Polarization-Dependent Optical Properties of Ce-Implanted Quartz

Nitrogen-Vacancy centers in diamonds and divacancies in SiC are the representative material systems composing solid-state spin qubits with a long coherence time at room temperature [1-4]. While various functionalities, e.g., remote entanglement, quantum sensing, and quantum communications, have been achieved, new qubit material systems may offer a new range of opportunities. One of the most critical material parameters for many quantum applications is the relaxation time <i>T</i>₂. Here, we focus on quartz as a host material, the most common mineral on the earth, and has been predicted to have a long <i>T</i>₂ without nuclear purification [5]. We choose the Ce³⁺ center as a spin center, which often composes a stable spin center in the widegap oxides, e.g., in Y₃Al₅O₁₂, whose spin state can be controlled through the circularly polarized laser [6].<br/>We implant Ce ion into a naturally abundant single-crystal quartz substrate at room temperature in the air. The implantation dose and energy are 1.0×10¹⁴cm^-2 and 100 keV, respectively. Subsequently, the samples are annealed in Ar for 0-9 hours at 700-1000^oC. First, we investigate the unpolarized optical properties of Ce-implanted quartz. The unpolarized photoluminescence (PL) spectra are measured with unpolarized He-Cd laser excitation at 325 nm. The unpolarized PL shows similar spectra with the previous studies on the sputter-deposited SiO₂:Ce [7]: unpolarized PL spectra have a prominent peak at around 445 nm, which corresponds to the 4<i>f</i>-5<i>d</i> transitions with different spin-orbit interaction of the Ce³⁺ center. We investigate the unpolarized PL of the samples with various annealing conditions. Up to 800oC, as the annealing temperature increases, PL intensity increases, suggesting the formation of luminescent Ce³⁺, while the further increase of the annealing temperature decreases the unpolarized PL intensity, suggesting the formation of the Ce clusters without bright luminescence.<br/>Next, we investigate the polarized optical properties with the samples annealed at 800^oC. In a single Ce³⁺ center of YAG:Ce³⁺, the polarization-dependent PL signal corresponding to the spin-dependent 4<i>f</i>-5<i>d</i> transition and the pulsed optically-detected magnetic resonance (ODMR) have been reported at 5 K [6]. We observe that a polarization-dependent optical signal of the Ce-implanted quartz is consistent with the 4<i>f</i>-5<i>d</i> transition of the isolated Ce³⁺ centers: σ⁺ excitation offers a larger σ⁺ PL emission than that with σ⁺ at the peak wavelength, and the more prominent σ^- emission at the shoulder wavelength (~560 nm). The degree of circular polarization (DOCP) reaches 15% (5%) at peak (shoulder) wavelength at 5 K. The DOCP monotonically decreases with increasing the measurement temperature, which suggests that the spin-lattice relaxation time <i>T</i>₁ limits <i>T</i>₂in this material system.<br/>[1] A. Gruber <i>et al</i>., Science <b>276</b>, 2012 (1997).<br/>[2] L. Childress <i>et al</i>., Science <b>314</b>, 281 (2006).<br/>[3] W. F. Kohel <i>et al</i>., Nature <b>479</b>, 84 (2011).<br/>[4] D. J. Christle <i>et al</i>., Nature Mater. <b>14</b>, 160 (2014).<br/>[5] S. Kanai <i>et al</i>., Proc. Natl. Acad. Sci. <b>119</b>, e2121808119 (2022).<br/>[6] P. Siyushev <i>et al</i>., Nat. Commun. <b>5</b>, 3895 (2014).<br/>[7] J. Weimmerskirch-Aubatin <i>et al</i>., J. Alloys Compd. <b>622</b>, 358 (2015).<br/><br/>Acknowledgment: We thank Hideo Ohno, Fumihiro Matsukura, F. Joseph Heremans, C. P. Anderson, S. E. Sullivan and G. Wolfowicz for the fluitful discussion. This work was supported in part by Shimadzu Research Foundation; Takano Research Foundation; RIEC Cooperative Research Projects; JSPS Kakenhi Nos. 19KK0130 and 20H02178; and JST-PRESTO No. JPMJPR21B2.