Observation of anomalous surface plasmon dispersion in a titanium nitride film

Surface plasmons (SPs) in refractory materials like titanium nitride (TiN) offer promising alternatives to noble metals for plasmonic applications due to their robustness and CMOS compatibility. However, the dispersion behavior of SPs at the surface of TiN in the non-retarded regime remains poorly understood. Here, we employ momentum-resolved high-resolution electron energy loss spectroscopy to investigate the collective electronic excitations of atomically flat TiN(111) thin films treated by Ar⁺ sputtering and annealing. Two distinct excitations are observed: a SP mode near 2 eV and a defect-induced interband transition (IT) mode around 1 eV. The SP exhibits an anisotropic, anomalous dispersion: negative at small momenta (q < 0.03 Å^-1 along ΓM and q < 0.025 Å^-1 along ΓK), transitioning to positive at larger momenta. The spectral weight of the IT shows an opposite momentum dependence to the SP energy. Assisted by a Drude-Lorentz dielectric model incorporating the momentum-dependent IT oscillator strength, we demonstrate that the anomalous SP dispersion arises from q-dependent screening by defect states. These findings highlight defect engineering as a strategy for tailoring plasmonic properties in refractory nitrides, with implications for integrated photonics and sensing applications.