3D Optical Trapping via Tapered Optical Fibre at Extreme Low Insertion Angles Presentation by: Steven Ross The GERI Weekly Seminar Friday 18th October 2013 Supervisors: Prof. D. R. Burton, Dr. F. Lilley & Dr. M. F. Murphy General Engineering Research Institute (GERI), Coherent & ElectroOptics Research Group (CEORG), Liverpool John Moores University (LJMU), GERI Building, Byrom Street, Liverpool, L3 3AF, UK. Email: [email protected] Introduction • • • • • • • • What is optical trapping? Optical trapping history & basic theory Optical trapping configurations Pros & cons of “classical” and fibre systems Tapered fibre optic tweezers(T-FOT’s) system Optical fibre insertion angle issues & solutions Maximum trapping range Conclusion What is Optical Trapping? • Exploitation of the forces produced during the interaction between light and matter • Allowing the deflection, acceleration, stretching, compression, rotation and confinement of organic & inanimate material • Ranging in sizes from the microscopic down to the atomic level • Optical forces can be in excess of 100 Pico Newton’s with sub-Nanometre resolution • Excellent force transducers The Origins of Optical Trapping • 1969 -Arthur Ashkin – Bell Laboratories • Effects of electromagnetic radiation pressure forces on microscopic particles • Witnessed Unusual phenomena • Expected – Particles driven in the direction of the laser beam’s propagation • Unexpected - Particles located at the fringes of the laser beam’s axis were drawn into the high intensity region of the axis Optical Forces Acting on a Particle Ashkin's initial observations Total forces acting on a particle Optical Trapping System Configurations Counter propagating laser beams Particle levitation trap Optical Trapping System Configurations Single Beam gradient force optical trap – “Optical tweezers” Multiple Optical Tweezers • Dual Optical tweezers – Splitting the beam – Two laser sources • Multiple trap systems – Fast Scanning time shared laser beam – Diffractive optical element (DOE) – Computer generated holographic optical trap’s Pros & Cons of “Classical” and fibre Based System Configurations “Classical” optical tweezers Fibre Based Optical Tweezers • Very large surface area required to mount the bulk optics • Physically large compared to the miniaturised arena which they were built to serve • Require a high numerical aperture (NA) microscope objective • Expensive • Poor solution for project design criterion • Reduced size and build costs • No bulk optics required • No high (NA) microscope objective required • Therefore it can be decoupled from the microscope • Optical fibre delivers the trapping laser light to the sample chamber • Basic system consists of a laser and an optical fibre • Ideal for project design criterion Disadvantages of Fibre Based Optical Trapping Systems Known Fibre Trapping Issues • Optical fibre is a physical entity • The light exiting a Fibre is divergent • Optical trapping efficiency of fibre systems < “classical” systems • Literature suggests that trapping cannot occur at fibre insertion angles below 20° Problem Relating to the Project • Requires fixing and manipulation • Fibre’s distal end requires shaping to focus the light • Requires higher optical powers to reach same level of forces • Design criterion requires an insertion angle of ≤ 10° to pass under the atomic force microscope (AFM) head Atomic Force Microscope (AFM) AFM Head Optical Lever Detection System Tapered Fibre Optic Tweezers (T-FOT’s) System 3D Trapping at 45° Insertion Angle 3D optical Trapping at 10° Insertion Angle • Initial attempt to trap at a 10° Insertion angle failed at low optical output powers • At extremely high optical output powers in excess of 500 mW 3D optical trapping was observed • Leading to investigations as to why trapping only occurred at high optical output powers at an insertion angle of 10° Investigation into Trapping Failure at Sub-45° Insertion Angles Investigation into Trapping Failure at Sub-45° Insertion Angles Fibre Taper Optimisation for 10° Insertion Angle Optical Trapping Tip 44 Tip 92 Tip 94 Tip 96 Maximum Trapping Range Maximum Trapping Range Maximum Trapping Range 92 94 96 Simulated Maximum Trapping Range (µm) 9 13 13 0-12  Tapered Tip Movement (µm) 9 9 10 Actual Particle Displacement (µm) 6.27 6.11 8.2 Percentage error % 30.3 32.1 18  Z. Liu, C. Guo, J. Yang and L. Yuan, “Tapered fiber optical tweezers for microscopic particle trapping: fabrication and application,” Opt. Express 14(25), 12510-12516 (2006) Conclusion • Brief explanation of optical trapping, its origins, basic theory behind the technique & the various system configurations • Provided an evaluation of the pros & cons for both classical and fibre based systems • Presented T-FOTs a 3D fibre based optical trapping system • Offered a hypothesis for trapping failure at a 10° insertion angle & provided a viable solution for the problem • Discussed the maximum trapping range Thank You Any Questions?