  Microwave absorption spectra of a single square of two-dimensional electrons
(2DES) have been investigated using an optical detection technique. Fundamental
dipole and harmonic quadrupole plasmon modes have been identified and compared
to those in the disk geometry. In the square-shaped 2DES, a strong interaction
is discovered between the neighboring plasmon modes, whereas no such
hybridization is observed in the disk-shaped geometry. We establish a rigid
theoretical platform to analytically describe the magneto-optical response of
confined two-dimensional systems. The developed theory provides a proper
description of the obtained experimental results.
