Optically actuated form birefringent microfluidic components

In the emergent field of microfluidics there is the desire to process ever decreasing amounts of analyte. This will challenge our ability to pump, move and mix small volumes of fluid and will require the controlled actuation of various microcomponents.¹²³⁴ A valuable tool for micromanipulation is optical tweezing. Here the forces due to the reflection and refraction of light at a dielectric interface can be used to pick up, move and rotate microscopic particles.⁵⁶⁷ A powerful method of rotating microparticles at high rates in optical tweezers is the use of birefringence. A birefringent particle can be rotated in a standard optical trap simply by manipulating the input light's state of polarisation, however this requires specialist materials that are not easily micromachined. Here we present the modelling, fabrication and rotation of form birefringent microgears.⁸ The birefringence comes from the shape of the microgear (a 1D photonic crystal etched into the center of the microgear) rather than the materials intrinsic properties allowing us to use materials convenient for microfabrication. We have demonstrated the orientation of microgears with the direction of polarisation of linearly polarised light and rotation by rotating the direction of polarisation of linearly polarised light and by illumination with circularly polarised light. We have modelled and experimentally measured the magnitude of the birefringence with good agreement. We have also demonstrated that one microgear can be used to rotate a second optically trapped microgear clearly demonstrating how form birefringent microgears could be used to actuate a micromachine such as a microgear pump.