Fig1_Dennis Lehr
Raster-electron-micrographs of fabricated samples: gold nanorings filled with lithium niobate.

Metallic nanorings are key structures for photonic nanomaterials. Their high degree of symmetry and their intriguing plasmonic properties have led to the development of many applications, such as spectroscopic filters, surface enhanced sensing and nanoantennas. At resonance, nanorings provide the unique advantage of a pronounced plasmonic field enhancement inside their core (Fig. 1) that should significantly enhance light-matter interactions, if filled with a polarizable medium.

Fig2_Dennis Lehr
Normalized Ex-component of the electric field at resonant illumination of gold nanorings filled with lithium niobate.

We validate this concept by filling gold nanorings with lithium niobate (LN) to significantly enhance second harmonic generation (SHG) in sub-wavelength volumes.
The implemented fabrication process allows efficient fabrication of the nanostructure with a throughput of dm² per hours. The main process steps comprise the creation of crystalline LN pillars by character projection electron beam lithography, i.e. imaging of a shaped aperture in the plane of the resist, and ion beam enhanced etching. Later, double patterning is utilized to form the gold nanorings around the LN pillars.

Fig3_Dennis Lehr
Measured SHG enhancement (SH1) and linear reflection spectrum (FH1). The inset depicts the chosen measurement geometry.

The fabricated nanorings with an inner diameter of 80 nm, an outer diameter of 120 nm, and a height of 100 nm are arranged periodically on a square lattice with a period of 260 nm on top of a crystalline LN substrate (Fig. 2). Such small dimensions allow excitation of the nanostructure at NIR wavelength and second harmonic generation at visible wavelength.

As predicted by simulations we obtained an enhancement of the second harmonic signal by a factor of 20 at oblique incidence (Fig. 3). In future experiments we will achieve a factor of 50 to 60 at normal incidence.

[1]   D. Lehr, J. Reinhold, I. Thiele, H. Hartung, K. Dietrich, C.Menzel, T. Pertsch,
       E.-B. Kley, A. Tünnermann, Nano Lett. 15 (2), 1025-1030 (2015).

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