# Nano- and Micropatterning on Optical Fibers by Bottom-Up Approach: The Importance of Being Ordered
## 1 Introduction  
Nature is both chaotic and ordered. Despite the usual and probably misleading association between entropy and disordered systems [1], in nature, we can find many examples of ordered patterns. The structure of a beehive, a spider's web, the stripes on a zebra's mantle, the ice crystals in a snowflake are just a few cases of perfect periodic repetitions among many others. Sometimes the aim of natural periodic patterns is to create specific color effects by light interference, leading to a diverse array of functions as mate attraction, UV protection, camouflage and sensory enhancement. Similarly, artificial ordered patterns at the nano- and microscale find large applications as photonic elements in different technological fields, and specifically, they play a central role in optical fiber (OF) technology. When coupled to photonic structures, in fact, the OF becomes a technological platform where the waveguiding propagation of light through the fiber is coupled with the light-matter interaction occurring on the photonic element. Consequently, a vast number of sensors and sensing probes based on OFs have been developed in the last decades, contributing to the vision of the lab-on-fiber concept [2, 3]. To make it possible, the effortof researchers is focused on one side to the development of new fabrication approaches able to control precisely the periodic structure at the nanoscale, and on the other side to adapt to the OF the already robust technologies usually applied on silicon wafers or, more in general, on planar surfaces. Two main approaches--top-down and bottom-up--can be followed to fabricate photonic nanostructures on the OF. The top-down approach is a subtractive process in which the nanostructures are produced by removing material, is generally the most expensive and time-consuming, but also the most precise manufacturing option. It typically produces highly ordered patterns by using robust technologies such as electron-beam lithography, focused ion beam, interference lithography, femtosecond laser writing and two-photon lithography, which have demonstrated its capability to implement complex designs and architectures. The bottom-up methods are additive processes normally based on self-assembly, in which the material elements are used to build up the desired objects. The self-assembly of nano-, micro- or macroscopic objects involves weak local interactions that can include van der Waals, electrostatic and hydrophilic/hydrophobic interactions, gravitational attraction, magnetic, capillary, and entropic forces. The consequence of these specific local interactions among the components, without external direction, is generally the formation of ordered state forms as the system approaches equilibrium, reducing its free energy. While the range of symmetries or structures is somewhat limited, the promise of high throughput and low-cost processing over large areas implies that, at least in principle, processing methods that leverage self-assembly will have a central role in the development of advanced nanotechnology. For the most part, their application to OFs provided so far randomly arranged or poorly ordered nanostructures. However, merging highly ordered nanostructures and bottom-up nanofabrication on the OFs would have many advantages, from reproducibility of fabrication to enhanced light-matter effects. Recently, we reviewed those nanofabrication methods, based on self-assembly, that have been used to create lab-on-fiber nanoprobes, covering papers published until the beginning of 2018 [4]. That collection of articles clearly indicated that decorating the OF with ordered patterns by exploiting the natural organization of matter, exactly in the same manner that nature does, is a challenging but achievable goal.