Far field image of a nanograting-based wave plate for the generation of radialazimuthal polarization
When ultrashort laser pulses are focused into transparent materials nonlinear absorption leads to localized energy deposition in the focal volume [MRS Bull. 31, 620-625 (2006)External link]. In glasses self-organized periodic sub-wavelength structures, so-called nanogratings, emerge in a certain parameter regime resulting in birefringent modifications embedded within the otherwise isotropic host material [Laser Photonics Rev. 6, 787-792 (2012)External link]. The nanogratings evolve under the irradiation of several hundred laser pulses and are always oriented perpendicular to the laser polarization. Hence their structural properties and thus their birefringence can be controlled by tuning the laser parameters [Appl. Phys. A 100, 1-6 (2010)External link].
By combining the formation of ultrashort pulse induced nanogratings with the 3D freedom of the laser writing technique manifold devices for the local control of the polarization can be fabricated. These range from waveplates e.g. for the generation of radial or azimuthal polarization to complex structures for advanced microscopy techniques or even chiral structures.
a) Sketch of the nanograting inscription process. b) Top view (parallel to the inscribing laser) of laser written lines illustrating the development of nanogratings with increasing number of laser pulses incident. c) Three-dimensional reconstruction of the fundamental structure of femtosecond pulse-induced nanostructures.