[Third harmonic enhancement and spectral analysis within plasma channel based on pump-probe effect].

The experiment realized third harmonic enhancement by using non-collinear dual plasma channels based on pump-probe effect. The coupling action between pump and probe beams can improve saturation effect within filament and overcome the limitation of harmonic enhancement due to intensity clamping. By non-collinearly focusing two ultrafast pulses in air which have respectively a single pulse energy of 4.4 and 10.2 mJ, both with duration of 60 fs and central wavelength of 810 nm, dual filaments as well as weak third harmonics can be generated. When strong beam is ahead of weak one, the former induces a plasma channel in advance which can modulate later weak beam and make obvious harmonic increase produced by probe beam. It was found that oscillation variation of spectral bandwidth in the region of harmonic energy increased significantly. When two beams intersect in a small angle of 27.3 mrad about 15 mm before geometric focus and the probe beam lags behind about 55 fs, the energy increase rate nearly reaches 70, and the corresponding spectral bandwidth is approximately 5 nm.