Optical Tweezers

The study of light phenomena is very old and it is one of the main motors for scientific researches. In particular, the creation and development of laser sources has allowed not only investigating the microscopic world in high details, but also its manipulation at our will. In 1970 Arthur Ashkin demonstrated for the first time that the pressure of radiation of a visible laser light can be used to move micro-sized particles in a fluid, thus paving the way for the future setting up of the optical tweezers. Optical tweezers exploit the linear momentum carried by light in order to create a potential well where microscopic objects can be efficiently trapped and manipulated. This technique require a proper balance between two types of forces: the scattering force, which push objects along the direction of propagation of light, and the gradient force, which pull objects along the spatial gradient of light. When gradient optical forces exceed those from scattering, an object is attracted to the point of highest intensity formed by focused light and can be stably trapped at this position in all three dimensions.
The Optical Tweezers setup at LaFSI labs use two counter-propagating near-infrared laser beams, which are tightly focused by a high-numerical aperture microscope objective. In this way the scattering forces are minimized and a micro-sized bead can be trapped in the focus of two laser beams: here the optical trap behaves as a linear “Hookean” spring and a controlled force can be exerted to the bead simply by displacing it from the center of the trap. With this instruments forces in the range of 0.5 to 100pN can be applied and the trapped object can be manipulated with a spatial resolution of 0.5nm.