Presentation is loading. Please wait.

Presentation is loading. Please wait.

Optical Tweezers A revolution in micro-manipulation Jonathan Leach University of Glasgow.

Similar presentations


Presentation on theme: "Optical Tweezers A revolution in micro-manipulation Jonathan Leach University of Glasgow."— Presentation transcript:

1 Optical Tweezers A revolution in micro-manipulation Jonathan Leach j.leach@physics.gla.ac.uk University of Glasgow

2 Todays talk What are optical tweezers? Dynamic movement and multiple particles Current research around the World

3 What are optical tweezers? Optical tweezers use light to trap, manipulate and position micron sized objects. Invented approximately 20 years ago by A. Ashkin et al. K.C. Neuman and S. M. Block, Optical Trapping, Rev. Sci. Inst., (2004) J. E. Molloy and M. J. Padgett, Lights, Action: Optical Tweezers, Cont. Phys., (2002)

4 What are optical tweezers? A tightly focused laser produces a force great enough to trap micron sized dielectric particles. Require…… 1. Laser 2. Lens 3. Object 4. Damping medium F scatt F grad

5 Optical tweezers in action

6 The equipment Optical tweezers are based on high magnification microscope lenses –produces tightly focussed beam –provides visualisation of image Samples suspended in fluid –provides cooling –provides buoyancy

7 The equipment Require tight focusing so need high numerical aperture, N.A. Magnification typically X100 N.A. = n sin( ) n is the refractive index of the medium between the objective lens and the sample. Using oil immersion lenses, n ~ 1.3 so N.A >1 is possible.

8 Optical Trapping - a>> Conditions for Mie scattering when the particle radius a is larger than the wavelength of the light. We can use a ray optics argument and look at the transfer of momentum a

9 Optical Trapping - a<< Condition for Rayleigh scattering when the particle radius a is smaller than the wavelength of the light. Scattering force and gradient force are separable F grad > F scatt requires tight focusing a

10 The scales Can trap 0.1 to 10s m 1 m is….. …the same as 1/100th diameter of a hair. In water, you can move a particle at about 20-30 m per sec. Require 10mW per trap. Can rotate at 100s of Hz.

11 If absorbed by particle of refractive index n, a beam of power P produces a reaction force F = nP/c (e.g. P = 1mW: F = 5pN) The Q factor of optical tweezers The efficiency Q, of optical tweezers is defined as Q = F actual / (nP/c) (typically Q 0.05-0.3)

12 Optical Trap Dynamics Equation of motion of particle in a potential well restoring force Brownian motion Newtonian force drag force

13 Particle in fluid Solution is of exponential decay Damping provided by water

14 Particle in ideal trap Solution is of simple harmonic motion Spring constant

15 Trapped particle in fluid Solution is of damped simple harmonic motion

16 The whole picture Time averaged effect is 0 Stochastic events introduce fluctuations in the particles position Add in the effect of Brownian motion

17 Trap dynamics Look at the movement of the particle in x and y

18 Power spectrum Trap strength Fourier transform to get the power spectrum Lorenzian

19 Real data

20 Coming next

21 Exam question? In groups of 3 or 4, create two exam questions, one long, (6 marks), one short (3 marks). 5mins

22 Collecting data How can we collect this data? Moving 100s nm at a few kHz!!! 3 options

23 Option 1 - Camera Camera placed in the image plane of the objective lens. Uses the light from the illumination source. Fast shutter speed to take clean image of particle.

24 Option 1 - Camera Advantages Easy to use Visual image of particle Multiple particles Disadvantages 2D measurement Bandwidth limitations <100Hz Very slow compared to f 0 Require very fast shutter so need a sensitive camera

25 Option 2 - Quadrant Photodiode A Quadrant photodiode placed in the image plane of the objective lens (exactly the same as the camera). Uses the light from the illumination source.

26 Option 2 - Quadrant Photodiode A Advantages Large bandwidth 100s kHz Very fast compared to f 0 Disadvantages Single particle Low light level, so small signal 2D measurement

27 Option 3 - Quadrant Photodiode B Quadrant photodiode collects the laser light transmitted through the condenser lens. Small changes in the transmitted and scattered light are measured.

28 Option 3 - Quadrant Photodiode B Advantages Large bandwidth 100s kHz Very fast compared to f 0 3D measurement High light level as collecting laser light Disadvantages Complex arrangement Single particle

29 Moving particles and multiple particles

30 Some background optics An angular shift in the object plane results in a lateral shift in the image plane. Collimated light is brought to a focus a distance f, from a lens of focal length f. Object plane Image plane

31 Some background optics If the beam is not collimated there is a shift in the axial position of the focus.

32 Moving objects around ffffff Relay lenses Beam steering mirror Angular deflection at mirror gives lateral shift of trap

33 Diffractive optics Placing a diffractive optical element in the object plane can generate a number of focused spots. Diffraction grating

