  For optical systems, states inside the light cone could be detected by
far-field measurement, while those below the light cone are not detectable by
far-field measurement. A new method for far-field detection has been developed
for observing states below the light cone with compound lattices. The basic
mechanism involved is that periodic weak scattering leads to band folding,
making the states out of the light cone to occur inside. By using
polarization-resolved momentum-space imaging spectroscopy the band structures
and iso-frequency contours of plasmonic lattices with different dimensions and
symmetries are experimentally mapped out, in good agreement with the
simulation.
