Figure 4
From: Octahedral faceted Si nanoparticles as optical traps with enormous yield amplification
Schematic view of the light trapping mechanism inside the Si-NP at the origin of the Raman signal increase as defined in eq. 3.
(top left) In bulk Si, the laser irradiation hits the bulk; Raman photons are emitted according to their cross section, but most of the incident photons are transmitted or undergo Rayleigh scattering; Similarly to a bulk system, in spherical Si-NP, the incident radiation, if not absorbed, exits at the same angle with respect to the radial direction as the incident radiation; therefore, amplification is absent. (top right) In octahedral Si-NPs, the incident radiation hits the Si-NPs with angles of 0° (top facet) or 70.5° (inclined facet), enters the Si-NPs and is efficiently trapped inside. The light travels back and forth inside a very small volume producing, by far, more Raman events, even though the single event cross section remains unaltered. The effective power and volume in the case of Si-NPs are, of course, much smaller than those of bulk Si. (bottom) Simulation of the radiation path inside octahedral and spherical Si-NPs showing a typical enhancement of scattering events in an octahedron as a function of the refractive index.
