Nonlinear effects

Birefringence and the structure and dynamics of biomacromolecules in solution

The area of research

The research work at  the  Department  of Molecular Biophysics is focused on the optical birefringence induced by electric field of a strong laser beam (the optical Kerr effect,  OKE) in solutions of nucleic acids  and that induced by a strong constant magnetic field in protein solutions (the Cotton-Mouton effect - CME). By these methods it is possible to obtain the information on  geometry, electric, optical and magnetic properties of polymers and biopolymers (tRNA, ferritin). The measurements permit drawing conclusions on the shape, conformation, dynamics and interactions of biomacromolecules in solution, in vivo and in vitro.

Natural birefringence

The natural birefringence is  the optical phenomenon observed in a transparent medium (gases, liquids, crystals), manifested as the medium ability to show double light refraction or a light beam splitting. The substances  for which this phenomenon is observed are called birefringent. The natural birefringence illustrated in the figure beside was discovered in 1669 in the crystal of calcite (CaCO3) by the Danish scientist Rasmus Bartholin and it was explained after the  English physicists Thomas Young propounded the wave theory of light, in 1801. The Dutch scientist Christiaan Huygens observed that the two  rays appearing in  the  crystal are linearly polarized in the mutually orthogonal planes. 

In the beginning of the 19th century, the French physicist Augustin J. Fresnel explained this phenomenon as follows: “In a birefringent crystal light has two different velocities depending on the orientation of the oscillation plane (polarization)”. A measure of birefringence Dn is the difference between the light refraction indices of the extraordinary ray, ne, and the ordinary one, no: Dn= ne- no, which depends on the microscopic properties of the birefringent medium.  A birefringent crystal has an optical axis, it is such a direction along which light does not split into two rays because its velocity does not depend on the direction of polarization. The presence of the optical axis in a birefringent crystal follows from the regular and identical arrangement of its elongated molecules, it is the symmetry axis of these molecules.

Induced birefringence

Birefringence can appear under the effect of external factors such as  electric field E (electro optical Kerr effect), including the electric field of photons (optical Kerr effect) and by magnetic field  H(Faraday effect, Cotton-Mouton effect). The explanation is  that the anisotropic molecules of a given substance may not be regularly arranged and can have charges at their ends so can be electric dipoles or magnetic dipoles. Then under the effect of the external  field E or H they assume the most beneficial energy positions so align  their dipoles  along the direction of the field. The disordered molecules can also be ordered by mechanical impact, under the effect of compression or stretching of a given material (like threads that get straighten up when stretched).


The phenomenon of birefringence is applied in polarizing objects (e.g.  in Nicola prisms, half-wave and quarter-wave plates) and in  LCD screens. This  phenomenon is  particularly important in nonlinear optics when it is induced by light of high intensity. It is also employed in birefringent fiber-optic cables, in sensors  of stress, bend, pressure and temperature, used in telecommunication and  fiber-optic monitoring.

The birefringence of minerals has substantial effect (along with the sample thickness) on their interference  colors  observed  in the so-called thin plates, used by geologists and petrologists. The interference colors and material birefringence allow identification of minerals.

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