  Plasmons in two-dimensional (2D) materials beyond graphene have recently
gained much attention. However, the experimental investigation is limited due
to the lack of suitable materials. Here, we experimentally demonstrate
localized plasmons in a correlated 2D charge-density-wave (CDW) material:
2H-TaSe2. The plasmon resonance can cover a broad spectral range from the
terahertz (40 {\mu}m) to the telecom (1.55 {\mu}m) region, which is further
tunable by changing thickness and dielectric environments. The plasmon
dispersion flattens at large wave vectors, resulted from the universal
screening effect of interband transitions. More interestingly, anomalous
temperature dependence of plasmon resonances associated with CDW excitations is
observed. In the CDW phase, the plasmon peak close to the CDW excitation
frequency becomes wider and asymmetric, mimicking two coupled oscillators. Our
study not only reveals the universal role of the intrinsic screening on 2D
plasmons, but also opens an avenue for tunable plasmons in 2D correlated
materials.
