The effect of generating short pulses, as it is seen in simulations, can be scaled to higher intensities using higher plasma densities and maintaining similar efficiency. Observing the wavefront of pulses generated by high intensities we find that they are capable of being focused to spot sizes corresponding to the shortest significant wavelengths in their spectra. Upon closer observation it has been determined that these well-defined wavefronts actually develop out of similarly small regions of intense focus in vacuum. Figure 1 shows the quality of the optical wavefront, (b), of the most intense attosecond pulse in (a). 3D simulation enables visualization of the moment of highest field strength and of the same field disturbance periods later.

Fig.1 Isolated attosecond pulses (a) generated by 5 fs pulses of intensity 3x10¹⁹W/cm² focused to a single wavelength dimensions are in wavelength and the wavefront error of the most intense pulse.

Fig.2 The insensified focus of an attosecond pulse driven by a 2x10¹⁹W/cm² focused to a single wavelength on a target of 4 times critical density. On the left the 50% isointensity contour identifies a region where the generated pulse is 15 times more intense than the driving pulse. On the right the pulse has propagated five period to develp a nearly spherical wavefront and neihboring pulses.

References:

1) Isolated attosecond pulses generated by relativistic effects in a wavelength-cubed focal volume
N. M. Naumova, J. A. Nees, B. Hou, G. A. Mourou, and I. V. Sokolov
Opt. Lett. 29, 778 (2004).

2) Relativistic generation of isolated attosecond pulses:
a different route to extreme intensity
J. Nees, N. Naumova, E. Power, V. Yanovsky, I. Sokolov,, A. Maksimchuk, S.-W. Bahk, V. Chvykov, G. Kalintcheko, B. Hou and G. Mourou
Journal of Modern Optics, 20 january-15 february 2005
vol. 52, no. 2-3, 305-319