Optical forces are a light-matter interaction phenomena originated by a momentum exchange between a light beam and a sample. When the light beam passes through a sample undergoes a global momentum change which is transferred to the sample resulting in a net force in the range of pN. If a high numerical aperture light beam is used over a microscopic particle (from tenths of nanometers to tenths of microns) with a refraction index greater than the surrounding medium, the resulting force field tends to move the particle towards an equilibrium position near the light focus. This kind of optical trap is usually called optical tweezer, which can be easily integrated with a conventional microscope allowing micro-manipulation of the sample in the microscope stage (view Optical setup).
Thanks to digital holography it is possible to sculpt the light beam in order to get arbitrary light patterns at the sample plane. This is achieved by introducing a Spatial Light Modulator (SLM), in the beam optical path (see detailed information about Spatial Light Modulators). Using this technique, the light spot can be moved with just a mouse click, multiple traps can be created from a single beam, and arbitrary light distributions can be obtained. For any desired trap pattern, the corresponding hologram must be computed and sent to the SLM (optically conjugated with the back focal plane of the objective lens). The calculation of the holograms can be done by means of different algorithms.
Optical tweezers can measure forces like a dynamometer. If the trap is well calibrated, the optical force as a function of the bead's position is known. When an external force is applied to the trapped bead, it is possible to measure the external force by looking at the bead move towards a new equilibrium position, where the optical force and external force compensate.
Position detection techniques:
Using this micro-dynamometer, an accurate measurement of the bead's position relative to the trap center will tell us which is the applied force. That is why position detection is such an important issue. The implemented position detection techniques are: back-focal-plane interferometry, and video analysis with image processing techniques.
Trap calibration methods:
The trap potential is usually assumed to be harmonic (F=-k·x), and there are several precise and reliable calibration methods to characterize it. Trap stiffness (k) must be calibrated in order to know the force as a function of the bead's position. Three calibration methods have been implemented in our lab:
- Equipartition theorem: it can give a fast estimation on the trap stiffness.
- Power Spectrum analysis: accurate measurements of trap stiffness can be obtained, but fast detectors are needed.
- Drag forces: changes on the beads' position are measured as a function of the (known) applied viscous force by moving the sample at a controlled speed.