  The resemblance between electrons and optical waves has strongly driven the
advancement of mesoscopic physics. However, electron waves have yet to be
understood in open cavity structures which have provided contemporary optics
with rich insight towards non-Hermitian systems and complex interactions
between resonance mode. Here, we report the realization of an open cavity
resonator in a two-dimensional electronic system. We studied the resonant
electron modes within the cavity and resolved the signatures of longitudinal
and transverse quantization, showing that the modes are robust despite the
openness of the cavity being highly open to the background continuum. The
transverse modes were investigated by applying a controlled deformation to the
cavity, and their spatial distributions were further analyzed using
magnetoconductance measurements and numerical simulation. These results lay the
groundwork to exploring electronic wavefunctions in the context of modern
optical systems, such as the dielectric microcavity.
