Optothermally controllable multiple high-order harmonics generation by Ge2Sb2Te5-mediated Fano clusters

Substantial enhancement of nonlinear high-order harmonics generation based on Fano-resonant nanostructures has received growing interest due to their promising potential for developing integrated and advanced next-generation nanophotonic devices. In this study, going beyond conventional subwavelength structures, we propose an optothermally functional hetero-metallodielectric asymmetric eight-member octamer cluster composed of a central silicon nanodisk and peripheral disks with a phase-change material (Ge2Sb2Te5). Using full electromagnetic calculations, we show that in the amorphous phase of the surrounding nanoparticles, the oligomer acts as an all-dielectric cluster, while in the crystalline regime, the octamer turns into a hybrid metallodielectric assembly. Exploiting the exquisite ability of supporting distinct Fano lineshapes at different wavelengths depending on the phase of Ge2Sb2Te5, we showed the generation of both second and third harmonics at amorphous and crystalline phases of GST nanodisks, respectively with the produced harmonic wavelengths of 425 nm and 317 nm, respectively. Our calculations for the corresponding conversion efficiencies revealed significant enhancements as ηSHG = 0.0081% and ηTHG = 0.012% for SHG and THG, respectively. Such an exquisite feature of multiresonant optothermally tunable cluster allows generation of several harmonics with substantial intensities using a single system for future photonics applications.