Optical tweezers are used to manipulate and characterize micrometer-sized colloidal particles in liquids. They are used in a variety of fields, ranging from fundamental physics to life sciences, to accurately measure forces down to piconewtons. Optical tweezers consist of a tightly focused laser beam in a microscope system. Because of the intensity gradient of the light, particles with a higher refractive index than the surrounding medium are attracted towards the focus. Nowadays a lot of research is devoted to developing new types of traps that are non-Gaussian. By using e.g. Laguerre-Gaussian beams, Bessel beams, optical solenoid beams, phase-gradient line traps, etc…, a wider range of particles can be optically manipulated, and the manipulation can be extended from simple trapping to the application of more general forces and torques. These possibilities open up new applications in which optical tweezers are a tool for the manipulation of microscopic objects.
Typically, optical tweezers are combined with back focal plane interferometry, which allows to accurately monitor the position of the trapped particle. In this technique, the scattered laser light is imaged on a quadrant photodiode and its fluctuations are analyzed. The use of non-Gaussian beams does not only extend the trapping possibilities, but can also increase the accuracy of these position measurements. It has for instance been shown that the sensitivity for position fluctuations of back focal plane interferometry with a Laguerre-Gaussian beam is higher than with a simple Gaussian beam.
In the LCP group, we have built a versatile state-of-the-art optical trapping setup. We own two spatial light modulators that can manipulate the phase front of the incident laser light, allowing us to create a wide variety of beam shapes. Your job during this master thesis will be to integrate these spatial light modulators into our setup (both for trapping and for detection), develop (numerical) methods to calculate the required holograms for a number of beam shapes, and explore experimentally the trapping and detection possiblities of these beam shapes. The master thesis thus consists of different aspects, ranging from optical design and hands-on construction of an optical setup, over programming in Matlab and Labview, to experiments on the full microscope setup.
In particular, we'd like you to experimentally demonstrate the use of Laguerre-Gaussian beams in back focal plane interferometry for more sensitive position detection than is nowadays available. This possibility has been demonstrated in theory, but has never been applied in a real experiment. In order to do so, you will optically trap a particle (of a few hundred nanometers), illuminate it with a second Laguerre-Gaussian beam, apply a known force to it (for instance by inducing a liquid flow), and record the signal of the scattered light on a quadrant photodiode. By comparing the results with traditional back focal plane interferometry, you can characterize the sensitivity of the measurement method.
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.