| Abstract [eng] |
Karolis Adomavičius Research of spectral and spatial properties of terahertz radiation generated in the air using femtosecond laser pulses During the research, the THz frequency radiation was generated focusing bichromatic femtosecond laser pulses in the air and then detected. The measured interference signals were processed and the terahertz radiation spectrum was calculated, with a range of 0,1 to 60 THz. Spatial measurements revealed the characteristic cone structure of the terahertz radiation. The measured angle of inclination is 5 degrees. Measurements with probing filament were also carried out. Performing spatial measurements with a probe impulse revealed that different processes occur in different plasma sites during the generation of THz: in the front of the filament probing plasma diffracts pump pulse, while at the end of the filament THz radiation is screened by probing plasma. Moreover, the spatial THz radiation profiles measured and depending on preformed plasma positionspatial asymetry between x and y axis was registered. Changing the delay between probing plasma and pump pulse terahertz radiation signal change was observed. Spectral attenuation of THz radiation dependence on delay of preplasma was registered and cut-off points were calculated. This approach led to a direct measurement of plasma frequency and particle concentration in the plasma. |
