Optical resonances (Mie modes) are visible in Figure 1 showing silicon nanowires etched in the silicon layer (100 nm) of a silicon on insulator substrate (SOI) Depending on the width of the nanowires (from 50 to 500 nm), the wavelength of the scattered light is modified.
Figure 1 : Darkfield microscopy image of Si nanowires etched in a SOI substrate. The light polarization is perpendicular to the main axis of the wires. © CEMES-CNRS
High index dielectric nanostructures can also be regarded as nano-antennas capable of transforming a freely propagating EM wave into an optical near-field localized in and around the nano-antenna. This property allows modifying the emission of light by emitters placed in the near field of the nanostructure. In Figure 2, silicon nanocrystals are implanted in a silica substrate 3 nm below a silicon nanowire. The photoluminescence map shows an enhancement of the signal in the presence of the nanowire for an excitation wavelength matching a Mie resonance of the nanowire.
Figure 2 : Left - Schematic representation of a Si nanowire above Si nanocrystals implanted in a silica layer. Center : Photoluminescence map showing the enhancement of the PL by the NW. Right : scattering spectrum of the Si NW revealing a Mie resonance at the excitation wavelength (532 nm). © CEMES-CNRS
The presence of resonances allows users to modify or improve the nonlinear properties such as second harmonic generation (SHG). Thus, no SHG is detected in the absence of resonance while a relatively intense emission is observed if the resonance wavelength coincide with the one of the fundamental and/or SHG (Figure 3).
Figure 3 : Left - experimental set-up for detection of Second Harmonic Generation from Si nanowires. Right : SHG maps for both incident polarizations. © CEMES-CNRS
The different resonances depending on the diameter silicon nanowires also provide access to the various contributions of the SHG. In Figure 4, NW of two different diameters excited in similar conditions emit SH with different origins. In the first case (small diameter), the SHG is due to strong electric field gradients in the volume whereas in the second case, it originates from the contribution of surfaces.
Figure 4 : Rotation of the SHG polarization from silicon nanowires of different diameters excited in TM configuration. © CEMES-CNRS