We study how the collective effects of nanoparticles arranged in rectangular arrays influence their temporal plasmon response and field enhancement property. By systematically changing the lattice constant for arrays containing identical metal nanorods, we experimentally demonstrate how grating induced effects affect the position and, more importantly, the broadening of extinction spectra. We correlate these effects with the achievable field enhancement and the temporal duration of plasmon transients and formulate criteria for the generation of enhanced few-cycle localized plasmon oscillations.

We show that the Floquet approach with a Sturmian basis means an efficient description of high-order harmonic generation with monochromatic excitation. This method, although involves numerical calculations, is close to analytic approaches with the corresponding deeper insight into the dynamics. As a first application, we investigate the role of atomic coherence during the process of HHG: as it is shown, different coherent superpositions of initial atomic states produce observably different HHG spectra. For linearly polarized excitation, we demonstrate that the question whether the constituents of the initial superpositions are dipole coupled or not, strongly influences the dynamics. By investigating time-dependent HHG signals, we also show that the preparation of the initial atomic state can be used for the control of the high-harmonic radiation.PACS numbers:

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