Toward Optical and Superconducting Circuit Integration

The physics of the interaction between coherent light and superconductivity is a very intriguing subject but the alignment of a laser beam with integrated superconducting systems (thin films, striplines or tunnel junctions) has represented so far a real bottleneck for the experimentalists. The integration of both junctions and waveguides on the same chip surely represents a possible path around this difficulty. In this paper we report on the combined integration of optical structures based on hybrid channel waveguides with superconducting Josephson junctions circuits. The optical films are grown on silicon substrates by a spin-coating process using the hybrid organic/inorganic sol-gel route. The channel waveguides are deposited on a buffer layer and realized by photolithographic and ion milling techniques. We find that the optical buffer is a good substrate for SIS Josephson junctions and circuitry based on the Nb/Al-AlOx/Nb trilayer processing. The junctions produced present good current-voltage characteristics with critical current density of the order of 1 kA cm(-2), high subgap resistance and good uniformity. Our results show that the quality of the Josephson junctions is well preserved even when they are grown on the sol-gel buffer layer.