Method of forming birefringent structures in an optical element

A method of fabricating an optical element comprising:

• providing a substrate of a transparent material in which is to be formed a plurality of birefringent nanostructures spaced apart in plane substantially parallel to a surface of the substrate in a first direction by a distance X and in a second direction by a distance Y;
• generating from the output of a source of femtosecond laser pulses a laser beam group comprising a plurality of focused seeding beams having a circular polarisation and spaced apart along a line by the distance X and a focused writing beam having a non-circular polarisation and spaced apart from one of the seeding beams by the distance Y or the distance X;
• directing the laser beam group onto the surface of the substrate at a first position and applying one or more femtosecond laser pulses from each beam to corresponding volumes in the substrate;
• repeatedly translating the laser beam group relative to the substrate parallel to the line of seeding beams and along the first direction by the distance X to a next position and applying one or more femtosecond laser pulses from each beam, in order to deliver a plurality of femtosecond laser pulses from the seeding beams to each corresponding volume to create substantially optically isotropic nanostructures in the corresponding volumes;
• translating the laser beam group relative to the substrate along the second direction by the distance Y and repeating the repeated translation and application of the femtosecond laser pulses along the first direction;
• wherein
• the relative translation of the laser beam group and the substrate aligns the writing beam with successive corresponding volumes in which substantially optically isotropic nanostructures have been created, so that the one or more femtosecond laser pulses applied from the writing beam transform the optically isotropic nanostructure in each corresponding volume into an optically anisotropic nanostructure with optical birefringence.