The optical activity of semiconductor nanostructures and nanostructured media
1. Perturbation of electronic subsystem
We examine the most general case in which there are not degenerate states in the nanocrystal electronic subsystem and all of these states are coupled to each other by the small perturbation. Since some of the transitions between the unperturbed states are electric or magnetic dipole allowed, an arbitrary transition between the perturbed states becomes optically active. In the first order of the perturbation, the rotatory strength of such transition is decomposed on electric dipole and magnetic dipole contributions, which correspond to the electric dipole allowed and magnetic dipole allowed transitions between the quantum states of the unperturbed nanocrystal. The rotatory strengths of these two kinds of transition are of the same order of magnitude, while the dissymmetry factors of the magnetic dipole allowed transitions exceed those of the electric dipole allowed transitions by orders of magnitude. We show that it is possible to achieve the total dissymmetry of the optical absorption upon the magnetic dipole allowed transitions and elucidate the conditions required to do this.
2. Ion doping
We apply the developed theory to describe optical activity of the ion-doped nanocrystals in the form of rectangular parallelepipeds. An impurity ion injected in the crystal lattice perturbs the electronic subsystem of the nanocrystal, which can make the nanocrystal optically active. We analyse the dependence of the rotatory strength on the position of an impurity ion inside a nanocrystal and show that the optical activity of doped nanorods and quantum dots can be 100 times stronger than that of typical chiral molecules.
Since chiral nanoparticles are much smaller than the optical wavelength, their enantiomers show little difference in the interaction with circularly polarized light. This scale mismatch makes the enhancement of enantioselectivity in optical excitation of nanoobjects a fundamental challenge in modern nanophotonics. Here we demonstrate that a strong dissymmetry of optical response from achiral nanoobjects can be achieved through their arrangement into chiral optically active superstructures with the length scale comparable to the optical wavelength. This concept is illustrated by the example of the simple helix supercrystal made of semiconductor quantum dots. We show that this supercrystal almost fully absorbs light with one circular polarization and does not absorb the other. The giant circular dichroism of such a supercrystal comes from the formation of chiral bright excitons, which are the optically active collective excitations of the entire supercrystal. Owing to the recent advances in assembly and self-organization of nanocrystals in large superparticle structures, the proposed principle of enantioselectivity enhancement has great potential of benefiting various chiral and analytical methods, which are used in biophysics, chemistry, and pharmaceutical science.