Exciton spin states in nanocrystal quantum dots revealed by spin-polarized resonant photoluminescence and raman spectroscopy

We performed spin-polarized resonant Raman and resonant photoluminescence excitation spectroscopy (also known as "fluorescence line narrowing") on ZnS-capped CdSe nanocrystal quantum dots in high magnetic fields to 33 Tesla and temperatures down to 1.7K, which allows detailed investigation of the excitonic spin states. In these experiments, spin-polarized electrons and holes are resonantly injected by circularly polarized light into colloidal quantum dots of specific size, using a narrowband tunable dye laser and a fiber-coupled probe that is specially-designed for use in high-field magnets. In addition to the expected broad features associated with excitonic recombination and Raman-like peaks associated with quantized acoustic phonons, the photoluminescence spectra measured at magnetic fields larger than 10 Tesla develop a sharp peak, which moves roughly linearly with applied magnetic field. Further, the energy of this high-field peak varies systematically as a function of nanocrystal size. However, unlike typical electron spin flip transitions, the mode energy extrapolates to a finite value at zero magnetic field, suggesting the existence of an additional size-dependent exchange mechanism.