Atomic force microscopy, first implemented in 1986, is a scanning force microscopy technique with which resolutions smaller than a nanometer can be achieved. It has proven to be a very powerful method, but is limited to probing surfaces. Photonic force microscopy is an innovative variation of the same principle, in which optical tweezers are used instead of a cantilever, and which is not limited to surfaces. The principle behind optical tweezers is that an object with a higher refractive index than the surrounding liquid is attracted towards the focus of a tightly focused laser beam. For a spherical microparticle, this force can be very well characterized. If another, unknown force is also acting on the particle, its equilibrium position will slightly shift away from the focus. By measuring the displacement of the particle, the unknown force can very accurately be determined. By scanning the particle along a surface, in the vicinity of another object, or just in the bulk of a liquid, a detailed three dimensional force landscape can be obtained.
Figure: Schematic representation of the implementation and principle of photonic force microscopy
The “Liquid Crystals and Photonics” (LCP) Group has built a double optical tweezers and accurate position detection setup, around a state-of-the-art confocal fluorescence microscope. We have already demonstrated that we can measure charge with a resolution better than the electron charge with this setup (see e.g. http://physics.aps.org/articles/v5/3 ). In this master thesis project, we want to extend the possibilities of our setup to photonic force microscopy. We will primarily focus on the measurement of electrical forces, to accurately map the charge distribution on surfaces or objects in a solution. To achieve this, charged microparticles will be used as a probe for the electric force. Your goal will be to implement this technique on our setup, characterize and calibrate the optical force on the charged microparticle probe, and demonstrate photonic force microscopy in a number of different situations. This is an ambitious project, which includes technological, experimental and theoretical work.
The staff of the LCP group 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.