Casimir pressure on a thin metal slab
  We consider the vacuum-field pressure on boundaries of a metal slab in the middle of a cavity with perfectly reflecting mirrors adopting the plasma model for the metal and paying special attention to the surface plasmon polariton contribution to the pressure. We demonstrate that, with increasing cavity length, the pressure on a thin (d<<\lambda_P) slab in this system decreases from the Casimir pressure F_C at zero slab-mirror distances to the non-retarded force per unit area F_nr=1.19 (d/\lambda_P)F_C in the case of an isolated slab. In the first case the pressure is entirely due to the photonic modes propagating through the metal whereas in the second case it is entirely due to the (nonretarded) surface plasmon modes supported by the free-standing thin slab. In either case the pressure decreases with the slab thickness. These considerations indicate that the vacuum-field pressure on a thin metal layer (and its modal structure) can be in a symmetric cavity significantly influenced when changing the cavity length. 
Marin-Slobodan Tomas and Zdravko Lenac