# Characteristics and Applications of Spatiotemporally Focused Femtosecond Laser Pulses
## 1 Introduction  
Femtosecond laser pulses have been widely used for three-dimensional (3D) micromachining on both micro and nano scales [1; 2; 3; 4; 5; 6]. The tightly-focused, intense, femtosecond laser pulses intrinsically provide capabilities of 3D micro and nano fabrication due to efficient confinement of the nonlinear interactions within the focal volume [2]. For many applications, such as writing optical waveguides or fabricating microchannels, a 3D isotropic resolution is necessary. However, because of the mismatch between the beam transverse radius and the Rayleigh length at the focus, the single-path transverse writing method (i.e., the writing direction perpendicular to the laser propagation direction) intrinsically creates asymmetric cross-sections with conventionally-focused femtosecond laser pulses [7]. To solve the problem, several pulse-shaping techniques have been proposed, including astigmatic beam shaping [8], slit beam shaping [9], and crossed-beam shaping [10], etc. However, none of these techniques can produce a true spherical focal spot with isotropic resolutions in all three dimensions of space. By applying the simultaneous spatial and temporal focusing (SSTF) technique in femtosecond laser micromachining, it is shown that such a problem can be settled appropriately.  
Simultaneous spatial and temporal focusing, which was originally developed to improve the depth resolution in the process of 3D-resolved imaging in 2005 [11; 12], has attracted much attention due to its potential applications in many research fields [13; 14; 15; 16; 17]. In 2010, [PERSON] et al. firstly applied the SSTF technique in femtosecond laser micromachining to achieve 3D isotropic resolution in fabrication of microfluidic channels with a circular cross-section regardless of the direction of sample translation [18]. Until now, the SSTF technique is the only method that allows a 3D isotropic fabrication resolution with a single focal lens [18; 19; 20], and the applications of such a technique in femtosecond laser micromachining have been extensively studied. Almost in the same time, SSTF has also extended to femtosecondlaser filamentation for reducing the nonlinear self-focusing which is the major obstacle to achieving high peak intensity in the filament [21; 22]. Shortly after the establishment of the technique, it was discovered that the SSTF spot has some unconventional characteristics, including a pulse front tilt (PFT) and an intensity plane tilt [23; 24]. Moreover, the interaction between the SSTF pulses and transparent media can induce some intriguing phenomena, such as a nonreciprocal writing effect, second harmonic generation in isotropic media, and curved plasma formation in fused silica glass [23; 25; 26]. Until now, the physics mechanism of the nonlinear interaction of SSTF pulses with transparent materials has not been fully understood.  
In this paper, we review the major experimental results related to characterization and application of SSTF pulses obtained by us in the past few years. The paper is organized as follows: In Section 2, we introduce the basic principles of the SSTF technique and the main characteristics of SSTF pulses. From Sections 3-5, we show the applications of SSTF pulses in femtosecond laser micromachining with improved axial resolution, femtosecond laser filamentation with enhanced peak intensity, and creating unconventional plasma behaviors, respectively. A summary and an outlook are given in Section 6.