Laser pulses with a few petawatt peak power is available from chirped-pulse-amplification, where the compression gratings of meter scale is vulnerable to the damage. For the exawatt or zettawatt lasers, as the diameter of gratings increased to hundreds of meters is challenging, a fundamentally different approach is required. One promising method can be found in plasma physics; as the plasma is damage-free and at the same time, strongly dispersive to electromagnetic waves. By properly designing the plasma structure, the powerful laser pulses can be compressed in a similar way to solid-based grating mirrors. Various methods exploiting the plasma have been investigated: Raman backward amplification, Brillouin amplification, and transient plasma gratings. Here I introduce a totally different idea to obtain high compression ratio with small energy loss. In the new method, a smooth gradient, high density (near-critical) plasma is used. Photons in a long negatively chirped incident pulse are reflected at different position inside the plasma, depending on their frequencies. The path difference of photons leads to concentration of the photons in a narrow region. From one-dimensional simulations, hundreds of times compression was obtained for the intensity orders of magnitude larger than the intensity sustainable by solid state of materials.
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