The influence of DC electric field-enhanced ionization on the self-focusing of femtosecond lasers

The critical power for self-focusing P cr is a reference value for determining the beginning of the nonlinear self-focusing effect. Its detection is crucial for the femtosecond filaments application. In this work, an experimental setup with a DC electric field applied at the filament position is proposed for measuring the P cr of femtosecond lasers. Measurements results show that the P cr is significantly reduced with a DC electric field applied, it decreases as the applied voltage increases. This is attributed to the enhanced ionization through the interaction between the applied electric field and the laser-ionized plasma. As both the applied voltage and incident laser power increase, filamentation occurs more easily, and focal shift becomes more pronounced. Additionally, the spectral signal of the filament was measured to clarify the mechanism of the interaction between the applied electric field and the laser-ionized plasma. This work is beneficial for understanding the interaction between DC electric field and femtosecond laser plasma.