Limits for light intensification by reflection from relativistic plasma mirrors

Ultraintense laser pulses might be capable of producing plasma mirrors through ultrarelativistic oscillating electrons. Two kinds of such mirrors with relativistic factors γ ∼ 100-300 were contemplated recently as a tool for producing huge frequency upshifts 4γ ² of reflected laser pulses as well as the compressing and focusing of those pulses. The combination of these effects would result in dramatic light intensification toward the vacuum breakdown intensities (Schwinger limit) [S. V. Bulanov et al., Phys. Rev. Lett. 91, 085001 (2003) and S. Gordienko et al., Phys. Rev. Lett. 94, 103903 (2005)]. The analysis performed in these publications was limited, however, to idealized situations of cold uniform plasmas and uniform laser intensities. The analysis here of effects of electron thermal motion and random inhomogeneities in the plasma density or laser intensity indicates that the largest relativistic factors allowed within these schemes are much smaller than those assumed in the idealized models, unless essentially new physical mechanisms are adduced in addition to those already considered.