Spectral diffusion in nitride quantum dots: Emission energy dependent linewidths broadening via giant built‐in dipole moments

Abstract

1 Introduction Quantum light sources exhibiting one-and two-photon emission are the key requirement for quantum cryptography ultimately leading to truly secure data transmission [1]. Physical realization can be obtained by parametric down-conversion [2], atomic [3], or defect related transitions [4,5] and quantum dot (QD) luminescence [6,7]. Tailoring the electronic structure of a highly integrable [8] semiconductor QD even leads to tunability of the resulting photon streams [9] based on variation of vital parameters as the exciton fine-structure splitting [10][11][12] and the biexciton binding energy [13][14][15][16].Hence, the precise determination of such parameters is mandatory for the design of quantum light sources, but the emission linewidth broadening due to spectral diffusion can make such task a tough challenge. The general phenomenon of spectral diffusion was first described in nu-

Manifestation of unconventional biexciton states in quantum dots

“…Owing to the large total spin of s ¼ ± 3, m-PL experiments within magnetic fields should reveal a field strength-dependent fourfold splitting of the hybrid-XX emission line, caused by the splitting of the hybrid-XX states and the dark X states, which will become optically active. Observing such a splitting will be challenging due to the large-emission linewidths 44 but seems achievable based on recent growth advances. However, even without the application of a magnetic field, one can already indirectly witness the anomalous cascade process via the dark states by a straight-forward polarization-dependent m-PL measurement.…”

“…We can observe a striking agreement between experiment and theory that already supports the identification of the hybrid-XX. The only parameters that had to be matched to the experiment for the simulation are the linewidth induced by spectral diffusion 44,45 and the particular excitation power and temperature dependencies, varying the intensity balance between X1, X2, XX as shown in Supplementary Figs 2 and 3. Neither material nor geometric QD parameters were varied to approach the measurement causing the apparent slight discrepancies for the luminescence energies.…”

“…Therefore, also the bright X states show a QD anisotropy-dependent smaller fine-structure splitting of E1 meV in the simulations and a slightly larger hybrid XX-binding energy. The visible emission energy variations of the measured emission lines (a) during the polarization angle scan are caused by the spectral diffusion in GaN/AlN QDs 44,45 , which can be suppressed by lowering the temperature, sacrificing the intensity balance of the bright exciton states (see Fig. 5).…”

“…ARTICLE equalize R1 and R2*, proving the existence of a phonon-limited hybrid-XX cascade including a transition from the dark to the bright X states. The apparent temporally broad (E ± 25 ns) bunching phenomenon in both correlation functions arises from spectral diffusion 44,45 and is modelled as described by Sallen et al 55 . Further standard techniques for the identification of QD emission lines are discussed in Supplementary Note 2.…”

“…The wurtzite GaN QDs grow by means of low-pressure metal-organic vapour deposition at a density of E6 Â 10 9 cm À 2 . A detailed description of the growth conditions can be found elsewhere 44,53,56,57 . The sample's mesa structures (200 nm-2 mm diameter) were processed by means of e-beam lithography and plasma etching to enable single-QD spectroscopy.…”