  The reflection and transmission of a few-cycle femtosecond Ti:Sa laser pulse
impinging on a metal nano-layer have been analysed. The thickness of the layer
was assumed to be of order of 2-10 nm, and the metallic free electrons were
represented by a surface current density distributed at the plane boundary of a
dielectric substrate. The target studied this way can be imagined, for
instance, as a semi-transparent mirror produced by evapotating a thin aluminum
layer on the surface of a glass plate. The exact analytic solution has been
given for the system of the coupled Maxwell-Lorentz equations decribing the
dynamics of the surface current and the scattered radiation fields. It has been
shown that in general a non-oscillatoty frozen-in wake-field appears following
the main pulse with an exponential decay and with a definite sign of the
electric field. The characteristic time of these wake-fields is inversely
proportional with the square of the plasma frequency and with the thickness of
the metal nano-layer, and can be larger than the original pulse duration. The
magnitude of these wake-fields is proportional with the incoming field
strength, and the definite sign of them governed by the cosine of the
carrier-envelope phase difference of the incoming ultrashort laser pulse. As a
consequence, when we let such a wake-field excite the electrons of a secondary
target (say an electron beam, a metal plate or a gas jet), we obtain 100
percent modulation in the electron signal in a given direction, as we vary the
carrier-envelope phase difference. This scheeme can perhaps serve as a basis
for the construction of a robust linear carrier-envelope phase difference
meter.
