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Enhanced nonlinear light generation in oligomers of silicon nanoparticles under vector beam illumination.

All-dielectric nanoparticle oligomers have recently emerged as promising candidates for nonlinear optical applications. Their highly resonant collective modes, however, are difficult to access by linearly polarized beams due to symmetry restraints. In this paper we propose a new way to increase the efficiency of nonlinear processes in all- dielectric oligomers by tightly focused azimuthally polarized cylindrical vector beam illumination. We demonstrate two orders enhancement of the third-harmonic gener- ation signal, governed by a collective optical mode represented by coherently excited out-of-plane magnetic dipoles. Crucially, the collective mode is characterized by strong electromagnetic field localization in the bulk of the nonlinear material. For comparison, we measure third-harmonic generation in the same oligomer pumped with linearly and radially polarized fundamental beams, which both show significantly lower harmonic output. We also provide numerical analysis to describe and characterize the observed effect. Our findings open new route to enhance and modulate the third-harmonic gen- eration efficiency of Mie-resonant isolated nanostructures by tailoring the polarization of the pump beam.