Filamentation of femtosecond laser pulse in transparent medium has attracted numerous attentions in the past decades owing to its potential applications in high-order harmonic generation, strong-field ionization, THz generation and remote sensing, etc. During the propagation, the high peak intensity of the femtosecond laser beam causes Kerr self-focusing effect in the transparent medium, meanwhile the medium will be ionized when the beam intensity increases, which leads to the defocusing effect because of the plasma channel. The dynamic balance between Kerr self-focusing and plasma defocusing will result in a laser filament that last several times longer than the diffraction length, leaving a row of plasma channel behind. In order to obtain longer filaments, different methods have been used to extend the laser filament lengths. Generally, the filament length is directly related to the energy of input pulse, filaments lasting over 2 m have been readily achieved using pulses with more than 5-mJ energy. However, the possibility of extending filament length with relatively lower pulse energy is still quite limited.

A novel scheme that lengthens the plasma channel during filamentation is proposed. Instead of one focal length introduced by a conventional convex lens, the multi-focal-length beam modulated by a spatial light modulator (SLM) produces a filament in an extended range with limited but strictly manipulated laser energy. The results show that the scheme is capable of doubling the filament length compared to a single-lens scheme with a 2-mJ input pulse. The filament location and length can be simply tuned by altering the spatial amplitude and phase or employing higher energies. Furthermore, the extended filament length leads to the generation of a broadened continuum ranging from visible (VIS) to infrared (IR) domain. This versatile scheme offers an efficient tool for the development of a variety of applications involving ultrafast nonlinear optics.

This work (Title: Extending plasma channel of filamentation with a multi-focal-length beam) was supported by the NNSF of China under grants 11574101, 11204095, 11234004and 61275126 and published on Optics Express 24, 4, 4029-4041 (2016).