We experimentally demonstrate Cooper pairs' drastic enhancement of the band-to-band radiative recombination rate in a semiconductor. Electron Cooper pairs injected from a superconducting electrode into an active layer by the proximity effect recombine with holes injected from a p-type electrode. The recombination of a Cooper pair with p-type carriers dramatically increases the photon generation probability of a light-emitting diode in the optical-fiber communication band. The measured radiative decay time rapidly decreases with decreasing temperature below the superconducting transition temperature of the niobium electrodes. Our results indicate the possibility to open up new interdisciplinary fields between superconductivity and optoelectronics. DOI: 10.1103/PhysRevLett.107.157403 PACS numbers: 78.60.Fi, 74.25.Gz, 78.66.Fd, 85.60.Jb Recent discoveries of new superconductors [1,2] boosted up the research fields with new experimental as well as theoretical possibilities. From a scientific viewpoint one great advantage of superconductivity is its long coherence time which is the most important feature for quantum information processing [3]. The combined system consisting of a coherent photon field and a superconducting (SC) condensate would be a promising candidate for realizing the quantum operation in solid state devices [4][5][6][7][8]. The Cooper pairs are preserved during these operations with photon energies smaller than the energy gap of superconductivity (on the order of meV). On the other hand, when photon energies become larger than the superconductivity gap, the absorption of high-energy photons only results in the destruction of Cooper pairs. This fact enables the application of superconductors as high-speed singlephoton detectors [9]. It is still unexplored what will take place with the counter process of photon emission from Cooper-pair states in this higher photon energy range.In this Letter, we demonstrate that electron Cooper pairs injected into a semiconductor by the proximity effect [10,11] can be highly involved in the interband transition and accelerate the photon generation processes. We measure the radiative recombination rate as a function of temperature across the SC transition temperature, T C . The results demonstrate drastic enhancement of the radiative recombination rate below T C . The temperature dependence of the radiative recombination rate can be explained by a theoretical model. Our new finding corresponds to experimental demonstration of the Cooper pair's gigantic oscillator strength [12].The light-emitting diode (LED) epitaxial layers were grown on a p-type (001) InP substrate by metalorganic vapor-phase epitaxy. The layers consist of a 500 nm thick p þ À InP buffer layer (Zn doping $1 Â 10 17 cm À3 ), a 30 nm thick n þ À In 0:53 Ga 0:47 As active layer (Si doping $5 Â 10 18 cm À3 ) lattice matched to InP, and a 10 nm thick n þ À In 0:7 Ga 0:3 As Ohmic contact layer (Si doping $5 Â 10 18 cm À3 ). Outside of the contact layer, we attached 20 m wide and 80 nm thick niobium (N...

A light-emitting diode (LED) in the optical-fiber communication band showed special features after replacing the n-type electrode with niobium (Nb) superconducting metal. Nb electrodes prepared on an InGaAs-based semiconductor surface formed a superconductor/semiconductor/superconductor junction, and the current-voltage characteristics exhibited both DC and AC Josephson junction properties. This was a result of the injection of electron Cooper-pairs into the n-InGaAs active layer of an LED. The drastic enhancement of the electroluminescence output observed below the Nb superconducting critical temperature, T c, demonstrates the active role of electron Cooper-pairs in radiative recombination. Lifetime measurements of this LED and accurate evaluation of the luminescence output made it possible to estimate the radiative recombination lifetimes. A theoretical formula derived for the Cooper-pair radiative recombination accurately describes both the measured steep reduction of the radiative recombination lifetime and the observed enhancement of the internal quantum efficiency below T c. This work will assist the development of interdisciplinary physics and new applications in superconductivity and optoelectronics.

Application fields of light emitting diodes (LEDs) are expanding in various fields. Development of LED-based single photon sources is expected to open a new possibility to expand the applications to quantum information communication and processing. The authors have proposed a photon-emitting LED combined with superconducting electrodes, which is expected to be an on-demand entangled photon pair source [1]. The main mechanism is based on the coherent spatial extension of the Cooper-pair states to the photon emitting layer, which is expected to enhance the oscillator strength of the radiative recombination processes by the Cooper-pair superradiance effect [2]. The preliminary operation was demonstrated with InGaAs quantum well (QW) LEDs and about 20-times enhancement of the electroluminescence (EL) was observed under the low-injection current regime [3]. This is the demonstration of the improved internal quantum efficiency (QE) under the low internal QE operation of the LED. In this paper, the enhancement of the radiative recombination processes with the superconducting (SC) effect is demonstrated by the lifetime measurements under the operation with high (~100%) internal quantum efficiency (QE).Electron Cooper pairs are injected into p-n junctions of n-InGaA/p-InP LEDs from Nb SC n-type electrodes [1] and will recombine with hole pairs injected from the p-side of LEDs. EL was observed from a Nb slit formed in the Nb electrode with the ~100-nm width. The main emission peak is around 1450nm at the measured temperature range of 2~12K. The SC critical temperature, Tc, of the Nb electrode was measured to be 7.3K in this sample. The diode built-up voltage remained low around 1.2V even at the low measurement temperature. The EL integrated intensity was linearly increased with the CW injection current and was temperature independent below and above the Nb Tc. This shows that the EL emission is dominated by the radiative recombination in this LED. The LED pulsed operation was measured with newly developed Hamamatsu Photonics Infrared StreakCamera. Consistent with the CW measurements, the EL intensity measured with the pulsed injection current of 1.6mA during the pulse duration was almost constant and was independent of the temperature, demonstrating the high internal QE. However, the recombination lifetime measured from the EL transient decay showed the critical temperature dependence below the Nb SC Tc of 7.3K. It was reduced from the constant value of ~1.9ns above 7.3K to 1.3ns at 1K. This shows that the radiative recombination lifetime is reduced below 7.3K, demonstrating the enhanced radiative recombinations with the SC effect, i.e., Cooper-pair superradiance effect. Since single photon emitters are usually operated under low injection currents where internal QE tends to be low, this new mechanism is extremely useful to realize on-demand photon emitting LEDs.This work was supported in part by the HINTS for nanofabrication. They are also grateful for all the collaborators related to this work.