Polarization-dependent photoluminescence of Ce-implanted MgO and MgAl₂O₄

The solid-state spin center is one of the systems that compose qubits used for quantum computing, quantum communications, and quantum sensing. ^1–13) Typical materials are nitrogen-vacancy (NV) center in diamond ^3,7–11) and divacancy (VV) center in silicon carbide, ^5,6) which have a long spin coherence time (T₂) even at RT and achieve high fidelity spin control ^14,15) and readout. ^2–4) Recently, using the polarization- and spin-dependent optical properties ^16–20) in the single substitutional lanthanoids defect in oxides, ¹⁹⁾ such as Y₃Al₅O₁₂:Ce³⁺ (YAG) with S = 1/2 ^16,17) and YAG:Pr³⁺ with S = 1, ²⁰⁾ optically detected magnetic resonance are investigated, demonstrating the operation, namely initialization, writing, readout of the spin qubit state. ¹⁷⁾ Since the qubit's performance of solid-state spin centers depends highly on the host material, spin centers using new host materials may offer new qubit applications.

In this study, we study Ce-implanted MgO and MgAl₂O₄, widely used in spintronics. ^21–27) The calculation based on the cluster correlation expansion (CCE) showed that MgO is expected to have a long T₂ comparable to diamond. ²⁸⁾ In addition, since there are relatively large differences in the ionic valence number and the ionic radius between Ce³⁺ and Mg²⁺, we also investigate MgAl₂O₄ as a host material with the advantage of having both divalent (Mg²⁺) and trivalent (Al³⁺) cations. We investigate the polarization-dependent properties of the spin centers for potentially composing optically accessible solid-state spin qubits.

We implant Ce ions into 0.5 mm-thick-MgO (001) and MgAl₂O₄ (100) substrates. The energy and dose of ion implantation are 100 keV and 1.0 × 10¹³ atom cm⁻² (1.0 × 10¹⁴ atom cm⁻²), respectively. The average depth of the induced ion is 50 nm below the surface. The density of the Ce ion in MgO is about 0.09 at% (0.9 at%) and that in MgAl₂O₄ is 0.02 at% (0.2 at%). After the ion implantation, the samples are annealed in an Ar atmosphere at T_a = 300 °C–1000 °C for 2 h. An optical system to measure the polarization-dependent photoluminescence under external magnetic fields at various measurement temperatures is shown in Fig. 1. A He–Cd laser at 325 nm (KIMMON KOHA IK3151R-E) is used for the excitation. The emission spectral signals are collected through an objective lens (Thorlabs LMU-5x-NUV with a numerical aperture of 0.12); and a dichroic mirror (Semrock Di01-R355 with a cutoff wavelength of 355 nm), and analyzed by a spectrometer (Teledyne-Princeton Instruments SpectraPro HRS-300 with a 150 g mm⁻¹) with a CCD array detector (Teledyne-Princeton Instruments PIXIS 100BRX). The samples are placed in a low-vibration cryostat (MONTANA INSTRUMENTS cryostat s-50) and cooled down to 4 K. We apply the laser and magnetic field in the direction of MgO (001) or MgAl₂O₄ (100), parallel to the main axis.

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Figure 2(a) shows the luminescence properties of Ce-implanted MgO with different implantation doses for annealing temperature T_a of 1000 °C measured at laser power I of 2 mW. The PL intensity increases as the dose increases, indicating that the implantation forms luminescent defects. The implantation dose slightly changes the peak position, possibly due to the formation of different centers with different fabrication conditions. ^29,30)

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Figure 2(b) shows the typical luminescence properties of the Ce-implanted MgO with different T_a for implantation dose of 1.0 × 10¹⁴ atom cm⁻² measured at RT. The sample at T_a = 600 °C–900 °C shows various emission intensities at around 450–800 nm. The emission spectra are different even with the different substrates with identical annealing conditions (not shown). At T_a = 1000 °C, however, all samples show a relatively strong emission at 460–490 nm, consistent with the 5d–4f emission spectra from Ce³⁺ in Ce-doped MgO fabricated by the spark plasma sintering method in a previous study. ³¹⁾ Thus, when the emission from the Ce-implanted MgO with T_a = 1000 °C in our study is that from 5d–4f emission of Ce³⁺, there is a possibility to show the polarization-dependent optical properties. We measure the polarization-dependent optical properties at 5 K for the samples prepared under the conditions above [Fig. 2(c)]. The degree of the circular polarization is defined as

$$\begin{eqnarray}&&{\rm{\Delta }}\equiv \displaystyle \frac{{\sigma }^{+}-{\sigma }^{-}}{{\sigma }^{+}+{\sigma }^{-}},\end{eqnarray} \tag{ 1 }$$

where ${\sigma }^{+}\left({\sigma }^{-}\right)\,$ denotes the right- (left-) handed circularly polarized emission intensity. At no polarization in emission, ${\sigma }^{+}={\sigma }^{-}$ and ${\rm{\Delta }}=0.$ 5d–4f emission of Ce³⁺ has two peaks in the photoluminescence with different signs of ${\rm{\Delta }}$ corresponding to the emissions from the 5d level to two different 4 f sublevels (²F_5/2, ²F_7/2) with different spin–orbit interaction. ¹⁶⁾ ${\rm{\Delta }}$ of these peaks in Ce-doped YAG was reported up to 36% and down to −15%. ¹⁶⁾ Our results on Ce-implanted MgO show no clear and finite ${\rm{\Delta }}$ at 5 K, unlike Ce center in YAG. ^16,17) This suggests that the PL signal from Ce-implanted MgO is not from the 5d–4f emission of Ce³⁺, but possibly from oxygen defects ^32,33) and/or from charge-transfer transitions by the hybridization of the 5d orbitals of Ce and 2p orbitals of adjacent oxygen. ^34–36)

