Ultrafast diffraction of tightly focused waves with spatiotemporal stabilization

Experimental studies of ultrafast beam shaping have come about from the need to compensate diffraction-induced dispersive effects in femtosecond laser beams. From a theoretical point of view, chromatic matching of diffracted spherical waves in the vicinity of the geometrical focus is attained by applying conveniently dispersive boundary conditions in the far-field zone, a subject thoroughly analyzed in the paraxial regime. For applications demanding high spatial resolution, however, high-numerical-aperture microscope objectives may be employed instead and would lead to nonparaxiality of the focal wavefields. These circumstances have motivated our investigation. Concretely we report on prerequisites for spectral invariance extended to wide-angle geometries, which provides stabilization of the spatiotemporal response in the Fourier plane. In this context, general boundary conditions are given in the frame of the Debye representation of wavefields. Features of this sort of dynamic apodization (spatial filtering) leading to perfect achromatization are described in detail. (C) 2008 Optical Society of America.