Issue
Eur. Phys. J. Appl. Phys.
Volume 94, Number 1, April 2021
Article Number 10403
Number of page(s) 5
Section Nanomaterials and Nanotechnologies
DOI https://doi.org/10.1051/epjap/2021200298
Published online 06 May 2021

© EDP Sciences, 2021

1 Introduction

Low refractive index polymers are used as materials for planar optical waveguides [18] and optical fibers [9]. The main requirements to polymers for planar optical waveguides used in optical microchips are low refractive index (1.5 or below for core and 1.45 for cladding at 1550 nm to match the refractive index of fused silica), good processing properties, low film birefringence on Si wafer, low optical losses, and ability to tune thermo-optic coefficient (TOC) depending on application purposes [7,10]. Low TOC is a requirement for passive optical waveguides used as a connection in optical microchips. Low refractive index and1 low optical losses at telecommunication wavelength (ca. 1550 nm) are normally obtained by substitution of hydrogen atoms by halogen (fluorine for lower refractive index and chlorine and bromine for higher refractive index) and by avoiding where possible O–H and N–H bonds, which have a strong absorption around 1550 nm. The required refractive index contrast between core and cladding polymers is attained by tuning of the polymer composition. Organic or hybrid polymers [11] are usually used for this purpose. However, a combination of inorganic and organic materials in the form of nanocomposites has received little attention so far for optical waveguide materials.

From the one hand, inorganic materials possess considerately lower thermal expansion coefficients compared to organic materials and consequently lower TOCs. Some of them (e.g. MgF2, CaF2, AlF3, SiO2) exhibit also low refractive indices. Therefore, these inorganic materials are promising for the fabrication of optical waveguiding composites with lower refractive index and lower TOCs. The refractive index of MgF2 is particularly low (1.37 at 1550 nm), therefore, this inorganic material has high potential for lowering the refractive index of polymer and, like other inorganic materials, a decrease of TOC. MgF2 nanoparticles (NPs) were introduced into polymer matrices before [12,13] and a decrease of the refractive index of nanocomposites was demonstrated [12]. From the other hand, organic polymers show good film building properties and the synergy of both materials combined into nanocomposites can be used for the fabrication of improved materials (Scheme 1). This approach could be a suitable method for tuning optical properties of optical waveguide materials. The change could be realized in the direction of reducing refractive index and TOC.

The impact of inorganic NPs on optical properties of polymers is not studied in detail so far. For example, fabrication of optical waveguides using polymer matrix filled with Au NPs has been reported recently [14], but no information concerning optical losses was presented in this work. While influence of NPs on refractive index and TOC (linked to thermal expansion coefficient) is theoretically clear [15], the influence of NPs distributed in polymer matrix on optical propagation losses is not well investigated. Inorganic NPs might have conflicting influence on optical losses of resulting waveguide, namely, while inorganic core of NP should decrease the optical losses (MgF2 does not absorb at 1550 nm), but the organic shell of NPs required for the compatibility with polymer matrix might increase optical losses depending on the chemical composition. In addition, possible scattering arising when NPs are not sufficiently small (larger than 1/20 of wavelength, namely ca. 80 nm) will increase optical losses. However, the impact of particular NPs on optical losses can be studied only experimentally while it depends on a number of factors, like size of NPs, type of the organic shell, and the NPs tendency for agglomeration. We have already tested this approach using SiO2 NPs [16]. SiO2 NPs possess higher refractive indices (1.44 at 1550 nm) compared to MgF2 but the advantage of commercial availability. In this recent work [16] we exploited nanocomposites containing mostly commercially available SiO2 NPs incorporated into copolymers with oxirane groups, which were then cross-linked photochemically on the substrate (incorporation of NPs into solid polymer matrix). We have shown that this approach could be successful from the point of view of lowering refractive index, lowering TOC, and sustaining relatively low optical propagation losses. However, we have a problem with the layer quality (cracks formation in thicker layers). In addition, SiO2 NPs allows smaller range of refractive index tuning. Therefore, in this work in house made MgF2 NPs were exploited in two ways, namely, in already prepared epoxy-containing copolymers (introduction into solid matrix) and in acrylate formulations (introduction into liquid mixture followed by photochemical cross-linking on the substrate, Scheme 1).

The copolymers of pentafluorostyrene and glycidyl methacrylate [17] (Scheme 2), and liquid acrylate mixtures (Scheme 3) were used as waveguiding materials with low optical losses. An introduction of other monomers in the mixture for polymerization (fluorinated or chlorinated) can be used to tune the refractive index of the final polymer according to general Scheme 2. Final cross-linking was achieved in already prepared layers by UV-curing with help of photoacid generator.

