  Modern optical nano-elements pursue ever-smaller sizes and individualized
functionalities. Those elements that can efficiently manipulate the magnetic
field of light boast promising future applications with a great challenge: the
magnetic near field is irretrievable from conventional optical far-field
characterization. Here we propose photoinduced magnetic force microscopy to
directly and exclusively sense the magnetic field of light at the nanoscale.
The proposed instrument exploits a magnetic nanoprobe with exclusive magnetic
excitation under structured light illumination. The magnetic nanoprobe detects
the photoinduced magnetic force, which is defined as the dipolar Lorentz force
exerted on the photoinduced magnetic dipole in the nanoprobe. Since the
resulting magnetic force is proportional to the incident magnetic field, the
measured force reveals the magnetic near-field distribution at the nanoscale.
The proposed instrument represents a fundamental step towards comprehensive
electric and magnetic near-field detection and/or manipulation in single
nano-element optical devices.
