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

2. 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

3. 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

4. What is Optical Trapping?

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

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

18. Results

19. Force/Optical Power Gradient
Results
Tip 44
45°
Tip 92
10°
Tip 94
Tip 96
Force/Optical Power Gradient
Trapping Efficiency Q
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

20. 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

21. 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

22. Thank You
Any Questions?