diameters between around $50\,$nm and $100\,$nm.

Lithographic SiNWs by Electron Beam Lithography  {#sec:methods_ebl_sinws}
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The rectangular NWs were fabricated by a top-down technique via electron beam lithography (EBL) and subsequent anisotropic plasma etching. [@han_realization_2011; @guerfi_high_2013] A transparent SOQ wafer with a $90\,$nm thick Si overlayer served as substrate. A RAITH 150 writer at an energy of 30 keV was used for EBL, employing a thin (60nm) negative-tone resist layer (hydrogen silsesquioxane, “HSQ”). After the EBL step, HSQ was developed by immersion in $25\,$% tetramethylammonium hydroxide (TMAH) for $1\,$min. The patterns in the resist were finally written into the SOQ silicon top layer by reactive ion etching (RIE). For the RIE step, a SF$_6$/C$_4$F$_8$ plasma was used. Etching was stopped when the layer was reached, following in-situ analysis. A residual layer of $\approx 20\,$nm of SiO$_2$ (developed resist) remains on the structures, which has no major impact on the optical properties due to its low index and small thickness. A comparison with a sample where the residual layer was removed using fluorhydric acid showed no significant difference in the spectra, but smaller structures were destroyed due to under-etching.

For SEM imaging, a second sample was prepared with identical process parameters on 