where A is the area of the unit cell, i.e., the average area that a particle occupies, and D is the beam diameter. Equation for the infinite array is chosen for our estimation because the Au NPs are dipersed over the whole TiO2 film that is much larger than the illumination area. Here, D is the width-half-maximum (FWHM) of the Gaussian beam. Note that we use the average area of randomly dispersed particles for A.