Optical micro-manipulation techniques
Optical micro-manipulation techniques use the transfer of momentum from light to microobjects. Such a transfer occurs, for example, during light scattering from the microobjects, which is accompanied by a change of direction of the light propagation. Optical micromanipulations make it possible to influence the motion of objects of sizes from tens of nanometers to tens of micrometers just by illumination with a laser beam. Optical tweezers represent an optical analogy to the classical mechanical manipulation tool, using a single tightly focused laser beam to confine objects in a contactless way. Since the objects are trapped in the vicinity of the beam focus, repositioning of the focus also causes repositioning of the objects, i.e. their controlled spatial micro-manipulation. Several laser beam foci can be placed simultaneously within the specimen in a controlled way, therefore enabling simultaneous confinement and controlled micro-manipulation with multiple objects. This tool is mainly used for manipulation of objects suspended in a liquid medium (living microorganisms or cells in water or appropriate solution, microobjects placed behind transparent obstacles etc.). Since such small objects can only be observed using optical microscope, both techniques are often combined.
Examples of optical micromanipulation application:
- A compact version of optical tweezers has been developed based on an adapter inserted between the microscope and lens and containing an integrated laser diode or optical fibre connector. Optical micromanipulation is therefore possible without disturbing the optical path of the microscope.
- Optical tweezers can be combined with several optical spectroscopic techniques (e.g. Raman microspectroscopy, fluorescent spectroscopy) to enable contactless and non-destructive characterization of the trapped microobjects.
- Highly promising is the combination of optical micromanipulation techniques with microfluidic systems (lab-on-a-chip) that can serve, for example, for the study of stress at the level of individual cells followed by cell sorting.
- In addition to tightly focused beams there are also other configurations of laser beams for regular microobject confinement and arrangement in the space or on the surface.
- Illumination of moving microobjects influences their motion and can cause rectification of their stochastic motion leading to optical sorting of different suspension components (e.g. different types of cells).
Examples of applications of laser beams focused down to micrometer spots
- The significant increase of the laser beam intensity near its focus initiates photopolymerization – a chemical reaction that solidifies the liquid monomer into a solid polymer. Complex microstructures can be created by controlled positioning of the focused laser beam within the monomer solution.
- Focused pulsed laser beams of suitable wavelength offer many opportunities of employing their destructive effects (microablation) to volume or surface modifications of objects including, for example, intervention inside living cells.
