which become perfect reflectors in the limit of zero frequency, Si is a semiconductor with a finite transverse plasma frequency $\omega_{0}$ corresponding to a cut-off wavelength $\lambda_{0}=2\pi c / \omega_{0}\sim 286$nm. For cavity length $L$ smaller than this cut-off wavelength, Si tends to become transparent. The associated optical thickness $\delta$ given in Eq.(\[Rslab\]) is large, so that the Si slab behaves like a bulk Si mirror with low reflectivity at high frequency. The Casimir force is then much smaller than the perfect reflection limit of Eq.(\[FEcas\]). On the other hand, at low frequencies $\omega\ll\omega_{0}$, one will have $\delta\ll 1$ together with $c\kappa\rightarrow 0$, low frequencies being predominant at large distances. In this latter case, the slab is transparent again, and the Casimir force between two Si slabs is decreased when $L\geq D$. This result can have interesting consequences for nanostructures as it opens a way to control the magnitude of the Casimir force and possibly eliminate an unwanted Casimir source of stiction. From a fundamental point of view, it also offers a new solution to study the comparison between experiment and theory of the Casimir force [@Chen07]

Geometry and the Casimir effect {#sec:6}
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Geometry effects are expected to lead 