# Nano- and Micropatterning on Optical Fibers by Bottom-Up Approach: The Importance of Being Ordered
## 3 Bottom-Up Fabrication of Periodic Structures on OFs
### Colloidal Self-Assembly
The same authors performed also a systematic study aimed at the integration of opal and inverse opal films onto bundles of OFs [50]. By using isothermal heating evaporation induced self-assembly and sol-gel co-assembly methods, respectively, they realized opal films and silica inverse opal films on several optical fibers simultaneously. The transaltion of the technique on multiple fibers (i.e., bundle) outlined the potential impact of such self-assembly techniques for the mass production of OF devices. As proof of concept, the fabricate probes were tested as humidity sensors, exploiting the effective refractive index changes due to the humidity concentration variations [51]. Recently, [PERSON] et al. used the self-assembly of a monolayer colloidal crystal as a basic building block to obtain different periodic structures onto the OF tip [52]. Specifically, PS nanospheres (with diameters ranging from 200 to 1000 nm) were self-assembled at the air-water interface into ordered hexagonal close-packed arrays and then used as basic building blocks to obtain different periodic structures after the transfer onto the OF tip (Figure 2a). The basic close-packed array (CPA) monolayer covered by a gold conformal layer was demonstrated to work as efficient surface-enhanced Raman spectroscopy (SERS) probe [53; 54]. Similar structures were obtained by adding a plasma etching step before the metal deposition in order to reduce the nanosphere diameter maintaining the initial pattern period (SA, sparse array) [52]. The nanospheres monolayer, arranged either in CPA or SA fashion, can be used as a mask to pattern the underlying substrate too. After metal deposition, indeed, the nanospheres, removed from the surface by sonication, leave behind metal triangles in the regions corresponding to the void volumes between nanospheres (CPA-SR) or holes in a metal substrate corresponding to the etched nanospheres (SA-SR). The plasmonic properties of such structures were efficiently exploited to create OF probes for label-free bio-sensing and SERS applications. By adopting a similar fabrication approach, [PERSON] et al. realized nanotriangles patterns (CPA-SR) on fiber tip using nanospheres with 450 nm diameter and different film thicknesses as SERS substrates. The probe functionality was verified by measuring the Raman spectrum of a mixed solution of diethylthiatricarbocyanine and crystal violet [55]. [PERSON] et al. exploited the same sensing configuration, featuring a period of 500 nm, as a plasmonic probe by investigating the effect of different gold thicknesses on the surrounding refractive index sensitivity [56]. [PERSON] et al. reported on the SERS performances of a CPA-SR probe using Rhodamine 6G (R6G) as target analyte (Figure 2b). In order to improve the SERS enhancement factor, they proposed a hybrid graphene/Au nanotriangle structure capable of exploiting the SERS chemical effect associated with the charge transfer between graphene and the target molecules [57]. In addition, they systematically investigated the optical coupling between the fiber and the Raman spectrometer (schematized in Figure 2(b-i)) by optimizing the intensity of Raman signal passing through the fiber probe. A similar optimization was carried out in refs [53; 54] for CPA sensing probes. Very recently, [PERSON] et al. demonstrated a plasmonic nanohole-patterned (SA-SR) multimode OF probe for real-time monitoring the specific binding of proteins [58]. In the same year, [PERSON] et al. proposed an alternative method to achieve similar structures by combining colloidal sphere self-assembly with ultraviolet (UV) nanoimprint lithography [59]. The whole fabrication process is shown in Figure 2c. The PS CPA was firstly formed on a glass substrate. Then, a thin layer of UV-curable film was coated on the OF tip. The PS CPA was embedded in the UV-curable resin by pressing it onto the OF end face onto the OF end face. Finally, after dissolving the PS spheres and depositing a thin metal film, a nanocavity-patterned fiber probe was achieved.