Single crystals fibers - Growth by micro-pulling down (µ-PD) process and applications

Modern engineering uses components made from crystals of controlled geometry, mainly in the form of plates, fibers or tubes, although sometimes the shapes are much more complicated. Crystals of specific formats and sizes, free from defects and impurities, are therefore desirable; they can be used as end products with minimal additional machining. Crystal growth processes for massive crystals (ingots) require very large crystallization crucibles of the order of a few liters, which presents a major disadvantage, as these containers made of rare metals such as iridium are expensive. What's more, such crucibles have a lifespan limited to a few runs, due to the chemical degradation to which they are subjected, which significantly increases the cost of single crystal crystallization. Since 2010, single-crystal fibers have been the focus of intense research, thanks to their remarkable characteristics for lasers and scintillators. The development of optical waveguides has stimulated the growth of single-crystal fibers for a wide range of applications.

The development of single-crystal fibers is motivated by optical applications that are not accessible to glass fibers or solid single-crystal forms. Single crystals in fibered form increase the interaction efficiency between the beam and the material. For laser applications, the fibered configuration also offers other advantages, including efficient dissipation of heat stored in the material thanks to the short distances between the pumping zone and the external thermostatic medium. In addition, by using a long interaction length, the concentration of activating cations (Nd ³⁺ , Yb ³⁺ ...) can be reduced. These two factors work together to minimize material heating, which is favorable for high-power laser applications. A low concentration of active cations also minimizes non-radiative de-excitations (extinction of light emission by energy transfer between ions). What's more, the efficiency of laser oscillations in a single-crystal host lattice is often much greater than in a glassy lattice, since in this case the structural disorder of the material reduces the effective cross-sections of stimulated emission, and thermal conductivity is lower. The small size of single-crystal fibers also minimizes the presence of defects responsible for the low mechanical strength of solid materials. Monocrystalline fibers can also be used for second-order interactions such as harmonic generation, frequency mixing, parametric oscillation and electro-optical modulation.

In the field of scintillators, single-crystal fibers are serious candidates for the development of new generations of scintillation calorimeters for high-energy physics. Since 2005, the...