  Controllable strong interactions between a nanocavity and a single emitter is
important to manipulating optical emission in a nanophotonic systems but
challenging to achieve. Here a three-dimensional DNA origami, named as DNA rack
(DR) is proposed and demonstrated to deterministically and precisely assemble
single emitters within ultra-small plasmonic nanocavities formed by closely
coupled gold nanorods (AuNRs). The DR uniquely possesses a saddle shape with
two tubular grooves that geometrically allows a snug fit and linearly align two
AuNRs with a bending angle <10{\deg}. It also includes a spacer at the saddle
point to maintain the gap between AuNRs as small as 2-3 nm, forming a
nanocavity estimated to be 20 nm3 and an experimentally measured Q factor of
7.3. A DNA docking strand is designed at the spacer to position a single
fluorescent emitter at nanometer accuracy within the cavity. Using Cy5 as a
model emitter, a ~30-fold fluorescence enhancement and a significantly reduced
emission lifetime (from 1.6 ns to 670 ps) were experimentally verified,
confirming significant emitter-cavity interactions. This DR-templated assembly
method is capable of fitting AuNRs of variable length-to-width aspect ratios to
form anisotropic nanocavities and deterministically incorporating different
single emitters, thus enabling flexible design of both cavity resonance and
emission wavelengths to tailor light-matter interactions at nanometer scale.
