Optical trapping in complex media
Begeleider(s): ir. Stijn Vandewiele en ir. Mathieu Balcaen


Optical tweezers are a powerful instrument to examine micro­meter-sized objects immersed in a fluid and are a widespread tool in various fields of research, ranging from molecular biology over fundamental cell research to colloidal science and nanotechnology. With optical tweezers it is possible to hold a micro­particle in place and manipulate its position by using a laser beam tightly focused by a microscope setup. Moreover, they allow for very sensitive force measurements to investigate various kinds of interactions between the micro­objects.
The performance of the optical tweezers relies mainly on the quality of the diffraction­ limited focus of the laser beam and on the intensity distribution of the light. Various aberrations, originating from the microscope optical components, the suspending fluid or the micro-object itself, deteriorate the beam quality and reduce the optical tweezers operation. The goal of this thesis is firstly to investigate novel ways to compensate for these aberrations by manipulating the phase front of the laser beam with a spatial light modulator (SLM). Secondly, this SLM will be used to generate arbitrary optical landscapes, enabling the trapping of complex structures.

As a proof of concept water droplets in a larger oil droplet (a double emulsion) will be trapped and manipulated. This allows us to investigate the fundamental principles behind the osmotic swelling/shrinking of these droplets under changing conditions. A better understanding of these processes in double emulsions is important for e.g. pharmaceuticals, food processing or water treatment. 

Figure: Trapped double water-in-oil-in-water particle.


This master project starts from an existing optical tweezing setup where a spatial light modulator is already introduced. The SLM creates a holographic image of the trapping beam using phase modulation of the beam. For this, various new algorithms will have to be developed and programmatically applied. Next, the algorithms will be used in a feedback loop resolving static and dynamic aberrations of the system. In a final step these algorithms will be used to generate optical landscapes, eventually resulting in the optical trapping of complex structures such as a water-in-oil-in-water double emulsion droplet.

The staff of the LCP group and of the PaInT  will provide you with all the necessary help and know-how, but we also encourage you to take the initiative to come up with your own ideas to tackle the project. Where feasible and within the scope of the project, we'll support you to develop these ideas.

Figure: Aberrations introduced by a suspending fluid deteriorate the performance of optical tweezers (left). This master thesis uses a spatial light modulator (SLM) to compensate for these aberrations and create optical landscapes to trap complex structures.