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