Figure 1

(a) By periodically turning on and off an external potential the Brownian diffusion of a dielectric particle is biased into one direction. (b) Top view of the plasmonic system: a periodic array of plasmonic structures formed by four optical dipole antennas. The geometrical asymmetry is characterized by angle $\theta$. (c) Side view of the periodic array unit cell: the plasmonic structure is placed on top of a glass substrate and embedded in water.Reuse & Permissions

Figure 2

(a) Trapping potential experienced by a PS bead above the plasmonic nanostructure (four dipole antennas). Several values of the geometrical asymmetry are considered. Inset panels: Evolution of the potential asymmetry parameter, $\alpha$ (upper), and depth of the potential well, $\Delta U$ (lower), as a function of $\theta$. The dashed vertical lines point to the antennas' boundaries for reference. (b) Forces experienced by the PS bead in the presence of a structure with $\theta ={18}^{\circ }$. Inset panel: The norm of the electric field upon illumination at $\lambda =1.5\mu$m is plotted at $z=90$ nm. The dipole antennas are designed to maximize the field enhancement at an infrared excitation wavelength: geometrical parameters $L=225$ nm, $d=40$ nm, $\text{w}=50$ nm, $g=25$ nm, and $h=30$ nm.Reuse & Permissions

Figure 3

Unit cell of the trapping potential generated by a periodic array of plasmonic structures ($\theta ={18}^{\circ }$). The anisotropic trapping potential is shown for different values of the period, $\Lambda$. Inset panel: Evolution of the potential depth with the array period.Reuse & Permissions

Figure 4

Plasmonic Brownian ratchet dynamics for $N=4000$ realizations of the system. One PS bead (radius $\sigma =50$ nm, density $\rho =1050$ $\text{kg}/{\text{m}}^3$) is embedded in water [viscous coefficient $\eta =1\times {10}^{-3}$ $\text{kg}/\left(\text{m}\text{s}\right)$] at room temperature and subjected to the time-modulated plasmonic potential. The dynamics is diffusive ($\gamma {\tau }_{\sigma }\sim {10}^6$, ${\tau }_{\sigma }={\sigma }^2/D$ being the time for the bead to diffuse its own radius). (a) Initial situation: The particle probability distribution is centered at the trapping position of the nanotweezer ($0\le y\le \Lambda$) after a time $t_{\text{on}}=2\times {10}^4{\tau }_0$, with ${\tau }_0=\sqrt{m{\sigma }^2/k_{B}T}=5.7\times {10}^{-7}$ s. Inset panel: Time evolution of the probabilities for the particles to move forward ($P_{\text{F}}$), backward ($P_{\text{B}}$), or to remain in the initial unit cell ($P_0$). (b) Final situation: After 16 on-off cycles ($t_{\text{off}}=8\times {10}^3{\tau }_0$), the probability for the particle to show a directed motion in the forward direction is $40\%$. The gray line represents the plasmonic potential in arbitrary units ($\Delta U=75k_{B}T$).Reuse & Permissions