Optical force mapping at the single-nanometer scale

Three-dimensional information on optical responses at the nanometre scale is vital for nanophotonics. Using photoinduced force microscopy (PiFM), the nano-scale optical field can be visualised via the optical gradient force between tip and sample. Here, we demonstrate 3D photoinduced force field visualisation around a quantum dot with single-nanometre spatial resolution using a heterodyne frequency modulation technique under ultra-high vacuum (UHV), which avoids photothermal expansion effects of the tip and sample. The 3D mapping reveals spatially localised photoinduced interaction potentials and force field vectors at the single-nanometre scale for composite quantum dots with photocatalytic activity. A spatial resolution of approximately 0.7 nm is achieved. Single-nanometre photoinduced field visualisation is crucial for applications in photocatalysis, optical functional devices, and optical manipulation.