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.

由于吸收光谱中激子跃迁的明显巨大的振荡器强度，二维材料被考虑用于需要强光-物质相互作用的应用。然而，这些转变的有效振荡器强度几乎没有报道，也没有对所获得值的一致解释。在这里，我们通过光诱导斯塔克效应 (OSE) 分析了 2D CdSe 纳米片中激子跃迁的跃迁偶极矩和随后的振荡器强度。有趣的是，我们发现激子吸收线对高强度光场的反应作为具有振荡器强度F _{S t a r k}的跃迁这比基于线性吸收系数的预期小 50 倍。我们建议将显着的激子吸收线视为多个低振子强度跃迁的总和，而不是单个高振子强度跃迁，这是我们赋予强激子质心定位的一项壮举。在激子的量子力学描述中，这两个振荡器强度之间的 50 倍差异对应于激子质心的相干面积与总面积之间的比率，其产生的相干面积仅为 6.1 nm ². 由于我们发现相干面积随着温度的降低而增加，因此我们得出结论，与晶格振动相关的热效应有助于激子局域化。为了进一步支持这种定位模型，我们表明F _{S t a r k}与纳米片面积无关，正确预测辐射寿命，并与强约束量子点系统对齐。