Ultrafast adaptive optical near-field control

Simultaneous control of the spatial and temporal properties of the optical near field in the vicinity of a nanostructure is achieved by illumination with broadband optimally polarization-shaped femtosecond light pulses. Here we demonstrate the spatial control of the local linear and nonlinear fluence, the local spectral distribution, and the local temporal intensity profile on a subdiffraction length scale. The boundary-element method is used for a self-consistent solution of Maxwell's equations in the frequency domain. Particular control objectives for spatial field distribution and temporal evolution are expressed as fitness functions in an evolutionary algorithm that optimizes adaptively the polarization-shaped input light pulses. Substantial control according to different goals is demonstrated and the limits of controllability are investigated. The dominating control mechanism is local interference of near-field modes that are excited with the two independent polarization components of the incident light pulses and hence polarization pulse shaping is essential to achieve substantial control in the optical near field. The influence of other control mechanisms is discussed and a number of applications are presented.