Abstract:We report the direct generation of linearly polarized single photons with a deterministic polarization axis in self-assembled quantum dots (QDs), achieved by the use of non-polar InGaN without complex device geometry engineering. Here, we present a comprehensive investigation of the polarization properties of these QDs and their origin with statistically significant experimental data and rigorous k·p modeling. The experimental study of 180 individual QDs allows us to compute an average polarization degree of 0.90, with a standard deviation of only 0.08. When coupled with theoretical insights, we show that these QDs are highly insensitive to size differences, shape anisotropies, and material content variations. Furthermore, 91% of the studied QDs exhibit a polarization axis along the crystal [1-100] axis, with the other 9% polarized orthogonal to this direction. These features give non-polar InGaN QDs unique advantages in polarization control over other materials, such as conventional polar nitride, InAs, or CdSe QDs. Hence, the ability to generate single photons with polarization control makes non-polar InGaN QDs highly attractive for quantum cryptography protocols.
We present here a combined experimental and theoretical analysis of the radiative recombination lifetime in a‐plane (11true2‾0) InGaN/GaN quantum dots. The structures have been grown by modified droplet epitaxy and time‐resolved photoluminescence measurements have been performed to gain insight into the radiative lifetimes of these structures. This analysis is complemented by multi‐band k·p calculations. To account for excitonic effects, the k·p theory is coupled with self‐consistent Hartree calculations. Special attention is paid to the impact of the quantum dot size on the results. Our calculations show that the residual built‐in fields in these nonpolar structures are compensated by the attractive Coulomb interaction, leading to the situation that the oscillator strength is almost unaffected by changes in the quantum dot size. Furthermore, our theoretical studies reveal that the radiative lifetimes are one order magnitude lower than values for c‐plane systems of identical size and shape. Our theoretical findings are consistent with experimental results. Also, the calculated lifetimes are comparable in magnitude to the measured values. The majority of the measured dots produce lifetime values of 250–300 ps, highlighting the potential of these nanostructures for future high‐speed single‐photon emitters.
Nonclassical light emission, such as entangled and single-photon
emission, has attracted significant interest because of its importance
in future quantum technology applications. In this work, we study
the potential of wurtzite (In,Ga)N/GaN quantum dots for novel nonclassical
light emission, namely, twin-photon emission. Our calculations, based
on a fully atomistic many-body framework, reveal that the combination
of carrier localization due to random alloy fluctuations in the dot,
spin–orbit coupling effects, underlying wurtzite crystal structure,
and built-in electric fields leads to an excitonic fine structure
that is very different from that of more “conventional”
zinc-blende (In,Ga)As dots, which have been used so far for twin photon
emission. We show and discuss here that the four energetically lowest
exciton states are all bright and emit linearly polarized light. Furthermore,
three of these excitonic states are basically degenerate. All of these
results are independent of the alloy microstructure. Also, our calculations
reveal large exciton binding energies (>35 meV), which exceed the
thermal energy at room temperature. Therefore, (In,Ga)N/GaN dots are
very promising candidates for achieving efficient twin photon emission,
potentially at high temperatures and over a wide emission wavelength
range.
We report the successful realisation of intrinsic optical polarisation control by growth, in solid-state quantum dots in the thermoelectrically cooled temperature regime (≥200 K), using a non-polar InGaN system. With statistically significant experimental data from cryogenic to high temperatures, we show that the average polarisation degree of such a system remains constant at around 0.90, below 100 K, and decreases very slowly at higher temperatures until reaching 0.77 at 200 K, with an unchanged polarisation axis determined by the material crystallography. A combination of Fermi-Dirac statistics and k·p theory with consideration of quantum dot anisotropy allows us to elucidate the origin of the robust, almost temperature-insensitive polarisation properties of this system from a fundamental perspective, producing results in very good agreement with the experimental findings. This work demonstrates that optical polarisation control can be achieved in solid-state quantum dots at thermoelectrically cooled temperatures, thereby opening the possibility of polarisation-based quantum dot applications in on-chip conditions.
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