title: Optical trapping at gigapascal pressures.
abstract: Diamond anvil cells allow the behavior of materials to be studied at pressures up to hundreds of gigapascals in a small and convenient instrument. However, physical access to the sample is impossible once it is pressurized. We show that optical tweezers can be used to hold and manipulate particles in such a cell, confining micron-sized transparent beads in the focus of a laser beam. Here, we use a modified optical tweezers geometry, allowing us to trap through an objective lens with a higher working distance, overcoming the constraints imposed by the limited angular acceptance of the anvil cell. We demonstrate the effectiveness of the technique by measuring water's viscosity at pressures of up to 1.3 GPa. In contrast to previous viscosity measurements in anvil cells, our technique measures absolute viscosity and does not require scaling to the accepted value at atmospheric pressure. This method could also measure the frequency dependence of viscosity as well as being sensitive to anisotropy in the medium's viscosity.
authors.affiliation: SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom. richard.bowman@cantab.net
authors.forename: Richard W
authors.initials: RW
authors.lastname: Bowman
authors.forename: Graham M
authors.initials: GM
authors.lastname: Gibson
authors.forename: Miles J
authors.initials: MJ
authors.lastname: Padgett
authors.forename: Filippo
authors.initials: F
authors.lastname: Saglimbeni
authors.forename: Roberto
authors.initials: R
authors.lastname: Di Leonardo
mesh_terms: D018130:Diamond / Q000737:chemistry; D052898:Optical Tweezers; D055095:Optics and Photonics / Q000295:instrumentation* / Q000379:methods*; D014783:Viscosity; D014867:Water / Q000737:chemistry
chemical_list: D014867:Water; D018130:Diamond