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
Optical trapping came about after Arthur Ashkin, investigating the forces of radiation pressure found that not only was a particle propelled along in the direction of the incident laser beams direction of propagation, he also observed that particles located at the fringes of the Gaussian beam, where pulled into the high intensity region of the beam.
When the laser was switched off the particles again drifted into the fringes of the beam but where pulled back on axis when the beam was switched back on.
This prompted the total force acting upon the particle to be divided into two force components.
The scattering force which pushes particles in the direction of the beams direction of propagation
And the gradient force which pulls particles located at the fringes of the beam into the high intensity region of the beams axis.

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
Ashkin went on to make his first 3-D optical trap by introducing a second counter propagating laser beam
The scattering forces of both the beams cancelled each other out creating a Net scattering force at an equilibrium point E the point in which a particle is trapped in three Dimensions.
Later he presented the optical levitation trap whereby the scattering force of a sing vertical laser beam was balanced using gravity at the equilibrium point 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
For a single laser beam to be able to trap a particle in three dimensions Ashkin later discovered that the gradient force must be greater than the scattering force. This is achieved by tightly focusing the laser beam through a high NA microscope objective.
Resulting in an axial equilibrium point, the point at which a particle is held in 3-D, located slightly beyond the focal point of the beam.
It is this single beam gradient force light trap that coined the phrase “optical tweezers”

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
Strong Field enhancement from light scattering at a metallic tip to generate a trapping potential deep enough to overcome Brownian motion and capture nano-metric particles
Combination of evanescent illumination from the Substrate and light scattering at a tungsten probe apex is used to shape the optical field into a localised 3D optical trap
However these two approaches require high powers for illumination (> 1 W) and prevent a difficult integration in conventional microscopy techniques such as fluorescence and confocal

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
The early 90’s witnessed the introduction of fibre based traps In an attempt to reduce build costs, decouple the trapping laser light from the microscope and reduce the size of an optical light trap. The first of which consisted of two counter propagating optical fibres two deliver the laser beams directly into the microscope sample chamber.
However to truly emulate “optical tweezers” the gradient force must be greater than the scattering force achievable through tightly focusing the laser light. This lead to modification of the fibre ends to create micro-lens’s to focus the beam.

8. Fibre Trapping Methods
Counter propagating fibres
Requires exact alignment
Difficult to manoeuvre
Requires particle to drift into trapping zone
Ideal for use in conjunction with a micro fluidic system

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
Read liberalle for more information
What is the dimensions of the bundle?

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
It is desirable to maintain the first order mode in the fibre without mode conversion to higher order modes or to radiation modes taking place.

12. Methods for Shaping Fibre Ends
Polishing
4 axis lapping machine
Laser micro-machining
Works in much the same way as a lathe
There are a number of techniques that allow the fabrication micro-lens’s on the fibre ends

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
Focused Ion beam milling-
The FIB is a scientific instrument that resembles a scanning electron microscope. However, while the SEM uses a focused beam of electrons to image the sample in the chamber, a FIB instead uses a focused beam of ions. Unlike an electron microscope, the FIB is inherently destructive to the specimen. When the high-energy gallium ions strike the sample, they will sputter atoms from the surface. Gallium atoms will also be implanted into the top few nanometres of the surface, and the surface will be made amorphous.
Because of the sputtering capability, the FIB is used as a micro-machining tool, to modify or machine materials at the micro- and nanoscale. FIB micro machining has become a broad field of its own, but nano machining with FIB is a field that still needs developing. The common smallest beam size is 4-6 nm. FIB tools are designed to etch or machine surfaces, an ideal FIB might machine away one atom layer without any disruption of the atoms in the next layer, or any residual disruptions above the surface. Yet currently because of the sputter the machining typically roughens surfaces at the sub micrometre length scales

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
Chemical etching-the fibre ends are dipped into a 40% Hydrofluoric acid solution with an organic solvent over-layer to control the height of the meniscus of the HF forming at the glass fibre 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