Figure 1
From: Origin of Shape Resonance in Second-Harmonic Generation from Metallic Nanohole Arrays

Experiment on SHG from nanostructured gold thin film.
(a) The schematic experimental setup. FM: flip mirror; CP: chopper; BBO: nonlinear crystal; f1 and f2: focusing lens. (b) The scanning electron microscope image of a typical sample of the rectangular hole array, whereby AR = 2. The AR of the rectangular hole is defined as the ratio between the y-axis edge length (260 nm here) and the x-axis edge length (130 nm here). (c) A high resolution image, with the x-y direction and the laser polarization (parallel to the y-axis) marked on the figure. (d) The schematic physical picture illustrating generation and transport of SHW mode
from surface gold atoms of the thin film under excitation of the FW mode
within the air hole (which can be seen as a channel).