The effect of molecular weight, molecular weight distribution and uniaxially stretching on the dielectric properties has been investigated over a broad range of temperature and frequency. The X-ray diffraction patterns and DSC curves demonstrate the coexistence of the multiple phases in the terpolymers. The dielectric spectra depict the local relaxation processes and relaxor ferroelectric behavior on the basis of the dielectric loss tangent as a function of temperature.

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Three kinds of nanometer-scale metal films (Cr, Ni and Ti) with different thicknesses are fabricated. The complex refractive indices of the three metal films are quantitatively measured by using THz differential time-domain spectroscopy (THz-DTDS). The orders of the complex refractive indices of the thin metal films are equal to those of the reported values. Our results validated that THz-DTDS can be used to study the features of the ultra-thin metal films.

In this paper, multilayer films Si/[TiO2/Al2O3]2TiO2 and Si/[TiO2/MgO]2/TiO2 with thickness values from microns to tens of microns are fabricated by spin-coating method. The transmission spectra of these films are obtained by terahertz time-domain transmission spectrum system (THz-TDS). The phase shifts of reflection and phase penetration depths of Si/[TiO2/Al2O3]2TiO2 and Si/[TiO2/MgO]2/TiO2 are simulated by the transfer matrix method. On this basis, two kinds of symmetrical THz microcavities each with a structure of DBR/LT-GaAs/DBR are designed and the radiation spectra are also simulated. The results show that the intensities of two microcavities are enhanced by 19 and 14 times at resonance wavelength, respectively. There are two resonance peaks in the emission spectrum of the structure Si/[TiO2/Al2O3]2TiO2/LT-GaAs (12 μm)/[TiO2/Al2O3]2TiO2, which are located at 208 μm and 248 μm, respectively. The reason is discussed based on the effective cavity length. The feasibility to regulate the emission properties of the THz source by introducing dielectric microcavities is discussed.

The complex refractive indices of the Cr film are obtained by terahertz time-domain spectroscopy. The penetration depth the Cr film is calculated based on the complex refractive indices, and then the effective cavity length and the emitted spectrum of the structure Cr/GaAs/Cr are simulated. The resonant frequencies are located at 0.32, 0.65, 0.98, 1.31 and 1.65 THz, respectively. The peak intensity of the cavity photo-conductive resource at 0.32 THz is 25 times higher than that of non-cavity one and the full width at half maximum is greatly narrowed. The relation between the emitting dipoles and the standing wave field in the cavity is also discussed. The results show that the emission intensity is enhanced when the emitting dipoles are located at the nodes of the standing wave field, but greatly suppressed at antinodes.