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Electro-optic effects of PbZr 0.52 Ti 0.48 O 3 (PZT) and BaTiO 3 (BTO) ferroelectric thin films in the terahertz frequency range are studied with high-sensitive terahertz time-domain spectroscopy (THz-TDS). A linear response of PZT film with the linear electro-optic coefficient rc=6.73×10–11 m/V, and a quadratic response of BTO film with the second-order electro-optic coefficient Rc=1.42×10–17 m2/V2, are observed at the frequency of 1 terahertz under the applied static electric field less than 80 kV/cm. The calculation of rc and Rc based on the Landau-Devonshire free energy theory explains the different electro-optic effects of PZT and BTO well.

Employing a novel polarization-resolved detection, we precisely measured the polarization of terahertz emission from ionized gases in two-color field composed of circular polarized fundamental beam and linear polarized second harmonic. It shows that the polarization of the THz emission rotates with the phase delay and forms an attoclock. The polarization angular deviations of THz emissions in different laser intensities and different atoms reveal the distinct tunneling time delays. The polarization-resolved THz detection can be used to study tunneling ionization.

We demonstrate an enhancement of terahertz wave generation from plasma produced by circularly polarized two-color fields when the laser intensity is high enough to saturately ionize the gas. Comparing to the linear two-color fields, the conversion efficiency of co-rotating circularly polarized two color-fields is about 40% higher.

We report the joint measurements of terahertz wave and high harmonic spectra generated from aligned nitrogen molecules. Our results show the yields of terahertz wave and high harmonics are sensitive to the alignment angle. This observation clarifies that electronic structure play an important role in the generation of terahertz and high harmonics from molecules.

A fourfold-azimuthal dependence of terahertz radiation from polished mono-crystalline (100) silicon is observed firstly. This phenomenon is explained by the electric quadrupole-magnetic dipole, which resemble second harmonic generation from (100) silicon.