Nanoscopic control and quantification of enantioselective optical forces

Circularly polarized light (CPL) exerts a force of different magnitude on left-and right-handed enantiomers, an effect that could be exploited for chiral resolution of chemical compounds(1-5) as well as controlled assembly of chiral nanostructures(6,7). However, enantioselective optical forces are challenging to control and quantify because their magnitude is extremely small (sub-piconewton) and varies in space with sub-micrometre resolution(2). Here, we report a technique to both strengthen and visualize these forces, using a chiral atomic force microscope probe coupled to a plasmonic optical tweezer(8-13). Illumination of the plasmonic tweezer with CPL exerts a force on the microscope tip that depends on the handedness of the light and the tip. In particular, for a left-handed chiral tip, transverse forces are attractive with left-CPL and repulsive with right-CPL. Additionally, total force differences between opposite-handed specimens exceed 10 pN. The microscope tip can map chiral forces with 2 nm lateral resolution, revealing a distinct spatial distribution of forces for each handedness.