  In this article we present calculations which indicate that nanoshells can be
used as a high pressure gauge in Diamond Anvil Cells (DACs). Nanoparticles have
important advantages in comparison with the currently used ruby fluorescence
gauge. Because of their small dimensions they can be spread uniformly over a
diamond surface without bridging between the two diamond anvils. Furthermore
their properties are measured by broad band optical transmission spectroscopy
leading to a very large signal-to-noise ratio even in the multi-megabar
pressure regime where ruby measurements become challenging. Finally their
resonant frequencies can be tuned to lie in a convenient part of the visible
spectrum accessible to CCD detectors. Theoretical calculations for a nanoshell
with a SiO2 core and a golden shell, using both the hybridization model and Mie
theory, are presented here. The calculations for the nanoshell in vacuum
predict that nanoshells can indeed have a measurable pressure-dependent optical
response desirable for gauges.However when the nanoshells are placed in
commonly used DAC pressure media, resonance peak positions as a function of
pressure are no longer single-valued and depend on the pressure media,
rendering them impractical as a pressure gauge. To overcome these problems an
alternative nanoparticle is studied: coating the nanoshell with an extra
dielectric layer (SiO2) provides an easy way to shield the pressure gauge from
the influence of the medium, leaving the compression of the particle due to the
pressure as the main effect on the spectrum. We have analyzed the response to
pressure up to 200 GPa. We conclude that a coated nanoshell could provide a new
gauge for high-pressure measurements that has advantages over current methods.
