Neutral shallow donors in ZnO: Contributions to the optical linewidth of the donor - bound excitonic transition*

Neutral shallow donors (D⁰) in ZnO such as In, Ga, and Al substituting for Zn are promising solid-state spin qubits for quantum technologies. The D⁰ are optically coupled to donor bound excitons (D⁰X). Studying donor ensembles, we have demonstrated long spin relaxation times (up to 0.48 sec at 1.75 T) [1], spin initialization via optical pumping [2], Hahn-spin-echo time of 50 μs [2], and all-optical coherent control [3]. In almost every quantum application, including remote entanglement generation, linear optics quantum computing, and quantum memories, a key figure of merit is the ratio of the optical transition linewidth to the Fourier-transformed limited linewidth. In this contribution, we present an experimental and theoretical study on the D⁰ /D⁰X optical linewidth properties of donor ensembles. In micro-PL measurements, we observed inhomogeneous linewidths as narrow as 6.5 GHz, only one order of magnitude broader than the expected lifetime-limited linewidth (~0.5 GHz). Temperature dependent linewidth studies are consistent with a one-phonon broadening mechanism based on population relaxation to an excited D⁰X state. At 2 K, the phonon contribution to the linewidth is negligible. Transient reverse spectral hole burning spectroscopy reveal a 4.3 GHz anti-hole that is similar to the inhomogeneous linewidth, suggesting a spectral diffusion mechanism. Finally, we propose that the linewidth is partially inhomogeneously broadened due to the isotope mass environment variation between disparate emitters in the ensemble.

 

[1] V. Niaouris, et al., Phys. Rev B 105, 195202 (2022)

[2] X. Linpeng et al., Phys. Rev. Appl. 10, 064061 (2018)

[3] M. L. K. Viitaniemi, et al., Nano Lett. 22, 5 (2022)