New J. Phys. 18, 023019 (2016)

Could superconductors turn out to be useful for building new types of photonic device? According to calculations performed by scientists in Israel, hybrid semiconductor–superconductor structures could, in theory, provide enhanced ultrafast two-photon gain (TPG). Raja Marjieh and collaborators from Technion have developed a theoretical quantum description of the ultrafast light–matter interaction in an electrically driven semiconductor–superconductor structure. They studied a structure that resembles an edge-emitting p–n junction laser diode made from an n-type ridge on top of a p-type substrate, but with the important innovation that the ridge is capped with a thin layer of superconductor. Their analysis focuses on TPG in the superconducting proximity region of the semiconductor for an ultrafast light pulse that is propagating along the direction of the ridge. Interestingly, the expressions for the two-photon emission exhibit interference terms between different two-photon transition paths corresponding to distinct spectral components of the ultrafast pulse: this indicates the possibility to realize coherent control of TPG. Further, the researchers evaluate the enhancement of TPG due to the presence of the superconductor by taking into account one-photon gain and loss. They find that an increase in the carrier density leads to a broader TPG spectrum, and that for moderately high seed intensities the TPG becomes comparable to one-photon gain.