Doubly-resonant enhanced second-harmonic generation and its wide-band tunability

The second-harmonic generation (SHG) from the doubly-resonant metasurfaces composed of the U-shaped and the rectangular apertures on an optically opaque gold film is investigated theoretically. By engineering the fundamental plasmonic resonances of the composite metasurfaces to match the excitation and the second-harmonic wavelengths, respectively, the doubly-resonant enhanced SHG intensity of almost an order of magnitude is obtained, compared with the singly-resonant U-apertures. In addition, the satisfaction of doubly-resonant condition can be extended to a wide wavelength range by changing the refractive index of the surrounding medium, which benefits from the two fundamental plasmonic resonances oscillating at frequencies of a two-fold relationship. Our work provides a new way to enhance the efficiency of nonlinear harmonic generation.