Role of Thermally Occupied Hole States in Room‐Temperature Broadband Gain in CdSe/CdS Giant‐Shell Nanocrystals

Abstract

Two decades ago, groundbreaking work by Klimov and Bawendi demonstrated that colloidal CdSe nanocrystals display optical gain and stimulated emission at densities of about 2 excitons per quantum dot, [1] despite efficient Auger recombination. [2] This finding sparked a vivid search for new solution-processed gain materials based on colloidal quantum dots, where subsequent progress produced CdSe/ CdS nanocrystals with suppressed Auger recombination, [3] microsecond pulsed [4] to continuous-wave lasing, [5] and most recently gain and lasing at sub-singleexciton densities using photo-or electrochemically charged nanocrystals. [6,7] Having reached these milestones, an outstanding challenge is to achieve such features over a wider gain bandwidth, in order to further broaden the opportunities that solution-processed gain materials Growing CdSe/CdS nanocrystals from a large CdSe core, and employing a giant CdS shell, a continuous, broadband gain spectrum, covering the spectral range between the CdSe and the CdS band edge, is induced. As revealed by k·p calculations, this feature is enabled by a set of closely spaced S-, P-and, for larger CdSe cores, D-state hole levels, which are thermally occupied at room temperature, combined with a sparse density of electron states. This leads to a range of bleach signals in the transient absorption spectra that persist up to a microsecond. By extending a state-filling model including relevant higher-energy states and a Fermi-Dirac distribution of holes at finite temperature, it is shown that thermal occupancy can lower the gain threshold for excited states. Inclusion of Gaussian broadening of discrete transitions also leads to a smoothening of the gain threshold spectrum. Next to a direct measurement of the gain threshold, a method is also developed to extract this from the gain lifetime, taking advantage that population inversion is limited by Auger recombination and recombination rates scale with the exciton density as 〈N〉·(〈N〉 − 1). The results should be readily extendable to other systems, such as perovskite or III-V colloidal nanocrystals.

Zero-Threshold Optical Gain in Electrochemically Doped Nanoplatelets and the Physics Behind It

“…Excitation of higher energy levels is followed by relaxation to the 1S­(e) and 1S 3/2 (h) levels, making QDs a three-level lasing system . For II–VI and III–V QDs this results in a gain threshold of 4/3 excitations per QD (∼1.54 excitations per QD when including Poisson excitation statistics), ,, and consequently, Auger recombination (AR) of multiple excitons is the main factor that prevents the buildup of gain using continuous wave excitation. Therefore, much work has focused on understanding and slowing down AR. − One way to decrease the gain threshold to below a single exciton per QD is by electronic doping.…”

Colloidal Semiconductor Nanocrystal Lasers and Laser Diodes

“…The same feature of the “giant” CdSe/CdS QDs was also explored earlier to enact three-band ASE which spanned the range of colors from red to green and occurred due to population inversion of the band-edge and higher-energy transitions (Figure c) . More recently, these dots were used to achieve ultrawide optical gain bandwidth of 700 meV and four-band ASE with spectral features ranging from ∼600 nm to ∼530 nm …”

“…60 The same feature of the "giant" CdSe/ CdS QDs was also explored earlier to enact three-band ASE which spanned the range of colors from red to green and occurred due to population inversion of the band-edge and higher-energy transitions (Figure 17c). 139 More recently, these dots were used to achieve ultrawide optical gain bandwidth of 700 meV 141 and four-band ASE with spectral features ranging from ∼600 nm to ∼530 nm. 142 Compositionally graded CdSe/Cd x Zn 1−x Se cg-QDs, discussed in Section 2.4.1 in the context of Auger decay manipulation, are also promising systems for realizing broadband optical gain.…”

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

“…In particular, the use of colloidal nanomaterials has pioneered this hybrid approach, showing optically pumped on-chip lasing, both with in and out-of-plane emission, − and even demonstrations of electrically driven amplification . Such results are proof of a continued pursuit of more effective optical gain metrics and materials, resulting in the development of a diverse range of nanocrystals with varying elemental compositions and structural configurations, e.g., by incorporating semiconductor shells to exploit charge separation, , or by changing the geometry from 0D to 2D, leading to different photophysical mechanisms for light amplification. , To understand what is lacking today, we can categorize these materials based on three nonlinear optical material parameters: the inverted-state (or gain −) lifetime, τ G , the threshold carrier density, n th , required for transparency, and the material gain coefficient, g i . A recent report by Tanghe et al provided a combination of record metrics on all fronts in a single material using bulk CdS nanocrystals .…”