The generation of terahertz(THz) emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model.While the gas is ionized by the two-color femtosecond laser-pulses composed of the fundamental and its second harmonic,a non-vanishing directional photoelectron current emerges,radiating a THz electromagnetic pulse.The gas ionization processes at three different laser-pulse energies are simulated,and the corresponding THz waveforms and spectra are plotted.The results demonstrate that,by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value,the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.
The absorption coefficient of magnesium-doped near-stoichiometric lithium niobate crystal is measured by terahertz time-domain spectroscopy in a frequency range of 0.2 THz–0.9 THz at room temperature.The absorption coefficient is modulated by external optical pump fields.Experimental results show that the absorption coefficient of near-SLN:Mg crystal is approximately in a range of 22 cm-1–35 cm-1in a frequency range of 0.2 THz–0.9 THz and tunable up to nearly15%.Further theoretical analysis reveals that the variation of absorption coefficient is related to the number of light-induced carriers,domain reversal process,and OH-absorption in this crystal.
