Axial as well as lateral color aberrations in broadband optical systems can efficiently be corrected by using diffractive optical elements (DOEs). However, DOEs such as kinoforms consisting of only one material are not suitable for high-quality optics because of straylight in spurious diffraction orders. The amount of stray light can significantly be reduced by using so-called efficiency-achromatized DOEs (EA-DOEs), which consist of a material pair whose refractive indices fulfill a specific material condition. Unfortunately, manufacturing of EA-DOEs is very challenging because the grating structures are subject to tight fabrication tolerances. Therefore, only few broadband optical systems with EA-DOEs are on the market. Here we show that DOEs in broadband optical systems can surprisingly be replaced by refractive doublets made of materials that fulfill the material condition for EA-DOEs. As opposed to the EA-DOEs themselves, these purely refractive replacements do not suffer from stray light. In addition, from a theoretical point of view, our result allows for understanding the effect of DOEs in optical designs by classical refractive optical design theory.
Modern optical systems are subject to very restrictive performance, size and cost requirements. Especially in portable systems size often is the most important factor, which necessitates elaborate designs to achieve the desired specifications. However, current designs already operate very close to the physical limits and further progress is difficult to achieve by changing only the complexity of the design. Another way of improving the performance is to tailor the optical properties of materials specifically to the application at hand. A class of novel, customizable materials that enables the tailoring of the optical properties, and promises to overcome many of the intrinsic disadvantages of polymers, are nanocomposites. However, despite considerable past research efforts, these types of materials are largely underutilized in optical systems. To shed light into this issue we, in this paper, discuss how nanocomposites can be modeled using effective medium theories. In the second part, we then investigate the fundamental requirements that have to be fulfilled to make nanocomposites suitable for optical applications, and show that it is indeed possible to fabricate such a material using existing methods. Furthermore, we show how nanocomposites can be used to tailor the refractive index and dispersion properties towards specific applications..
Daniel Werdehausen, Steinn Agustsson, Minjae Kim, Parmida Shabestari, Emily Huang, Amrit Pokharel, Timofei Larkin, Alexander Boris, Tomohiro Takayama, Yangfan Lu, Andreas Rost, Hao Chu, Alexander Yaresko, Marc Höppner, Armin Schulz, Dirk Manske, Bernhard Keimer, Hidenori Takagi, Stefan Kaiser
The excitonic insulator (EI) is an intriguing phase of condensed excitons undergoing a Bose-Einstein-Condensation (BEC)-type transition. A prominent candidate has been identified in Ta2NiSe5. Ultrafast spectroscopy allows tracing the coherent response of the EI condensate directly in the time domain. Probing the collective electronic response we can identify fingerprints for the Higgs-amplitude equivalent mode of the condensate. In addition we find a peculiar coupling of the EI phase to a low frequency phonon mode. We will discuss the transient response on multiple energies scales ranging from the exciton dynamics to the coherent THz response of the gap.
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