  In the material SmLa$_{1-x}$Sr$_x$CuO$_{4-\delta}$ with alternating intrinsic
Josephson junctions we explain theoretically the relative amplitude of the two
plasma peaks in transmission by taking into account the spatial dispersion of
the Josephson Plasma Resonance in $c$ direction due to charge coupling. From
this and the magnetic field dependence of the plasma peaks in the vortex solid
and liquid states it is shown that the electronic compressibility of the
CuO$_2$ layers is consistent with a free electron value. Also the London
penetration depth $\lambda_{ab} \approx 1100 {\rm \AA}$ near $T_c$ can be
determined. The voltage response in the $IV$-curve of a
Bi$_2$Sr$_2$CaCu$_2$O$_8$ mesa due to microwave irradiation or current
injection in a second mesa is related to the nonequilibrium charge imbalance of
quasiparticles and Cooper pairs and from our experimental data the relaxation
time $\sim 100 {\rm ps}$ is obtained.
