Long considered a problem in search of a solution, lasers have now – more than 60 years after their first practical demonstration – moved into our everyday lives where they are ubiquitous. They revolutionized lighting technology, offer diverse applications in consumer electronics and data storage, enable novel medical treatments and therapies, and have become indispensable in research and for many industrial applications. For decades, one has observed a furious development in laser technology, realizing a "Moore's Law" analogous to electronics, especially for the available output power levels of ultrashort pulsed lasers. In laser-based materials processing, this has triggered a wild race to ever greater powers and machinable surfaces and with ever smaller structure sizes down to the nanometer range. This race, along with the question of fundamental limits, is the subject of the new book "Ultrafast Laser Nanostructuring — The Pursuit of Extreme Scales" [1]. It brings together 33 overview chapters, written by more than 140 authors. Here, we tell the background of the editors and how this new book came to be.

About the Editors.

It was the late nineties in Berlin, Germany, when the two book editors, Razvan Stoian and Jörn Bonse, first met - at the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Arkadi Rosenfeld's group, one of the important German research groups with an early and continued focus on the fundamentals and applications of ultrashort laser pulses in materials processing. They were both PhD students, starting their research work in the early, pioneering times of ultrafast laser structuring, one employed at the MBI, the other at the German Federal Institute for Materials Research and Testing (BAM), which was also already researching in this area since the early nineties. At that time, the research landscapes of the two parts of the formerly divided Germany were growing together sustainably, and a particularly visible and dynamic example could be seen in Berlin-Adlershof, the location of the MBI and also one of the branches of BAM.

At that time, both book editors were particularly interested in so-called "incubation effects", by which a usable effect of the laser radiation occurs only after a certain minimum number of applied laser pulses as material ablation of the workpiece, and which, moreover, still strongly depends on the material. However, both did this with different methods: while Razvan Stoian's focus at the MBI was on in-situ optical and spectroscopic methods and investigated phenomena such as the "Coulomb explosion" of transparent glasses and crystals, Jörn Bonse's work at BAM was mainly devoted to ex-situ analyses of topographical, structural and chemical effects that occurred during the material processing of semiconductors and ceramics. Both then received their doctorates at the beginning of the new millennium, one at the Free University of Berlin in 2000, the other at the Technical University of Berlin the following year. And then they went their separate ways again for the meantime, as both were drawn to other European countries such as France and Spain.

However, the common interest in the field of laser-material interaction with ultrashort pulses on surfaces and in the volume of workpieces remained. Both continued to meet regularly at conferences and in scientific exchanges, keeping in touch via the working group at the MBI.

Razvan Stoian established his own research group at the Laboratoire Hubert Curien in St.-Etienne, France, where he worked, among other things, on temporal and spatial beam shaping and engineering of ultrashort pulsed laser radiation and its application for direct writing of three-dimensional optical waveguides in transparent materials – involved with effects that could also be used for future high-capacity optical data storage. In particular, non-diffractive Bessel beams opened up novel processing possibilities even for relatively thick glass samples, such as those used in displays in cell phones.

Jörn Bonse conquered various varieties of fs-time-resolved microscopy on surfaces and in the volume of transparent materials through research at the Spanish National Research Council (CSIC) in Madrid and at the MBI in Berlin and also devoted himself in depth to the topic of "laser-induced periodic surface structures" (LIPSS) over the past 18 years - then again at BAM as a senior scientist since 2009. Initially seen only as an annoying side-effect, such nanometer-scale LIPSS currently represent one of the cornerstones for the fast, robust and inexpensive generation of large-area surface structures with periods below the optical diffraction limit using laser radiation. Given the rapid success in laser technology, LIPSS processing is now on the verge of a breakthrough in industrial applications.

A look into the book.

The directionality of laser beam allows its tailored localization, while the coherence of laser radiation additionally enables near-field or far-field scattering and interference effects that widen significantly the capabilities of controlling and tracking laser-matter interactions in space and time. In laser materials processing both properties finally enable the capabilities of micro- and nanostructuring of surfaces or even in the bulk of the material. This can render new functions and properties that are impacting the mechanical, electrical, or optical characteristics of the processed materials.

