Optical Imaging
Mattias Goksör, Experimental Physics

Optical manipulation is a laser based manipulation technique that can be applied on a single cell level. The technique was originally designed for manipulations of neutral atoms and molecules in vacuum, but was found to apply also for microscopic particles, such as cells and bacteria. Optical manipulation is achieved by focusing the light from a laser using the objective of an ordinary light microscope to a diffraction limited focus in the sample. A transparent particle with a refractive index different from the surrounding medium will then refract the light, causing a change in momentum. This gives rise to a force acting on objects in the sample near the focus. If the index of refraction of the surrounding medium is less than that of the object, the change in momentum will force the object to the region of highest intensity, thus trapping it in the centre of the beam. Any displacements of the trapped cell will be counteracted which brings it back to the centre of the beam again. The usefulness of optical manipulation applied in biological research was demonstrated in 1989 when Arthur Ashkin showed the possibility of trapping membranes inside plant cells, pulling out slender visco-elastic filaments and altering the structure of the cell as a form of internal cell surgery without damaging the cell membrane. Since the sample under the microscope is never in physical contact with any manipulators, the cells can be prepared, studied and manipulated under sterile conditions. Using different lasers the light can trap (optical tweezers), heat or dissect (optical scalpel/laser scissors) the studied cells. The different manipulation techniques thus allowed the user of the microscope not only to be a passive bystander, but to take full control over the studied objects. The results from the early experiments clearly show that optical manipulation is an excellent tool for single cell studies. It allows the user of the microscope to follow a single cell under a long period of time and study how that specific cell responds to different conditions. It is therefore of great importance to be able to combine optical manipulation and imaging of fluorescent probes in different optical setups for studies on a cellular level. This aim of this project is to develop new instrumentation and techniques for optical manipulation and imaging that can be applied in microbiological research. A genuine interest in optical construction and laser physics are required.