Fig. 1. (a) Schematic diagram for a typical TERS system involving a molecule immersed within a mode that is excited by an incident p-polarized laser. (b) Schematic diagram showing the molecule affected by the gradient force coming from the highly localized and enhanced electric field hot spot within the nanogap.

Fig. 2. (a) Schematic diagram of classical optical tweezers formed by a tightly focused Gaussian laser beam trapping a molecule. (b) Calculated optical gradient forces component in the x-axis, y-axis, and z-axis directions in the xy plane of the focus point.

Fig. 3. (a), (b) The xy plane (z=1  nm) and xz plane electromagnetic field distribution of an Ag tip (curvature radius of 6.25 nm). (c), (d) The xy plane (z=1  nm) and xz plane electromagnetic field distribution of an Au tip (curvature radius of 6.25 nm). (e), (f) The xy plane (z=1  nm) and xz plane electromagnetic field distribution of an Ag tip (curvature radius of 25 nm).

Fig. 4. Electric field and gradient force distribution in the xy plane located 1 nm below the Ag tip. (a) Electric field intensity distribution. Optical gradient force distribution for (b) x-axis, (c) y-axis, and (d) z-axis components.

Fig. 5. xz plane field and gradient force distribution of the TERS system. (a) Electric field intensity distribution. Optical gradient force distribution for (b) x-axis, (c) y-axis, and (d) z-axis components.

Fig. 6. yz plane field and gradient force distribution of the TERS system. (a) Electric field intensity distribution. Optical gradient force distribution for (b) x-axis, (c) y-axis, and (d) z-axis components.

Fig. 7. (a) One-dimensional distribution of the gradient force of the molecule along the z direction of the hot spot. One-dimensional gradient force distribution in the (b) x and (c) y directions of the hot-spot center (z=1.65  nm).

Fig. 8. One-dimensional distribution of the gradient force of a CH4 molecule along the z directions of the hot spot; (a) curvature radius of 6.25 nm, (b) curvature radius of 25 nm.