biophysical research group
The Group has several laboratory facilities - all located in the basment of the historic buildings of The Niels Bohr Institute.
The facilities spans several technical disciplines and allows for research in the fields of optical physics, advanced microscopy and biochemistry. The five microscope are:
To understand how laser light is capable of trapping a particle, it is necessary to understand, that the light carries a momentum, that can be transferred, when it interacts with physical matter. When the laser light is scattered by a translucent particle, the change in momentum is transferred to this particle.
The resulting force on the particle can be divided into two contributions:
For succesful trapping, the gradient force must overpower the scattering force. This is accomplished with a high intensity laser beam that is focused very tightly, resulting in a sharp 3-dimensional intensity profile.
The light is collected after trapping and focused on a photo diode for detection. The diode is either a position sensitive device or a quadrant photodiode, and will produce a raw voltage output in the x and y dimension - depending on the position of the laser spot on the diode.
It has been shown empirically, that the response from the photo diode is linearly proportional to the displacement of the bead in the trap. This linearity is dependent on the setup, but it has been shown to extend to half a bead radius away from the trap center.