The progress in laser engineering with major breakthroughs, notably in pulse duration and average output power, always closely accompanied the industrial efforts in material structuring, typically with two milestones in sight: (1) yield and (2) resolution. Every significant reduction of the laser pulse duration led to a subsequent improvement in process precision – particularly when the ultrashort pulse durations underran the fundamental electron-phonon relaxation times. Thus, minimizing heat diffusion into the laser processed surrounding, ultrashort laser pulses have managed to confine the spatial resolution to the optical diffraction limit and sometimes even beyond. The nanoscale was already in sight at the turn of the millennium. An important question is raised here: is there any fundamental limit in the processing resolution, a barrier defined by the intrinsic properties of light and matter? The answer has an inherently multidisciplinary nature and is in focus of the present book that is arranged in three parts.

The first part of the book (“Fundamental Processes”) offers a perspective into the fundamentals of laser-matter interaction on extreme spatial scales, with a description of the most advanced modeling efforts in understanding energy deposition in matter, a plethora of material excitation and relaxation pathways, as well as advanced concepts for probing and observing excited matter in motion. Roadmaps for energy localization are developed, and the atomistic perspective of laser ablation visualized. Theoretical modelling enables in-depth insights on ultrafast quantum processes at the nanoscale. Laser-driven self-organization at surfaces is dissected regarding the question of how light drives material periodic patterns down to the nanoscale, explored and transmitted to its ultimate limits of an atomic printer, and immediately complemented by the unprecedented capabilities of ultrafast in-situ observation approaches for tracking the laser-induced material response with extreme spatial and temporal resolution.

In the second part of the book (“Concepts of Extreme Nanostructuring”), distinct concepts are developed. A special focus lies on optical near-field related approaches for localizing light on scales even below the optical diffraction limit and on plasmonic printing. Spatial and temporal beam-shaping and tailored interference techniques are discussed in the context of ultrashort laser pulses, and insights into some extreme states of matter realized by the tight confinement of laser energy are presented. The ultimate limits of writing waveguides in the bulk of dielectrics and for manifesting 3D-nanolithography are elucidated.

Finally, the third part of the book (“Applications”) leads us to a number of resuming applications, unveiling the tremendous capabilities of surface functionalization through laser micro- and nanostructuring, assessing the 3D-writing of waveguides in the bulk of dielectrics or semiconductors for enabling new branches of micro-integrated photonics, and summarizing related applications ranging from nanophotonics to nanofluidics and from optical sensing to biomedical applications, including the latest capabilities of refractive eye surgery. This part analyzes the applications’ compatibility in yield and reproducibility with current industrial requirements, costs, and intellectual property aspects. It expands the involved spatial scales by more than eight orders of magnitude, when extending extremely small structures featuring sizes of few tens of nanometers to larger dimensions in the meter range.

How the book came to live.

In early 2020, a contact with Springer Nature Publisher was established and the idea was born to jointly organize a book "Ultrafast Laser Nanostructuring — The Pursuit of Extreme Scales" of about 300 pages as part of the book series "Springer Series in Optical Sciences", which has been established for more than three decades. Quickly, the respective scientific colleagues were asked and by overwhelming agreement from the entire laser materials processing community, the vast majority of the 38 requests for review chapters were successful and also assigned to carefully planned and selected topics. Over the next three years, the contents of the book were then created, written by the world's leading experts in their respective fields and with the aim of providing an up-to-date and complete overview of each topic, historically versed, international, balanced between theory, experiment, and practice, professionally complete and richly illustrated! For quality assurance, the chapters were also subjected to a peer-review process, and the page count of the book exploded compared to the initial planning.

In 2023, three years later, the mission is now completed by the publication of a comprehensive book with more than 1,200 pages and 586 figures [1]. Several thousand e-mail communications had been exchanged between the publisher, editors, and authors. In the end, it took a year longer than originally planned, but the present result is impressive: the book is suitable both as an introduction for students as well as a reference work for specialists. It comprehensively reflects the current state of knowledge and, in the view of the editors and the publisher, has the potential to establish itself as a standard work.

Mission and goal of this book.

From surfaces to the bulk, from subtractive to additive manufacturing approaches, from advanced theoretical frames to practical technological processes – we invite the readers to an exciting journey into the varicolored landscape of extreme laser nanostructuring.

[1] Razvan Stoian, Jörn Bonse (Eds.) Ultrafast Laser Nanostructuring — The Pursuit of Extreme Scales; Springer Nature Switzerland AG: Cham, Switzerland, 2023. ISBN: 978-3-031-14751-7, DOI: https://doi.org/10.1007/978-3-031-14752-4
https://link.springer.com/book/9783031147517