Optical Tweezing at Extremes Graham M. Gibson, a Richard W. Bowman, a,b Filippo Saglimbeni, c Roberto Di Leonardo c and Miles J. Padgett a a SUPA, School of Physics and Astronomy, University of Glasgow, G12 8QQ, UK; b Department of Physics, Cavendish Laboratory, University of Cambridge, CB3 0HE, UK; c CNR-IPCF c/o Dipartimento di Fisica, Universit`a di Roma “La Sapienza”, P.le A. Moro, 2, 00185, Roma, Italy ABSTRACT Diamond anvil cells can be used to study the behavior of materials at high pressure by compressing small samples up to hundreds of GigaPascals. There is no mechanical access to the sample once the cell is pressurized but it is possible to observe the sample through the diamond windows. Optical tweezers can be used to measure the mechanical properties of fluids, such as viscosity, by trapping and monitoring micron sized spheres suspended in the fluid. We use a diamond anvil cell within a modified optical tweezers instrument to measure the viscosity of water as a function of pressure up to 1.3GPa. Development of this technique will allow investigations of the mechanical changes in biological cells and other soft materials placed under high pressure. Keywords: High Pressure, Diamond Anvil Cell, Optical Tweezers, Spatial Light Modulator, Particle Tracking, Rheology 1. INTRODUCTION Diamond Anvil Cells (DACs) enable small samples of liquid to be compressed to very high pressures by squeezing the sample between the small “cutlets” of two diamonds. 1–3 Pressure applied to the larger bases of the diamonds, usually driven using a gas membrane, is amplified at the cutlets. Gas pressures of up to 20MPa on an area of around 30 cm 2 can result in hundreds of GPa at the sample. 1 Applications of DACs have mainly been restricted to those that require only optical access to the sample, as physical access is impossible once the cell is pressurized. We combine a DAC with optical tweezers to directly probe the mechanical properties of materials. Optical tweezers, 4, 5 are an established tool for trapping, manipulation, and force measurements of micron sized objects. A small dielectric sphere suspended in a fluid is trapped in the focus of a laser beam and can be used to probe the mechanical properties of the surrounding fluid. By using a high-speed camera it is possible to track an optically trapped sphere and extract the viscosity from a time-series measurement of the sphere’s motion. 6 This technique has been extended to measure the more general frequency-dependent viscoelastic moduli. 7, 8 The most common design of optical trap is the single beam gradient trap 4 that requires a high Numerical Aperture (NA) microscope objective (NA greater than 1 when working in water). Unfortunately, the short working distance associated with a high NA objective is not compatible with the relatively large dimensions (and hence, long working distance) of the DAC. In addition, the NA of the DAC is typically restricted to about 0.5. However, it is also possible to manipulate particles in counterpropagating traps, formed by laser beams propagating in opposite directions. 9, 10 The two traps can be formed using two lower NA, longer working distance, objectives which makes them an ideal choice for working in a DAC. We utilize a DAC in a modified holographic optical tweezers system to measure the viscosity of water as a function of pressure up to 1.3 GPa. In the last few years, holographic techniques have been used to form counterpropagating optical traps more easily. Holographic optical tweezers 11–15 use spatial light modulators (SLMs) as dynamic computer-controlled diffractive optical elements, to manipulate many objects independently. They can manipulate objects in 3D, in real-time, and have found many applications in microscopy, and they are Further author information: (Send correspondence to G.M.G.) G.M.G.: E-mail: Graham.Gibson@glasgow.ac.uk Invited Paper Optical Trapping and Optical Micromanipulation X, edited by Kishan Dholakia, Gabriel C. Spalding, Proc. of SPIE Vol. 8810, 881009 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2027152 Proc. of SPIE Vol. 8810 881009-1 Downloaded From: http://spiedigitallibrary.org/ on 03/31/2014 Terms of Use: http://spiedl.org/terms