Quantum Optoelectronics

Abstract:

The surprising recent observation of highly emissive triplet-states in lead halide perovskites accounts for their orders-of-magnitude brighter optical signals and high quantum efficiencies compared to other semiconductors. This makes them attractive for future optoelectronic applications, especially in bright low-threshold nanolasers. While nonresonantly pumped lasing from all-inorganic lead-halide perovskites is now well-established as an attractive pathway to scalable low-power laser sources for nano-optoelectronics, here we showcase a resonant optical pumping scheme on a fast triplet-state in CsPbBr₃ nanocrystals. The scheme allows us to realize a polarized triplet-laser source that dramatically enhances the coherent signal by 1 order of magnitude while suppressing noncoherent contributions. The result is a source with highly attractive technological characteristics, including a bright and polarized signal and a high stimulated-to-spontaneous emission signal contrast that can be filtered to enhance spectral purity. The emission is generated by pumping selectively on a weakly confined excitonic state with a Bohr radius ∼10 nm in the nanocrystals. The exciton fine-structure is revealed by the energy-splitting resulting from confinement in nanocrystals with tetragonal symmetry. We use a linear polarizer to resolve 2-fold nondegenerate sublevels in the triplet exciton and use photoluminescence excitation spectroscopy to determine the energy of the state before pumping it resonantly.

Abstract:

We have characterized the carrier dynamics of the excitonic emission emerging from a monolayer of graphene grown on a Cu(111) surface. Excitonic emission from the graphene, with strong and sharp peaks both with a full-width at half-maximum of 2.7 meV, was observed near ~3.16 and ~3.18 eV at 4.2 K. The carrier recombination parameters were studied by measuring both temperature-dependent and time-resolved photoluminescence. The intensity variation with temperature of these two peaks shows an opposing trend. The time-resolved emission was modelled using coupled differential equations and the decay time was found to be dominated by carrier trapping and Auger recombination as the temperature increased.

Abstract:

Crystallographic point defects (PDs) can dramatically decrease the efficiency of optoelectronic semiconductor devices, many of which are based on quantum well (QW) heterostructures. However, spatially resolving individual nonradiative PDs buried in such QWs has so far not been demonstrated. Here, using high-resolution cathodoluminescence (CL) and a specific sample design, we spatially resolve, image, and analyze nonradiative PDs in InGaN/GaN QWs at the nanoscale. We identify two different types of PDs by their contrasting behavior with temperature and measure their densities from 10¹⁴ cm^–3 to as high as 10¹⁶ cm^–3. Our CL images clearly illustrate the interplay between PDs and carrier dynamics in the well: increasing PD concentration severely limits carrier diffusion lengths, while a higher carrier density suppresses the nonradiative behavior of PDs. The results in this study are readily interpreted directly from CL images and represent a significant advancement in nanoscale PD analysis.