  We present microwave and infrared measurements on SmLa0.8Sr0.2CuO4-d, which
are direct evidence for the existence of a transverse optical plasma mode,
observed as a peak in the c-axis optical conductivity. This mode appears as a
consequence of the existence of two different intrinsic Josephson couplings
between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a
(La,Sr)O block layer. From the frequencies and the intensities of the
collective modes we determine the value of the compressibility of the two
dimensional electron fluid in the copper oxygen planes.
