Surface exciton polariton in monoclinic HfO2: an electron energy-loss spectroscopy study

Surface exciton polaritons (SEPs) were mostly expected in materials displaying sharp excitonic absorptions. Using electron energy-loss spectroscopy with a spatial resolution of 0.2-2 nm and associated calculations, we demonstrated SEPs upon rather weak excitonic oscillator strengths (broad interband transitions) in insulating, monoclinic HfO2 above its optical band gap. Broad interband transitions exist in many semiconductors and insulators above the band gap, and our work could stimulate future explorations of SEPs in a wide spectrum of materials and corresponding applications in optics.