Augmenting Second Harmonic Generation Using Fano Resonances in Plasmonic Systems

Abstract

Significant augmentation of second harmonic generation using Fano resonances in plasmonic heptamers made of silver is theoretically and experimentally demonstrated. The geometry is engineered to simultaneously produce a Fano resonance at the fundamental wavelength, resulting in a strong localization of the fundamental field close to the system, and a higher order scattering peak at the second harmonic wavelength. These results illustrate the versatility of Fano resonant structures to engineer specific optical responses both in the linear and nonlinear regimes thus paving the way for future investigations on the role of dark modes in nonlinear and quantum optics. 1 One such property is the generation of hot spots, whose presence in nanostructures, such as optical antennae, is promising for applications in nonlinear optics, since they require a strong electric near-field.2 Recent studies have reported the observation of different nonlinear optical phenomena in such nanosystems, like, for example, second harmonic generation (SHG), 3 multiphoton luminescence, 4,5 third harmonic generation, 6,7 higher harmonic generation, 8 and four-wave mixing. 9 SHG is significantly dependent on the symmetry of both the material being used and the structure being studied and therefore is a sensitive tool for characterizing plasmonic structures.10−13 SHG from centrosymmetric materials is due to the breaking of inversion symmetry at their surfaces and is therefore used as a surface probe.14 In recent years, strong SHG has been observed in metallic nanostructures such as sharp metallic tips, 15 multiple resonant nanostructures, 16,17 split-ring resonators, 18 metamaterials 19,20 and nanocups, 21 underlining the fast growing interest in the area of nonlinear plasmonics. 22,23 Efficient SHG requires the presence of strong SHG sources, that is, nonlinear polarization currents oscillating at the second harmonic frequency, at the nanostructure surface as well as an efficient scattering of the SHG signal into the far-field. Several strategies have been developed to enhance SHG from metallic nanostructures including using multiple resonances at both the fundamental and the second harmonic wavelengths, 16 breaking the centrosymmetry using noncentrosymmetric nanostructures 20 and even enhancing the electric fields using nanogaps. 24 However, the efficiency of SHG is restricted by two fundamental classes of losses that electromagnetic waves suffer in such metallic structures, namely radiative "losses" like optical scattering and nonradiative losses like the generation of heat. 25 Therefore, to increase the detected SHG in the far-field the structures must decrease the radiative losses at the fundamental wavelength while increasing them at the second harmonic wavelength and at the same time increase the near-field at the fundamental wavelength. Because the SHG yield increases as the square of the fundamental field intensity, 26 a higher near-field at the fundamental is required to increase the near-field at the second harmonic. The wid...

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