We focus on the Ce-implanted MgAl₂O₄. We prepare and measure the MgAl₂O₄ sample using the same procedure as MgO [Fig. 1]. The atomic state of the Ce³⁺ center is [Xe] 4f¹, and the [Xe] 4f¹ orbital splits into two levels (²F_5/2, ²F_7/2) with spin–orbit interactions. In the oxides with tetrahedral or octahedral coordination, these ²F_5/2 and ²F_7/2 levels further split into the sublevels with crystalline fields with S = 1/2. Spinel has tetrahedral or octahedral coordinations depending on the cation sites, ³⁶⁾ and the spin state is S = 1/2.

Figure 3(a) shows the annealing temperature dependence of the PL spectra for the Ce-implanted MgAl₂O₄. Unlike the Ce-implanted MgO, with increasing T_a, the PL intensity at around 430–460 nm increases from T_a = 300 °C to 600 °C, decreases from T_a = 600 °C to 700 °C, and increases again from T_a = 700 °C to 1000 °C. The temperature of spectra shift with the annealing seen in Fig. 3(a) is consistent with the temperature of structural change of spinel at T_a = 600 °C–700 °C. ³⁷⁾ We compare the Ce-implanted MgAl₂O₄ with T_a = 1000 °C and Ce-implanted MgO with T_a = 1000 °C [Fig. 3(b)]. As can be seen, we observe a 14 times larger PL intensity in Ce-implanted MgAl₂O₄. The emission peak wavelength is around 430 nm, consistent with the previous study of the 0.01% Ce-doped MgAl₂O₄ fabricated by the spark plasma sintering method. ³⁸⁾

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Figure 3(c) shows the polarization-dependent PL spectra at 4 K with different μ₀ H. ${\rm{\Delta }}$ increases with increasing H and reaches 2% at μ₀ H = 500 mT. This behavior is consistent with Ce-doped YAG, ^16,17) whose ${\rm{\Delta }}$ increases with increasing H due to the decoupling through the Zeeman energy of the ground states coupled through the surrounding nuclear spins. ¹⁶⁾

The magnitude of ${\rm{\Delta }}$ is mainly determined by ${T}_{2}$ ¹⁾ and emission lifetime T_emi. When the sample shows finite polarization-dependent PL, ${T}_{{\rm{emi}}}$ satisfies ${T}_{{\rm{emi}}}\lt {T}_{2}.$ In general, spin-lattice relaxation time (T₁) decreases with increasing temperature. In addition, T₁ and T₂ increase with an external magnetic field. ^39–41) As for Ce³⁺ in YAG, T₂ is 240 ns at 3.5 K, ¹⁷⁾ which is mainly limited by the spinful nucleus ⁸⁹Y (I_n = −1/2, 100% abundance) and ²⁷Al (I_n = +5/2, 100% abundance), and thus, T₂ in MgAl₂O₄ is expected to be longer than 240 ns. The calculation of the spin coherence based on the CCE, which ignores the effect of T₁, predicts that T₂ of MgAl₂O₄ is in the order of 0.1 ms. ²⁸⁾ T_emi of Ce in the oxides is typically in the order of about 60 ns. ⁴²⁾ Thus, the magnetic field and temperature dependences of the polarization-dependent optical properties of Ce-implanted MgAl₂O₄ suggest that the spin-dependent emission is hindered by the short T₂ limited by T₁ (${T}_{2}\leqslant 2{T}_{1}$ ¹⁾) at H = 0 while T₂ becomes longer than T_emi with H > 0. The energy level system and the emission mechanism of Ce³⁺ centers in the widegap oxide are almost host-agnostic, as known in the scintillator research. ^43,44) In YAG:Ce³⁺, polarization-dependent PL is observed due to the spin polarization into S_z = 1/2 or S_z = −1/2 states (depending on the helicity), ^16,17) corresponding to the spin state initialization. The results suggest that the PL of Ce-implanted MgAl₂O₄ is consistent with that from Ce³⁺, and the observed polarization-dependent PL suggests the spin-dependent PL, and the operation corresponding to the initialization and readout in the quantum manipulation has been achieved in this system.

In conclusion, we have investigated the optical properties of the Ce-implanted MgO and MgAl₂O₄, which have been attracting attention in spintronics for the application of the potential solid-state spin center with optical access to the spin properties. We have investigated those samples' dose and annealing condition dependences and found that only Ce-implanted MgAl₂O₄ shows polarization-dependent photoluminescence up to 2% at 4 K under the magnetic field of 500 mT, which is consistent with the previous work on the Ce³⁺ in YAG. This indicates that Ce-implanted MgAl₂O₄ is a potential material for an optically accessible solid-state spin center.

Acknowledgments