SUPERCONDUCTING TUNNEL JUNCTION AS POSSIBLE SOURCE OF COHERENT PHOTONAND PHONON GENERATION

In this report we analyse the possibility of utilising superconductors as active media for infrared lasers and as generators of high-frequen cy coherent acoustic fields. The main problem in this direction is to obtain the inverse population in the charge-carrier system. Tunnel inj ection of electrons is one of the most promising methods to create non equilibrium states in superconductors. The deviation of electrons from equilibrium changes the shape of single electron distribution functio n. Despite rather large observed shifts from equilibrium, the inverse population of charge carriers in tunnel junctions is not yet registere d experimentally. After discovery of oxide high-temperature supercondu ctors (HTSC) new possibilities in this field emerge. The analysis show s that to obtain the inverse population is much more promising in HTSC , which have anisotropic crystalline structure and non-Debye phonon sp ectrum. We present the results of theoretical investigation of non-equ ilibrium distribution function both for ''narrow'' and for ''wide'' re gimes of tunnel injection. We show that the inverse population can be obtained more easily in the case when the transverse Bose-fields are i nvolved in the mechanism of Cooper pairing. In ''dirty'' samples, cont aining sufficient amount of elastic scatterers, the inverse population may initiate the ''phonon'' instability. Hence, the nonequilibrium ju nction may serve as the generator of coherent phonon field. For crysta lline samples with sufficiently perfect structure the additional possi bility emerges, because the absorption coefficient of high-frequency e lectromagnetic radiation may become negative. This suggests a possibil ity to create new family lasers: superconductor generators of far and middle range infrared radiation.