Large enhancement of second-harmonic generation in subwavelength metal-dielectric-metal plasmonic waveguides

Plasmonic waveguides have shown the potential to guide subwavelength optical modes, the so called surface plasmon polaritons, at metal-dielectric interfaces. In particular, a metal-dielectric-metal (MDM) structure supports a subwavelength propagating mode at a wavelength range extending from DC to visible. Thus, such a waveguide could be important in providing an interface between conventional optics and subwavelength electronic and optoelectronic devices. Nonlinear processes such as second-harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this paper, we show that field enhancement in MDM waveguides can result in large enhancement of SHG. We first consider a structure consisting of a MDM waveguide filled with lithium niobate, which is sandwiched between two high-index-contrast dielectric waveguides. Such a structure forms a Fabry-Perot resonant cavity and can be designed to have a resonance at both the first and second harmonic. We show that this doubly resonant device results in more than two orders of magnitude enhancement in SHG compared to a uniform slab of lithium niobate. We also consider structures in which multisection tapers are used to couple light in and out of the MDM waveguide. We optimize the tapers so that their transmission efficiency is maximized at both the first and second harmonic. For such structures the field enhancement is due to the squeezing of the optical power from the wavelength-sized dielectric waveguide to the deep subwavelength MDM waveguide.