Collective excitations in antidots

Antidot structures have been prepared by etching arrays of 100-nm holes into a two-dimensional electron gas of ${\mathrm{Ga}}_{\mathit{x}}$ ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As quantum wells. In the far-infrared response we observe the unique collective excitation spectrum of antidots. It consists of a high-frequency branch which starts, in a magnetic field B, with a negative B dispersion and then increases in frequency with B. A second low-frequency branch corresponds at high B to edge magnetoplasmons which circulate around the holes. For small B this branch approaches the cyclotron frequency, where the electrons perform classical cyclotron orbits around the holes.