biophysical research group


Facilities Optical Tweezers



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:

Setup 1
6W single trap capabilities
Setup 2
6W single trap, confocal capabilities
Setup 3
?W single trap. Mostly student applications
3W Dual trap capabilities
Olympus Confocal
No trapping laser installed
Setup 1-3 is housed in the old laboratories, along with a small workshop and biological lab for preparation of samples. The two latter microscopes are in the newer laboratories in the old cyclotron hall, along with a fully equipped biochemical lab.

Introduction to Optical Tweezers

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:

The scattering force:
Light reflected on a particle pushes this particle in the direction of propagation.
The gradient force:
Light refracted from a particle, will push the particle towards the region of highest intensity.
The direction and magnitude of the gradient force is dependent on the refractive index of the particle and the surrounding medium. Only if the former is higher than the latter will the particle be pushed towards the highest intensity.

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.


Response signal from a bead, moved lateral through the trap.

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.