Implementing optical tweezers at high pressure in a diamond anvil cell

Diamond anvil cells allow us to study the behaviour of materials at pressures up to hundreds of gigaPascals in a small and convenient instrument, however physical access to the sample is impossible once it is pressurised. Optical tweezers use tightly focussed lasers to trap and hold microscopic objects, and their ability to measure nanometric displacements and femtonewton forces makes them ubiquitous across the nano and bio sciences. We show that optical tweezers can be used to hold and manipulate particles in such a cell, in the ``macro tweezers'' geometry allowing us to use objective lenses with a higher working distance. Traps are structured to overcome the limitations imposed by the sample cell. Wedemonstrate the effectiveness of the technique by measuring water's viscosity up to 1.2 GPa. The maximum pressure reached was limited by the water crystallising under pressure.