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.
Highlights
The surprising recent observation of highly emissive triplet-states in lead halide perovskites accounts for their orders-ofmagnitude brighter optical signals and high quantum efficiencies compared to other semiconductors
We study colloidal PNCs in the tetragonal phase where the linear dipoles are polarized along two symmetry axes
The measurement is performed at cryogenic temperatures to resolve the splitting, since its energy is small compared to the thermal bath at room temperature
Summary
We study colloidal PNCs (see Materials and Methods) in the tetragonal phase where the linear dipoles are polarized along two symmetry axes. Since carriers excited by a near-resonant pump occupy lower-lying energy states closer to the band-edge, it follows that upon recombination their emission wavelength is red-shifted relative to that generated by states occupying higher energy levels. As the excitation power increases, a sharp peak emerges and dominates the spectrum This is attributed to stimulated emission (SE), corroborated by a characteristic S-shaped curve for the lasing intensitydependence on pump fluence (Figure 4D) above a threshold of 15 μW (∼23 mJ/cm2) for lasing onset. These states are quickly repopulated, and as the pump power increases, higher-energy states lying further away from the band edges are populated These hot carriers thermalize by scattering with phonons and other hot carriers until they reach the crystal temperature with energies characterized by a Boltzmann distribution, resulting in the observed blue-shift of the emission.[21−23]. The fact that the on-resonance intensity continues to rise linearly with excitation power up to the maximum excitation power available at this energy (80 μW) indicates that heating effects are not important over the range of powers used and little degradation of the signal is seen for long excitation times indicating that these systems would be suitable for real-word applications
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