Optical tweezers

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Optical tweezers

• Manipulating the microscopic world

Tom Lummen, June 2004
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Introduction History

• 1609 Johannes Kepler noticed Suns radiant
  pressure
• 1970 Arthur Ashkin of Bell Labs builds
  levitation trap
• 1978 Ashkin builds two-beam trap
• 1986 Ashkin builds single-beam gradient force
  trap Optical tweezers

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Working principle of optical tweezers

• One photon carries momentum p h/ ?
• photon refraction momentum change

• Transparent particle of large refractive
• index lens
• Gaussian beam intense center
• momentum conservation
• Lateral trapping refraction of Gaussian
• beam gradient force (Fgr) and a
• scattering force (Fscat).
• The lateral gradient force pulls particle
• to beam center

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Working principle of optical tweezers

• Scattering force (radiant pressure)
• pushes the particle

• Strongly focused beam axial intensity
  
• gradient axial gradient force
• 3D optical trapping axial gradient force
• (Fgrad) gt scattering force
• Strong enough focusing Fgrad gt Fscat
  
• fullfilled
• Optical forces in nN-pN range

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Working principle of optical tweezers

• Trapped objects - Bose-Einstein condensates
• - chromosomes
• - bacteria
• Specific designs optically
• induced rotation
• Variations/additions other
• functionalities

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Unconventional optical tweezers

• Variants different modes of light

• Optical vortices donut intensity
  pattern
• they trap dark-seeking particles
• absorbing, reflecting or
  low-refractive-index
• Laguerre-Gaussian mode
• helical phase profile
• angular momentum
• optical rotation

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Unconventional optical tweezers
Variants different modes of light

• Laguerre-Gaussian mode (index l) and Gaussian
• beam superposed spiral pattern
• Variation of relative phase optical
  rotation

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Multiple dynamic optical tweezers

• Multiple optical tweezers several methods

• Time-shared optical tweezers computer
  controlled mirrors
• trap periodically scanned
• arbitrary trapping patterns
• - restricted by minimum required
• scanning period
• - only formation of 2D patterns possible

The Chinese character for light
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Multiple dynamic optical tweezers
Multiple optical tweezers several methods

• Dynamic holographic optical tweezers
  computer-addressed
• spatial light modulator (SLM) splits
  incident beam
• specific pattern specific spatial
  light modulation
• (phase hologram)
• phase holograms calculated beforehand
• Also 3D trapping patterns can be generated

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Multiple dynamic optical tweezers
Multiple optical tweezers several methods

• The generalized phase contrast (GPC) method SLM
  
• spatial phase profile conversion to
  spatial intensity profile
• No need to calculate phase holograms
  efficient dynamic control
• Only 2D trapping patterns possible

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Multiple dynamic optical tweezers

• Multiple dynamic optical tweezers
  microfluidic pumps
• Rotating lobe-pump rotating lobes
  laminar flow
• - reversing the rotation directions
  flow reversed

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Multiple dynamic optical tweezers
Multiple dynamic optical tweezers
microfluidic pumps

• Peristaltic pump propagating sine wave
  laminar flow
• - changing propagation direction
  reversed flow

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Conclusions/Future prospects

• Optical tweezers unique non-invasive
  control of wide variety of microscopic particles
• Variants field of applicability even
  further expanded
• also optical rotation
• Multiple dynamic optical tweezers
  dynamic reconfiguration of arbitrary trapping
  patterns
• functional micromachines lab-on-a-chip
• technologies

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Questions/comments