Band-edge exciton states in bulk lead chalcogenides are 64-fold degenerate. In quantum dots (QDs), the degeneracy is lifted by the valley mixing and the electron-hole exchange interaction. To investigate their interplay we calculate excitonic states in PbS QDs within the tight-binding method. This allows one to trace the genesis of the bright excitonic states from the valley-degenerate states of the direct exciton which may be described within the effective mass model. We compute optical absorption spectra fully accounting for the exciton fine structure within the tight-binding method and extend the effective-mass model to include description of the inter-valley coupling.
We show that mechanical properties of atomically thin crystals, such as graphene and transition metal dichalcogenides can be efficiently controlled by optical excitation. Illumination by a plane electromagnetic wave with the frequency close to plasmon or exciton resonance affects directly the membrane tension. Depending on the sign of the frequency detuning from the resonance, the membrane is either stretched or crumpled by light. In the latter case, the optomechanical crumpling force competes with the rigidity and the radiation pressure that try to flatten the membrane. When the excitation intensity surpasses the critical value, transition to the crumpled phase occurs.
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