Optical observation of superconducting density of states in luminescence spectra of InAs quantum dots

Abstract

We study luminescence spectra observed from InAs quantum dots (QDs) embedded in an n-type InGaAs-based heterostructure, where electron Cooper pairs penetrate from an adjacent niobium (Nb) superconductor with the proximity effect. Below the superconducting (SC) critical temperature of Nb, we observe substantial luminescence intensity enhancement and a sharp edge in luminescence spectra of InAs QDs. We explain the observed sharp edge in the luminescence spectra with the proximity effect, that is, with the consideration of opening of SC gap and modification of density of states (DOS) near the electron Fermi level in the n-type semiconductor heterostructure. We demonstrate that the sharp edge luminescence spectra are well reproduced by the SC DOS, with quasiparticle lifetime broadening and a Gaussian distribution of lowest QD state emission lines. We discuss the reason why it has been difficult to observe the sharp edge luminescence spectra in the previous quantum well-based SC light emitting diodes.

Time-resolved measurements of Cooper-pair radiative recombination in InAs quantum dots

“…Then QDs, whose conduction-band lowest energy states are near resonance with the energy of the SC-DOS, experience enhanced photon emission by the Cooper-pair recombination. 27,28 Concerning the excitation power dependence, the minority-hole population in the valence-band lowest QD states changes with the excitation power. When the excitation remains low, the hole population among QDs is determined mainly by the capture process of photo-generated carriers at low temperature.…”

Andreev reflection enhancement in semiconductor-superconductor structures

“…Many types of hybrid semiconductor-superconductor devices have been successfully fabricated [1,2,3,4] and characterized, with some of the experimental results showing interesting phenomena, such as asymmetry in the conductance spectrum [1]. Moreover, various applications involving semiconductor-superconductor structures have been shown [5,6,7,8,9,10,11]. These include superconducting light emitting diodes (SLEDs) [5,6,7,9] and superconducting quantum dots capable of emitting entangled photon-pairs [8], Bell-state analyzers [10] and semiconductorsuperconductor based waveguide amplifiers [11].…”

“…Moreover, various applications involving semiconductor-superconductor structures have been shown [5,6,7,8,9,10,11]. These include superconducting light emitting diodes (SLEDs) [5,6,7,9] and superconducting quantum dots capable of emitting entangled photon-pairs [8], Bell-state analyzers [10] and semiconductorsuperconductor based waveguide amplifiers [11]. This wide range of novel devices takes advantage of Andreev reflection [12] at the superconductor-semiconductor interface [1,2,3,4], to directly inject electron Cooper-pairs into the semiconducting side of the junction.…”

Andreev Reflection in a Superconducting Light-Emitting Diode

“…Hybrid superconductor−semiconductor devices are a rapidly growing field of research, with a wide range of applications, including a variety of quantum devices, − such as quantum dot (QD) based light-emitting diodes (LEDs), − Bell-state analyzers, and superconducting two-photon amplifiers . One of the most promising directions in these hybrid devices is generation of entangled-photon pairs through Cooper-pair luminescence in compact semiconductor devices, crucial for practical quantum technologies.…”

“…Cooper-pair injection into the normal material can be detected experimentally through the phenomenon of Andreev reflection appearing as enhancement of conductance within the vicinity of the superconducting gap, a zero-bias conductance peak (ZBCP) . Several superconducting LED (SLED) devices have been studied in the past − ,,, demonstrating Josephson effects and enhanced light emission. While Josephson effects are interesting and promising for future superconducting optoelectronics, − they occur in a wide range of superconductor-normal metal or superconductor–insulator junctions, (including vacuum as an insulator) and thus do not exhibit unique signatures of Cooper pair injection into the semiconductor.…”