Superconductor- Semiconductor Hybrid Devices

Hybrid devices combining superconductors with semiconductors offer a wide range of applications, particularly in the growing field of quantum information processing. This is due to their ability to take advantage of both the extensive knowledge gathered in the field of semiconductors and the unique quantum properties of superconductors. This results in novel device concepts, new sources of quantum light, hybrid solid-state-photonic quantum gates, two-photon amplification, Bell-state detection, and many others.

I developed a theoretical approach for modeling a wide range of semiconductor-superconductor structures with arbitrary potential barriers and a spatially-dependent superconducting order parameter. A key result is that the Cooper-pair injection process can be significantly enhanced through resonant tunneling with appropriate barrier configuration, incorporating the Schottky barrier as a contributing component of the device.

I then experimentally demonstrated enhanced Andreev reflection in a Nb/InGaAs/InP-based superconductor-semiconductor hybrid device resulting in increased Cooper-pair injection efficiency, achieved by Cooper-pair tunneling into a semiconductor quantum well resonant state. This work demonstrated the validity of my previously developed theoretical approach.

Radiative recombination of Cooper pairs in superconducting light-emitting diodes (SLED) results in superconductor-enhanced light emission. The theory behind this effect is based on a two-photon interaction with Cooper pairs. I demonstrated two-photon emission in a GaAs/AlGaAs SLED, enabled by efficient Cooper-pair injection due to a specially-designed superlattice structure. The measured electroluminescence spectra revealed unique superconducting proximity features below the critical temperature (T_c). Moreover, I performed photon-pair correlation experiments (g⁽²⁾), demonstrating temperature-dependent time coincidences below T_c between photons emitted simultaneously from Cooper pairs, in good agreement with theory.

During my research I also utilized high-T_c superconductors. Andreev reflection was observed in a YBCO/GaN junction through differential conductance spectroscopy, with a strong characteristic zero-bias peak persisting up to the critical temperature of the superconductor (>80K). Efficient injection of Cooper pairs into direct bandgap semiconducting structures, together with the high transition temperature of YBCO, can pave the way to novel optoelectronics and quantum optical studies of high-T_c materials.

Finally, I researched additional applications of hybrid devices as well. One such application is a gate based on giant Cooper-pair-based optical nonlinearity predicted to exist in a semiconductor-superconductor structure, selectively introducing a phase to the Bell-state. I theoretically demonstrated this scheme on a practical device based on a superconducting contact coupled to a GaAs/AlGaAs waveguide structure.

Ph.D. Under the supervision of Prof. Alex Hayat.