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Terahertz (THz) gas photonics uses gas as THz emitter and sensor for time-domain spectroscopy. Unique properties of the gas promise scalable, strong THz wave generation with broad spectral range covering the entire THz gas (0.3 THz to 35 THz). The systematic study of THz wave generation and detection in different gases shows that the generation efficiency is monotonically decreasing with the ionization potential of the gas molecules while the detection efficiency is linearly proportional to the third order nonlinear coefficient of the gas molecules. We also discuss the development of THz wave detection using laser-induced fluorescence and coherent control with THz gas photonics.

In this paper, using the collisions between ion and free electron, we analyze the conditions of electron collision equilibrium (ECE) for laser-induced plasma (LIP). Considering the collision excitation and de-excitation, we discuss the principle for temperature diagnostic for the three level ions for LIP. Taking the Ne v ion spectral lines in infrared as an example, for the intensity ratio of the (3-2) transition to the (2-1) transition, we compare the temperature in local thermal equilibrium and electron collision equilibrium when the intensity ratio varies from 0 to 1.8, and we obtain some important conclusions. This discussion is useful in the LIP diagnostic under the conditions of electron collision equilibrium.

For simple plasma diagnostics for laser-induced plasma (LIP) under the condition of optically thin, taking the Cu I spectral lines produced by the laser-induced copper plasma, we investigate a simple method for temperature and electron density diagnostics, and we obtain the plasma temperature which has 10⁴ K order of magnitude and the averaged electron density is $3.8\times 10^{18}{\mathrm{\text{cm}}}^{-3}$, which are in agreement with that obtained by other methods. This investigation will be significant for spectral diagnostics for LIP.

This paper presents the experimental investigation of the changes in the geometrical shape of femtosecond laser-induced plasma in air under different laser power, and its effects on supercontinuum white light generation and conical emission. When a femtosecond laser is focused into a tiny spot in air, optical breakdown of air molecules occurs and this leads to the generation of plasma filament whose geometrical size and shape depend on laser power. This process is then followed by two light-emitting processes, namely supercontinuum white light generation and conical emission, both of which scatter light that reveals the characteristics of the plasma filament. Our experiment shows that the laser-induced plasma becomes thinner and longer at high average laser power but appears thick and round at lower laser power. At higher laser power, conical emission which scatters laser light in the forward direction dominates the scattering process while at lower laser power, it is the scattering of supercontinuum white light in all directions that plays a bigger role. The intricate rainbow-like pattern formed on a white screen located far away in the forward direction reveals sophisticated nonlinear optical processes that take place in conical emission which slowly diminishes as the laser power is gradually reduced.

Terahertz (THz) gas photonics uses gas as THz emitter and sensor for time-domain spectroscopy. Unique properties of the gas promise scalable, strong THz wave generation with broad spectral range covering the entire THz gas (0.3 THz to 35 THz). The systematic study of THz wave generation and detection in different gases shows that the generation efficiency is monotonically decreasing with the ionization potential of the gas molecules while the detection efficiency is linearly proportional to the third order nonlinear coefficient of the gas molecules. We also discuss the development of THz wave detection using laser-induced fluorescence and coherent control with THz gas photonics.