Phenomenon of photoinduced superconductivity: prediction and discovery

Over twenty five years ago, we predicted the occurrence of photoinduced superconducting phase transition in some complex systems. In a series of papers, conditions and constraints were determined for the realisation of a photoinduced non-equilibrium superconducting state under transient or metastable situations. Owing to the involvement of two boson modes (one with photons or photoexcitations and the other the virtual mode of the system, namely phonon or any other electronic boson mode) in the interaction mechanism, the transition temperature $(T_c)$ is found to depend on the photon density or photoexcitation dosage impressed on the sample within the absorption depth. Thus $T_c$ can be controlled and tuned as required. After giving an overview of the theoretical concepts and the current status of experiments in this field, we present an extended model in which both effects of intraband and interband transitions caused by the appropriate boson modes are taken into account. The intraband pairing thus obtained consists of two parts, the usual single boson induced pairing and the new radiation induced two boson pairing. The latter leads to the enhancement of $T_c$. The dual role of the radiation field quanta is emphasized. The recent experimental discovery of photo-induced superconductivity in some complex oxide (cuprate) systems in transient or metastable state are in accord with the predicted phenomena. These aspects along with some possible applications are discusse