  We report on high-order magnetoplasmon resonances detected in photoresistance
in high-mobility GaAs quantum wells. These resonances manifest themselves as a
series of photoresistance extrema in the regime of Shubnikov-de Haas
oscillations. Extending to orders above 20, the extrema exhibit alternating
strength, being less (more) pronounced at even (odd) order magnetoplasmon
modes. This experimental technique provides sensitive and elegant means to
detect and investigate multiple magnetoplasmon modes and could be applied to
other systems.
