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 (F gr ) and a scattering force (F scat ). 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 (F grad ) > scattering force Strong enough focusing F grad > F scat 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
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