3D Optical Trapping via Tapered Optical Fibre at Extreme Low

Insertion Angles

Presentation by: Steven RossThe GERI Weekly Seminar

Friday 18th October 2013

Supervisors: Prof. D. R. Burton, Dr. F. Lilley & Dr. M. F. MurphyGeneral Engineering Research Institute (GERI), Coherent & Electro-Optics Research Group (CEORG), Liverpool John Moores University

(LJMU), GERI Building, Byrom Street, Liverpool, L3 3AF, UK.Email: s.ross@2002.ljmu.ac.uk

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

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

Single Beam gradient force optical trap – “Optical tweezers”

Pros & Cons of “Classical” and fibre Based System Configurations

“Classical” 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

Fibre Based Optical Tweezers

• 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 96

Tip 92

Tip 94

Maximum Trapping Range

Maximum Trapping Range

Maximum Trapping Range92 94 96 Simulated

Maximum Trapping Range (µm) 9 13 13 0-12 [1]

Tapered Tip Movement (µm) 9 9 10

Actual Particle Displacement (µm) 6.27 6.11 8.2

Percentage error % 30.3 32.1 18

[1] 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?