Double Resonant Local Fields Appreciably Enhance Third Harmonic Generation in Core-shell Nano-cavity

We simulate the intensity of third harmonic generation (THG) in a metallic core-shell nanostructure in double resonant conditions, where the corresponding wavelength of two cavity modes are tuned in to couple with the wavelength of the pump light and the THG signal simultaneously by adjusting the refractive index of the dielectric. It turns out that the double resonant condition is crucial to our discovery.

Numerical simulations show that if the fundamental wavelength of the pump light is adjusted to 1390nm, which is the corresponding eigen wavelength of TM₂₁ mode, and the frequency of a higher-order cavity mode, say, TE₃₂ mode was also adjusted to the frequency of THG, the THG signal was more intensified than the single resonant condition on which only TM₂₁ mode was tuned in to the fundamental frequency. What’s more, when the two cavity modes involved, e.g., TM₂₁ and TM₂₃ mode, have the same multipolar moment (as indicated by the first number), the final THG efficiency will be enhanced up to 3 magnitudes, much more than the enhancement in situations where two cavity modes have different multipolar moment, for instance, TM₂₁ and TM₁₃ mode. It can be believed that this phenomenon is important and will open new opportunities in nonlinear optics.