Ion-Implanted Erbium in X-cut Thin-film Lithium Niobate: Luminescence and Low-Temperature Response

This study investigates the integration of Erbium (Er³⁺) ions into X-cut thin-film lithium niobate (TFLN) using focused ion beam (FIB) implantation to enable applications in quantum photonics and telecom-based devices. The research explores how deterministic doping with Er³⁺ ions in TFLN can enhance the material’s performance in areas such as quantum memory, amplification, and single-photon emission in the telecom band.

Key Findings:

1. Ion Implantation Process:

• The study uses FIB implantation for precise, sub-100 nm spatial control of Er³⁺ ions in the TFLN material. The implantation doses range from 10¹³ to 10¹⁵ ions/cm², allowing for controlled doping with minimal damage to the material's surface, which is crucial for integrated nanophotonics.

• The ion implantation was modeled and analyzed using SRIM simulations, which showed a peak depth of approximately 25 nm for the Er⁺ ions.

2. Photoluminescence (PL) Characteristics:

• PL spectra were measured for samples implanted with varying Er³⁺ doses. At low temperatures (around 50 K), the spectra showed Stark-split transitions similar to bulk Er-doped lithium niobate, confirming that the ions were occupying lattice sites comparable to those in bulk Er-doped materials.

• The intensity of the PL response was found to be proportional to the implantation dose, with higher doses leading to stronger emission. The study also noted that no detrimental ion-ion interactions occurred at the highest doses tested.

3. Low-Temperature Behavior:

• The PL intensity and decay times were studied as a function of temperature. As the temperature decreased, the PL intensity increased, stabilizing below 100 K, then rapidly decreased below 50 K.

• This decrease in PL intensity and lifetime at lower temperatures is linked to the pyroelectric effect in LiNbO₃. At low temperatures, the pyroelectric response in lithium niobate leads to changes in the local electric field, which in turn affects the emission characteristics of Er³⁺.

4. Polarization Dependence:

• Polarized PL spectra revealed that the emission transitions were primarily σ-polarized, with certain transitions (e.g., Y1 → Z5) showing a distinct π-polarized characteristic. This supports the hypothesis that pyroelectric effects affect the luminescence more significantly for certain polarized states.

5. Physical Mechanism of Low-Temperature Effects:

• The observed low-temperature anomalies in emission intensity and lifetime are speculated to be caused by changes in the ferroelectric polarization of LiNbO₃. These changes lead to electric fields that influence the luminescence of Er³⁺ ions.

• The behavior is consistent with known pyroelectric effects in lithium niobate, where the spontaneous polarization decreases with temperature, leading to a dramatic decrease in the pyroelectric response at low temperatures, affecting the optical properties.

6. Conclusion:

• The study demonstrates that FIB implantation is an effective method for integrating Er³⁺ ions into TFLN, achieving high precision and compatibility with integrated photonic devices.

• The results offer important insights into the low-temperature behavior of Er³⁺-doped TFLN and highlight the influence of pyroelectric effects on quantum emitters in LiNbO₃ at cryogenic temperatures.

• The findings contribute to the development of quantum memory, single-photon sources, and other quantum photonic applications, providing a path for the deterministic integration of rare-earth ions in integrated photonic circuits.

This work paves the way for utilizing Er³⁺ ions in TFLN for scalable, multifunctional photonic circuits in both quantum and classical applications.