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 generation signal, governed by a collective optical mode represented by 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 a new route to enhance and modulate the third-harmonic generation efficiency of Mie-resonant isolated nanostructures by tailoring the polarization of the pump beam.

to their plasmonic counterparts, alldielectric nanoparticles do not suffer from nonradiative losses, and they allow novel functionalities. Owing to a strong localfield confinement in the volume of such nanostructures, as well as high stability under strong laser radiation, Mie-resonant nanoparticles are very attractive for nonlinear nanoscale photonics. [10][11][12][13] As an example, enhanced nonlinear optical harmonic generation has been predicted and demonstrated for a number of alldielectric systems, ranging from single resonant nanoparticles [14][15][16][17][18] and nanoparticle arrays [19][20][21] to more special structures supporting subradiant anapole modes [22][23][24][25] and bound states in the continuum. [26] When resonant nanoparticles are placed close to each other to form subwavelength clusters (also known as oligomers), novel effects such as the mode hybridization [27] and formation of electric and magnetic hot spots [28] emerge.

Single photon sources based on semiconductor quantum dots are one of the most prospective elements for optical quantum computing and cryptography. Such systems are often based on Bragg resonators, which provide several ways to control the emission of quantum dots. However, the fabrication of periodic structures with many thin layers is difficult. On the other hand, the coupling of single-photon sources with resonant nanoclusters made of high-index dielectric materials is known as a promising way for emission control. Our experiments and calculations show that the excitation of magnetic Mie-type resonance by linearly polarized light in a GaAs nanopillar oligomer with embedded InAs quantum dots leads to quantum emitters absorption efficiency enhancement. Moreover, the nanoresonator at the wavelength of magnetic dipole resonance also acts as a nanoantenna for a generated signal, allowing control over its radiation spatial profile. We experimentally demonstrated an order of magnitude emission enhancement and numerically reached forty times gain in comparison with unstructured film. These findings highlight the potential of quantum dots coupling with Mie-resonant oligomers collective modes for nanoscale single-photon sources development.

We demonstrated two orders of magnitude enhancement of the third-harmonic intensity for isolated nanoclusters of silicon nanoparticles illuminated by normally incident azimuthally polarized cylindrical vector beams at the wavelength of oligomer’s out-of-plane magnetic mode.

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