Single Tapered Fibre “Optical Tweezers” Steven Ross GERI-CEORG Supervisors: Professor D. Burton, Dr. F. Lilley & Dr. M. Murphy

Contents Project Aim What is Optical Trapping? Optical Forces & Optical Trapping Theory Classical “Optical Tweezers” V Fibre Based System Fibre Trapping at Low Insertion Angles; The Problem, Solution & the Resultant Change in Trap Dynamics Experimental methods, Particle Tracking, Force Determination & Results Conclusion

Project Aim To develop a 3D-single beam optical trap To aid investigations of the mechanical properties of cells Capable of easy integration with other microscopy applications for example AFM Therefore some degree of portability required Leading to the development of an optical fibre based configuration

What is Optical Trapping?

Optical Forces Forces generated up to 200 x 10-12 N Scattering force - Due to reflection, points in the direction of the beams propagation Gradient force - Due to refraction, points in the direction of the beams high intensity region

Optical Trapping Theory Scattering force must be cancelled – counter propagating beams gravity Single beam trapping gradient force must be greater than scattering force Achieved by strongly focusing the laser Creating a high intensity Gaussian profile 1 2 3

Classical “Optical Tweezers” First demonstrated by Arthur Ashkin in 1986 High NA microscope objective used for focusing the laser and imaging Photo detectors for position detection Multiple trapping sites Particle transit in X and Y plane Large surface are required

Single Tapered Fibre “Optical Tweezers” Advantages Reduced size and build costs Decoupled from the microscope No position detection Capable of sample elevation Disadvantages Complexity increases for multiple trapping sites Fibre tips Prone to damage

Fibre Trapping “to be or not to be”? Literature suggests optimum trapping forces found at fibre insertion angles between 45-55° Oh dear major problem AFM has a head above the sample chamber Insertion angle limited to about 10° or less

Trapping at Low Insertion Angles First Fibre tip (tip 44) trapped at 45° Insertion angle But not at 10° Why is this? After all its just a beam of light incident upon a spherical object Lead to investigations of the tip profile

Trapping at Low Insertion Angles Taper profile was such that the particle had to be elevated This required very high optical powers ~ 600 mW Dangerous levels for biological samples Lead to the development of new tips with different profiles Requiring a longer taper and a smaller diameter tip

Newer Slimmer Models To begin with the new tips appeared to fail Created 3 new working tips Tip 92, 94 & 96 3D trapping Tested at 45°, 30° & 10° insertion angles Tip 96 trapping focal point beyond fibre end Start Frame End Frame

Change in trap Dynamics At 10° insertion angle the introduction of a trapping range was observed Particles within the range are trapped and repelled if beyond The trapping range is tuneable, varying with tip profile Ranges recorded between 6-13 µm

Experiment 1 Force Determination Dynamic measurement method Particle trapped, laser deactivated Fibre tip moved in (–ve) x direction, Laser reactivated High speed video records the particles position, as it is drawn into the trap Repeated for various laser outputs, tips and insertion angles Characterise the trap as a function of the optical power with respect to the trapping force

Particle Tracking Particles trajectory is tracked using particle tracking image processing package developed in the IDL platform Provides particle co-ordinate data in both pixel and µm formats Allows the origin to be set at the fibre end

Force Determination Fopt = Trapping force η = Viscosity of the medium r = Radius of the particle S =Particles position m = Mass of particle Re = Reynolds number ρ = Density of the medium V = Maximum velocity D = Particle diameter η = Viscosity f the medium Trapping force calculated using the stokes equation and Newton's second law Calculated from the first and second derivatives of the particles position as a function of time Inertia force can be ignored due to the Reynolds number (Re)<< 1

Results 60% ~ 630 mW 55% ~ 550 mW 50% ~ 470 mW 45% ~ 380 mW

Results

Force/Optical Power Gradient Results Tip 44 45° Tip 92 10° Tip 94 Tip 96 Force/Optical Power Gradient 0.00386 0.00848 0.00816 0.00724 Trapping Efficiency Q 0.00114 0.00234 0.00173 0.00170 Force/Optical Power Gradient = F/P (N/W) Trapping Efficiency QMAX n= Fc/nP F = Optical force acting on the particle, P = Incident laser power, c = Speed of light, n = Refractive index of the medium

Experiment 2 Optical Force Field Mapping Particles trapped from various points about the fibre tip Knowing the optical trapping force for each trajectory will allow the mapping of the optical force field distribution Providing information such as focal point, beam spot size, beam waist position and divergence angle

Conclusion Project Aims Optical Trapping Forces Basic Optical Trapping Theory Differences Classical and Fibre Based Optical Traps Low Insertion Angle Hurdles and Solutions Novel Trap Dynamics Observation Experimental Procedures Particle Tracking & Force determination Results

Thank You Any Questions?