Preface
Laser-induced photorefractivity discovered and studied in the late 1970s was the prelude to emerging laser-based materials growth and device processing technologies in photonics. Tailoring the refractive index of optical media by intense short wavelength laser radiation paved the way for periodic Bragg structures used as selective reflectors, optical fibre filters and sensor devices, all available today as off-the-shelf components for industry and telecoms. These prime developments have been followed by a rich portfolio of science and technology, with thousands of scientific articles and a plethora of patents addressing materials fundamentals and technical solutions to long-standing industrial problems. Novel materials compositions and phases become manipulated by laser radiation effects and drive this now blooming technology to its nanoscale limits, by taking advantage of ultrashort, femtosecond, laser pulses and their extremely high peak power. Internal structure manipulation is achieved with extreme precision, enabling tailoring of photonic crystal fibres and other complex devices with novel functional properties.
Pulsed laser deposition by use of high power excimer lasers achieved transparent epitaxial films of ferroelectrics (BaTiO3) and paraelectrics (Bi12GeO20) in the late 1980s and the start of the 1990s. Such success reinforced with optimism those first apprehensive steps and pointed to the capacity for epitaxial materials growth of complex compositions with high optical quality. The successful growth of waveguide lasers (Ti:Sapphire and Nd:GGG) became reality after a couple of years’ effort. ‘Lasers-make-Lasers’ fabrication approaches revealed a unique potential for the development of new materials for photonics applications demanding extreme structural and functional qualities. It was not only the congruent nature of laser ablative deposition that enabled hard-to-achieve structures and advanced stoichiometries. Mastering the laser-growth techniques allowed in-growth structural manipulation, yielding active photonic nanocomposites and multi-layered structures with functionalities not available by other means.
Novel functional properties come about from miniaturization and new dramatic nanoscale effects unravel a palette of new functionalities for photonics. Along these lines, tailoring photonic interfaces by laser light would permit the evolution of yet unknown phenomena and light–matter interactions at the molecular level. Highly energetic photons ‘hammer’ the interphase neatly, with no contamination, building micro- and nano-structures in direct ablative or, alternatively, subtle reactive processing modes. Ultraviolet laser micro-etching and micro-printing approaches are now complemented by multiphoton lithographic and other photoreactive processing approaches for surface tailoring and photonic structure fabrication. Taking advantage of the materials’ optical nonlinearities and the good beam quality, the processing resolution at sub-wavelength levels offers an unsurpassed potential for three-dimensional nano-structuring.
Laser growth and processing of photonic devices thus emerges as a new global technological approach addressing both materials and device concepts. This book aspires to provide a comprehensive outline of the field, an overview of fundamentals and a focus on the latest developments and future trends. It is within its scope to assist a deeper understanding of laser–matter interactions and related phenomena aiming to trigger and support new frontier research lines. Novel functional photonics structures and devices offer a horizontal technology platform targeting applications of multidisciplinary interest, from information processing and sensing to bio-photonics and energy harvesting. Even though overall not at the level of maturity and the compatibility required by microelectronics industrial foundries, I am confident that, with its unique qualities and penetrating character, this technology will find its way to industrial implementation and the marketplace.
The book comprises a balanced content of science fundamentals and technology. It highlights important aspects and currently open issues, also pointing to the emerging and future trends. The introductory chapter overviews the fundamental phenomena and the basic processes driving this emerging field. By providing some review and pioneering references it aims to guide the reader through the contents without exhausting the subject. Some more background details on laser ablative processing are given to cover the topic. The following three main parts are presented by eminent contributors in the field, respectively addressing materials and surface structuring, three-dimensional structures and materials structure tailoring. In Chapter 2, Eason et al. detail the latest developments in the field of pulsed laser deposition for photonics applications, presenting novel approaches and hardware which will make feasible materials composition and structural tuning. In Chapter 3, Medvid’ reviews the fundamentals and the applications of nanocone formation on semiconductor interfaces by laser-induced self-assembly, pointing to a potential future sustainable technology in the microelectronics environment. In Chapter 4, Zhao discusses the deployment of femtosecond laser multiple-beam interference methods for the fabrication of photonic nanostructures, enabling a range of unique functions. In Chapter 5 Fourkas elaborates a detailed discussion on fundamentals and three-dimensional fabrication by nonlinear, multiphoton effects using ultrashort pulses, addressing lithographic and alternative photochemical methods, as well as novel materials for micro- and nanostructure fabrication. In Chapter 6 by Schmidt, the laser becomes a versatile lithographic and rapid prototyping tool for multi-dimensional photonics, giving practical components and devices of high potential and prospective impact in several areas. In Chapter 7, a novel approach of three-dimensional microstructuring by applying laser radiation gradient forces is introduced by Athanasekos et al., clarifying its reversibility and its distinct differences to 3D lithographic processing. In Chapter 8, Wang discusses laser assembly of photonic devices, a range of versatile laser-based fabrication techniques, including microwelding and packaging, methods of direct industrial relevance. In Chapter 9, Canning and Bandyopadhyay introduce materials structural modifications for photonics, detailing thermal glass processing in the nanoscale with recent results unravelling a unique potential and fabrication flexibility. In Chapter 10, Ams et al. thoroughly address laser-induced refractive index manipulation, from the fundamentals to novel concepts and a complete range of photonic devices produced by these methods. In Chapter 11, Eaton et al. present in detail the thermal writing methods for photonic device fabrication in glass and polymers and investigate the role of laser exposure parameters in refractive manipulation for waveguide formation, a topic of crucial practical importance. Finally in Chapter 12, Pissadakis reviews exhaustively laser processing of optical fibres, presenting thoroughly index engineering and microstructuring approaches and concluding with emerging topics relating to tailoring of photonic crystal fibres and applications.
I am hopeful that this book will become a useful resource for the academic and the industrial reader, an aid to their future research and endeavours.
The constant guidance, encouragement and support of Woodhead Publishing’s editorial staff throughout this project have been invaluable. I am especially grateful to the Commissioning Editor Ms Laura Pugh, Senior Project Editor Ms Cathryn Freear and Publications Coordinators Ms Rachel Cox and Ms Anneka Hess. Their devotion to our targets and their patience with my, unavoidably, delayed responses has been remarkable. Many thanks also go to my colleagues Miltos Vasiliades, Loukas Athanasekos and Dimitris Alexandropoulos, for their useful discussions, support and contributions in the preparation of this volume.
Finally, I would like to express my deepest and sincerest gratitude to all authors of this book. Without their authority and eminent contributions this work would not have been realised.