  Casimir forces, related to London-van der Waals forces, arise if the spectrum
of electromagnetic fluctuations is restricted by boundaries. There is great
interest both from fundamental science and technical applications to control
these forces on the nano scale. Scientifically, the Casimir effect being the
only known quantum vacuum effect manifesting between macroscopic objects,
allows to investigate the poorly known physics of the vacuum. In this work, we
experimentally investigate the influence of self-assembled molecular bio and
organic thin films on the Casimir force between a plate and a sphere. We find
that molecular thin films, despite being a mere few nanometers thick, reduce
the Casimir force by up to 14%. To identify the molecular characteristics
leading to this reduction, five different bio-molecular films with varying
chemical and physical properties were investigated. Spectroscopic data reveal a
broad absorption band whose presence can be attributed to the mixing of
electronic states of the underlying gold layer and those of the molecular film
due to charge rearrangement in the process of self-assembly. Using Lifshitz
theory we calculate that the observed change in the Casimir force is consistent
with the appearance of the new absorption band due to the formation of
molecular layers. The desired Casimir force reduction can be tuned by stacking
several monolayers, using a simple self-assembly technique in a solution. The
molecules - each a few nanometers long - can penetrate small cavities and
holes, and cover any surface with high efficiency. This process seems
compatible with current methods in the production of micro-electromechanical
systems (MEMS), which cannot be miniaturized beyond a certain size due to
`stiction' caused by the Casimir effect. Our approach could therefore readily
enable further miniaturization of these devices.
