  GaN has recently been shown to host bright, photostable, defect single photon
emitters in the 600-700 nm wavelength range that are promising for quantum
applications. The nature and origin of these defect emitters remain elusive. In
this work, we study the optical dipole structures and orientations of these
defect emitters using the defocused imaging technique. In this technique, the
far-field radiation pattern of an emitter in the Fourier plane is imaged to
obtain information about the structure of the optical dipole moment and its
orientation in 3D. Our experimental results, backed by numerical simulations,
show that these defect emitters in GaN exhibit a single dipole moment that is
oriented almost perpendicular to the wurtzite crystal c-axis. Data collected
from many different emitters shows that the angular orientation of the dipole
moment in the plane perpendicular to the c-axis exhibits a distribution that
shows peaks centered at the angles corresponding to the nearest Ga-N bonds and
also at the angles corresponding to the nearest Ga-Ga (or N-N) directions.
Moreover, the in-plane angular distribution shows little difference among
defect emitters with different emission wavelengths in the 600-700 nm range.
Our work sheds light on the nature and origin of these GaN defect emitters.

Materials Science;Optics