Solving the puzzle of non-reciprocity in ultrafast laser writing

The nanostructuring of transparent media with ultrashort laser pulses has attracted interest due to its unique applications. However, little is understood with respect to the physical mechanisms responsible for the peculiarities of the dielectrics inscribing with high intensity laser beams. It has been shown that spatio-temporal couplings (STC) inherent to the ultrashort pulses make inscribing sensitive to the writing direction [1, 2] and that anisotropic photosensitivity [3] originates from the pulse front tilt (PFT). More recently, a greater appreciation of STC in focussing has been gained, shedding light on previously unknown parameters of the pulse such as a lighthouse-like wavefront rotation [4] emphasizing the need for a better understanding of the STC effects in light-matter interaction. Nevertheless, understanding of the microscopic processes responsible for the modifications of dielectrics with ultrashort laser pulses and control of the writing process via STC is still lacking. Here we show with control of STC through the use of grating compressors, allows one to control and understand ultrafast phenomena associated with material modification. We reveal unambiguously that PFT gives rise to the non-kreciprocity during femtosecond laser writing in transparent media and induces either an isotropic damage-like structure or a self-assembled nanostructure depending on the writing direction. This phenomenon is known as the “quill-writing effect” (Fig. 1a). A switching of the modification regime is observed when the translation of the beam is in the direction of the tilt, which can be qualitatively described in terms of a first-order phase transition.