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In this work, we characterize the transmission properties of the focusing-laser-induced filament by using terahertz (THz) time domain spectroscopy. Significant resonance absorption of the THz wave is identified, which is attributed to the dynamic screening of the ionized electrons. The polarization and intensity of the filament-generated pulse have been varied to study their influences on the resonance absorption. Our results suggest that the electron density of the filament can be determined by fitting the phase shifts and absorption properties around the absorption. This method provides not only an alternative to the plasma diagnostics with plasma frequency at the THz range, but also a supplement to the existing generation theory of THz waves from plasma.

Terahertz radiation from tunneling ionization of gaseous atoms and molecules in the two-color laser fields with various polarizations has been investigated. We experimentally demonstrate that the efficiency of terahertz emission in the circularly polarized laser fields with the same helicity is 5 times higher than that with linearly polarized two-color femtosecond pulses in high laser intensity. By solving time-dependent Schrodinger equation, this enhancement is well explained based on the analysis of electron tunneling ionization and subsequent dynamics.

Electro-optic effects of PbZr0.52Ti0.48O3 (PZT) and BaTiO3 (BTO) ferroelectric thin films in the terahertz frequency range are studied with high-sensitive terahertz time-domain spectroscopy. A linear response of PZT film with the linear electro-optic coefficient rc = 6.73 × 10−l1 m/V and a quadratic response of BTO film with the second-order electro-optic coefficient Rc = 1.42 × 10−l7 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 the multi-state multiconfiguration second-order perturbation theory (MS-CASPT2) and complete active space self-consistent field (CASSCF) methods, the geometries, relative energies (Tv′) to the ground state (X3Σg−), adiabatic excited energies, and photodissociation mechanisms and corresponding kinetic energy releases for the lower-lying 14 electronic states of the CO22+ ion are studied. The Tv′ values are calculated at the experimental geometry of the ground state CO2 molecule using MS-CASPT2 method and highly close to the latest threshold photoelectrons coincidence and time-of-flight photoelectron photoelectron coincidence spectrum observations. The O-loss dissociation potential energy curves (PECs) for these 14 states are drawn using MS-CASPT2 partial optimization method at C∞v symmetry with one C–O bond length ranging from 1.05 to 8.0 A. Those 14 states are confirmed to be correlated to the lowest four dissociation limits [CO+(X2Σ+) + O+(4Su), CO+(A2Π) + O+(4Su), CO+(X2Σ+) + O+(2Du), and CO+(X2Σ+) + O+(2Pu)] by analyzing Coulomb interaction energies, charges, spin densities, and bond lengths for the geometries at the C–O bond length of 8.0 A. On the basis of these 14 MS-CASPT2 PECs, several state/state pairs are selected to optimize the minimum energy crossing points (MECPs) at the CASSCF level. And then the CASSCF spin-orbit couplings and CASPT2 state/state energies are calculated at these located MECPs. Based on all of the computational results, the photodissociation mechanisms of CO22+ are proposed. The relationships between the present theoretical studies and the previous experiments are discussed.Employing the multi-state multiconfiguration second-order perturbation theory (MS-CASPT2) and complete active space self-consistent field (CASSCF) methods, the geometries, relative energies (Tv′) to the ground state (X3Σg−), adiabatic excited energies, and photodissociation mechanisms and corresponding kinetic energy releases for the lower-lying 14 electronic states of the CO22+ ion are studied. The Tv′ values are calculated at the experimental geometry of the ground state CO2 molecule using MS-CASPT2 method and highly close to the latest threshold photoelectrons coincidence and time-of-flight photoelectron photoelectron coincidence spectrum observations. The O-loss dissociation potential energy curves (PECs) for these 14 states are drawn using MS-CASPT2 partial optimization method at C∞v symmetry with one C–O bond length ranging from 1.05 to 8.0 A. Those 14 states are confirmed to be correlated to the lowest four dissociation limits [CO+(X2Σ+) + O+(4Su), CO+(A2Π) + O+(4Su), CO+(X2Σ+) + O+(2Du), and CO+(X...

Employing an orientation-modulated bias field, a polarization-sensitive scheme for terahertz air-biased-coherent-detection (THz-ABCD) is presented to directly measure the amplitude and polarization angle of THz field in the time domain. It can provide all characteristics of arbitrarily polarized THz wave with one single-scan measurement. Measuring convenience, broad bandwidth, and high angular resolution have been achieved. Polarization-sensitive THz time-domain spectroscopy can surely be developed based on this technology. Many other applications in the THz spectral region are also believed.