Enhancement of Second-Harmonic Generation from Metal Nanoparticles by Passive Elements

We prepare arrays of gold nanoparticles that include both noncentrosymmetric particles with a second-order nonlinear optical response (active particles) and centrosymmetric particles with no second-order response (passive particles). The plasmon resonances of the active and passive particles are at distinct wavelengths, yet the passive particles modify the electromagnetic modes of the structure in such a way that second-harmonic generation from the active particles is enhanced. Our results provide a completely new concept for optimizing the nonlinear responses of metamaterials.

Figure 1

Designs and SEM pictures of the samples. (a) Sample 175L, active $L$’s with 175 nm arm length; (b) sample 275L, active $L$’s with 275 nm arm length; (c) sample bar, passive bars with 300 nm length; (d) the coordinate system for the samples and scale bar; (e) sample 175L_xbar, active $L$’s with 175 nm arm length and passive bars in $x$ orientation; (f) sample 275L_xbar, active $L$’s with 275 nm arm length and passive bars in $x$ orientation; (g) sample 175L_ybar, active $L$’s with 175 nm arm length and passive bars in $y$ orientation; (h) sample 275L_ybar, active $L$’s with 275 nm arm length and passive bars in $y$ orientation.

Figure 2

Polarized extinction spectra of samples: (a) 175L and bar, (b) 275L and bar, (c) 175L_xbar, (d) 275L_xbar, (e) 175L_ybar, and (f) 275L_ybar.

Figure 3

Experimental setup for second-harmonic measurements. L, lens; P, Glan polarizer; HWP, half-wave plate; VISF, visible blocking filter; IRF, infrared blocking filter; A, analyzer; PMT, photomultiplier tube.

Figure 4

SHG signals from the samples normalized to the stronger signal of $L$ nanoparticles ($yxx$ for 175L and $yyy$ for 275L). (a) $L$’s with 175 nm arm length (175L, 175L_xbar, and 175L_ybar). The signals $yyy$ are much weaker than $yxx$ (less than 0.01) and thus are not visible on the graph. (b) $L$’s with 275 nm arm length (275L, 275L_xbar, and 275L_ybar).

Figure 5

Experimental [$x$ polarization, cf. Figs. 2c, 2d; measured, blue curve] and calculated (green curve) from the coupled dipole model optical density spectra for the samples (a) 175L_xbar and (b) 275L_xbar.