Abstract
Plasmonic structures can precisely localize electromagnetic energy to deep subwavelength regions resulting in significant field enhancement useful for efficient on-chip nonlinear generation. However, the origin of large nonlinear enhancements observed in plasmonic nanogap structures consisting of both dielectrics and metals is not fully understood. For the first time, here we probe the third harmonic generation (THG) from a variety of dielectric materials embedded in a nanogap plasmonic cavity. From comprehensive spectral analysis of the THG signal, we conclude that the nonlinear response results primarily from the dielectric spacer layer itself as opposed to the surrounding metal. We achieved a maximum enhancement factor of more than six orders of magnitude compared to a bare gold film, which represents a nonlinear conversion efficiency of 8.78 × 10-4%. We expect this new insight into the nonlinear response in ultrathin gaps between metals to be promising for on-chip nonlinear devices such as ultrafast optical switching and entangled photon sources.
Highlights
The efficiency of nonlinear generation can be enhanced by taking advantage of nanostructures that mold the local electromagnetic fields, such as all-dielectric nanodisks [1,2], metallic nanoantennas [3,4,5], and patterned gold film [6,7]
A schematic of the sample structure is shown in Fig. 1(a), consisting of a thin dielectric layer sandwiched between a 75 nm gold film and gold nanostripes fabricated by electron-beam lithography (EBL)
As a first step to elucidate the main source of the third harmonic generation (THG) enhancement in this plasmonic structure, we explore the fundamental resonance for various widths of the stripes on samples with four different dielectric materials (1 nm Al2O3, 1 nm HfO2, 5 nm ITO and 3 nm TiO2)
Summary
The efficiency of nonlinear generation can be enhanced by taking advantage of nanostructures that mold the local electromagnetic fields, such as all-dielectric nanodisks [1,2], metallic nanoantennas [3,4,5], and patterned gold film [6,7]. R. Smith, “Third-harmonic generation enhancement by film-coupled plasmonic stripe resonators,” ACS Photonics 1(11), 1212–1217 (2014). We utilize spectral analysis and a variety of dielectric materials to investigate the main source of the THG response in a hybrid plasmonic structure, namely the film-coupled gold nanostripe resonator, as well as demonstrate a large nonlinear enhancement and high conversion efficiency.
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