Optical Tweezing with Optical Tweezing with Adaptive OpticsAdaptive Optics

Canada France Germany - Young Photonic Researchers – Canada France Germany - Young Photonic Researchers – Munich, 2009Munich, 2009

Shaun Bowman

Supervisors - Dr. Colin Bradley, Dr. Rodolphe ConanAdaptive Optics Laboratory

The Adaptive Optics Laboratory – University The Adaptive Optics Laboratory – University of Victoriaof Victoria

Lab facts:

•Founded 1998

•Dr. Colin Bradley, director

•2 post docs

•2 PhD candidates

•2 Masters students

•Co-op student / interns

•Contract professionals

Key equipment:

•Deformable mirrors: 1k,

64, 52, and 32 actuator

•Steer mirrors

•HASO Beam profiler

•Zygo interferometer

•Micro EDM mill

•Optics room

The Adaptive Optics Laboratory – University The Adaptive Optics Laboratory – University of Victoriaof Victoria

Astronomy Collaboration

•European Space Agency

•ACURA

•University of California

•Thirty-Meter-Telescope Project

•NRC Hertzberg Institute of

Astrophysics

•University of Toronto

•Canada-France-Hawii

Telescope

•Caltech Astronomy

•... and a lot I don't know!

Optical Tweezers – Ray optics regimeOptical Tweezers – Ray optics regime

Light as a manipulator

•Particle:

•Index of refraction

> medium

•Diameter ~ 5x

wavelength to

100um

Optical Tweezers – Force applicator / Optical Tweezers – Force applicator / dynomometerdynomometer

Gauging and applying forces

•Overdamped 2nd order

system

•Brownian motion gives

Stochastic forcing function•=> Langevin equation

•=>Use power spectral density of

position to deduce stiffness

Science Case for Optical TweezersScience Case for Optical Tweezers

Force extension

•Study of DNA uptake by

bacteria

•Observe uptake

•Measure stall-force

(7 – 40 pN typ.)

P. Johnson, Simon Fraser University, 2007

Background on adaptive opticsBackground on adaptive optics

Adaptive Optics using deformable mirrors

•Wave description of light:

•A(x,y) => Amplitude

•ζ(x,y) => Phase

•Wave at trap = F( wave at aperture or deformable mirror)

The mirrors shape DIRECTLYdescribes the phase!

Sensing phase, wavefront sensor – Sensing phase, wavefront sensor – the eyesthe eyes

Optical Tweezing

•Need phase for closed-loop

•Cant measure the phase

•Can measure focal position

•Tilted phase causes position shift

•Can measure SLOPE of the phase

•Shack-hartmann wavefront sensor

Controlling phase, deformable mirrors – Controlling phase, deformable mirrors – the the handshands

Deformable mirrors

•Peizo:

•0.5 – 2 um stroke

•> 1khz bandwidth

•Voice coil

•5 – 100 um stroke

•> 400hz bandwidth

DM SHAPE = -0.5 x IN shape

IN OUT

Closed loop control – Closed loop control – the brainsthe brains

Closed loop controllers

•Calibrate:

•Phase vs mirror

voltages

•Trap position vs

phase

•Invert

•Command new position

•Generate new phase reference

•Controller removes phase error

•Particle moves to new location

Uvic Optical Tweezer ApperatusUvic Optical Tweezer Apperatus

Closed loop controllers

•Calibrate:

•Phase vs mirror

voltages

•Trap position vs

phase

•Invert

•Command new position

•Generate new phase reference

•Controller removes phase error

•Particle moves to new location

Using particle position in wavefront Using particle position in wavefront controllercontroller

Command position in real units

•Particle detection by

symeteric-phase-only-matched-

filter (SPOMF) method

•Known Tip/Tilts applied to

relate wavefront and trap

position

Using particle position in wavefront Using particle position in wavefront controllercontroller

Modified controller

[R] [W]t k(DM)Z-1

(WFS)Z-1

(PC)Z-1

DesiredTrap Location

Reconstruction Matrix

P

ReferenceWFS

Measurements

sr

+

ReconstructionWFS

Offsets

rr

+

+

+

+

-

+

+

Using particle position in wavefront Using particle position in wavefront controllercontroller

Demonstration

•20 mW optical power at

objective

•15 um polystyrene bead

•30 x 30 um range of motion

•50 um max wavefront tilt

Current workCurrent work

Calibration

•Stiffness as a function of trap

position

•2x2 Traps controlled by one

deformable mirror

totrap formingMicroscopeobjective

afterDMM andWFS beamsplitter

2x2 lenslets

collimating lens

Thank youThank you

Questions?

Shaun Bowman

Adaptive Optics Laboratory

University of Victoria

BC, Canada

Ph: 1 250 721 8624

sbowman@engr.uvic.ca