  The Mott-insulator \kappa-(BEDT-TTF)_2Cu_2(CN)_3 is the prime candidate of a
quantum spin liquid with puzzling magnetic properties. Our THz and infrared
investigations reveal that also the charge dynamics does not follow the
expectations for a Mott insulator. The frequency-dependent conductivity
exhibits a power-law behavior $\sigma_1(\omega)\propto \omega^n$ that grows
stronger as the temperature decreases and extends all the way through the
far-infrared. With $n\approx 0.8$ to 1.5 we obtain a significantly smaller
exponent than predicted by Ng and Lee [Phys. Rev. Lett. {\bf 99}, 156402
(2007)]. We suggest fluctuations becomes important in the spin-liquid state and
couple to the electrodynamic properties differently compared to the
antiferromagnetic Mott insulator \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl. We discuss
the various possibilities of how charge fluctuations are influenced by the
presence or absence of magnetic order.