The objective of this work was to introduce MgF2 NPs having lowest refractive index into polymer waveguiding materials and investigate their influence on optical properties (refractive index, TOC, and optical propagation losses).

thumbnail Scheme 1

Synthesis and modification of MgF2 NPs and nanocomposite fabrication.

thumbnail Scheme 2

Polymerization and photochemical cross-linking of optical waveguiding copolymers.

thumbnail Scheme 3

Examples of fluorinated acrylate and diacrylate and schematics of viscous polyether acrylates used for the formulation of liquid monomer waveguiding materials.

2 Experimental

2.1 Materials and film preparation

Fluorinated monomers (ABCR), glycidyl methacrylate (Sigma-Aldrich), dioxane (Carl Roth), cyclopentanone (ABCR), 2-ethoxyethyl acetate (Fluka) were used as received. Benzoyl peroxide (Merck) was dried over CaCl2 in vacuum desiccator. Highly viscous acrylate oligomers were obtained as free samples from Sartomer by Arkema Group (France) and MIWON Specialty Co. Ltd. (South Korea). Fluorinated acrylates for monomer mixtures (typical examples see Scheme 3) were purchased from ABCR (Germany) and Fluorochem (UK).

For copolymer synthesis according to Scheme 2, glycidyl methacrylate, pentafluorostyrene, trifluoroethyl methacrylate (or other fluorinated or chlorinated acrylates, ca. 10 g) in dioxane (50 ml) with dibenzoyl peroxide (0.2 g) were purged with nitrogen for 15 min under stirring, then inserted in oil bath to achieve 80 °C inside, and maintained at this temperature for 6 h. After cooling, the solution was poured into water, the polymer separated, washed with ethanol, and dried at 50 °C in vacuum. Then, the polymer was dissolved in a small amount of hot ethyl acetate and after cooling precipitated with ethanol and dried at 50 °C in vacuum. Proprietary acrylate monomer mixtures were formulated by mixing fluorinated acrylates and diacrylates (Scheme 3) with viscous acrylate polyether oligomers in different combinations to achieve target refractive index and viscosity.

Powders of MgF2 nanoparticles, functionalized with trifluoracetic acid, were obtained from Nanofluor GmbH. Further information can be found in [12]. The NPs were aged for fortnight and had a size of ca. 12 nm (hydrodynamic diameter according to dynamic light scattering [18]). The NPs were spray dried and kept as a powder for the application. Mg (Chempur, 99.998%), dried methanol, and trifluoroacetic acid (Aldrich, 99%) were used for NPs synthesis. MeOH was dried using standard procedure and was kept over molecular sieves. MgF2 NPs modified with trifluoroacetic acid were dispersed in 2-ethoxyethyl acetate and then the polymer was dissolved in this solution by gentle heating and/or sonication. The same NPs were introduced with an addition of small quantity of 2-ethoxyethyl acetate into liquid monomer mixture by sonication. The viscosity suitable for spin-coating was achieved by addition of solvent if required. Thin layers of polymers and nanocomposites were prepared by spin-coating using a P6700 spin-coater (specialties coating system) or by casting onto Si wafer or fused silica substrate. The films were exposed to UV light using an UV-H 200 mercury lamp (Panacol-Elosol) and then post-baked at 110–130 °C for cross-linking.

2.2 Measurements

Optical properties of polymer films (refractive index and thickness, TOC, optical propagation losses) were measured using m-line spectroscopy performed on a Metricon MODEL 2010/M Prism Coupler System (USA) with polymer films on Si for refractive index and TOC measurements and SiO2 substrate for optical propagation loss measurement. Optical losses of the planar waveguides were measured by a technique involving measurement of transmitted and scattered light intensity as a function of propagation distance along the waveguide [19]. This technique has been also used recently for the characterization of polymer optical waveguides [16,2022].

3 Results and discussion

The NPs made of MgF2 were prepared by fluorolytic sol-gel method [12] and then modified with trifluoroacetic acid for stabilization against agglomeration. The size of NPs (ca. 12 nm in diameter [18]) theoretically allowed obtaining nanocomposites without scattering if agglomeration can be avoided. The NPs were crystalline with a crystallite size of ca. 5 nm [18], so one can expect a refractive index of crystalline MgF2 (1.37 by 1550 nm) not considering quantum size effects. An introduction of MgF2 NPs into polymer matrix was performed in two ways, namely, into solution of polymer synthesized according to Scheme 2 and using liquid acrylate formulations (Scheme 3) adding NPs with a help of a solvent. Then, polymer or monomer mixture were cross-linked in the layer on a substrate by exposure to UV-light using photoacid generator or radical initiator, for epoxy-containing polymer (Scheme 2) and acrylate mixture (Scheme 3), respectively.

