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

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) lumines-cence [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 [10–12] and the biexciton binding energy [13–16]. 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 phe-nomenon of spectral diffusion was first described in nu-clear magnetic resonance experiments [17, 18] and was then applied to light-emitting systems such as rare-earth ions [19], nitrogen vacancies in diamond [20], and semi-conductor QDs [21–23]. For the case of a semiconductor QD the emitter can be situated in a semiconductor matrix material containing defects that stochastically trap and re-lease charges and hence generate a fluctuating Coulomb field [24]. As a consequence an exciton inherent to the QD senses the charged defects in the matrix material mediated by a basic electrostatic interaction resulting in a continuous change of the associated emission energy. If the typical time scale of this as spectral diffusion known phenomenon is shorter than the integration time of the detection system, then the natural linewidth is obscured, and an inhomoge-neous emission linewidth broadening is observed [25].