J. Eur. Opt. Society-Rapid Publ. 21, 15( 2025) 163
Figure 3. Left( right): Thickness of the deposited layer of PMMA versus the diameter of the fiber for a deposition speed of 2.5 10 �3 m / s( 8 10 �3 m / s). Square: experimental data, line: model prediction. SEM Image in insert: Rayleigh-Plateau instability along the longitudinal axis of a ~ 5 lm diameter fiber.
A consequence of( 1) is that the deposited thickness is expected to be larger at the taper location than at the waist of the fiber. To verify this law experimentally, we used the fact that the two tapered sections, whose diameter varies from 125 lm to that of the ONF, are also coated during the deposition process. By cleaving the fiber at different locations in the tapers it is thus possible to measure the thickness of the deposited layer for several different diameters using a single sample. For each cut, we measured the local diameter of the fiberandthicknessofthedeposited layer using SEM imaging. We also tested several deposition speeds. Figure 3 shows the measured PMMA layer thickness as a function of the fiber diameter, for two pulling speeds, 2.5 10 �3 m / s( see Fig. 3 left) and 8 10 �3 m / s( see Fig. 3 right). The expected PMMA thickness, as predicted by( 1) is also shown on the graphs. Note that no parameter is adjusted to obtain the fit lines. It can be observed in both cases that the deposition thickness calculation model is representative of the thickness actually deposited on the fibers. However, it also seems that the difference between the calculation and the experimental results is greater when the deposition speed is higher( 8 10 �3 m / s). It is also observed that, for a given deposition speed, the discrepancy between experiment and calculation increases with larger fiber diameters( and consequently greater deposited polymer thicknesses). This behavior is attributed to a modulation of the deposition thickness longitudinally to the fiber axis, as shown in the SEM image in Figure 3, right. This phenomenon is not considered in the thickness calculation and is due to an instability of the liquid film during deposition known as the Rayleigh-Plateau instability [ 17 ]. As the liquid film is deformable, the system tends to minimize the surface area of the interface by forming droplets before the evaporation of the solvent. A complete study of this effect would be complex and is beyond the scope of this work, in which we focused on the solutions to minimize it. We observed that this modulation is not visible at low deposited thicknesses, typically a few tens of nm, corresponding to ONF diameters around 1 lm. To increase this thickness to reach a few hundreds of nm for such small diameters as targeted we have carried out successive depositions of thin layers.
The process developed for a single layer deposition was used for a multiple deposition of PMMA on an ONF at a speed of 2.5 10 �3 m / s. Six successive depositions were made with a vacuum drying step between each deposition. At the end of the process, the fiber was prepared for SEM observation. The SEM images showed no Plateau-Rayleigh instability, and the deposited thickness was clearly very large( see Fig. 4, right). A study of the dependency between fiber diameter and polymer thickness shows a linear relationship( see Fig. 4, left), highlighting that the deposition process performed on a previously coated fiber does not induce the solubilization of the film. The successive coating process allows large thicknesses to be reached. For example, a thickness of 100 nm can be obtained on an ONF having a diameter of 1.5 lm( vs. 10nmwithasingledeposition pass). We can also notice that the final thickness tends to be slightly greater than predicted by the calculation, even considering each increase of fiber diameter after the previous coating. The origin of this difference will need further experiments to be understood and may require improved experimental precision. Current hypotheses include the presence of Plateau-Rayleigh instability – although not seen on the SEM images – or a possible influence of the modification of the surface chemistry of the fiber after the first coating. At first glance, the deposited layers appear homogeneous, but further studies will be needed for precise measurement.
We have also applied this technique successfully to the deposition of thin layers of PMMA doped with a nonlinear dye( Disperse Red 1). This shows that we can extend further the possibilities offered by these coatings as for example proposed in [ 18 ] to excite second-order nonlinearities. Other polymers can also be investigated for developing novel applications [ 19 ].
In addition, we have studied the feasibility of encapsulating an ONF coated with PMMA in a low index medium. The challenge is to achieve encapsulation while preserving the PMMA layer and maintaining good optical transmission. We have chosen silicone RTV elastomer as the encapsulation material since silicone resin has the advantage of being bulk polymerized for millimeter scale deposition thicknesses and is not a solvent of PMMA. We measured