Volume 10, Issue 1 - Diffractive Optics / Guest Editor: Reinhard Voelkel

Augmented reality (AR) enhancing the existing natural environment by overlaying a virtual world is an emerging and growing market and attracts huge commercial interest into optical devices which can be implemented into head-mounted AR equipment. Diffractive optical elements (DOEs) are considered as the most promising candidate to meet the market’s requirements such as compactness, low-cost, and reliability. Hence, they allow building alternatives to large display headsets for virtual reality (VR) by lightweight glasses. Soft lithography replication offers a pathway to the fabrication of large area DOEs with high aspect ratios, multilevel features, and critical dimensions below the diffractive optical limit down to 50 nm also in the scope of mass manufacturing. In combination with tailored UV-curable photopolymers, the fabrication time can be drastically reduced making it very appealing to industrial applications. Here, we illustrate the key features of high efficiency DOEs and how the SMILE (SUSS MicroTec Imprint Lithography Equipment) technique can be used with advanced imprint photopolymers to obtain high quality binary DOEs meeting the market’s requirements providing a very versatile tool to imprint both nano- and microstructures.

Diamond is an exceptional material that has recently seen a remarkable increase in interest in academic research and engineering since high-quality substrates became commercially available and affordable. Exploiting the high refractive index, hardness, laser-induced damage threshold, thermal conductivity and chemical resistance, an abundance of applications incorporating ever higher-performance diamond devices has seen steady growth. Among these, diffractive optical elements stand out—with progress in fabrication technologies, micro- and nanofabrication techniques have enabled the creation of gratings and diffractive optical elements with outstanding properties. Research activities in this field have further been spurred by the unique property of diamond to be able to host optically active atom scale defects in the crystal lattice. Such color centers allow generation and manipulation of individual photons, which has contributed to accelerated developments in engineering of novel quantum applications in diamond, with diffractive optical elements amidst critical components for larger-scale systems. This review collects recent examples of diffractive optical devices in diamond, and highlights the advances in manufacturing of such devices using micro- and nanofabrication techniques, in contrast to more traditional methods, and avenues to explore diamond diffractive optical elements for emerging and future applications are put in perspective.

Nanostructured filter arrays on image sensors are promising for miniature spectrometers and spectral imagers. In this work, we report on resonant waveguide gratings fabricated by UV nanoimprint lithography and conformal dielectric-plasmonic coatings. Optical measurements in accordance with numerical simulations report on a resonance bandwidth of 20 nm in transmission in the visible range. The impact of cladding thickness and filter lateral size on the resonance properties is investigated with the help of numerical calculations. Finally, it is shown that the proposed geometry based on conformal coatings has a very efficient blocking rate compared to other nanostructured filter approaches.

Diffractive optical elements are ultra-thin optical components required for a variety of applications because of their high design flexibility. We introduce a gradient-based optimization method based on a step-transition perturbation approach which is an efficient approximation method using local field perturbations due to sharp surface profile transitions. Step-transition perturbation approach be available to calculate the gradient of figure of merit straightforwardly, we implemented optimization method based on this gradient. This fast and accurate inverse design creates binary (2-level) diffractive elements with small features generating the wide angle beam arrays. The results of the experimental characterization confirm that the optimization based on the perturbation method is valid for 1-to-117 fan-out grating generating beam pattern of linear array.

Information projection using laser-based illumination systems in the automotive area is of keen interest to enhance communication between road users. Numerous work on laser-based front end projection employing refractive and reflective optics has been reported so far, while for rear end illumination efforts are more scarce and a different optical design concept due to limited volumetric size and field of view regulations is required. Here, we report on a new and versatile approach for a laser-based rear end lighting system for automotive application which enables projection of information or signals to support other road users. The design is based on thin diffractive optical elements projecting the desired patterns upon illumination. Also, for protection of the road users from the steering laser beam, a diffusive back projection screen is designed to project information while fulfilling both the field of view and safety requirements. The projection system is based on a periodic diffusive structure made of an array of biconic lenses with sizes in the millimeter range. The field of view (FOV) from the simulated lens arrays complies with the angular requirements set by the Economic Commission for Europe (ECE). As a proof of concept, the diffusive screen is fabricated using microfabrication technology and characterized. In future, the screen will be combined with thin diffractive optical elements to realize an entire integrated projection system.

The continuous miniaturization of components and devices along with the increasing need of sustainability in production requires materials which can fulfill the manifold requests concerning their functionality. From an industrial point of view emphasis is on cost reduction either for the materials, the processes, or for both, along with a facilitation of processing and a general reduction of resource consumption in manufacturing. Multifunctional nanoscale materials have been widely investigated due to their tunable material properties and their ability to fulfill the increasingly growing demands in miniaturization, ease of processes, low-cost manufacturing, scalability, reliability, and finally sustainability. A material class which fulfills these requirements and is suited for integrated or waferscale optics are inorganic–organic hybrid polymers such as ORMOCER ® s [ORMOCER ® is registered by the Fraunhofer Gesellschaft für Angewandte Forschung e.V. and commercialized by microresist technology GmbH under license since 2003]. The combination of chemically designed multifunctional low-cost materials with tunable optical properties is very attractive for (integrated) optical and waferscale applications via a variety of different nano- and microstructuring techniques to fabricate micro- and nano-optical components, typically within less than a handful of process steps. The influence of photoinitiator and cross-linking conditions onto the optical properties of an acrylate-based inorganic–organic hybrid polymer will be discussed, and its suitability for being applied in waferscale optics is demonstrated and discussed for miniaturized multi- and single channel imaging optics.

Waveguide technology has great prospects of development in optical see-through near-eye displays with larger field of view, lower thickness and lighter weight than other conventional optical technologies. However, the stray light is usually inevitable in current optical design and manufacturing, causing a poor imaging quality. In this paper, the principle and structures of stray light generation are analyzed, and the causes are discussed by non-sequential ray-tracing with mass precision calculation. From the ray-tracing, the suppression of stray light by optimizing design and manufacturing are achieved. A 2 mm-thickness geometrical waveguide with partially reflective mirror array is designed. The field of view of the optimized geometrical waveguide reaches 47° with 10  mm at exit pupil diameter and 20  mm at eye relief.