“Nonlinearity Engineering” &
“Nonlinear Feedback-Driven Laser-Material Interactions”
Can we engineer outcomes of nonlinear and stochastic processes to achieve desired, pre-planned functionalities with minimal intervention?
Nonlinearity Engineering approach gladly relinquishes absolute control over the many degrees of freedom of the system under study, in return for global control by setting the terms and parameters of its governing equations.
While the concept is general and independent of specific implementations, we first conceived of and demonstrated Nonlinearity Engineering in the field of ultrafast lasers (first theoretical proposition, Ilday, et al., J. Opt. Soc. Am. B, 2002, first experimental demonstration, Ilday, et al., Phys. Rev. Lett., 2004 and later, Ilday, et al., Nature Photon., 2010, also see recognition of our approach in the review of J. Dudley, et al., Nature Photon., 2012), and most recently in laser-induced nanostructuring of surfaces (Ilday, et al., Nature Photon., 2013). From laser physics to self-organized nanostructures and more recently to self-assembled nanomaterials (Nature Commun., 2017), this is already a very diverse set of systems with no physical relationship to each other.
Ultrafast material processing in the ablation-cooled regime
We have recently invented a new regime of laser-material processing (Nature, 2016), which is accessed with ultrafast pulse repetition rates in the range of multi-GHz, in contrast to the traditional ultrafast regime using kHz repetition rates. As a result, we achieved 10-100 times higher ablation efficiency (volume per incident energy). At the GHz repetition rates, there is so little time between the pulses that heat diffusion becomes negligible compared to cooling by the ejection of material during ablation, hence the name, ablation cooled. This is the fundamental reason for increased efficiency. Also, instead of using 10s to 100s of microjoule pulse energy, several orders of magnitude lower energies are used. This is so because, in the ablation-cooled regime, thousands of pulses interact collectively with the material.
Precision material processing with record-high efficiency
The benefits of the ablation-cooled regime has been proven on a diverse and rapidly expanding set of industrially important materials, including various metals, semiconductors, heat-sensitive piezoelectric and magnetic materials, different medical implants, glasses and other transparent materials.
Ultrafast and ultra-efficient laser surgery
The most important future application of this regime will likely be laser surgery. UFOLAB has already reported record-high speeds in the cutting of brain, corneal tissue, and dentine (Nature, 2016). UFOLAB is working with medical doctors to develop ultrafast and ultra-efficient laser surgeries.
Nonlinear physics of mode-locked lasers
Mode-locked fibre lasers enable ultrafast science with enormous scientific, industrial and medical impact. However, to date, there is no general theory of mode-locked operation that allows to design a mode-locked laser, in other words, to determine, a priori, a sequence of optical cavity elements for desired laser operation.
Starting circa 2000, we have been applying Nonlinearity Engineering to discover new mode-locking regimes. This has already led to the discovery of two of the five well-known mode-locking regimes, namely the similariton (Ilday et al., Physical Review Letters, 2004) and soliton-similariton lasers (Oktem et al., Nature Photon., 2010).
We are particularly interested in developing a theoretical framework that unifies all the known mode-locking regimes, namely, soliton, dispersion-managed soliton, dissipative soliton, similariton, soliton-similariton and predict new ones.
Laser-controlled self-organization and self-assembly
Imagine, as a very distant goal, the Replicator technology from the sci-fi series Star Trek. Can we aim for a capability that resembles the Replicator via laser-driver self-organization? Both our fundamental understanding of driven self-organization and present technology comes woefully short. At the same time, there is no physical law that prevents it; our long-term motivation is to develop a 3D material synthesizer of complex materials with pre-programmed structure through laser-driven self-organization. There is no physical law that precludes this, since any biological organism accomplishes all of the above, but our fundamental understanding as well as technology is completely inadequate.
We have recently developed an approach, Nonlinear Laser Lithography, that exploits the nonlocal nonlinear interference of the incoming laser beam and its scatterings from the surface (Oktem, et al., Nature Photonics, 2013). This way, growth of the nanostructures is initiated from surface roughness (i.e., fluctuations) through a nonlinear and nonlocal positive feedback mechanism. The total optical field at any point is determined by the incident laser field and the scattered light from the surrounding surface, in a mathematical form similar to that of a hologram.
The height and shape of the structures are regulated by a negative feedback that kicks in as the structures grow in height. This approach has allowed us to demonstrate high-speed fabrication of nanostructures with sub-1-nm uniformity over millimeters.