Ultrafast optics is well deservedly associated with strong nonlinear responses. In fact, the entire field of ultrafast optics is unthinkable without nonlinear interactions of light with matter, starting with mode-locking of lasers, the process that generates ultrashort pulses in the first place, to the majority of their applications, from material processing and nonlinear microscopy to high-field physics and frequency metrology. However, when the nonlinear effects are too strong, they cause instabilities, even catastrophic damage. It is, perhaps, ironic that the majority of efforts into improving mode-locked lasers has been dedicated to limiting these nonlinearities. Starting in early 2000’s, we have been developing the concept of Nonlinearity Engineering (J. Opt. Soc. Am. B, 2002), which kicked-started the exploration of particular nonlinear waveforms that are resistant to strong nonlinearities inside mode-locked laser cavities, rather than the traditional approach of weakening the nonlinear effects. Albeit being initially met with disbelief, this approach has led to the demonstration of multiple record-breaking lasers, such as the wave-breaking-free (Opt. Lett., 2003), the similariton (Phys. Rev. Lett., 2004) and the soliton-similariton (Nature Photon., 2010) lasers. 

Around 2013, we have started applying what we learned from mode-locking to laser-material interactions — despite the physical system (a material vs light field inside a laser cavity) being entirely different, the interactions share essential mathematical similarities. Following this approach, we showed that we could create laser-induced spatial nanostructures on various material surfaces with unprecedented uniformity (Nature Photon., 2013) by locking the modes in space. The commonality between mode-locking and this approach is to harness feedback mechanisms that arise intrinsically: When a laser beam modifies the properties of a material, this, in turn, changes how the next laser pulse interacts with the same material. Such two-way interactions lead to a feedback mechanism, whereby, akin to mode-locking, one only has to arrange for any desired pattern to have higher feedback gain for it to emerge spontaneously and effortlessly. This concept was later generalized to other applications, such as creation of self-organized 3D structures deep inside semiconductors (Nature Photon., 2017), self-assembly of colloidal nanoparticles (Nature Commun., 2017), even 3D hologram generation (Nature Photon., 2019), a new type of nonlinear feedback-driven optical tweezers (Nature Commun., 2019), and a new and ultra-efficient regime of laser-material interactions (Nature, 2016). In each demonstration, we achieved striking results that were previously thought to be impossible in their respective fields. 

The common theme to all the activities of UFOLAB is judicious exploitation of nonlinear processes.