We examine plasma excitations with linear dispersion in a system of superconducting electrons in thin NbN disks. Using frequency-domain terahertz spectroscopy, we study the evolution of plasmons up to critical temperature $T_c$. The observed excitations tend to shift to the lower frequencies while temperature is increasing and the spectral features vanish at $T>T_c$. We study the dependence of the plasma velocity on superconducting electron density $n_s$, a NbN layer thickness $t$, and disk diameter $d$. The velocity appears to be independent from the disk diameter and period while having a square-root dependency on the parameter $n_{s}t$.

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• Optical & microwave phenomena
• Plasmonics
• Plasmons
• Superconductors
• Thin films
• Terahertz spectroscopy