Light-induced current in Cooper pair insulators in NbTiN films.

The usual macroscopic Cooper pairs systems, the superconductors, very weakly interact with the visible light since self-existing superconducting currents screen the light penetration into the samples. However, the superinsulators whose superinsulating behavior is caused by localization of Cooper pairs demonstrate significant interaction with light. As a result, superinsulating films, in particular, the NbTiN films, offer a remarkable and unique platform for constructing high sensitivity light detector working at superlow millikelvin temperatures. Irradiating the NbTiN film with a visible light when the film is in the insulating state results in the emergence of a measurable current through this NbTiN-based insulator. It was found that this light-induced current depends on the radiation intensity and thus opens an opportunity for high precision measurements of the light intensity. The induced current disappears with an increase of temperature and light wavelength.