Plasmonic Coherent Drive of an Optical Trap

We demonstrate that optical trapping can be driven by delocalized surface plasmon modes resonantly excited within a standing wave trap. Dynamical modifications are shown to be determined by the near-field symmetry of the plasmonic modes with negligible thermal effect. With low trapping powers and polarization control, remarkable stiffness enhancements are recorded, the larger the smaller the particle. The results can be simply modeled accounting for a coherent interaction between the plasmon field and the Gaussian standing wave of the trap.

Figure 1

(a) Schematics of the SWT above a 1D array. (b) PSD in trapping conditions with Lorentzian fitting (black line) for a $d=2\mu \mathrm{m}$ bead above an array (parameters: $P=400\mathrm{nm}$, $s=50\mathrm{nm}$, $w=150\mathrm{nm}$).

Figure 2

(a) Reflection spectra of the array ($s=100\mathrm{nm}$, $w=150\mathrm{nm}$) varying $P$ under perpendicular polarization (the last (red) curve showing no resonance corresponds to a parallel polarization). Red line corresponds to ${\lambda }_L=785\mathrm{nm}$. (b) Normalized trap stiffnesses and (c) reflectivities under perpendicular polarization varying $P$ for $s=100\mathrm{nm}$ (red dots) and $s=50\mathrm{nm}$ (black dots). All spectra are normalized by the reflectivity of a flat Au film.

Figure 3

(a) Calculated intensity map of the GSW with $w_0=2{\lambda }_L$ on a flat Au film. In the SWT configuration, the bead is typically trapped at the first antinode from the surface (white dashed circle). Calculated intensity changes $\delta I$ for (b) a symmetric and (c) an antisymmetric SP mode on a periodic array ($P=500\mathrm{nm}$, $w=150\mathrm{nm}$). We take $\rho =0.98$ and $\varphi =-0.85\pi$ for Au at ${\lambda }_L$, a typical $\beta =0.5$ value, and fix $\gamma$ and $\mathrm{Im}(k_z)$ to an SP propagating ca. $2\mu \mathrm{m}$ on the array and extending 100 nm above. (d) QPD measured (red dots) lateral position distribution of a trapped bead ($d=400\mathrm{nm}$) in an unperturbed SWT trap and (e) in a SWT operating above the array with an excited symmetric SP mode. (f) The same with an antisymmetric SP mode. Gaussian fits are shown as continuous black lines.

Figure 4

(a) Reflection spectra varying $w$ under perpendicular polarization with $s=100\mathrm{nm}$. The period $P$ is set to 500 nm such that the laser excites the low-energy ${\omega }_{<}$ SP mode. Red line corresponds to ${\lambda }_L=785\mathrm{nm}$. (b) Normalized lateral trap stiffnesses $k_{\mathrm{trap}}/k_0$ under perpendicular (black dots) and parallel (red dots) polarization for $d=400\mathrm{nm}$. Continuous (green) line scales as the spectral factor (${\epsilon }_0^{-1}+{\epsilon }_2^{-1}$) with $w$ for $w<P/2$. This curve is set to one when the SP mode becomes antisymmetric for $w>P/2$. (c) Same for perpendicular and parallel polarization with $d=2\mu \mathrm{m}$.