Optical gecko toe: Optically controlled attractive near-field forces between plasmonic metamaterials and dielectric or metal surfaces

On the mesoscopic scale, electromagnetic forces are of fundamental importance to an enormously diverse range of systems, from optical tweezers to the adhesion of gecko toes. Here we show that a strong light-driven force may be generated when a plasmonic metamaterial is illuminated in close proximity to a dielectric or metal surface. This near-field force can exceed radiation pressure and Casimir forces to provide an optically controlled adhesion mechanism mimicking the gecko toe: At illumination intensities of just a few tens of nW/$\mu$m${}^2$ it is sufficient to overcome the Earth's gravitational pull.

Figure 1

Gecko toes and their optical analog. (a) Gecko toes sticking to a smooth glass wall (Ref. 24). (b) Artistic impression of a metamaterial film attracted by a beam of light to a dielectric surface.

Figure 2

Optical forces between a plasmonic metamaterial and a dielectric surface. (a) Reflection $R$, transmission $T$, and absorption $A$ spectra of a 50 nm thick gold metamaterial located at a distance $h$ = 20 nm from the surface of a dielectric with refractive index $n$ = 2.5 for $y$-polarized light normally incident from free space. The inset shows the metamaterial unit cell geometry. (b) Total optical force $F$ acting on a metamaterial illuminated from free space as illustrated inset. Dashed lines show the evanescent $F_e$ and light pressure $F_r$ components of $F$. (c) Total optical force acting on a metamaterial illuminated through the dielectric. For comparison, the total force acting on an unstructured gold film in place of the metamaterial is shown in both (b) and (c). In all cases, positive values denote forces acting in the direction of incident light propagation.

Figure 3

Optical forces between a plasmonic metamaterial and a dielectric surface. Spectral dispersion of the total optical force [under illumination from free space as illustrated inset to Fig. 2b] for different values, as labeled, of (a) dielectric refractive index $n$ (at $h=20$ nm) and (b) gap size $h$ ($n$ = 2.5). Insets show peak optical force as a function of $n$ and $h$, respectively.

Figure 4

Optical forces between a plasmonic metamaterial and a metallic surface. (a) Normal incidence reflection $R$ and absorption $A$ spectra of a 50 nm thick gold metamaterial located at a distance $h$ = 20 nm from a gold surface (incident light propagation in the $-z$ direction as defined inset). The insets show the metamaterial unit cell geometry and a map of the normalized magnetic field intensity distribution at the 1095 nm resonance wavelength for a cross-section along the dashed line in the y-z plane. (b) Evanescent $F_e$, radiation pressure $F_r$, and total optical force $F$ acting on the metamaterial. The inset shows peak optical force as a function of gap size $h$ and the corresponding dependence on $h$ of the Casimir force between two perfectly conducting plates (scaled assuming $I$ = 50 mW/$\mu$m${}^2$).