Electron-Beam Fabrication of Micron-Scale Birefringent Quartz Particles for Optical Torque Wrenches

Torque plays a fundamental role in biological processes such as transcription, replication, and repair. A number of techniques have been established for the measurement of torque in single-molecule processes, amongst them the optical torque wrench [1]. This approach relies on the incorporation of birefringent particles into optical tweezers, using control of the laser polarization to apply torque, and measurement of the polarization following trapping to measure torque.Such birefringent particles, which are also employed in micron-scale pumps to generate flow in microfluidics [2], and in microrheology as sensors that measure the local properties of surrounding fluid [3,4], would benefit from carefully-controlled geometries. This has been demonstrated by Deufel et al. [5] and Gutierrez-Medina et al. [6] in the context of optical lithography. Here, we demonstrate the ability of fabricate such birefringent particles of controlled geometry using electron-beam writing and discuss the relative merits of this approach.[1] La Porta, A. and M.D. Wang, Physical Review Letters 92, 190801 (2004).[2] Leach, J., et al., Lab on a Chip 6(6), pp. 735–739 (2006).[3] Bishop, A.I., et al., Physical Review Letters 92(19), 198104 (2004).[4] Parkin, S.J., et al., Physical Review E 76(4): 041507 (2007).[5] Deufel, C., et al., Nature Methods 4(3): pp. 223–225 (2007).[6] Gutierrez-Medina, B., et al. in Methods in Enzymology:Single Molecule Tools, Pt B. 474: pp. 377–404 (2010).