Other very interesting applications are in miniaturization of quantum functional devices, such as single or entangled photon sources [2,9]. Such applications require coupling of a single quantum emitter to a metal nanostructure. This is not an easy task, because it requires extremely precise control over the mutual position and separation of the emitter and the metal nanostructure. This is crucial, as too small separation will result in quenching of the emitter due to nonradiative recombination at the metal interface and too large separation will cause, that the plasmonic effects will be negligible [5]. That is why truly nanometer scale precision is required, which rules out usage of all top-down fabrication approaches and leaves only bottom-up processes. In case of colloidal QDs, chemical self-assembly based on long organic molecules that bind to the functionalized surface of a metal nanoparticle is often used [10]. Also for solid-state systems such as molecular beam epitaxy (MBE) grown InGaAs/GaAs QDs, a very promising approach exists, which is based on strain-driven adatom migration. So far, it was mostly used for forming complex semiconductor nanostructures like QD molecules or for positioning of QDs on strain-engineered substrates [11,12]. It has been demonstrated that the same principle works also for the alignment of metal nanocrystals on top of QDs, provided that the metal recognizes the underlying crystalline structure of the substrate [13]. Since the present-day MBE technology offers very precise control of growth rates and other deposition parameters, strain-driven migration can be used for extremely precise lateral and vertical positioning of metal nanocrystals with respect to underlying QDs, which allows full utilization of the plasmonic effects.

By choosing suitable deposition conditions, it is possible to obtain low densities of In nanocrystals, as seen in Fig. ​Fig.1c.1c. Such configurations are very promising for the investigation into coupled QD–SPR emission at the single QD level. It is well known that the SPR-related local field enhancement is extremely short-ranged [5], especially in high-index media like III-V semiconductors. Hence, only QDs that are in close proximity of an In nanocrystal can interact with its SPR. As a QD placed too close or too far away from the In nanocrystal either will be quenched or will not experience any effect from the SPR, the one-dimensional ordering of the QDs is very beneficial, because it implies that there always is a QD at the optimal distance from the In nanocrystal. Finally, a sufficiently low density of the In nanocrystals is necessary, so that there are only few of them in the laser spot, enabling observation of single QD–SPR coupling using standard micro-PL spectroscopy.

The results of the optical characterization of the investigated sample are presented in Fig. ​Fig.2.2. DR measurements reveal a distinct peak due to the SPR of the In nanocrystals around 1 μm (Fig. ​(Fig.2a).2a). Tuning of the SPR into the near-infrared spectral region is most likely due to the high refractive index of GaAs and also the rather complicated shape of the In nanocrystals. Room-temperature PL reveals a clear emission peak centered around 1.25 μm from the QD array (Fig. ​(Fig.2b).2b). It has an additional feature in the 1 μm region, which is attributed to coupled QD–SPR emission. No such feature is visible for the reference sample without In nanocrystals. It is also not observed for samples with thicker GaAs capping layer, indicating that the observed effect is near-field-related. At lower temperature, the SPR-related peak becomes more pronounced, most likely due to lower resistive losses in the metal. Micro-PL measurements show that the SPR-related peak around 1 μm is composed of sharp lines due to emission from individual QDs (Fig. ​(Fig.2c).2c). No such splitting is resolved for samples without In nanocrystals as the QD density is too large. This evidences that only emission from single QDs close to the sparse In nanocrystals is enhanced, reconfirming that the enhancement is near-field-related. Hence, coupled QD–SPR emission at the single QD level is observed for the first time in an all solid-state system.