2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength F_Stark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton’s center of mass and the total area, which yields a coherence area of a mere 6.1 nm². Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that F_Stark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems. [Figure not available: see fulltext.]

Ultrathin two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications because of their pronounced room-temperature excitonic features, leading to unique properties in terms of light-matter interaction. However, only a few studies focus on light amplification and the complex photophysics at high excitation density. The beneficial nature of strong excitonic effects on optical gain remain hence unquantified, and despite the increased binding energies of the excitonic species, it remains unclear what the involvement of 2D excitons would be in optical gain. Here, we use colloidal CdSe nanoplatelets as a model system for colloidal 2D materials and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several excitation density-dependent optical gain regimes exist. At low density, optical gain originates from excitonic molecules delivering large material gains up to 20 000 cm ^-1 with an Auger limited lifetime of a few hundred picoseconds. At increasing pair density, we observe a persistence of this excitonic gain regime and the unexpected coexistence of blue-shifted and significantly enhanced optical gain up to 10 ⁵ cm ^-1 . We show that this peculiar situation originates from a carrier cooling bottleneck at high density that limits further exciton formation from unbound charge carriers. The insulating (multi-)exciton gas is found to coexist with the conductive phase, indicating the absence of a full Mott transition. Our results shed a new light on the photophysics of excitons in strongly excited 2D materials and pave the way for the development of more efficient (broadband) optical gain media and/or high exciton density applications.

Owing to their widely tuneable optical properties and strong light-matter interaction, colloidal quantum dots[1] (QDs) are considered for next-generation photonic devices such as solution processable lasers[2] and all-optical wavelength converters. Using QDs for this last application, i.e. wavelength conversion, is limited by either slow interband (106-109 s-1) or energy consuming multi-exciton dynamics (109-1012 s−1). Here we show, using white light pump-probe spectroscopy, that the interplay between two intrinsic material properties of PbS QDs, intraband absorption and interband bleach can lead to a very strong modulation of near-infrared light on an ultrafast, picosecond, timescale: A peculiar dynamic arises at the matching wavelength: a strong burst of absorption induced by the pump lasts for roughly 1 picosecond and vanishes, leaving the dots as they were before the pump pulse arrived.

Colloidal quantum dots (QDs) raise more and more interest as solution-processable and tunable optical gain materials. However, especially for infrared active QDs, optical gain remains inefficient. Since stimulated emission involves multifold degenerate band-edge states, population inversion can be attained only at high pump power and must compete with efficient multi-exciton recombination. Here, we show that mercury telluride (HgTe) QDs exhibit size-tunable stimulated emission throughout the near-infrared telecom window at thresholds unmatched by any QD studied before. We attribute this unique behaviour to surface-localized states in the bandgap that turn HgTe QDs into 4-level systems. The resulting long-lived population inversion induces amplified spontaneous emission under continuous-wave optical pumping at power levels compatible with solar irradiation and direct current electrical pumping. These results introduce an alternative approach for low-threshold QD-based gain media based on intentional trap states that paves the way for solution-processed infrared QD lasers and amplifiers.

All-optical approaches to change the wavelength of a data signal are considered more energy-and cost-effective than current wavelength conversion schemes that rely on back and forth switching between the electrical and optical domains. However, the lack of cost-effective materials with sufficiently adequate optoelectronic properties hampers the development of this so-called all-optical wavelength conversion. Here, we show that the interplay between intraband and band gap absorption in colloidal quantum dots leads to a very strong and ultrafast modulation of the light absorption after photoexcitation in which slow components linked to exciton recombination are eliminated. This approach enables all-optical wavelength conversion at rates matching state-of-the-art convertors in speed, yet with cost-effective solution-processable materials. Moreover, the stronger light-matter interaction allows for implementation in small-footprint devices with low switching energies. Being a generic property, the demonstrated effect opens a pathway toward low-power integrated photonics based on colloidal quantum dots as the enabling material.

The cooling dynamics of hot charge carriers in colloidal lead chalcogenide nanocrystals is studied by white light transient absorption spectroscopy. We demonstrate a transient accumulation of charge carriers at a high-energy critical point in the Brillouin zone. Using a theoretical study of the cooling rate in lead chalcogenides, we attribute this slowing down of charge carrier cooling to a phonon scattering bottleneck around this critical point. Our approach allows for the first ever determination of hot carrier cooling rates, relevant in e.g. modeling of multiple exciton generation.

A spectroscopic study of the optical non-linearities of PbSe colloidal solutions was performed with the Z-scan technique, at wavelengths between 1200 and 1750 nm. No non-linear absorption is observed, while the third-order non-linear refractive index n₂ shows clear resonances, somewhat blueshifted relative to the exciton transitions in the absorbance spectrum. The occurrence of thermal effects is ruled out by time-resolved measurements. At resonance, measured n₂ values exceed typical bulk semiconductor values by two orders of magnitude. At high optical intensity, the refractive index change saturates, indicating that state filling lies at the origin of the observed effect.

A spectroscopic study of the optical nonlinearity of PbSe colloidal solutions was performed with the Z -scan technique at wavelength intervals of 1200-1350 and 1540-1750 nm. While nonlinear absorption remains below the detection threshold, the third order nonlinear refractive index n2 shows clear resonances, somewhat blueshifted relative to the exciton transitions in the absorbance spectrum. The occurrence of thermal effects is ruled out by time-resolved measurements. At 1.55 μm, measured (resonant) n2 values exceed typical bulk semiconductor values by two orders of magnitude. At high optical intensity, the refractive index change saturates, indicating that statefilling lies at the origin of the observed effect.