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Quantum technologies

Dieter Meschede's research group
Home Optical microfibres Surface passivation

Surface passivation of optical microfibers

The strong evanescent field of tapered optical microfibres (OMF) makes them an excellent tool for light-matter interaction experiments. However, their high sensitivities to surface adsorbed molecules impose a problem: Microfibre transmission can be strongly reduced by surface adsorbates and chemical reaction of dangling bonds with the surrounding medium, especially in a chemically aggressive environment such as hot caesium vapour.

Fig. 1. Isolated silanol group

For silica being amorphous, it has a randomly arranged bond structure, resulting in open, reactive, or dangling bonds throughout the material. At the surface of silica, silanol groups (≡Si-OH) are formed at these bonds in air (see Fig. 1).

The hydroxyl group (-OH) makes the silanol group very reactive. The silanol groups are hydrophilic, leading to the formation of water layers. Organic and inorganic molecules and atoms can bind physically, chemically, or electrostatically to the water molecules or the silanol groups themselves under ambient conditions. With the high intensity of light at the surface, the transmission of an OMF is very sensitive to surface adsorbates and reaction products which absorb or scatter light.

Fig. 2. Silanol group and TMCS  chemical reaction.

To overcome this problem, we have implemented a method for surface passivation of tapered optical microfibres by silylation with methyl chlorosilanes.The principal chemical reaction scheme is shown in Fig. 2. The hydroxyl group reacts with the chlorine from the trimethylchlorosilane (TMCS), forming HCl and a trimethylsiloxane, ≡Si-O-Si(CH3)3, end group strongly chemically bound to the silica surface by the ≡Si-O-Si≡ siloxane group. The methyl groups arrange above the former silanol group and cover a certain surface area.

Although the exact properties of the methyl end groups depend on the rest of the molecule, methyl groups are in general chemically inert up to a few hundred degree Celsius, hydrophobic and have a low surface binding energy. The change from a hydrophilic to a hydrophobic silica surface of a flat fused silica mirror substrate by a treatment with trimethylchlorosilane is shown in Fig. 3. The contact angles of a water drop on the substrate increase significantly.

Fig. 3. Water contact angle before and after treatment on silica substrates with TMCS.

Our method allows us to passivate the surface of an optical microfibre with waist diameters of a few hundred nanometers while maintaining the optical properties of the microfibre (see Fig. 4). This chemical surface passivation should extend the range of environments in which optical microfibres can be used. More information can be found in the Diploma thesis of Jan Hartung.

Fig. 4. Transmission after silylation indicated by red curve