Coherent control of ultrafast nano-optical excitations of a corrugated silver surface is demonstrated by means of predetermined few-parameter scans and adaptive polarization laser pulse shaping. "Hot spots" in the multiphoton photoemission signals are enhanced and manipulated with a high contrast. Switching between separated and closely spaced hot spots is shown. The latter allows controlling the shape of hot spots and yields improved nanofocusing and "purification" of the photoemission signals. Complex pulse shapes were obtained in adaptive optimizations whose features were reproducible in repeated runs. Predetermined few-parameter control scans provide insight into the interpretation of optimal pulse shapes. The results indicate the existence of long coherence lifetimes on a corrugated silver surface. This combination of collective strong nanoplasmonic near-field enhancement and long-lived coherence may be used to achieve an even stronger field enhancement ("superenhancement") making these hot spots ideal candidates for future nanophotonic, spectroscopic, sensor and quantum information applications. In addition the observation of such long coherence lifetimes is relevant to the understanding of surface-enhanced spectroscopies such as single-molecule Raman spectroscopy.

We introduce a spectroscopic method that determines nonlinear quantum-mechanical response functions beyond the optical diffraction limit and allows direct imaging of nanoscale coherence. In established coherent two-dimensional (2D) spectroscopy, four-wave-mixing responses are measured using three ingoing and one outgoing wave; thus, the method is diffraction limited in spatial resolution. In coherent 2D nanoscopy, we use four ingoing waves and detect the final state via photoemission electron microscopy, which has 50 nanometer spatial resolution. We recorded local nanospectra from a corrugated silver surface and observed subwavelength 2D line shape variations. Plasmonic phase coherence of localized excitations persisted for about 100 fs and exhibited coherent beats. This was analyzed with a model of coupled oscillators leading to Fano-like resonances in the hybridized dark- and bright-mode response.

The most general investigation and exploitation of light-induced processes require simultaneous control over spatial and temporal properties of the electromagnetic field on a femtosecond time and nanometer length scale. Based on the combination of polarization pulse shaping and time-resolved two-photon photoemission electron microscopy, we demonstrate such control over nanoscale spatial and ultrafast temporal degrees of freedom of an electromagnetic excitation in the vicinity of a nanostructure. The time-resolved cross-correlation measurement of the local photoemission yield reveals the switching of the nanolocalized optical near-field distribution with a lateral resolution well below the diffraction limit and a temporal resolution on the femtosecond time scale. In addition, successful adaptive spatiotemporal control demonstrates the flexibility of the method. This flexible simultaneous control of temporal and spatial properties of nanophotonic excitations opens new possibilities to tailor and optimize the light–matter interaction in spectroscopic methods as well as in nanophotonic applications.

We present general analytic solutions for optical coherent control of electromagnetic energy propagation in plasmonic nanostructures. Propagating modes are excited with tightly focused ultrashort laser pulses that are shaped in amplitude, phase, and polarization (ellipticity and orientation angle). We decouple the interplay between two main mechanisms which are essential for the control of local near-fields. First, the amplitudes and the phase difference of two laser pulse polarization components are used to guide linear flux to a desired spatial position. Second, temporal compression of the near-field at the target location is achieved using the remaining free laser pulse parameter to flatten the local spectral phase. The resulting enhancement of nonlinear signals from this intuitive analytic two-step process is compared to and confirmed by the results of an iterative adaptive learning loop in which an evolutionary algorithm performs a global optimization. Thus, we gain detailed insight into why a certain complex laser pulse shape leads to a particular control target. This analytic approach may also be useful in a number of other coherent control scenarios.

We show through simulations how to control the spatial field distribution of a tightly focused Gaussian beam of polarization-shaped femtosecond laser pulses. The field in the focus is calculated employing a decomposition into plane-wave components with appropriate incidence angles. Both polarization directions of the shaped pulse are treated separately and then superposed coherently. The incident polarization shape can be used to control the spatial and temporal evolution of the longitudinal field component.

Adaptive shaping of the phase and amplitude of femtosecond laser pulses has been developed into an efficient tool for the directed manipulation of interference phenomena, thus providing coherent control over various quantum-mechanical systems. Temporal resolution in the femtosecond or even attosecond range has been demonstrated, but spatial resolution is limited by diffraction to approximately half the wavelength of the light field (that is, several hundred nanometres). Theory has indicated that the spatial limitation to coherent control can be overcome with the illumination of nanostructures: the spatial near-field distribution was shown to depend on the linear chirp of an irradiating laser pulse. An extension of this idea to adaptive control, combining multiparameter pulse shaping with a learning algorithm, demonstrated the generation of user-specified optical near-field distributions in an optimal and flexible fashion. Shaping of the polarization of the laser pulse provides a particularly efficient and versatile nano-optical manipulation method. Here we demonstrate the feasibility of this concept experimentally, by tailoring the optical near field in the vicinity of silver nanostructures through adaptive polarization shaping of femtosecond laser pulses and then probing the lateral field distribution by two-photon photoemission electron microscopy. In this combination of adaptive control and nano-optics, we achieve subwavelength dynamic localization of electromagnetic intensity on the nanometre scale and thus overcome the spatial restrictions of conventional optics. This experimental realization of theoretical suggestions opens a number of perspectives in coherent control, nano-optics, nonlinear spectroscopy, and other research fields in which optical investigations are carried out with spatial or temporal resolution.

The tight focus of Gaussian beams is commonly used to trap dielectric particles in optical tweezers. The corresponding field distribution generates a well-defined trapping potential that is only marginally controllable on a nanometre scale. Here we investigate the influence of a metal nanostructure that is located in the vicinity of the trapping focus on the trapping potential by calculating the corresponding field and force distributions. Even for an excitation wavelength that is tuned far from the plasmonic resonance of the nanostructure, the presence of the latter alters significantly the trap potential. For the given nanostructure, a ring of spheres that is illuminated in the axial direction, a smaller focus volume is observed in comparison to free focus. The superposition of this non-resonant Gaussian field with a planar wave illumination that is tuned to the plasmonic resonance gives a handle to modify the trapping potential. Polarization and intensity of the resonant illumination allows modifying the equilibrium position of the trapping potential, thus providing means to steer dielectric particles with nanometre precision.

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.

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.

We propose and analyze a scheme for ultrafast spectroscopy with nanometer spatial and femtosecond temporal resolution. The interaction of polarization-shaped laser pulses with a nanostructure allows us to control the spatial and temporal evolution of the optical near field. Employing a learning algorithm, the field is tailored such that pump and probe excitation occur at different positions and at different times. Both excitations can be restricted to subdiffraction extensions and are separable on a nanometer length scale. This enables the direct spatial probing of nanoscale energy transfer or charge transfer processes.