Simultaneous Ultrastrong Coupling of Multiple Material Excitations Within a Single Nano‐Slot Mode Enabled by Dispersion Engineering

ABSTRACT

Strong coupling between a material excitation and a nanophotonic resonance boosts light‐matter interactions in various nonlinear and quantum optical applications. Extending the scheme to simultaneous strong coupling of multiple, distinct material excitations within a single nanophotonic structure may greatly improve the functionality of such devices. While multiresonant photonic platforms seem ideal for such purposes, most current designs suffer from poor spatial overlap between distinct electromagnetic mode profiles of the different resonances, limiting the potential for coherent coupling between the different material excitations. Here, we propose a scheme to simultaneously couple two distinct material excitations within a single nano‐slot mode by leveraging material dispersion properties. Nano‐slots generally support a single resonance mode whose wavelength is proportional to the refractive index of the slot‐filling medium. Because materials with strong optical excitations exhibit significant dispersion in their refractive indices, filling the nano‐slots with such materials allows multiple resonances to coexist within the single photonic mode, thereby enabling simultaneous strong coupling of the distinct material excitations. We demonstrate the feasibility of our scheme with a hybrid perovskite MAPbI ₃ and observe the simultaneous coupling of two optical phonon modes that are nearly one octave apart from each other.