Manipulation of the properties of single quantum emitters is an important topic of current research. It has been traditionally achieved in various systems by coupling of the emitter to a cavity [1
]. Recently, there is an increasing interest in achieving this goal by coupling the emitter to metal nanostructures that act as an optical analog of an antenna at radiofrequencies [4
]. Quantum dots (QDs) are an important class of quantum emitters as they can have engineered optical response. Composite metal–semiconductor QD structures enable even more control over the optical properties. The controllable properties include radiative lifetime, absorption cross-section and nonlinear susceptibility [5
]. Controlling them is not only of interest in the context of basic science, but also has numerous applications. The most obvious are increasing the efficiency of light emitters, detectors and solar cells. Apart from that, metal optics or plasmonics enables confinement of light at truly sub-wavelength scales. Combining metal nanostructures with active materials can lead to ultimate scaling down of coherent light sources and other active devices, which could be potentially used for ultra-fast, nanoscale photonic integrated circuits that could compete with their electronic counterparts [7
Other very interesting applications are in miniaturization of quantum functional devices, such as single or entangled photon sources [2
]. 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
]. 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.
In this article, we report the observation of coupling of single near-surface InGaAs QDs with the surface plasmon resonance (SPR) of an In nanocrystal. Micro-photoluminescence (micro-PL) measurements at low temperature reveal intense sharp lines due to single QD emission in the spectral region of the SPR. Reference samples with no In nanocrystals measured under the same conditions reveal only a much weaker broad background emission. Thus, absolute QD emission efficiency enhancement due to coupling to the In SPR is observed proving that the In nanocrystals act as efficient nanoantennae. The In nanocrystals are aligned on top of the QDs using the principle of strain-driven adatom migration, which enables precise control over the QD–metal separation.