0.35%THz pulse conversion efficiency achieved by Ti:sapphire femtosecond laser filamentation in argon at 1 kHz repetition rate

In this study,an optical setup for generating terahertz(THz)pulses through a two-color femtosecond laser filament was carefully designed to achieve a precise overlap of two-color laser pulses in space and time.β-barium borate(BBO),α-BBO,and a dual-wavelength plate were used to compensate the phase delay of the two-color lasers.Tilting ofα-BBO could further realize the precise spatial overlap of the two beams by counteracting the walk-off effect.The maximum out-put THz pulse energy reached 21μJ in argon gas when using a commercial Ti:sapphire laser with a pulse energy of 6 mJ at a 1 kHz repetition rate.The corresponding conversion efficiency exceeded 0.35%.

Rapid diagnosis of femtosecond laser filament by single laser shot-induced acoustic pulses[Invited]

Due to the promising applications of femtosecond laser filamentation in remote sensing,great demands exist for diagnosing the spatiotemporal dynamics of filamentation.However,until now,the rapid and accurate diagnosis of a femtosecond laser filament remains a severe challenge.Here,a novel filament diagnosing method is proposed,which can measure the longitudinal spatial distribution of the filament by a single laser shot-induced acoustic pulse.The dependences of the point-like plasma acoustic emission on the detection distance and angle are obtained experimentally.The results indicate that the temporal profile of the acoustic wave is independent of the detection distance and detection angle.Using the measured relation among the acoustic emission and the detection distance and angle,a single measurement of the acoustic emission generated by a single laser pulse can diagnose the spatial distribution of the laser filament through the Wiener filter deconvolution(WFD)algorithm.The results obtained by this method are in good agreement with those of traditional point-by-point acoustic diagnosis methods.These findings provide a new solution and idea for the rapid diagnosis of filament,thereby laying a firm foundation for femtosecond laser filament-based promising applications.

Anti-Correlated Plasma and THz Pulse Generation during Two-Color Laser Filamentation in Air

The THz generation efficiency and the plasma density generated by a filament in air have been found anti-correlated when pumped by 800 nm+1600 nm two-color laser field.The plasma density near zero delay of two laser pulses has a minimum value,which is opposite to the trend of THz generation eficiency and contradicts common sense.The lower plasma density cannot be explained by the static tunneling model according to the conventional photocurrent model,but it might be attributed to the electron trapping by the excited states of nitrogen molecule.The present work also clarifies the dominant role of the drifting velocity accelerated by the two-color laser field during the THz pulse generation process.The results promote our understanding on the optimization of the THz generation efficiency by the two-color laser filamentation.