Optical tweezers are based on a strongly focussed laser beam which allows contact free holding and manipulating of objects by means of forces arising from the scattering and reflection of light. Since the development of optical traps they have found a broad range of applications in physical, chemical and biological sciences. Applications include single molecule force measurements, determination of microscale viscosity and sophisticated manipulation of objects in the micro- and nano-size range including the manipulation of aerosols, vesicles and even living cells. Radiative damage to tweezed objects and especially cells and objects containing (bio)polymers can be largely avoided by the use of infrared lasers which emit light in a absorption minimum between the strong absorptions of bio-molecules and water. At these wavelengths, cells and vesicles can be trapped as well as manipulated without extensive photo-damage.
Combinations of optical traps with Raman spectroscopy are also an established technique whose applications span from particle characterisation to chromosome characterisation and experiments on living cells. The combination of Raman spectroscopy and optical tweezing opens the opportunity to use the trapped object as optical cavity. The quantitative analysis of the standing waves that are forming in this cavity (whispering gallery modes, WGMs or morphology dependent resonances. MDRs) allows the determination of the size of tweezed micro-objects with nanometer precision. Combining this analysis with conventional Raman spectroscopy allows the precise sizing of tweezed particles whilst simultaneously chemical processes can be monitored via the Raman signal.
Fluorescence spectroscopy can be combined with optical tweezing to use the full potential of the technique and even fluorescent particles can be trapped directly providing the use of suitable trapping laser wavelengths.