Superradiant single-phonon emission from a finite-sized patch of two-dimbensional material

Strong light-matter coupling from a large transition dipole moment is highly desirable for solid-state single photon emitters. For an exciton with a coherent spread over a spatial region larger than its own binding radius, its transition dipole moment is enhanced by a factor proportional to the size of the region, a phenomenon known as single photon superradiance. Here we discuss the theoretical upper limit of the transition dipole moment of an exciton confined in a finite-sized patch of 2D material, given that its lowest bright eigenstate have a level spacing of over 4k_BT from other eigenstates, sufficient for preserving its single photon nature. This limit goes beyond the particle-in-a-box model due to the electron-hole exchange interaction, which in the continuous limit induces linear instead of parabolic dispersion in the direction of the transition dipole. For a realistic 2D material such as black phosphorus, we show that the transition dipole moment at 30K can be over 10 times larger than a typical defect-bound quantum emitters.