Higher-order optical harmonics entered the realm of nanostructured solids being observed recently in optical gratings and metasurfaces with a subwavelength thickness. Structuring materials at the subwavelength scale allows us toresonantly enhance the efficiency of nonlinear processes and reduce the size of high-harmonic sources. We report the observation of up to a seventh harmonic generated from a single subwavelength resonator made of AlGaAs material. This process is enabled by careful engineering of the resonator geometry for supporting an optical mode associated with a quasi-bound state in the continuum in the mid-infrared spectral range at around λ = 3.7 μm pump wavelength. The resonator volume measures ~0.1 λ3. The resonant modes are excited with an azimuthally polarized tightly focused beam. We evaluate the contributions of perturbative and nonperturbative nonlinearities to the harmonic generation process. Our work proves the possibility to miniaturize solid-state sources of high harmonics to the subwavelength volumes.
Bibliographical noteFunding Information:
Y.K. thanks D. Reis for highly insightful and stimulating discussions of the results. This work was supported by Australian Research Council (DE210100679 and DP210101292), Samsung Research Funding and Incubation Centre of Samsung Electronics (SRFC-MA2001-01), Air Force Office of Scientific Research (FA9550-19-1-0032 and FA9550-21-1-0463), the International Technology Center Indo-Pacific (ITC IPAC) via Army Research Office (FA520921P0034), and the National Research Foundation of Korea (NRF) funded by the Korean government (2021R1A2C3006781).
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