The European Physical Society Forum
Photonic applications, for example sensing and quantum computing, demand increasing integration density. Therefore, research is dedicated to reduce the size as well as costs of optical systems. Following this trend, a multitude of photonic micro-platforms has recently been introduced, e.g. based on the hybrid integration approach in which functional components are miniaturized and inserted in cavities on a chip, connected by waveguides. For such a platform, wavelength- or polarization dependent splitting or spectral filtering are examples of functionalities needed in applications. Multilayer thin-film interference coatings are state of the art components for such tasks and can be designed for a wide spectral range. Produced by means of optically monitored and controlled plasma based vacuum coating processes , such thin-film coatings fulfill high precision requirements. However, the substrate on which the coating is applied has so far limited widespread use i n highly integrated systems. The substrate typically makes up for 90 \% or more of the thickness of the component without contributing to the spectral transfer properties. This is particularly detrimental for the simple case of transmission from one waveguide on the platform to another through a thin-film element. Within the thin-film element, the beam is no longer guided and diverges quickly, increasing the coupling losses, as illustrated in Fig. 1 (a). To overcome these disadvantages, we have developed substrate-free miniaturized multilayer thin films, as shown in Fig. 1 (b). Fig. 1 (a) Schematic sketch of hybrid integration of a multilayer thin-film element: The red curves correspond to a Gaussian intensity profile, emitted from a waveguide, which diverges inside the element, thereby reducing the coupling efficiency to the subsequent waveguide. The effect is minimized by a substrate-free thin film. (b) Miniaturized, substrate-free multilayer thin film placed on top of a match head for scale. We utilize a sacrificial substrate approach, which is based on a water-soluble layer between the substrate and the high precision optical coating . In a water bath, we separate the coating from the substrate. Since we also miniaturize in the areal extend, achieving edge lengths between 25 µm and 1 mm by segmentation of the coating with a laser dicing process, we produce thousands of elements within a single run of the coating process on a standard wafer size. Additionally, substrate-free coating elements are well suited for an automated and high throughput photonics assembly process by transferring them to a releasable tape. With these benefits and our ongoing research towards realizing more functionalities, e.g. active switching, miniaturized substrate-free multilayer thin films can be considered as a highly relevant concept for hybrid integrated photonics. References  H. Ehlers and D. Ristau in, Optical Thin Films and Coatings, A. Piegari, F. Flory eds. (Woodhead Publishing, 2018).  A. K. Rüsseler, F. Carstens, L. Jensen, S. Bengsch, and D. Ristau, "Applying sacrificial substrate technology to miniaturized precision optical thin-film coatings," Adv. Opt. Thin Films VII, Proc. SPIE 11872, 48–55 (2021).