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 Schrödinger equation, this enhancement is well explained based on the analysis of electron tunneling ionization and subsequent dynamics.

New instrumentation and calibration procedures for terahertz time domain spectroscopic ellipsometry (THz-TDSE) are demonstrated. The THz-TDSE is capable of simultaneous measurement of two orthogonal components of reflected THz electric fields with no need to rotate a polarizer. In the calibration, the TDSE response function was obtained via the simultaneous polarization measurements reflected by a flat metal mirror, adapted in conventional TDRS, and used here for THz-TDSE without problems of position accuracy. The calibration could be used to determine accurate ellipsometric parameters with a high tolerance of imperfect polarizer extinction ratios and of non-ideality in the THz reflection components. Results are presented for an opaque Si wafer with heavy doping. The simultaneous measurements rejected significant common-mode noise from the laser, and it extended reliable THz spectra into the frequency range with a low dynamic range of a photoconductive-antenna THz source, which is a fundamental breakthrough for reflection-based measurements and overcomes the hurdle of phase uncertainty.

We demonstrated the generation and characterization of 9.7 fs, 180 μJ pulses centered at 385 nm via the frequency doubling of few-cycle near-infrared pulses. Both moderate conversion efficiency (9.5%) and broad phase matching bandwidth (20 nm) were achieved by shaping the spectra of the fundamental pulses. The strong intensity dependence of second-order harmonic generation and well controlled material dispersion ensured the inexistence of satellite pulses, which was confirmed by the self-diffraction frequency resolved optical gating measurement.

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.

We investigate the terahertz radiation from a filament in two-color laser fields with various polarizations. It has been experimentally demonstrated that the efficiency of terahertz emission in co-rotating two-color circularly polarized laser fields is eight times as strong as that in linearly polarized two-color laser fields with parallel polarization. This enhancement is explained based on the polarization-dependent clamping intensity inside a filament and the dynamics of electrons ionizing from a single atom or molecule. Our finding provides a simple method to improve the THz intensity from an air filament in two-color fields.

Using a controlled optical bias and balanced geometry, we propose a new scheme for broadband terahertz detection by laser-gas interaction without high-voltage manipulation. Compared to the conventional optical bias scheme, the common noise is reduced and the dynamic range as well as the signal-to-noise ratio are doubled. It provides a simple alternative for coherent broadband terahertz detection. The influence of optical bias on terahertz waveform is also investigated, and the evolution of the terahertz-induced second harmonic with probe delay is further revealed. This new detection scheme for broadband terahertz will boost the application of terahertz time-domain spectroscopy for its miniaturization and integrability.