We are building an experiment to investigate excitons in cuprous oxide. Excitons are an electron and hole in a semiconductor that are bound by coloumb attraction. We are interested in high principle quantum number (n > 10) excited states or "Rydberg states" of these excitons. On the right is a spectrum showing the absorbance peaks caused by these excited states up to a principle quantum number of n = 11.
The size of the wavefunction of the exciton scales as n2 and so at the limit of the most recent measurements, the orbital is in the order of µm wide. These extended wavefunctions exhibit large dipole moments that shift the energy levels of nearby lattice sites. If this shift is larger than the linewidth of the laser used to excite the Rydberg excitons, then no more excitons can be generated within that region of the crystal. This leads to a "Rydberg blockade" effect, where only one Rydberg exciton can exist within a given volume. On the left is a diagram showing the energy level of a second instance of an exciton shifting as a function of its distance from the first instance. The distance RB is defined as the blockade radius. If R<RB then the system is in the blockaded regime where only one Rydberg exciton can exist for a given laser linewidth.
For high n Rydberg excitons the blockade radius is larger than the diffraction limited spot size of the excitation laser. Using a two photon excitation scheme, we plan to exploit the blockade effect to create a single photon emitter. Our hope is that Rydberg excitons in this solid state system, cooled to 3.1 K in a cryostat, should exhibit many of the characteristics of atomic Rydberg systems, which are widely studied here in Durham. This experiment is therefore an exciting new opportunity at the interface of gas-phase atomic spectroscopy and solid-state physics.