To produce a one-dimensional photonic crystal, thin film layers of different dielectric constant may be periodically deposited on a surface which leads to a band gap in a particular propagation direction (such as normal to the surface). A Bragg grating is an example of this type of photonic crystal. One-dimensional photonic crystals can include layers of non-linear optical materials in which the non-linear behaviour is accentuated due to field enhancement at wavelengths near a so-called degenerate band edge. This field enhancement (in terms of intensity) can reach N 2 where N is the total number of layers. However, by using layers which include an optically anisotropic material, it has been shown that the field enhancement can reach N 4 , which, in conjunction with non-linear optics, has potential applications such as in the development of an all- optical switch.