The optical properties of the lowest-energy excitons in nanocrystal quantum dots (NQDs) are central to applications and our understanding of NQDs. The most important properties of the associated transitions are their size-dependent energies and oscillator strengths. The effective-mass approximation (EMA) has successfully described the energies of the confined electronic states of CdSe NQDs. However, the EMA predicts a size-independent product of exciton energy and oscillator strength (hωf) for the lowest-energy optical transitions, whereas several reports over nearly 20 years have indicated that the oscillator strength of the 1S3/21Se peak of strongly confined CdSe NQDs is substantially lower than the EMA prediction. We have demonstrated a metrology based on the optical Stark effect (OSE) that allows us to directly determine the oscillator strength of the lowest-energy excitons in NQDs. Red-detuned, non-resonant light produces an optical Stark shift of an optical transition to higher energies. This shift depends only on the intensity of the non-resonant excitation, the detuning of the Stark field, and the squared magnitude of the dipole transition matrix element. Therefore, a measurement of the optical Stark shift (OSS) is directly related to the magnitude of the transition matrix element, which determines the oscillator strength of the transition. Contrary to previous reports, as shown in Figure 1, we find that for CdSe NQDs of diameter 2.5 to 6.7 nm, the 1S3/21Se oscillator strength does follow the predictions of the EMA. This has important implications for applications and coherent control of excitons.

Optical Stark metrology of CdSe nanocrystal quantum dots: On the size-dependence of the oscillator strength

253rd National Meeting (2017)

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