The electrodynamic response of organic spin liquids with highly-frustrated triangular lattices has been measured in a wide energy range. While the overall optical spectra of these Mott insulators are governed by transitions between the Hubbard bands, distinct in-gap excitations can be identified at low temperatures and frequencies which we attribute to the quantum spin liquid state. For the strongly correlated $beta^{prime}$-EtMe$_3$-Sb-[Pd(dmit)$_2$]$_2$, we discover enhanced conductivity below $175~{rm cm}^{-1}$, comparable to the energy of the magnetic coupling $Japprox 250$ K. For $omegarightarrow 0$ these low-frequency excitations vanish faster than the charge-carrier response subject to Mott-Hubbard correlations, resulting in a dome-shape band peaked at 100~cm. Possible relations to spinons, magnons and disorder are discussed.
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