Reducing the Optical Gain Threshold in Two-Dimensional CdSe Nanoplatelets by the Giant Oscillator Strength Transition Effect

Abstract

Two-dimensional CdSe nanoplatelets are promising lasing materials. Their large lateral areas reduce the optical gain threshold by increasing the oscillator strength and multiexciton lifetimes but also increase the gain threshold by requiring multiple band-edge excitons (>2) to reach the optical gain. We observe that the optical gain threshold of CdSe nanoplatelets at 4 K is ∼4-fold lower than that at room temperature. Transient absorption spectroscopy measurements indicate that the exciton center-of-mass coherent area is smaller than the lateral size at room temperature and extends to nearly the whole nanoplatelets at 4 K. This suggests that the reduction in the optical gain threshold at a low temperature can be attributed to exciton coherent area extension that reduces the saturation number of band-edge excitons to enable biexciton gain and increases the radiative decay rate, consistent with the giant oscillator strength transition effect. This work demonstrates a new direction for lowering the optical gain threshold of nanomaterials.

Localization-limited exciton oscillator strength in colloidal CdSe nanoplatelets revealed by the optically induced stark effect

“…F A b s is proportional to the total platelet area and determines the pronounced exciton feature in the absorption spectrum, while F S t a r k is proportional to the coherence area of the localized exciton and is the relevant quantity to understand the radiative lifetime of the exciton. This result questions previously published estimates of the exciton area at room temperature of 96 nm 2 or 21 nm 2 for similar 4.5 ML nanoplatelets 19 , 20 . Such large coherence areas would yield significantly shorter room temperature radiative lifetimes than measured experimentally.…”

“…Similar observations were made in the case of epitaxial quantum wells 17 , 18 , and attributed to the large in-plane coherence area of the exciton center-of-mass motion in these systems. Intriguingly, recent reports based on state-filling models proposed that even at room temperature, this coherence might be close to 100 nm 2 , a number that seems incompatible with the nanosecond radiative lifetime reported by various authors 19 – 21 . In addition, several studies indicated the potential for strong coupling of excitonic transitions with the light field at room temperature using 4.5 monolayer CdSe nanoplatelets, a feat that requires narrow transition lines with large oscillator strength 8 , 9 .…”

“…Having rationalized the ratio between both oscillator strengths as the ratio between the exciton coherence area and the total nanoplatelet area, we obtain an exciton coherence area of 6.1 nm 2 at room temperature. Importantly, opposed to commonly used state-filling models, we can use the optical Stark measurement to calculate the exact radiative lifetime, without any assumptions 19 , 20 . This internally consistent picture indicates that the coherence area of excitons in 4.5 ML CdSe nanoplatelets is considerably smaller than the total nanoplatelet area.…”

“…Using OSE spectroscopy, one pumps the material using a femtosecond pump pulse detuned relative to the exciton transition and measures the induced energy shift of the exciton using a broad, white-light probe pulse. This method alleviates the need for electrical contacting 29 and does not rely on real charge carriers, thereby eliminating any spurious effects of defect trapping and assumptions on state-filling or electron-hole overlap 19 , 20 . Recent work by Diroll showed that also CdSe nanoplatelets display such a Stark effect and dipole moments in the range 15–23 D were extracted, numbers which are very much in line with other 2D materials 30 .…”

Low‐Threshold Amplified Spontaneous Emission and Lasing from Thick‐Shell CdSe/CdS Core/Shell Nanoplatelets Enabled by High‐Temperature Growth

“…Unlike in core‐only NPLs, thick‐shell NPLs could further extend the spatial separation of excitons in thickness dimensions, which lengthens the multiexciton state and gain lifetimes. As a result, the extremely long optical gain lifetime (>800 ps) of CdSe/CdS NPLs is ≈8 times larger than those previously reported core‐only NPLs (≈100 ps) and also near two times greater than those of conventional CdSe/CdS QDs (>400 ps) . Recently, such long optical gain lifetime (≈800 ps) is also reported in this novel CdSe/CdS@CdZnS core/crown@gradient‐alloyed shell colloidal quantum wells .…”