Optically Tunable Surfaces with Trapped Particles in Microcavities

We introduce optically tunable surfaces based upon metallic gold nanoparticles trapped in open, water-filled gold cavities. The optical properties of the surfaces change dramatically with the presence and location of the particles inside the cavities. The precise position of the particles is shown to be controllable through optical forces exerted by external illumination, thus leading to all-optical tunability, whereby the optical response of the surfaces is tuned through externally applied light. We discuss the performance of the cavity-particle complex in detail and provide theoretical support for its application as a novel concept of a large-scale optically tunable system.

Figure 1

(a) Cavity-particle system under study, including definitions of the cavity dimensions, and particle radius $R_p$ and length $L$. The cavity, excavated in an otherwise infinite planar gold-water interface, is illuminated by $p$-polarized (TM) light under 45° incidence with respect to the planar surface. The gold nanorod is assumed to be placed in different positions along the cavity axis ($z$ axis). (b),(c) Normalized absorption cross section ${\sigma }_{\mathrm{abs}}$ of the single cavity and a single nanorod of dimensions $20\mathrm{nm}\times 100\mathrm{nm}$, respectively. Near-field-intensity plots at the resonance wavelengths 985 and 970 nm are shown in the insets.

Figure 2

(a) Absorption spectra of the rod-cavity composite system for different rod diameters $2R_p$ (see labels) and fixed rod length $L=100\mathrm{nm}$. The rod is placed along the $z$ axis with its center 210 nm below the cavity opening. The spectra are vertically displaced for clarity. The dashed lines indicate the position of resonance maxima. The dotted curve shows the cavity without a particle. (b) Vertical force acting on the rod under the same conditions as in (a) for an external light intensity of $1\mathrm{mW}/\mu {\mathrm{m}}^2$. The different curves are displaced for clarity with the open symbols denoting the zero-force points.

Figure 3

(a) Resonance modes of the rod-cavity system extracted from Fig. 2a as a function of the particle aspect ratio $\delta =L/2R_p$ (solid curves). Dotted curves: single-cavity and single-rod resonance modes. Dashed curves: analytical-model approximation (see text). (b),(c) Near-field-intensity plots for the $16\mathrm{nm}\times 100\mathrm{nm}$ rod [see open circles in (a)] at the resonance wavelengths 960 and 1090 nm of the composite system.

Figure 4

(a) Absorption spectra of the rod-cavity system for a $20\mathrm{nm}\times 100\mathrm{nm}$ rod placed at different depths from the cavity opening: $-60$, $-230$, and $-330\mathrm{nm}$ (solid, dashed, and dotted curves, respectively). The single-cavity absorption is shown for reference. (b) Forces along the $z$ axis acting on the rod for the cases considered in (a). The vertical dashed lines are guides to the eye (see text).

Figure 5

(a) Vertical force acting on the rod as a function of light wavelength and particle position (depth) below the cavity opening. Solid and dashed curves denote stable and unstable equilibrium (zero-force) points, respectively. The thick arrows visualize the direction of the optical force in the vicinity of the wavelength 1030 nm, indicated by the dotted vertical line, for which the detailed form of the force is presented in (b). Points A and B show stable and unstable trapping positions inside the cavity, respectively.