The copolymers containing glycidyl methacrylate were used as a polymer matrix for following photochemical cross-linking (Scheme 2). These copolymers have relatively low refractive index and relatively low TOC of −(1.0 − 1.7) × 10−4K−1, depending on the composition. Trifluoroacetic acid modified MgF2 NPs were introduced into polymer solution, which affords relatively good quality films of few micrometers thickness. However, polymer-NPs formulations were not stable and were prone to gelation with a time. Unfortunately, some visible scattering appeared after exposure of nanocomposite films to UV-light to affect cross-linking (agglomeration of NPs due to polymer network building). Despite some scattering, it was possible to measure refractive index and TOC (but not optical propagation losses) using m-line spectroscopy (Fig. 1, left). It is clear from an example in Figure 1, left that introduction of MgF2 NPs decreases both refractive index and TOC, which was a goal of this investigation.

Better results were obtained when MgF2 NPs were introduced into liquid acrylate formulations. Such proprietary liquid formulations are composed of different acrylate monomer/oligomers (Scheme 3); the resulting nanocomposite-formulations are cross-linked photochemically using radical photoinitiator on the substrates. It is clear from the example in Figure 1, right that also in the case of nanocomposites based on acrylate mixtures, an introduction of MgF2 NPs leads to the decrease in both refractive index and TOC. However, in this case, it was also possible to produce thick (10–20 μm) transparent films of nanocomposites without visible scattering (Fig. 2). Neither cracks nor other visible defects were observed even in thicker layers, which seems to be an advantage over recent results with SiO2 NPs [16].

While relatively thick transparent films were obtained in this case (Fig. 2) optical propagation losses were also measured. The level of optical losses is rather important for practical application in planar waveguides. Unfortunately, despite transparency, optical propagation losses increase significantly as it is shown in Figure 3 for two different acrylate mixtures. While NPs are rather small (ca. 12 nm [18]) the scattering of light by NPs as a reason for an increase in optical losses is hardly plausible. Therefore, an absorption of trifluoroacetic acid at working wavelength (1547 nm) used for modification of MgF2 NPs could contribute into increase of optical propagation losses. The modification of MgF2 NPs is successful by using acids (e.g. trifluoroacetic or phosphonic acid [12,18]). Therefore, other modifiers not absorbing around 1550 nm have to be found in a future work to avoid an increase of optical propagation losses.

It means that despite achieved reduction of refractive index and TOC high optical losses makes it impracticable to use MgF2 nanocomposites in present form for the fabrication of planar optical waveguides. Further chemical modifications studies are required.

thumbnail Fig. 1

Temperature dependencies of refractive index of polymer and nanocomposite films (left) and acrylate formulation and nanocomposite films (right) measured by m-line spectroscopy (TOC are the slopes of the lines).

thumbnail Fig. 2

Image through the samples on fused silica substrate for optical loss measurement of acrylate-MgF2 nanocomposite (left) and for the comparison of original acrylate core mixture with n = 1.48 at 1547 nm.

thumbnail Fig. 3

Comparison of optical propagation losses at 1550 nm between polymer material and polymer composites with MgF2 NP (solid lines are exponential fits).

4 Conclusions

Successful introduction of MgF2 NPs into optical polymer layers without visible scattering or defects has been demonstrated. The transparent nanocomposites were prepared by direct introduction of modified MgF2 NPs into polymer matrix or into acrylate monomer mixtures. This results in decrease of thermo-optic coefficient and refractive index of the material but also leads to a significant increase in the optical propagation losses. This increase of planar waveguide optical losses is probably due to the absorption of organic shell of the NPs.

Author contribution statement

All the authors have participated in the preparation of the manuscript. They have read and approved the final version.

Acknowledgments

The work was financially supported by German Federal Ministry of Education and Research (Project PolyPhotonics, Support Code 03VKCT1C).

