a silicon on insulator (SOI) substrate (Si-Layer: $90\,$nm, BOX layer: $145\,$nm)

Forward/Backward Confocal Darkfield Spectroscopy {#sec:methods_df_spectroscopy}
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#### Forward, “FW”: {#forward-fw .unnumbered}

Transmission confocal optical dark-field (DF) microscopy was performed on an inverted optical microscope. A broadband white lamp was focused through the sample substrate on the individual SiNWs using a DF condenser (Nikon, NA$0.8-0.95$). The FW scattered light was collected using a Nikon NA$0.75$ microscope objective, spatially selected via a confocal hole and guided through a polarization filter ($\parallel$ or $\perp$ to the NW axis for TM and TE geometry, respectively). Finally, the light was dispersed by a $150\,$grooves per mm grating on a highly sensitive CCD (Andor Newton).

#### Backward, “BW”: {#backward-bw .unnumbered}

Reflection confocal optical DF experiments were carried out on a separate spectrometer (Horiba XploRA). A white lamp was focused on individual SiNWs by a $\times 50$ dark-field objective (NA$0.5$, condenser: NA$0.8-0.95$). The backscattered light was filtered by a confocal hole and a polarization filter and dispersed by a $300\,$grooves per mm grating on a highly sensitive CCD (Andor iDus 401).

In both setups (FW and BW), for practical reasons we put the polarization filters in the detection path (detection direction normal to the NW axis). 