Terahertz wave generation from air plasma induced by ultrashort laser pulses has been widely studied in the past decade. We report the study of terahertz wave generation from the laser induced plasma where there is a preformed air plasma. We found that the power of the terahertz wave generated by the main pump pulse decreases in the presence of the preformed plasma. The amount of the power drop increases with the power of pulse that generates preformed plasma. The result confirms the key role of tunneling ionization in the terahertz generation mechanism.
Indium Phosphide (InP) has attracted great physical interest because of its unique characteristics and is indispensable to both optical and electronic devices. In this paper, we study the optical properties of undoped (100) InP wafer in the ultra-broad terahertz frequency range (0.5-18 THz) by using air-biased-coherent-detection (ABCD) system. It is observed that InP wafer is opaque at the frequencies spanning from 6.7 THz to 12.1 THz. In the frequency regions of 0.8-6.7 THz and 12.1-18 THz it has relatively low absorption coefficient. Meanwhile, the refractive index increases monotonously in the 0.8-6.7 THz region and 12.1-18 THz region. These findings will contribute to the design of InP based component for nonlinear terahertz devices.
Terahertz (THz) emission from laser-induced air plasma is a well known and widely used phenomenon. We report that when two laser beams from the laser creating two plasma filaments interact with each other, THz absorption is observed. We believe that a change in the refractive index of the plasma causes the THz-wave absorption. The following experimental results reveal that the THz absorption becomes more pronounced with increasing pump power and that the gas species surrounding the femtosecond laser filament can also influence the THz absorption rate.