  Vertical plasmonic coupling in double-layer graphene leads to two hybridized
plasmonic modes: optical and acoustic plasmons with symmetric and
anti-symmetric charge distributions across the interlayer gap, respectively.
However, in most experiments based on far-field excitation, only the optical
plasmons are dominantly excited in the double-layer graphene systems. Here, we
propose strategies to selectively and efficiently excite acoustic plasmons with
a single or multiple nano-emitters. The analytical model developed here
elucidates the role of the position and arrangement of the emitters on the
symmetry of the resulting graphene plasmons. We present an optimal device
structure to enable experimental observation of acoustic plasmons in
double-layer graphene toward the ultimate level of plasmonic confinement
defined by a monoatomic spacer, which is inaccesible with a
graphene-on-a-mirror architecture.
