Reflection of a few-cycle laser pulse on a metal nano-layer: generation of phase dependent wake-fields

Abstract

The reflection and transmission of a few-cycle femtosecond Ti:Sa laser pulse impinging on a metal nano-layer have been analyzed. The thickness of the layer was assumed to be of the order of 2 – 10 nm, and the metallic free electrons were represented by a surface current density 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 evaporating a thin aluminum layer on the surface of a glass plate. The exact analytical solution has been given for the system of the coupled Maxwell-Lorentz equations describing the dynamics of the surface current and the scattered radiation field. It has been shown that in general a non-oscillatory 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 then the original pulse duration. The magnitude of these wake-fields is proportional with the incoming field strength, and the definite sign of them is 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 depth in the electron signal in a given direction. This scheme can perhaps serve as a basis for the construction of a robust linear carrier-envelope phase difference meter.