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Combining superconductor and semiconductors in nanostructure junctions was achallenging technological problem that attracted attention for long time [vanWees]. The radiative recombination of Cooper pairs was demonstrated, using aNb/n-InGaAs/p-InP heterostructure [EL_Hayashi, PL_Hayashi], called Cooper pairLED [PL_Suemune, R_Suemune]. It has been suggested that the junction couldproduce entangled photon pairs [Benson, Gywat] needed for quantum informationprocessing and communication. Here we demonstrate an enhanced radiativerecombination of electron Copper pairs in inhomogeneousK$_{0.75}$Fe$_{1.75}$Se$_{2}$ (KFS) subjected to laser light upon cooling belowsuperconducting transition temperature $T_{c}$ 28 K. The observation of thisphenomena is possible due to fulfillment of the following three conditions: (1)Phase separation in superconducting KFS crystals is realized via naturallyoccurring heterostructure [mic_Charnukha, TEM_Wang]; (2) Partial Fe-vacancyordered $n$-type semiconducting regions, sandwiched between Fe-vacancy free SCand vacancy-ordering AFM structure, forming active layers. The electronicstructure of those active layer is tuned to induce Cooper pairs by theproximity effect [de Gennes] and/or to accept Cooper pair tunneling from the SCphase. In the active layer, the electron Cooper pair can radiatively recombinewith two p-type holes [Asano] produced by laser photoexcitation; (3) In KFSnature provides at least 10$^{9}$ SC/$n$-type semiconducting/AFM insulatingjunctions per cm$^{3}$. For laser excitation energy of 1.92 eV ($\lambda$=647.1 nm) and power 0.68 mW focused to the spot of 40x80 $\mu $m$^{2}$ theestimated internal quantum efficiency of the natural heterostructure at 10 K inthe luminescence range of 700 to 1300 nm is close to 100% and is likely limitedby availability of the $p$-type holes.

Photo luminescence of Cooper pairs in a naturally occurring heretostructure K$_{0.75}$Fe$_{1.75}$Se$_{2}$