Adaptive ultrafast nano-optics in a tight focus

We theoretically demonstrate the control of electromagnetic field properties on a sub-diffraction length scale, by polarization shaping of tightly focused femtosecond laser pulses. The field distribution in a tight focus is represented as a superposition of plane waves. The near-field of a model nanostructure is then obtained as a sum of the near-field distributions induced by the planar waves components. A self-consistent solution of Maxwell's equations in the frequency domain yields the near-field distributions for planar wave illumination. Adaptive optimization of the incident polarization pulse shape using an evolutionary algorithm allows controlling of a number of observables, such as local nonlinear flux, simultaneous spatial and temporal control of the intensity evolution, and control of the local spectrum. The tight focusing reduces the controllability of the flux distribution in comparison to plane wave illumination. However, it is still possible to control the spatial and temporal field evolution for particular locations in the vicinity of the nanostructure.