Laser-induced in-fiber capillary instability and material engineering

Abstract

The emerging of multimaterial fibers has been found with widespread applications in various areas. Towards the concept, preform-to-fiber thermal drawing method has been well-established which allows the fabrications of glass or polymer-based fibers with multifunctional internal structures. Besides, a wide range of materials, such as semiconductor, metal, doped-glass and functional polymer, have been drawn into fiber successfully by this method thereby researchers achieve photonics bandgap fiber, optoelectrical fiber, energy harvesting fiber, in-fiber chemical synthesis, etc. However, typical multimaterial fibers are fixed in an axially-invariant core/cladding structure after the drawing process. To increase the number of functional units in one multifunctional fiber, fabricating more complex structures in both fiber cross-section axial and fiber length axial is needed, in other words, ways to achieve micro/nano structures in-fiber are strongly needed. In addition, multifunctional fibers request more advanced materials with preferable properties, such as extremely low-loss optical materials and high-efficiency energy-harvesting crystal materials. But methods to modify the material properties of as-drawn fibers have not been well established, which limits multifunctional fibers to benefit from today’s material sciences or to be used in wider areas. Both in-fiber fabrication and materials engineering request innovations in multimaterial multifunctional fiber’s development. In this thesis, we develop a new thermal treatment system based on a CO2 laser as the heat source. The laser features (1) precisely controlled and stable heating profile; (2) gradient heat distribution; (3) adjustable heating temperature to fit various functional materials; (4) limited heating area. Utilizing the system, we can produce uniformly-sized and globally ordered in-fiber micro/nano-structures and to achieve direct material properties modifications inside the as-drawn fibers. Moreover, the precisely controlled laser heating process enables plenty of interesting in-fiber phenomena such as built-in stress, Plateau-Rayleigh capillary instability, laser-induced recrystallization and laser trapping of in-fiber objects accordingly at the heterogeneous interfaces along the entire length. XII Once occurred on melted fiber materials during the laser heating process, surface-tension-driven capillary instability causes the initially continues core to break up into a periodic particles array with uniform sizes. This capillary instability provides the possibility for the fabrication of in-fiber micro- and nanoscale spherical particles and structures, which is comprehensively studied in this thesis. We demonstrate that the size-controllable fabrications of micro to nanoscale spherical particles could be applied onto a wide range of functional materials (metal, doped glass, semiconductor, and functional polymer) with different melting temperatures from 400 to 2400 K. Optical properties of the fabricated spherical particles are also characterized, showing that an ordered array of silicon-based whispering gallery mode resonators with high Q factor can be achieved following our method. Furthermore, by tuning the laser heating process through the controlling of multiple fabrication parameters, we gain the ability to engineer the properties of in-fiber materials. Indeed, we develope a recrystallization process to directly modify the in-fiber functional material. The CO2 laser selectively heats a limited zone of the fiber and scans through the fiber to recrystallize the core by precisely controlling the cooling rate and heating temperature. Eventually, we realize a stable laser-induced recrystallization method and achieve a pressure-induced structural transition of single crystalline Tin Selenide (SnSe) core fiber in Fm3̅m cubic rock-salt phase with outstanding thermoelectric performance. Next, more laser induced in-fiber phenomena that could be utilized for in-fiber fabrication and materials engineering are discussed. The laser-based system developed by us has adjustable parameters including laser heating power, spot size, and fiber feed-in velocity to control the in-fiber heating temperature and the cooling rate. This laser heating process can induce built-in stress, laser recrystallization, in-fiber capillary instability, and laser trapping into the multimaterial fiber. We also study the laser induced heating process in fundamental by theoretical modeling and software simulations. The study offers us the capability of fabricating electric bandgap adjustable devices, high-quality single crystal fiber devices, uniform or asymmetry shaped micro/nano-particles as well as spheres PN molecules.