# Characteristics and Applications of Spatiotemporally Focused Femtosecond Laser Pulses
## 4 Applications of SSTF Technique in Femtosecond Laser Filamentation  
Another subject of great interest for applying SSTF technique is in the field of femtosecond filamentation. Owing to the interplay between Kerr focusing and plasma defocusing, a femtosecond laser beam can transform itself into a thin and long light string, which propagates to a remote place without divergence [35]. The intensity inside is generally clamped at \(10^{13}\)-\(10^{14}\) W/cm\({}^{2}\) as plasma can be efficiently generated at such intensities which, in turn, induces strong defocusing of the light in the filament [35; 36]. The clamped intensity in the filament can be enhanced by tight focusing, as the strong geometric focusing can compensate the plasma-induced defocusing to some extent. Unfortunately, the filament cannot be launched far away with the tight focusing, which limits its applications in remote sensing.  
In 2011, [PERSON] et al. demonstrated that the difficulties can be settled using the SSTF technique [21]. Under the same operating distance, the clamped peak intensity in the core of the filament has reached a higher level with the SSTF pulses than that obtained with the conventional focusing scheme (see Figure 8a for more details). This is mainly due to the suppression of the self-focusing before the geometric focus which allows the pulses the approach the geometric focal plane in a more linear fashion. Only near the geometric focal plane, both the optimal conditions for spatial and temporal focusing can be simultaneously reached to give rise to a sharp increase of the peak intensity. The plasma defocusing effect is, therefore, hampered owing to the relatively lower peak intensity of the SSTF pulses away from the geometrical focal plane. Additionally, the length of the filament generated by SSTF pulses is nearly 25 times shorter than that generated by the conventional focusing scheme, which is a clear indication of the suppressed self-focusing effect (see Figure 8b,c for details). Another manifestation of the suppressed nonlinear propagation is the suppression of supercontinuum generation in the filament, which is verified by investigating the optical breakdown spectra in water [14].  
Figure 8: (**a**) Optical intensity distribution generated by SSTF pulses (blue solid line) and conventionally-focused laser pulses (red dashed line). Filament profiles generated by (**b**) the SSTF scheme; and (**c**) the conventional focusing scheme. Reproduced with permission from [21], Copyright American Physics Society, 2011.  
Due to the improvement of the clamped intensity inside the filament, the SSTF technique can be applied in remote sensing spectroscopy. Researchers show that significant enhancement of fluorescence signals can be achieved in filament-induced breakdown spectroscopy. The typical spectrum achieved on iron and aluminum samples from 22 m away using SSTF pulses are demonstrated in Figure 9 [37]. By using SSTF pulses, approximately three- and four-fold enhancements of the fluorescence signals were observed from iron and aluminum targets, respectively [37]. The results can be attributed to the nonlinear dependence of the fluorescence signals on the laser intensities at the targets.