Toward optical and superconducting circuit integration

Abstract

The physics of the interaction between coherent light and superconductivity is a veryintriguing subject but the alignment of a laser beam with integrated superconductingsystems (thin films, striplines or tunnel junctions) has represented so far a real bottleneckfor the experimentalists. The integration of both junctions and waveguides onthe same chip surely represents a possible path around this difficulty. In thispaper we report on the combined integration of optical structures based on hybridchannel waveguides with superconducting Josephson junctions circuits. The opticalfilms are grown on silicon substrates by a spin-coating process using the hybridorganic/inorganic sol–gel route. The channel waveguides are deposited on a buffer layer andrealized by photolithographic and ion milling techniques. We find that the opticalbuffer is a good substrate for SIS Josephson junctions and circuitry based on theNb/Al–AlOx/Nb trilayer processing. The junctions produced present good current–voltagecharacteristics with critical current density of the order of1 kA cm−2, high subgap resistance and good uniformity. Our results show that the quality of theJosephson junctions is well preserved even when they are grown on the sol–gel buffer layer.