References

  1. R. Yoshimura, M. Hikoto, S. Tomaru, S. Imamura, J. Light. Technol. 16, 1030 (1998) [Google Scholar]
  2. T. Matsuura, S. Ando, S. Sasaki, F. Yamamoto, Macromol. 27, 6665 (1994) [Google Scholar]
  3. H.J. Lee, M.H. Lee, M.C. Oh, J.H. Ahn, S.G. Han, J. Polym. Sci., Part A: Polym. Chem. 37, 2355 (1999) [Google Scholar]
  4. J.W. Kang, J.P. Kim, W.Y. Lee, J.S. Kim, J.S. Lee, J.J. Kim, J. Light. Technol. 19, 872 (2001) [Google Scholar]
  5. G. Fischbeck, R. Moosburger, C. Kosttzewa, A. Aeben, K. Petermann, Electron. Lett. 33, 518 (1997) [Google Scholar]
  6. M-C. Oh, K-J. Kim, W-S. Chu, J-W. Kim, J-K. Seo, Y-O. Noh, H-J. Lee, Polym. 3, 975 (2011) [Google Scholar]
  7. N. Keil, H.H. Yao, C. Zawadzki, F. Beyer, O. Radmer, M. Bauer, C. Dreyer, S-G. Han, H-J. Lee, Proceedings of International Cooperative of Plastic Optical Fibres −ICPOF:13th International Plastic Optical Fibres Conference 2004. Nürnberg, Germany, 27–30 September 2004 [Google Scholar]
  8. C. Dreyer, J. Schneider, K. Göcks, B. Beuster, M. Bauer, N. Keil, H. Yao, C. Zawadzki, Macr. Symp. 199, 307 (2003) [Google Scholar]
  9. O. Ziemann, J. Krauser, P.E. Zamzow, W. Daum, POF Handbook, in Optical Short Range Transmission Systems (Springer-Verlag, Berlin Heidelberg, 2008), pp. 901 [Google Scholar]
  10. C. Zawadzki, M. Schirmer, Photonik 1, 43 (2018) [Google Scholar]
  11. T. Ishigure, S. Yoshida, K. Yasuhara, D. Suganuma, IEEE Electronic Components & Technology Conference, 768 (2015) [Google Scholar]
  12. J. Noack, C. Fritz, C. Flügel, F. Hemmann, H.-J. Gläsel, O. Kahle, C. Dreyer, M. Bauer, E. Kemnitz, Dalton Trans. 42, 5706 (2013) [Google Scholar]
  13. J. Noack, L. Schmidt, Gläsel, M. Bauer, E. Kemnitz, Nanoscale 3, 4774 (2011) [Google Scholar]
  14. M. Signoretto, I. Suárez, V.S. Chirvony, R. Abargues, P.J. Rodríguez-Cantó, J. Martínez-Pastor, Nanotechnology 26, 475201 (2015) [Google Scholar]
  15. H. Zou, S. Wu, J. Shen, Chem. Rev. 108, 3893 (2008) [Google Scholar]
  16. L.M. Goldenberg, M. Köhler, O. Kahle, C. Dreyer, Opt. Mater. Express 10, 2987 (2020) [Google Scholar]
  17. C. Pitois, S. Vukmirovich, A. Hult, D. Wiesmann, M. Robertsonn, Macromol. 32, 2903 (1999) [Google Scholar]
  18. J. Noack, F. Emmerling, H. Kirmse, E. Kemnitz, J. Mater. Chem. 21, 15015 (2011) [Google Scholar]
  19. N. Nourschargh, E.M. Starr, N.I. Fox, S.G. Jones, Electron.Lett. 21, 818 (1985) [Google Scholar]
  20. V. Prajzler, P. Nekvindova, P. Hyps, V. Jerabek, Opt. Commun. 24, 442 (2015) [Google Scholar]
  21. V. Prajzler, P. Hyps, R. Mastera, P. Nekvindova, Opt. Commun. 25, 230 (2016) [Google Scholar]
  22. V. Prajzler, M. Neruda, P. Nekvindova, J. Mater. Sci.-Mater. Electron. 29, 5878 (2018) [Google Scholar]

Cite this article as: Leonid M. Goldenberg, Mathias Köhler, Christian Dreyer, Tohralf Krahl, Erhard Kemnitz, Optical nanocomposites containing low refractive index MgF2 nanoparticles, Eur. Phys. J. Appl. Phys. 94, 10403 (2021)

All Figures

thumbnail Scheme 1

Synthesis and modification of MgF2 NPs and nanocomposite fabrication.

In the text
thumbnail Scheme 2

Polymerization and photochemical cross-linking of optical waveguiding copolymers.

In the text
thumbnail Scheme 3

Examples of fluorinated acrylate and diacrylate and schematics of viscous polyether acrylates used for the formulation of liquid monomer waveguiding materials.

In the text
thumbnail Fig. 1

Temperature dependencies of refractive index of polymer and nanocomposite films (left) and acrylate formulation and nanocomposite films (right) measured by m-line spectroscopy (TOC are the slopes of the lines).

In the text
thumbnail Fig. 2

Image through the samples on fused silica substrate for optical loss measurement of acrylate-MgF2 nanocomposite (left) and for the comparison of original acrylate core mixture with n = 1.48 at 1547 nm.

In the text
thumbnail Fig. 3

Comparison of optical propagation losses at 1550 nm between polymer material and polymer composites with MgF2 NP (solid lines are exponential fits).

In the text

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