J. Eur. Opt. Society-Rapid Publ. 21, 15( 2025) 161
Figure 1. Principle of an ONF drawn from a standard fiber( grey) and coated with a thin layer of nonlinear material( blue). The component is possibly encapsulated in a low-index material( orange).
of ONF coated with thin layers of polymers or oxide materials has been proposed for their nonlinear optical properties in the literature. The problematic described here is much more challenging, firstly due to the very small aspect ratio of the devices we aim to fabricate( typically a uniform diameter of 1 lm over a length of a few cm). Secondly several constraints must be considered regarding the guidance of the optical mode. As highly nonlinear materials have generally a high refractive index, thin layers of a few tens of nm to a few hundreds of nm should be deposited to maintain a propagating light mode. However, if the layers are too thin, the overlap of the evanescent field with the nonlinear material will to be too low to achieve a sufficientnonlineargain. In addition, there are technological constraints since the reachable thickness of the layers is also determined by the coating process, itself depending on the nonlinear material. In this work, two materials well-known for their nonlinearities have been studied: Titanium dioxide( TiO 2) and Polymethyl metacrylate( PMMA). The theoretical design of the coated ONF for the realization of SRS in these nonlinear materials has been discussed elsewhere [ 7 ]. We have shown that the thickness of TiO 2 should be of a few tens of nanometers and the thickness of PMMA should be of a few hundreds of nanometers to get SRS in the sub-nanosecond regime with ONF diameter of typically 1 lm. Modal Raman gains of 0.2 m � ¹ W � ¹ are expected, which would enable the realization of Raman converters with centimeterlength ONF. These diameters and thicknesses will be our targets in the following. One of the challenges is to adapt already existing processes for the deposition of thin layers of controlled thickness on an ONF, keeping an optimized optical transmission. This paper is organized as follows.
After a general presentation of the component, we will present the coating of ONF by thin layers of TiO 2 by Atomic Layer Deposition. In a second part, the deposition of thin layers of PMMA will be discussed. We have developed an original method based on multiple layer deposition by a modified dip-coating technique inspired by Velázquez- Benítez et al. [ 8 ]. The encapsulation by silicone of a tapered fiber coated with PMMA has also been realized successfully. The results are discussed, and perspectives of this work are given.
2 General design of the component
The devices we aim to fabricate are depicted in Figure 1. The ONF central part and possibly its tapers are coated by a thin layer of nonlinear material. The coated ONF can be encapsulated in a surrounding medium for mechanical protection or let in air.
The ONF pulling rig is described in [ 9 ]. A small butane flame softens the central part of a standard telecommunication fiber. Two translation stages controlled by computer stretch it following the classical“ pull and brush” technique, creating the ONF and the tapers. This fabrication process is performed in a class-5 clean room to avoid the deposition of dust on the ONF. Our pulling system enables us to fabricate ONF with“ on-demand” radii and lengths that can be respectively as small as 200 nm and as long as 8 cm( limited by the travel of the translation stages). Optical transmissions of the ONF with its tapers are currently around 70 – 90 %.
After its fabrication, the component is fixed from its two untapered parts using a double-side tape piece on a glass holder. The holder is put in a clean box to be protected during the transit to a clean room used for the coatings.
3 TiO 2 coating
The first selected material is TiO 2. This is a semiconductor material with a wide band gap, and a relatively high refractive index( around 2.45 at a wavelength of 1.5 lm but depending on the crystallinity and the crystalline phase). This material has been largely studied for nonlinear applications [ 10, 11 ]. In the following, the target thickness is 50 nm.
A technique that is particularly well suited to metal oxide depositions is Atomic Layer Deposition [ 12 ]. This technique is used in a primary vacuum chamber. A thin layer is progressively built on all the surfaces of the chamber and of the sample exposed to successive metal precursor vapor and oxidizing gas. Its thickness can be very precisely controlled. ALD is particularly adapted to the case of ONF since the deposition is non-directional and a homogeneous