Photonic molecules with a tunable inter-cavity gap

Optical micro-resonators have broad applications. They are used, for example, to enhance light-matter interactions in optical sensors or as model systems for investigating fundamental physical mechanisms in cavity quantum electrodynamics. Coupling two or more micro-cavities is particularly interesting as it enlarges the design freedom and the field of application. In this context, achieving tunability of the coupling strength and hence the inter-cavity gap is of utmost importance for adjusting the properties of the coupled micro-resonator system. In this paper, we report on a novel coupling approach that allows highly precise tuning of the coupling gap of polymeric micro-resonators that are fabricated side by side on a common substrate. We structure goblet-shaped whispering-gallery-mode resonators on an elastic silicone-based polymer substrate by direct laser writing. The silicone substrate is mechanically stretched in order to exploit the lateral shrinkage to reduce the coupling gap. Incorporating a laser dye into the micro-resonators transforms the cavities into micro-lasers that can be pumped optically. We have investigated the lasing emission by micro-photoluminescence spectroscopy, focusing on the spatial localization of the modes. Our results demonstrate the formation of photonic molecules consisting of two or even three resonators, for which the coupling strengths and hence the lasing performance can be precisely tuned. Flexibility and tunability are key elements in future photonics, making our approach interesting for various photonic applications. For instance, as our coupling approach can also be extended to larger cavity arrays, it might serve as a platform for tunable coupled-resonator optical waveguide devices.