The following bullet points give a brief overview for the work conducted during the PhotoHeatEffect project, the corresponding results and their exploitation and dissemination.
- Development of a unique optical setup that combines (time-resolved) µPhotoluminescence (µPL) with 2-laser Raman thermometry (2LRT), comprising mapping capabilities for both functions:
It was demonstrated that 2LRT can be integrated into a modern µPL setup, which allows for mappings, polarization resolved studies, time-resolved, and even photon correlation analyses. Throughout the PhotoHeatEffect project the corresponding setup was designed and built at the host institution. In addition, all required software for the data acquisition and analysis was developed, which sets the basis for a fruitful use of the setup beyond the time-frame of the PhotoHeatEffect project. The employed thermal technique was carefully calibrated by recording temperature-dependent Raman shifts of the most relevant Raman modes for state-of-the-art bulk III-nitride material. This unique spectroscopic setup formed and will form the basis for numerous publications.
- Processing of III-nitride membrane structures and nanobeam lasers along with the development of a quantum dot (QD) positioning approach especially suited for III-nitrides:
InxGa1-xN/GaN QW structures (x = 15%) were grown on silicon, which allows fabricating underetched membrane structures. Such freestanding, two-dimensional III-nitride structures are ideal for analysing the in-plane heat transport, which can directly be monitored by the aforementioned conjunction of µPL and 2LRT spectroscopy. These III-nitride membranes were etched into one-dimensional nanobeam structures, which form the basis for nanolasers. Furthermore, the positioning of GaN/AlN QDs was addressed in order to establish the basis for future three-dimensional thermal crystals. Consequently, the entire set of growth and processing efforts forms the basis for publications.
- In-detail characterization of the optical properties of all employed optical media, which mainly comprises InGaN/GaN QW structures:
InxGa1-xN samples were studied by PL, µPL, and time-resolved µPL in order to, e.g. determine the exciton diffusion lengths and the internal quantum efficiency depending on the indium content x. Knowledge of such parameters proved important for detangling the different contributions to thermal transport evoked by thermal phonons, excitons, exciton-polaritons, and free carriers. The characterization results of the PhotoHeatEffect project have been published in peer-reviewed journals. Here, a publication in Physical Review X (Callsen et al., PRX 9, 031030, 2019) represents a dissemination highlight of the PhotoHeatEffect project, which will be followed by further future publications.
- Two-dimensional 2LRT mappings of heat distributions across nanostructures were obtained with a spatial resolution of down to 500 nm under operating conditions of the selected photonic membranes (optical injection):
The developed experimental techniques were showcased for dedicated III-nitride membrane structures that form the basis for nanolasers. As a result, highly spatially resolved temperature distributions could be recorded for varying ambient temperature, injection levels, and sample geometries. Clear signatures of different phonon propagation regimes were attested by the employed 2LRT spectroscopy, which will be the topic of an upcoming publication. Furthermore, the direct link in between local heating and the detrimental effects on the µPL signal was established. Clearly, the entire set of these measurements will in the future lead to novel device designs that consider both, the optical and thermal prerequisites of a photonic membrane structure like a III-nitride nanobeam laser.
