We study the donor-bound exciton optical linewidth properties of Al, Ga and
In donor ensembles in single-crystal zinc oxide (ZnO). Neutral shallow donors
(D0) in ZnO are spin qubits with optical access via the donor-bound exciton
(D0X). This spin-photon interface enables applications in quantum
networking, memories and transduction. Essential optical parameters which
impact the spin-photon interface include radiative lifetime, optical
inhomogeneous and homogeneous linewidth and optical depth. The ensemble
photoluminescence linewidth ranges from 4-11 GHz, less than two orders of
magnitude larger than the expected lifetime-limited linewidth. The ensemble
linewidth remains narrow in absorption measurements through the 300
μm-thick sample, which has an estimated optical depth up to several
hundred. Homogeneous broadening of the ensemble line due to phonons is
consistent with thermal population relaxation between D0X states. This
thermal relaxation mechanism has negligible contribution to the total linewidth
at 2 K. We find that inhomogeneous broadening due to the disordered isotopic
environment in natural ZnO is significant, ranging from 1.9 GHz - 2.2 GHz.
Two-laser spectral anti-hole burning measurements, which can be used to measure
the homogeneous linewidth in an ensemble, however, reveal spectral anti-hole
linewidths similar to the single laser ensemble linewidth. Despite this
broadening, the high homogeneity, large optical depth and potential for isotope
purification indicate that the optical properties of the ZnO donor-bound
exciton are promising for a wide range of quantum technologies and motivate a
need to improve the isotope and chemical purity of ZnO for quantum
technologies.Comment: 22 pages, 12 figure