Diffractive optical elements for 3D display architectures: Materials and processing: Presentation held at Huawei Material and Processing Summit 2018, 15.-16.10.2018, Munich, Germany

The optical technologies require advanced materials and new processing methods creating small, thin, efficient and multifunctional optical elements and devices. The Fraunhofer-IAP developed optical materials and processing procedures suitable for the holographic fabrication of diffraction gratings with different and variable properties creating novel concepts for sensors, passive or active DOE, security features and functional diffraction elements for 3D displays. Examples are materials for the all-optical fabrication of surface relief gratings, electrically tuneable volume Bragg gratings, stretchable elastomer gratings or organic-inorganic nanocomposite gratings possessing additional functionalities like luminescence, SPR, etc. Novel optical architectures for 3D displays require special materials and processing steps. Active tunable/switchable diffractive optical elements with high diffraction efficiency (DE) and high deflection angles are a strong demand for such displays. Two appropriated material and element approaches were developed in IAP. The first is highly efficient electrically switchable diffraction gratings based own photo-curable LC-containing composites. The LC-containing composites were patterned by holographic exposure forming alternating stripes of planar aligned LCs and isotropic crosslinked polymer resulting in gratings of high DE. The orientation of the LCs can be changed by external fields, thus, they are potential candidates for the development of smart tunable/switchable DOE. In contrast to the known electrically controllable holographic polymer dispersed liquid crystals (H-PDLCs) characterized by a scattering state without application of an electric field, the new polymer-LC gratings are transparent in both field states and allow not only a fast On/Off light switching but also a fine tuning of the signals intensity. Such non-slanted as well as slanted gratings, are characterized by a droplets-free morphology, show DE up to 100% and perform excellent electro-optical performance with switching times in the µs range. These 10 µm thick elements combine two functions in one layer: light deflection and electrical On/Off switching. Another type of efficient thin diffraction gratings, which we call Bragg circular polarization gratings, CPG, was very recently developed for new architectures of 3D holographic displays. The bulk photo-alignment of a photosensitive liquid crystalline polymer allows creating of very efficient symmetric and slanted CPGs with periods < 1 µm. The gratings provide steering angles up to 42° in combination with the DE of about 100% in one order of the whole visible range at grating thickness < 1.5 µm. The developed material and the holographic processing provide thermally stable thin CPGs with spatial periods down to 700 nm which possess the features of classical Bragg diffraction: only one beam, either transmitted or diffracted is observed, depending on the circular polarization handiness of the input beam; maximal DE can be obtained only at the designed incident angle of light. Both types of gratings are attractive for numerous applications in optics and photonics and especially in new 3D displays as switchable wide-angle beam-steering, angular/wavelength-addressing RGB elements, etc. Some possible applications in holographic head-mounted and head-up displays are presented