  A femtosecond laser pulse is able to switch the magnetic state of a 3d-4f
ferrimagnetic material on a pico-second time scale. Devices based on this
all-optical switching (AOS) mechanism are competitive candidates for ultrafast
memory applications. However, a large portion of the light energy is lost by
reflection from the metal thin film as well as transmission to the substrate.
In this paper, we explore the use of dielectric coatings to increase the light
absorption by the magnetic metal layer based on the principle of constructive
interference. We experimentally show that the switching energy oscillates with
the dielectric layer thickness following the light interference profile as
obtained from theoretical calculations. Furthermore, the switching threshold
fluence can be reduced by at least $80\%$ to 0.6 mJ/cm$^2$ using two dielectric
SiO$_2$ layers sandwiching the metal stack, which scales to 15 fJ of incident
energy for a cell size of $50^2$ nm$^2$.
