Holographic optical tweezer
Holographic optical tweezers with Twisted Nematic (TN) liquid crystal display
Holographic optical tweezers with TN-LCD
Optical tweezers are instruments that are able to manipulate dielectric particles with a diameter of a few nanometers up to several micrometers by means of a focused laser beam. The particles are kept in focus by forces along the gradient of the field. The forces acting here are only a few piconewtons, which, however, is sufficient to hold and shift the particles against Brownian motion. A frequent field of application is microscopic methods, e.g. in biological research.
Conventional optical tweezers use mechanical-optical components to change the position of the "optical trap". Holographic optical tweezers (HOT) use an electrically addressable spatial light modulator (EASLM) to generate the desired intensity distribution, e.g. several focuses simultaneously, with the aid of a hologram. Some optical and mechanical components are replaced by intelligent software and mathematics. The often used "phase only" EASLMs are very cost-intensive.
Since 2009, a research project "ZAFH-Photon" at the Faculty E+I at the Offenburg University of Applied Sciences is working on a holographic 3D tweezer with conventional TN-LCDs (twisted nematic liquid crystal displays). Picture 1 shows the successful construction of these holographic optical tweezers. With the structure shown it is possible, for example, to capture polystyrene particles with a diameter of approx. 2.8 µm and to move them at a speed of up to 10 µm/sec. Video 1 shows a particle on a circular path while in Video 2 the particle moves to a user-defined target position.
The presented method is not limited to the manipulation of single cells but can also be used to measure forces in the range of some piconewtons, cells can be separated and sorted and much more.
Picture 1: Successful construction of holographic optical tweezers. With the structure shown, it is possible, for example, to capture polystyrene particles with a diameter of about 2.8 μm and move them at a speed of up to 10 μm/s.
Video 1: A polystyrene particle is moved along a predefined circular path.
Video 2: A polystyrene particle is set to a target position by the user.