Figure 3
(a) Schematic representation of a BTO nanocrystal modelled as a 2D disk of tetragonal symmetry (in red) embedded in a cubic shell (in blue). The tetragonal crystal is of 4 mm symmetry class and its main axis orientation is denoted by the polar angles (θ, φ, ψ). The cubic crystal is centro-symmetric (m3m symmetry class) and has no SHG response. The schematic representation of the corresponding unit cell atoms are colored in red for barium, green for titanium, and orange for oxygen. The interface (black rim) is of 1 pixel thickness (1 px = 40 nm) and modelled as nonlinear 1D dipoles oriented along the surface normal (n). (b) Representation of three diameter sizes: 100 nm (2.5 px), 300 nm (7.5 px) and 500 nm (15 px), with an identical core-to-size ratio r = 0.4 (r = c/s with c the core diameter and s the total diameter size of the nanocrystal). (c) Schematic representation of the 2D Gaussian optical focus (250 nm size) scanned to simulate diffraction-limited P-SHG imaging. (d) Resulting theoretical P-SHG images for a 100 nm size nanocrystal, at different θ angles, without the shell (left) and in presence of the shell (right, using r = 0.4). Scale bars: 1000 nm. The surface-to-core nonlinear efficiency ratio is η = 0.5.