34 Spatial Light Modulators Calculated pattern Video signal Incoming beam reflected/diffracted beam SLM optical addressing Spatial light modulator = computer-controlled hologram –Liquid crystal (introduce phase or amplitude modulation) –Optically addressed SLMs convert intensity pattern to phase –diffraction efficiency >50% –>VGA resolution

35 Holograms at work split the beam focus the beam transform the beam combinations of the above

36 Whole beam path SLM beam- steering mirror mirror imaged on microscope entrance pupil microscope objective SLM imaged on beam-steering mirror also: plane waves conserved

37 Holographic optical tweezers can do (just about) anything! Hologram Incident beam Diffracted beams Curtis et al. Opt. Commun. 207, 169 (2002) Holographic beam generation can create –multiple beams –modified beams –focussed beams –or all these at the same time REAL TIME control of the beams

38 Dynamic multiple traps Eriksen et al. Opt. Exp. 10, 597 (2002) Use spatial light modulator to create multiple traps –Lateral displacement –Axial displacement Update trap positions –Video frame rate

39 Rotating cube

40 Coming next ?

41 Exam question? In groups of 3 or 4, create two exam questions, one long, (6 marks), one short (3 marks). 5mins

42 Applications of optical tweezers

43 Bio-applications The size of particles that can be trapped is ~0.1 m to 10s m Approximately the same size as many biological specimen. e.g. Blood cells, stem cells, DNA molecules Either trapped directly, or beads used as handles to reduce optical damage. Ashkin et al. Nature. 330, 768 (1987) Block et al. Nature. 338, 514 (1989)

44 Measuring force/motion Molloy et al. Biophys J. 68, S298 (1995) biological object trapped bead quadrant detector imaging lens Image trapped bead (handle) onto quadrant detector Measure movement of shadow –nm accuracy! –kHz response Adjust trap to maintain position gives measurement of force –sub-pN accuracy!

45 e.g. Observation of myosin binding Handles attached to actin filament Intermittent binding to myosin suppresses thermal motion of beads due to stiff physical bond

46 e.g. Stretching/twisting of DNA Perkins et al. Science. 264, 822 (1994) Wang et al. Science. 282, 902 (1998) Attach handles to ends of DNA molecule Pull, let go and observe what happens! –understanding of protein folding

47 Work at Glasgow 5 microns Jordan et al., J. Mod. Opt.,2004 Permanent micro- structures Use SLM to create tweezers arrays Trap pseudo 2D crystals (100) (Curtis 2002) What happens when you turn the light off? –Fix structure in gel Permanent micro- structures Use SLM to create tweezers arrays Trap pseudo 2D crystals (100) (Curtis 2002) What happens when you turn the light off? –Fix structure in gel

48 Physical applications

49 Transfer of angular momentum Angular momentum per photon = hbar Angular momentum per photon = -hbar If the particle Is birefringent it will absorb angular momentum and rotate. Half- waveplate Circularly polarised light Direction of propagation

50 Physical applications Polarisation vectors rotate (circular polarisation) Spin angular momentum Phase structure rotates (helical phase fronts) Orbital angular momentum Padgett and Allen, Contemp. Phys. 41, 275 (2000)

51 Absorption of orbital and spin angular momentum Orbital AM Spin ONeil et al. Phys. Rev. Lett. 053601 (2002)

52 Microfluidic applications

53 Micro-machines driven by optical tweezers optical micro-pump Terray et al. Science. 296, 1841 (2002) Translational (or rotational) control Fluid pumps Optically driven stirring

54 Vortex Arrays Ladvac and Grier, Optics Express, 2004

55 Work at Glasgow Optically driven pump using two counter rotating birefringent particles

56 Work at Glasgow Flow Flow meter v = d/t Turn laser on and off and measure particle displacement. d

57 Work at Glasgow Flow meter

58 Work at Glasgow Flow meter results

59 A few of the (many) active groups World-wide –Grier et al. NY USA –Glückstad et al. Risø Denmark –Rubinstein-Dunlop et al. Queensland, Australia UK –Us! Glasgow –Dholakia et al. St Andrews –Molloy et al. National Institute for Medical Research, London

60 Conclusions Trap dynamics and mechanisms Positioning, manipulation and control Bio, micro, physical applications

61 Constants N.A. = numerical aperture n = refractive index = angle a = radius of particle = wavelength of light I 0 = intensity n m = refractive index trapping medium n p = refractive index particle m = n p /n m (in the F scatt, F grad equation) c = speed of light m = mass (in the equation of motion) P = power Q = trapping efficiency a = acceleration v = velocity x = position t = time T = temperature k B = Boltzmanns constant S = power spectrum = 6 a = viscosity = trap strength

62 Exam question? In groups of 3 or 4, create two exam questions, one long, (6 marks), one short (3 marks). 5mins


Download ppt "Optical Tweezers A revolution in micro-manipulation Jonathan Leach University of Glasgow."

Similar presentations


Ads by Google