Nonresonant Local Fields Enhance Second-Harmonic Generation from Metal Nanoislands with Dielectric Cover

We study second-harmonic generation from gold nanoislands covered with amorphous titanium oxide (${\mathrm{TiO}}_2$) films. As the ${\mathrm{TiO}}_2$ thickness increases, the plasmon resonance of the nanoislands shifts away from the second-harmonic wavelength of 532 nm, diminishing the resonant enhancement of the process at this wavelength. Nevertheless, the second-harmonic signal is enhanced by up to a factor of 45 with increasing ${\mathrm{TiO}}_2$ thickness. This unexpected effect arises from the scaling of local fields at the fundamental wavelength of 1064 nm—which is at the far tail of the resonance—due to a change in the dielectric environment of the nanoislands.

Figure 1

Absorption spectra of the samples with gold nanoisland film coated with ${\mathrm{TiO}}_2$ layers of different thicknesses (indicated near the curves). The spectra of the 51.9 and 97.9 nm samples, which demonstrate irregularities, are dotted and dashed, respectively. Inset: SEM image of the nanoisland film.

Figure 2

SH response from gold nanoisland films coated with ${\mathrm{TiO}}_2$ layers as a function of the ${\mathrm{TiO}}_2$ layer thickness. The resonance offsets $\mathrm{\Delta }\lambda$ (nm) between the LSPR and SH wavelength are indicated near the corresponding data markers. The SH intensity is normalized by the SH signal from the nanoisland film without the ${\mathrm{TiO}}_2$ cover. The line is a guide for the eye only. Inset: SHG Maker-fringe patterns from representative samples.

Figure 3

Influence of outer medium permittivity (${ϵ}_{\text{out}}$) on the dispersion of the imaginary part of the polarizability of a spherical gold nanoparticle of radius $R$.

Figure 4

(a) Schematic of a truncated gold nanosphere on a glass substrate and covered with a ${\mathrm{TiO}}_2$ layer, $\theta$-truncation angle. (b) Influence of the ${\mathrm{TiO}}_2$ (${ϵ}_{\text{coat}}=5.5$) cover of different thickness ($h$) on the imaginary part of the polarizability of a truncated gold sphere (truncation angle $\sim 50°$) of radius $R$ placed on a substrate with ${ϵ}_{\text{sub}}=2.25$; the $h/R$ ratio is labeled near the curves. Inset: Polarizability at the fundamental wavelength 1064 nm vs the $h/R$ ratio. Dispersions of the substrate and the cover are neglected.

Figure 5

Calculated SH intensity for truncated spherical gold nanoparticles (50° truncation angle) on a glass surface (${ϵ}_{\text{sub}}=2.25$) as a function of the ${\mathrm{TiO}}_2$ (${ϵ}_{\text{coat}}=5.5$) coating thickness. The intensity is normalized by the SH signal from bare gold nanoparticles ($h=0$). Inset: Calculated LFFs at the fundamental and SH frequencies. Dispersions of the substrate and the cover are neglected.