# Characteristics and Applications of Spatiotemporally Focused Femtosecond Laser Pulses
## 6 Summary and Outlook  
To summarize, we have given a comprehensive review on the characteristics of SSTF pulses, and the applications of SSTF in femtosecond laser micromachining, femtosecond laser filamentation, and manipulation of plasma dynamics. In femtosecond laser micro and nano-fabrication, the SSTF technique has been applied for enhancing the axial resolution which is particularly useful for 3D processing inside glass and crystals. When the SSTF pulses are used in femtosecond laser filamentation, they naturally give rise to the enhancement of peak intensity within the filament and suppression of the nonlinear self-focusing during propagation. Remarkably, the interactions of the SSTF pulse with solid and gaseous media show unexpected behaviors, such as nonreciprocal writing in dielectric materials and second harmonic generation in centrosymmetric gas media. The physical mechanisms behind these unconventional phenomena are still under debate.  
Since the SSTF technique was proposed, a lot of progress has been achieved in the research of interaction of the spatiotemporally focused femtosecond laser pulses with transparent media. However, it is worth noting that there are some unsettled problems in practical applications. For instance, the  
Figure 11: Upper panel: The second harmonic spectrum obtained in SSTF with the fundamental laser polarization along the \(y\) axis (SF–PY), \(x\) axis (SF–PX) and conventional focusing with the fundamental laser polarization along the \(x\) axis (CF–PX) schemes. Lower panel: Spatial patterns of second harmonic generated by the SSTF scheme with the fundamental laser linearly polarized along (**a**) \(y\) axis; and (**b**) \(x\) axis. The incident pulse energy is \(\sim\)1 mJ and the focal length of the focusing lens is 75 cm. Reproduced with permission from [25], Copyright The Optical Society, 2014.  
Figure 10: Time-resolved shadowgraphs of transient plasma induced by SSTF pulses inside fused silica glass. The polarization direction of the femtosecond laser pulse is parallel to spatial chirp in (**a**–**c**) and perpendicular to spatial chirp in (**d**–**f**). The pulse energies are 6 \(\upmu\)J in (**a**,**d**); 4 \(\upmu\)J in (**b**,**e**); and 2.5 \(\upmu\)J in (**c**,**f**), respectively. The direction of laser propagation (\(k\)) and the FFT are indicated by the arrows in the lower panel of the figure. Reproduced with permission from [26], Copyright The Optical Society, 2015.  
nonreciprocal writing effect, which is unfavorable to the fabrication of complex 3D structures, should be avoided when the SSTF technique is employed for direct writing in transparent materials. In order to propose a method to overcome such difficulty, systematically theoretical calculations, as well as further experimental investigation, are necessary in the follow-up work. Additionally, with the merits of the SSTF pulses, the SSTF technique has shown its advantage in high-throughput 3D femtosecond laser fabrication and high-speed biomedical imaging. Moreover, such a technique is also expected to be widely applied in surface nano/micro structuring and tissue engineering in the near future.  
The work is supported by National Basic Research Program of China (2014 CB921300), Key Scientific Research Project of Henan Province (17A140023) and Foundation for Fostering National Project of Luoyang Normal University (2015-PYJJ)-001).  
[PERSON] and [PERSON] prepared the manuscript under the supervision of [PERSON].  
The authors declare no conflict of interest.