In figure 1, nanostructures of complex shaped optimized by an inverse deign approach (evolutionary algorithm coupled to numerical simulations) show tunable optical light scattering properties as function of the light wavelength and polarization.
Figure 1 : Figure 1 : Si nanostructures modeled by evolutionary algorithm coupled to numerical simulations for optimizing scattering efficiency at two wavelengths and two polarizations. Left : pixel design (simulations and scanning electron microscopy images of the structures produced by electron beam lithography). Right : polarization-filtered images in dark field optical microscopy of pictograms fabricated from pixels obtained by inverse design approach. © CEMES-CNRS
The high-refractive index dielectric nanostructures behave as nanoantennas capable to transform a light wave propagating in the space in a wave strongly localized inside and around the nanoantenna. This property allows modifying the light emission of emitters positioned on the nanostructure near field (Purcell effect). In Figure 2, the light emission of a thin film doped with rare-earth ions (Eu3+) deposited on a Si nanostructure is strongly influenced by this nanostructure. Furthermore, the photoluminescence map corresponding to the magnetic dipole emission at 590 nm is very different from the electric dipole emission at 610 nm. This remarkable result is due to the modification of the electric and magnetic density of photon states around the nanostructure.
Figure 2 : (a) raster-scan a Eu3+-doped film by a laser beam for photoluminescence (PL) mapping ; (b) comparison of PL images of MD and ED transitions with the corresponding local density of photon states ; (c) PL spectrum. The ED and MD bands for images are located at about 590 nm (MD) and 610 nm (ED). © 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
Fundings : ANR HiLight (2020-24), EUR NanoX 2DLight (2019-2021)
[1] P. R. Wiecha et al., Nature Nanotech. 12 , 163–169 (2016).
[2] P. R. Wiecha et al., App. Optics 58, 1682-1690 (2019).
[3] P. R. Wiecha et al., Phys. Rev. B 93, 125421 (2016).
[4] P. R. Wiecha et al., Phys. Rev. B 91, 121416 (2015).
