Versatile excitation of nanoparticles

A single pulse of twisted light can induce transitions to energy levels not accessible using Gaussian beams.

Gaussian beams can directly induce electronic transitions between a fraction of all possible states: For example, according to the vertical transition approximation, a well collimated beam will only induce transitions between states with the same linear momentum.

In contrast, twisted light can promote electrons to a variety of states, as we discuss in the following for the case of quantum dots.

Collimated beam

Let us imagine that we shine circularly polarized twisted light on a quantum dot. The light's parameters are: topological charge $\ell>0$ and circular polarization index $\sigma=1$. Under these conditions, the electric field perpendicular to the QD dominates, and the interaction can promote electrons from the heavy-hole band to many different levels in the conduction band: the final state differs from the initial one in the amount of orbital angular momentum $\hbar \ell$. Note that this mode of excitation requires that Sign($\ell$)=Sign($\sigma$), what we call a parallel angular momenta beam. See this post for details.

Focused beam: Electric interaction

A different set of states can be addressed by using the strong longitudinal component of the electric field ($E_z$) of an antiparallel beam, with $\ell=\pm 1$. 

Close to the phase singularity, located at the beam axis, the longitudinal component dominates and can produce light-hole excitons with zero microscopic angular momentum.

This is shown in the energy-level diagram below, where one can see the differences in excitation between Gaussian beams (black arrow) and twisted light (red and blue). See this post for details.

Focused beam: Magnetic interaction

Moreover, by still using the antiparallel beam configuration but with $\ell=\pm 2$, the magnetic interaction can become dominant. This peculiar effect can help address a different set of states in the conduction band. Now we can generate excitons with zero microscopic angular momentum but with orbital angular momentum $m=\pm 1$ (red arrows); the previous modes of excitation are shown with grey arrows.

 See this post for details.

To conclude, we can achieve with a single pulse at normal incidence the selective excitation of particular states in the quantum dot --the same ideas apply to other nanostructures. We need control on the topological charge, polarization and degree of focusing.