  Collective coherent scattering of laser light induces strong light forces
between polarizable point particles. These dipole forces are strongly enhanced
in magnitude and distance within the field of an optical waveguide so that at
low temperature the particles self-order in strongly bound regular patterns.
The stationary configurations typically exhibit super-radiant scattering with
strong particle and light confinement. Here we study collective excitations of
such self-consistent crystalline particle-light structures as function of
particle number and pump strength. Multiple scattering and absorption modify
the collective particle-field eigenfrequencies and create eigenmodes of
surprisingly complex nature. For larger arrays this often leads to dynamical
instabilities and disintegration of the structures even if additional damping
is present.
