Download (Tom) Lummen

Optical tweezers
Manipulating the microscopic world
Tom Lummen, June 2004
Introduction: History
• 1609: Johannes Kepler noticed Sun’s
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
Working principle of optical tweezers
• One photon carries momentum p = h/ λ
• 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
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) > scattering force
•
Strong enough focusing
fullfilled
•
Optical forces in nN-pN range
Fgrad > Fscat
Working principle of optical tweezers
• Trapped objects: - Bose-Einstein condensates
- chromosomes
- bacteria
• Specific designs
induced rotation
• Variations/additions
functionalities
optically
other
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
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
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’
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
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
› Only 2D trapping patterns possible
efficient dynamic control
Multiple dynamic optical tweezers
Multiple dynamic optical tweezers
microfluidic pumps:
• Rotating lobe-pump: rotating lobes
- reversing the rotation directions
laminar flow
flow reversed
Multiple dynamic optical tweezers
Multiple dynamic optical tweezers
microfluidic pumps:
• Peristaltic pump: propagating sine wave
laminar flow
- changing propagation direction
reversed flow
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
technologies
lab-on-a-chip
Questions/comments