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The manipulation of light-frequency upconversion on the nanoscale at the optical regime can benefit a wide variety of existing applications,[1] enhancing (bio)imaging resolution,[2] increasing optical sensing sensitivity,[3,4] improving solar light harvesting,[5,6] and bettering control of optically triggered intracellular drug delivery mechanisms.[7,8] Benefiting from large intrinsic nonlinearities, low absorption, and high field enhancement abilities, all-dielectric nanoantennas are considered essential for efficient nonlinear processes at subwavelength volumes.[9-11] We present an all-dielectric germanium nanosystem exhibiting a strong third order nonlinear response and efficient third harmonic generation in the optical regime. A thin germanium nanodisk shows a pronounced valley in its scattering cross section at the dark anapole mode, while the electric field energy inside the disk is maximized due to high confinement within the dielectric. We investigate the dependence of the third harmonic signal on disk size and pump wavelength to reveal the nature of the anapole mode. Each germanium nanodisk generates a high effective third order susceptibility of χ(3) = 4.3 × 10−9 esu, corresponding to an associated third harmonic conversion efficiency of 0.0001% at an excitation wavelength of 1650 nm, which is 4 orders of magnitude greater than the case of an unstructured germanium reference film.[12]
Furthermore, we demonstrate a higher-order anapole mode in a 200 nm thick germanium nanodisk that delivers the highest THG efficiency on the nanoscale at optical frequencies. We observe a highly improved electric field confinement effect within the dielectric disk at this higher-order mode, leading to THG conversion efficiencies as large as 0.001% at a third harmonic wavelength of 550 nm.[13] In addition, by mapping the THG emission across the nanodisk, we are able to unveil the anapole near-field intensity distributions, which show excellent agreement with numerical simulations. A similar nonlinear optical response is observed in the case of degenerate FWM where two different pump wavelengths are coupled to a single high-order resonant mode. However, when the two pump wavelengths are coupled to different high-order modes, the FWM process is partially suppressed due to a diminished near-field spatial overlap of the mixed wavelengths within the disk.[14] These findings not only open new possibilities for the optimization of upconversion processes through the appropriate engineering of suitable dielectric materials, but also remarkably expand contemporary knowledge on localized modes in dielectric nanosystems.
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