Strip-loaded waveguide on titanium dioxide thin films by nanoimprint replication

Significance Statement

Light-matter interaction enhancement by reduction of waveguide cross-section, in a majority of the nanophotonic wave guide devices, leads to additional losses due to increased sensitivity to the surface and sidewall roughness. Utilization of the well-known strip loaded waveguide concept while maintaining the light confinement in a small volume has been noted to be a viable method of overcoming this obstacle. Decreased propagation losses in strip-loaded waveguides that are confined inside the film under the strip has been seen to be almost not influenced by the quality of the loading strip.

One of the setbacks in integrated optics has been the production of nanophotonic structures in large scale quantities, since they are not fully compatible with fast and low-cost fabrication methods. However, a combination of varying techniques may open novel avenues that lead to standardization of photonic and nanophotonic structures based on a panel of materials.

In a recent paper published in Optics Letters Leila Ahmadi and colleagues at the Institute of Photonics in University of Eastern Finland proposed and demonstrated empirically a waveguide structure based on a combination of two low cost techniques: nanoimprinting and the atomic layer deposition. Their aim was to develop an etchless, facile and repeatable fabrication technique that could realize low-loss and low-cost waveguides based on the strip-loaded waveguide concept and also be compatible to large-area mass production methods such as the roll to roll technique.

First, they opted to consider only the fundamental quasi-TE mode since higher orders of the mode were observed to be less confined under the strip, seemed lossy and coupled naturally to the entire slab, yielding diverging modes that could be considered as background noise. A 200-nm thick titanium dioxide slab waveguide was fabricated on an oxidized silicon wafer by atomic layer deposition. The silicon wafer was then coated with a 1 micrometer AR-P 661 positive tone resist and prebaked at 170° for 5 minutes. Two nickel stamps were then grown by electro-forming and ultra violet imprinting done to transfer the structure into a polymer film. The replicated structure was peeled off from the nickel stamp after being cured for 60 seconds in a UV oven, was washed under running water and dried with nitrogen. Eventually, the sample was diced to the desired dimensions.

The research team was able to realize an excellent replication of the structure into the UV curable polymer. The bias layer was also noted to remain even after the imprinting process and its lowering could be observed between the two-consecutive waveguide after lamination onto the titanium dioxide film. To overcome this obstacle a ring resonator was used whose structure allowed the researchers to determine the various parameters as well as the effective index of the wave guide. Values of excellent agreement with their theoretical predictions were recorded when a resonator of small radius was used since only a few modes were allowed to resonate.

The results prove that light is well guided in the 200-nm titanium oxide layer and therefore the loading strip made by replication is operating as desired. The only drawback of the techniques applied is the high bend loss. The discussed fabrication method leads the way to the integration at a large scale production scale of the nanostructures. The demonstrated wave-guides allow more freedom in terms of fabrication since the critical point is the guiding layer that can be efficiently curbed by the atomic layer deposition process.

Reference

Leila Ahmadi, Ville Kontturi, Janne Laukkanen, Jyrki Saarinen, Seppo Honkanen, Markku Kuittinen, Matthieu Roussey. Strip-loaded waveguide on titanium dioxide thin films by nanoimprint replication. Optics Letters Volume 42, No. 3 (2017) pages 527-530

Institute of Photonics, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland.

Go To Optics Letters

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