Birefringence
Created | Updated Oct 10, 2002
Introduction
Birefringence is an optical effect based on dispersion. In the case of birefringent materials, the refractive index of light1 travelling through the material varies depending on the polarisation of the light ray.
How does it work?
In a normal material, the response of the atoms to light is isotropic2. However, some materials (for example, calcite crystals) have refractive indices that vary depending on the polarisation state of the light that enters them. The refractive index of light which is vertically polarised may be different to that which is horizontally polarised. This occurs because the susceptability of the material is different in different directions, therefore the response of the material varies.
How can it be used?
Birefringence can be used in a number of different ways.
Waveplates
Birefringent elements can be used to rotate the polarisation of a light beam. When using lasers, some optical elements are polarisation sensitive, and so a way of getting the correct polarisation is required3. By using birefringent elements the polarisation can be tuned and rotated to that which is required.
Frequency Conversion
One problem where birefringence comes into its own is in harmonic generation and parametric oscillators, in nonlinear optics. Here, the use is called phasematching. The physics in this is a little complicated, but through dispersion it is found that the bluer the light the slower it travels in a medium. In frequency conversion both beams should travel at the same speed, so a birefringent nonlinear crystal is used, and set up so that the original beam and the generated light travel at the same speed through the crystal. This allows for better conversion.
1And hence the speed of the light.2This means that the material behaves in the same way, no matter what the polarisation of the light3Of course, one could use a polaroid sheet, but in practice this is not a good idea, for two reasons. One, this would remove power from the beam, and we want all the power we can get. Two, using lasers can melt polaroid.
