The Impact of Partial Carrier Confinement on Stimulated Emission in Strongly Confined Perovskite Nanocrystals.

Semiconductor lead halide perovskites are excellent candidates for realizing low threshold light amplification due to their tunable and highly efficient luminescence, ease of processing, and strong light-matter interactions. However, most studies on optical gain have addressed bulk films, nanowires, or nanocrystals that exhibit little or no size quantization. Here, we show by means of a multitude of optical spectroscopy methods that small CsPbBr3 nanocrystals (NCs) exhibit a progressive red shift of the band-edge transition upon addition of electron-hole pairs, at least one carrier of which occupies a 2-fold degenerate, delocalized state in agreement with strong confinement. We demonstrate that this combination results in a threshold for biexciton gain, well below the limit of one electron-hole pair on average per NC. On the other hand, both the luminescent lifetime and the optical Stark effect of 4.7 nm CsPbBr3 NCs indicate that the oscillator strength of the band-edge transition is considerably smaller than expected from the band-edge absorption. We assign this discrepancy to a mixed confinement regime, with one delocalized and one localized charge carrier, and show that the concomitant reduction of the oscillator strength for stimulated emission accounts for the surprisingly small material gain observed in small NCs. The conclusion of mixed confinement aligns with studies reporting small and large polarons for holes and electrons in lead halide perovskite nanocrystals, respectively, and creates opportunities for understanding multiexciton photophysics in confined perovskite materials.

Two-Dimensional Electron-Hole Plasma in Colloidal Quantum Shells Enables Integrated Lasing Continuously Tunable in the Red Spectrum.

Combining integrated optical platforms with solution-processable materials offers a clear path toward miniaturized and robust light sources, including lasers. A limiting aspect for red-emitting materials remains the drop in efficiency at high excitation density due to non-radiative quenching pathways, such as Auger recombination. Next to this, lasers based on such materials remain ill characterized, leaving questions about their ultimate performance. Here, we show that colloidal quantum shells (QSs) offer a viable solution for a processable material platform to circumvent these issues. We first show that optical gain in QSs is mediated by a 2D plasma state of unbound electron-hole pairs, opposed to bound excitons, which gives rise to broad-band and sizable gain across the full red spectrum with record gain lifetimes and a low threshold. Moreover, at high excitation density, the emission efficiency of the plasma state does not quench, a feat we can attribute to an increased radiative recombination rate. Finally, QSs are integrated on a silicon nitride platform, enabling high spectral contrast, surface emitting, and TE-polarized lasers with ultranarrow beam divergence across the entire red spectrum from a small surface area. Our results indicate QS materials are an excellent materials platform to realize highly performant and compact on-chip light sources.

Decoding Excimer Formation in Covalent-Organic Frameworks Induced by Morphology and Ring Torsion.

A thorough and quantitative understanding of the fate of excitons in covalent-organic frameworks (COFs) after photoexcitation is essential for their augmented optoelectronic and photocatalytic applications via precise structure tuning. The synthesis of a library of COFs having identical chemical backbone with impeded conjugation, but varied morphology and surface topography to study the effect of these physical properties on the photophysics of the materials is herein reported. The variation of crystallite size and surface topography substantified different aggregation pattern in the COFs, which leads to disparities in their photoexcitation and relaxation properties. Depending on aggregation, an inverse correlation between bulk luminescence decay time and exciton binding energy of the materials is perceived. Further transient absorption spectroscopic analysis confirms the presence of highly localized, immobile, Frenkel excitons (of diameter 0.3-0.5 nm) via an absence of annihilation at high density, most likely induced by structural torsion of the COF skeletons, which in turn preferentially relaxes via long-lived (nanosecond to microsecond) excimer formation (in femtosecond scale) over direct emission. These insights underpin the importance of structural and topological design of COFs for their targeted use in photocatalysis.

Short-Wave Infrared Colloidal QD Photodetector with Nanosecond Response Times Enabled by Ultrathin Absorber Layers.

Ultrafast short-wavelength infrared (SWIR) photodetection is of great interest for emerging automated vision and spatial mapping technologies. Colloidal quantum dots (QDs) stand out for SWIR photodetection compared to epitaxial (In,Ga)As or (Hg,Cd)Te semiconductors by their combining a size-tunable bandgap and a suitability for cost-effective, solution-based processing. However, achieving ultrafast, nanosecond-level response time has remained an outstanding challenge for QD-based SWIR photodiodes (QDPDs). Here, record 4 ns response time in PbS-based QDPDs that operate at SWIR wavelengths is reported, a result reaching the requirement of SWIR light detection and ranging based on colloidal QDs. These ultrafast QDPDs combine a thin active layer to reduce the carrier transport time and a small area to inhibit slow capacitive discharging. By implementing a concentration gradient ligand exchange method, high-quality p-n junctions are fabricated in these ultrathin QDPDs. Moreover, these ultrathin QDPDs attain an external quantum efficiency of 42% at 1330 nm, due to a 2.5-fold enhanced light absorption through the formation of a Fabry-Perot cavity within the QDPD and the highly efficient extraction (98%) of photogenerated charge carriers. Based on these results, it is estimated that a further increase of the charge-carrier mobility can lead to PbS QDPDs with sub-nanosecond response time.

Integrated PbS Colloidal Quantum Dot Photodiodes on Silicon Nitride Waveguides.

Colloidal quantum dots (QDs) have become a versatile optoelectronic material for emitting and detecting light that can overcome the limitations of a range of electronic and photonic technology platforms. Photonic integrated circuits (PICs), for example, face the persistent challenge of combining active materials with passive circuitry ideally suited for guiding light. Here, we demonstrate the integration of photodiodes (PDs) based on PbS QDs on silicon nitride waveguides (WG). Analyzing planar QDPDs first, we argue that the main limitation WG-coupled QDPDs face is detector saturation induced by the high optical power density of the guided light. Using the cladding thickness and waveguide width as design parameters, we mitigate this issue, and we demonstrate WG-QDPDs with an external quantum efficiency of 67.5% at 1275 nm that exhibit a linear photoresponse for input powers up to 400 nW. In the next step, we demonstrate a compact infrared spectrometer by integrating these WG-QDPDs on the output channels of an arrayed waveguide grating demultiplexer. This work provides a path toward a low-cost PD solution for PICs, which are attractive for large-scale production.