Figure 22.7 — Optical traps based on plasmonic nanoapertures Figure 22.7 — Optical traps based on plasmonic nanoapertures. (a) Set-up for illumination of a rectangular plasmonic nanoaperture in a gold film supported on a silicon membrane. (b) Calculation of the enhanced field distribution inside the aperture showing maxima at the mid-point of the long sides due to excitation of the Fabry–Pérot resonance at λSPP/2. (c) Plasmonic double nanohole: when excited, the double nanohole shows an enhanced field strongly confined in the small gap between the two tips separating the holes. (d) Time-evolution of the intensity transmitted through the double nanohole shown in (c), showing two discrete transmission levels, T1 and T2, corresponding to the trapping of single BSA protein molecules in the native (N-state) and unfolded (F-state) states.Figures (a) and (b) are adapted with permission from Chen et al., Nano Lett. 12, 125–32. Copyright (2012) American Chemical Society.Figures (c) and (d) are adapted with permission from Pang and Gordon, Nano Lett. 12, 402–6. Copyright (2012) American Chemical Society.