Localization of light induced in ordered colloidal suspensions: powerful sensing tools

We study the light-matter coupling by Raman scattering in colloidal suspensions composed by core-shell TiO₂@Silica (Rutile@Silica) nanoparticles suspended in ethanol and water solutions. Strong enhancement of the Raman signal per particle is observed as [TiO₂@Silica] is increased above a threshold, being stronger in ethanol suspensions. Moreover, above this [TiO₂@Silica] threshold, the optical transmittance of the ethanol suspension starts to be considerably lower than in water, despite scattering strength being higher in water. These results are attributed to localization of light induced by strong correlation in the scatterers' position as a consequence of the long-range Coulomb interaction between the TiO₂@Silica nanoparticles. Light diffraction in TiO₂@Silica suspensions (water and ethanol) shows strong correlation in the scatterers' position (structure seemingly cubic), being stronger in ethanol than in water (longer-range Coulomb interaction). As a result, we demonstrate in these colloidal suspensions for the first time, to our knowledge, strongly enhanced light-matter coupling through correlation-induced localization with kl_T much higher than unity and in an ordered colloidal-photonic structure. This strong enhancement of light-matter coupling by localization of light opens an avenue for manufacturing powerful sensing tools.