Shaping Fibre for Optical Trapping

1. Shaping Fibre for Optical Trapping Steven Ross GERI-CEORG Supervisors: Prof. D. Burton, Dr. F. Lilley & Dr. M. Murphy

2. Introduction • Optical Trapping Theory • Non “Classical” Methods • Why Fibre Based Trapping? • Fibre based Trapping Methods • Methods for Shaping Fibre Ends • Further work • Conclusion

3. Optical Trapping Theory • Gradient force – pulls particles into the high intensity region of the beams axis • Scattering force - propels particles in the direction of the beams propagation

4. Optical Trapping Theory Counter propagating dual beam trap Net opposing scattering force at E Optical levitation trap Scattering force balanced with gravity at E

5. Optical Trapping Theory • “Optical tweezers” - single beam gradient force Optical trap • Gradient force greater than Scattering force • Axial equilibrium position is located slightly beyond the focal point

6. Non “Classical” Methods of Optical Trapping Metallic probes to trap small particles Strong field enhancement from light scattering at a metallic tip Generate a trapping potential deep enough to overcome Brownian motion High optical powers required Difficult integration in conventional microscopy Fibre based optical trapping

7. Why Fibre Based Trapping? • Decoupled from the microscope • Reducing the build costs • Reduction in system size • No requirement for position detection equipment • However, for single fibre trapping, a new way of focusing the emitted light is required

8. Fibre Trapping Methods • Counter propagating fibres • Requires exact alignment • Difficult to manoeuvre • Requires particle to drift into trapping zone

9. Fibre Trapping methods • Single fibre traps • Lensed optical fibre tips • Highly efficient for optical trapping • Only 2D trapping • Difficult end face processing • Expensive

10. Fibre Trapping methods • Fibre bundles • Focusing with high NA using internal reflection • 3D trapping • Complicated end face process • Requires coupling Laser into 4 fibres • Resulting with large system optical losses

11. Fibre Trapping methods • Adiabatic optical fibre Tapers • Convert the optical mode size • Capable of 3D trapping • Taper must operate under strict conditions • Radiation-loss-free • Mode conversion free

12. Methods for Shaping Fibre Ends • Polishing • 4 axis lapping machine • Laser micro-machining • Works in much the same way as a lathe

13. Methods for Shaping Fibre Ends • Focused Ion beam milling • FIB is destructive to the specimen • High energy gallium ions strike and sputter atoms from the surface

14. Methods for Shaping Fibre Ends • Chemical etching • 40% Hydrofluoric acid solution • Organic over layer controls the height of the meniscus of the HF forming at the Fibre • HF is an extremely hazardous material

15. Sutter P2000/F Micropipette Puller • Heating & Drawing • Fibre heated with 20W CO2 laser • Microcontroller controlled allowing a wide range of tapers • Simple, rapid & repeatable fabrication of taper • Core cladding ratio maintained

16. Further Work Work continues to find the optimum optical fibre taper Trapping of dielectric particles Define optical trap parameters Trapping of non adhered cells Integrate Laser trap with other microscopy systems

17. Conclusion Optical Trapping Theory Non “Classical” Methods Why Fibre Based Trapping? Fibre based Trapping Methods Methods for Shaping Fibre Ends Further work Conclusion

18. Thank You