(a) 2D color contour plot for the absorptance (A) as a function of frequency f and surrounding refractive index n, for the single-rod case shown in Figure 2a. (b) 2D color contour plot for the absorptance (A) as a function of frequency f and relative vertical position (y − y_bottom/L_y) in the case where the single rhodium rod of radius 14 nm is placed at a vertical position y in a graded index slab of thickness L_y, bottom surface at y_bottom, and index variation between 1.2 and 2.2.