Research Topics

  1. Bullet Laser physics,

  2. Bullet Dynamics of mode-locked fiber lasers

  3. Bullet Ultrafast and high-power fiber lasers

  4. Bullet Laser-controlled self-organization and self-assembly

  5. Bullet Laser-material processing and laser-surgery in the ablation-cooled regime

  6. Bullet Nonlinear, dissipative, non-equilibrium systems

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. Later, we have applied the same concept to extremely efficient laser-material ablation (Nature, 2016), creation of self-organized 3D structures deep inside silicon (Nature Photon., 2017), self-assembly of colloidal nanoparticles (Nature Commun., 2017), among others.

Recently, our focus is shifting back to mode-locking. We would like to apply what we learned from complex laser-material interactions to answer some difficult questions in self-organizing, far-from-equilibrium systems. Mode-locking, itself, is a self-organized far-from-equilibrium state, involving the cooperative action of hundreds of thousands of electromagnetic modes, which interact strongly with matter making up the laser cavity. Often overlooked, fluctuations (in the form of laser noise) plays an essential role when nonlinearities are strong. And unlike many other systems, we have excellent control over the experimental parameters, including the amount and spatial distribution of dissipation, nonlinearity, we can externally inject tailored fluctuations into a cavity, we have a precise theoretical description at hand and we can measure most experimental quantities with dynamic ranges spanning multiple orders of magnitude.

In addition, a large fraction of the research effort of UFOLAB is dedicated to physics and practical development of fiber lasers, in particular high-power ultrafast fiber lasers, along with their numerous biomedical and industrial applications.

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 disocver 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 Photonics, 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.

Nonlinear physics of mode-locked lasers

We were the first laboratory to demonstrate ultrafast burst-mode fiber lasers (Kalaycioglu et al., Optics Letters, 2011) and also the first to uncover the ablation-cooled laser-material processing regime (Kerse, et al., Nature, 2016). In burst mode, the laser produces a group of (typically few 100’s of) pulses with high repetition rate (>1 GHz) and high total energy (typically 100-250 µJ) at a low overall repetition rate (100-500 kHz). The average power is kept between 10 to 50 W, but we also developed high-power versions with more than 150 W using Doping Management, a laser amplifier scheme which we have pioneered. This unique mode is extremely interesting for material and tissue processing, as the entire burst behaves almost like a single pulse.

Ultrafast material processing in the ablation-cooled regime

The ablation-cooled laser processing regime that we have pioneered (Kerse et al., Nature, 2016) is well-suited to high-precision, high-speed ablation of tissue while avoiding thermal damage. This is an area of intense effort within UFOLAB.

We were also the first to use a fiber laser for nanosurgery (Yavas, et al., Biomedical Optics Express, 2012). A custom femtosecond laser was developed and integrated to a high-resolution microscope system with which 3D ablation with sub-micron resolution was achieved.

Laser surgery in the ablation-cooled regime

Our long-term goal is a 3D material synthesizer of complex materials with pre-programmed structure, self-organized/assembled in non-equilibrium over multiple spatio-temporal scales as a result of nonlinear & stochastic dynamics — partially inspired by the Replicator technology from the sci-fi series Star Trek. 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.

Supported by an ERC Grant, we are now developing new ideas based on this concept to achieve higher-level of control over these self-organized nanostructures.

Laser-controlled self-organization and self-assembly

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., 2009, 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 (currently under review), this is already a very diverse set of systems with no physical relationship to each other.

Since July 2014, we are funded through an ERC Consolidator Grant to further explore these ideas with the goal of developing hitherto unimaginable levels of control over matter via laser light.

“Nonlinearity Engineering”

Every pattern, structure or functionality in nature is self-organized or self-assembled. While several scientific disciplines like complexity or network science target understanding of the underlying dynamics, and rapid progress is made, still our understanding remains quite limited.

Here, we’d like to make a few observations:

  1. 1.Nonlinear feedback mechanisms are often the key. Fractals are excellent examples to this, and spatiotemporally extended systems with nonlocal feedback result in truly rich behavior.

  2. 2.Fluctuations or stochastic effects are essential. Fluctuations usually instigate pattern formation and nonlinear systems exhibiting advanced features such as self-healing, robustness, adaptability incorporate stochastic dynamics

  3. 3.Interesting behavior is observed far from equilibrium.

  4. 4.Complex behavior does not require complex rules. Consider cellular automata, e.g., Game of Life.

We further note that many, many systems, which are physically completely distinct from each other are found to exhibit similar behavior. Usually the similarity is mathematically subtle, but when it is manifested geometrically, it is easy to recognize, as the collage to the right shows.

Nonlinear, non-equilibrium and stochastic dynamics





Femtosecond burst-mode fiber laser setup with integrated OCT for imaging