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.

We experimentally demonstrate third-harmonic generation (THG) in gases ionized by a femtosecond laser pulse superimposed on its second-harmonic (SH). The mechanism of THG has been investigated, and it demonstrates that a third-order nonlinear process dominates at low pump intensity. Asymmetric third-harmonic (TH) spectra are observed at different time delays in two color fields, which are attributed to the process of the four-wave mixing (FWM) of the broad spectrum of pump pulses. A joint measurement on the terahertz (THz) and the TH is performed. It reveals that the optimized phase for the THG jumps from 0 to 0.5π as the pump intensity increases, which is different from the THz being a constant, and indicates that the THG arises from the nonlinearity of the third-order bound electrons to the tunnel-ionization current.

We demonstrate an all-optical technique for in situ monitoring of strong-field ionization (SFI) dynamics. The method relies on coherent scattering of a probe pulse from a transient plasma grating (TPG) formed by two interfering femtosecond pulses in a gas target. The diffracted signal provides a real-time, relative measure of the ionization yield. We demonstrate the method’s sensitivity by resolving the attosecond-scale coherent control of the ionization rate. Unlike conventional diagnostics that directly measure charged particles, our ensemble-based optical method is non-invasive and particularly suited for non-vacuum environments. To demonstrate its utility, we apply the technique to investigate the relationship between ionization and terahertz (THz) generation in a two-color ( ω + 2 ω ) field. We measure a phase offset of (0.18±0.05) π between the maxima of the ionization yield and the THz emission. This result provides a quantitative benchmark for theoretical models and establishes the TPG technique as a practical tool for probing SFI dynamics in ensemble systems.