Post on 13-Dec-2015
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 Fgrad > Fscat
fullfilled
• Optical forces in nN-pN range
Working principle of optical tweezers
• Trapped objects: - Bose-Einstein condensates
- chromosomes
- bacteria
• Specific designs optically
induced rotation
• Variations/additions other
functionalities
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
• Laguerre-Gaussian mode (index l) and Gaussian
beam superposed spiral pattern
Variation of relative phase optical rotation
Variants different modes of light
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
• 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 optical tweezers: several methods
Multiple dynamic optical tweezers
• 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
Multiple optical tweezers: several methods
Multiple dynamic optical tweezers Multiple dynamic optical tweezers microfluidic pumps:
• Rotating lobe-pump: rotating lobes laminar flow - reversing the rotation directions flow reversed
• Peristaltic pump: propagating sine wave laminar flow - changing propagation direction reversed flow
Multiple dynamic optical tweezers Multiple dynamic optical tweezers microfluidic pumps:
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
Questions/comments