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The ongoing drive towards the miniaturization of nonlinear optics is motivated by the number of functionalities it could bring about in integrated devices, such as frequency conversion, information processing, and holography. The main hindrance to the technological deployment of nonlinear optics is the perturbative character of nonlinear interactions, whose intrinsic weakness is compounded at the nanoscale by the small volume of matter involved. The exploitation of the resonant electromagnetic modes supported by nanostructures in order to increase the strength of light–matter coupling is therefore a thriving area of research.
In this work, we investigate gold dimers, where a V-antenna tuned to resonate at a telecom frequency ω (corresponding to a 1550 nm wavelength) is electromagnetically coupled to a rod resonating at 2ω. The structures were patterned lithographically out of a 40 nm-thick monocrystalline gold flake with a focused-ion (Ga) beam. In our experiment, a pump pulse at ω is mixed with its frequency-doubled replica, resulting in SFG at frequency ω+2ω = 3ω. A thorough experimental characterization discloses a rich phenomenology as the SFG is ruled by the resonant response. Such sensitive dependence on the geometry of the system and the excitation is unraveled through a systematic comparison to full-vectorial numerical simulations.
In our dimers, on the one hand the doubly resonant design boosts both pumps of the SFG; on the other hand, the non-centrosymmetric shape allows interference of SFG with third-harmonic generation (THG), which also occurs at ω+ω+ω = 3ω. Delay traces of the signal at 3ω exhibit strong interference fringes (power modulation above 400%) with a 2ω periodicity. The small dephasing between pumps required to achieve such sizable modulation can be imparted by either mechanical or electro-optic means; along with the instantaneous (electronic) character of the materials response, this suggests enticing perspectives for ultrafast modulation and coherent control.
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