Light is a very important aspect of our everyday life: it allows us to receive information about the surrounding world through our eyes and from the streams of data flowing through optical fibers. However, light can also exist as perfectly "dark" states, which no longer propagate as waves, but are confined within structures even 100 times smaller than the thickness of a hair. Moreover, such states may be "topological", which means that they remain confined regardless of small structural changes, which makes them immune to defects. In this project, researchers will develop new ideas to make the "dark states of light" even more robustly confined, unlocking their true potential in advanced light-based research and applications. New experimental techniques will also be established in this project to directly study such states using so-called cascaded nonlinear frequency conversion, which will enable probing even perfectly "invisible" states from the outside using propagating light waves.