We have developed a low-loss waveguide platform for erbium-doped rare-earth oxide thin films grown on silicon substrate and investigated optical properties of erbium ions in waveguides at cryogenic temperature, aiming for integrated quantum photonic devices.
Coherent quantum devices play a central role in quantum science and engineering. Rare-earth ions such as erbium in solids feature numerous 4f- intra-shell transitions that are effectively shielded from their crystalline surroundings by closed outer shells, allowing for long spin coherence times and narrow optical transitions. Recent ensemble experiments have established the rare-earth doped crystals as the leading materials for optical quantum memories. However, the development of large-scale quantum devices based on rare-earth doped materials has remained a challenge. Here we describe a scalable quantum photonic platform based on epitaxially grown oxide thin films doped with trivalent ions on silicon substrates. The platform builds upon the recent developments of high quality epitaxial rare-earth doped oxides on silicon, and highlights the integration of quantum coherent materials with existing silicon technologies for ultimate device scalability. The proposed device platform builds upon our capability to epitaxially grow high quality, near-lattice-matched Y2O3 thin films on silicon. The epitaxial thin film is of high structural quality and offers several distinct advantages: (1) significantly less optical loss and better quantum emitter coherence; (2) atomic precision placement of rare-earth emitters (via delta doping) with respect to the photonic mode for controllable ion-photon coupling; (3) scalable top-down device fabrication using standard lithography and etching techniques; (4) heterogeneous integration with silicon takes full advantage of existing silicon photonics technologies for routing, tuning, and modulation of the quantum emission from rare-earth ions.
Rare-earth ion ensembles doped in single crystals are a promising materials system with widespread applications in optical signal processing, lasing, and quantum information processing. Incorporating rare-earth ions into integrated photonic devices could enable compact lasers and modulators, as well as on-chip optical quantum memories for classical and quantum optical applications. To this end, a thin film single crystalline wafer structure that is compatible with planar fabrication of integrated photonic devices would be highly desirable. However, incorporating rare-earth ions into a thin film form-factor while preserving their optical properties has proven challenging. We demonstrate an integrated photonic platform for rare-earth ions doped in a single crystalline thin film lithium niobate on insulator. The thin film is composed of lithium niobate doped with Tm 3+ . The ions in the thin film exhibit optical lifetimes identical to those measured in bulk crystals. We show narrow spectral holes in a thin film waveguide that require up to 2 orders of magnitude lower power to generate than previously reported bulk waveguides. Our results pave the way for scalable on-chip lasers, optical signal processing devices, and integrated optical quantum memories.
The obtention of quantum-grade rare-earth-doped oxide thin films that can be integrated with optical cavities and microwave resonators is of great interest for the development of scalable quantum devices. Among the different growth methods, chemical vapor deposition (CVD) offers high flexibility and has demonstrated the ability to produce oxide films hosting rare-earth ions with narrow linewidths. However, growing epitaxial films directly on silicon is challenging by CVD due to a native amorphous oxide layer formation at the interface. In this manuscript, we investigate the CVD growth of erbium-doped yttrium oxide (Er:Y2O3) thin films on different substrates, including silicon, sapphire, quartz, or yttria stabilized zirconia (YSZ). Alternatively, growth was also attempted on an epitaxial Y2O3 template layer on Si (111) prepared by molecular beam epitaxy (MBE) in order to circumvent the issue of the amorphous interlayer. We found that the substrate impacts the film morphology and the crystalline orientations, with different textures observed for the CVD film on the MBE-oxide/Si template (111) and epitaxial growth on YSZ (001). In terms of optical properties, Er3+ ions exhibit visible and IR emission features that are comparable for all samples, indicating a high-quality local crystalline environment regardless of the substrate. Our approach opens interesting prospects to integrate such films into scalable devices for optical quantum technologies.
The development of quantum technologies is generating great interest, as they promise major breakthroughs in computational power, measurement sensitivity and precision, as well as communication security, with potential applications acrossdiverse fields such as healthcare or electronics. In this context, rare-earth-doped oxide thin films represent a competitivesolid-state quantum platform for light–matter interactions. This is due to the ability of rare-earth ions to exhibit quantumcoherent states with long coherence times, combined with the nanometric thickness of the films, which makes them compatible with diverse photonic architectures. Chemical vapor deposition (CVD) offers a fast and cost-effective techniquefor producing thin films with good crystalline quality. However, compared to bulk crystals and ceramics, their propertiesare often degraded, likely due to surface proximity, a higher density of point defects, and strain. The use of epitaxial thinfilms, with a reduced defect density, is expected to lead to improved coherence times. The aim of this PhD project istherefore to obtain epitaxial CVD thin films and to enhance their crystalline properties in order to achieve light–matterinterfaces with long coherence times. Thin films were grown on various substrates, including silicon, oxides (quartz, sapphire, yttria-stabilized zirconia, YSZ), and hybrid substrates with a first layer deposited by molecular beam epitaxy (MBE).In parallel, the influence of post-deposition annealing treatments on the crystalline and optical properties of the films wasinvestigated. Epitaxy on hybrid substrates and YSZ was confirmed, with optical coherence times (T2) of 1.1 µs and 0.8µs, respectively, more than twenty times longer than those measured in polycrystalline films of equivalent composition.This epitaxial architecture paves the way for nanostructuring and integration into photonic devices such as resonators andwaveguides.
Development of a Scalable Quantum Memory Platform -- Materials Science of Erbium-Doped TiO2 Thin Films on Silicon
Rare-earth ions (REI) have emerged as an attractive candidate for solid-state qubits, particularly as a quantum memory. Their 4f-4f transitions are shielded by filled 5s and 5p orbitals, offering a degree of protection from external electric fields. Embedded within a thin film oxide host, REIs could enable a qubit platform with significant memory capabilities. Furthermore, a silicon-compatible thin film form factor would enable the use of standard semiconductor fabrication processes to achieve chip-based integrability and scalability for functional quantum networks. Towards this goal, we have carried out optical and microstructural studies of erbium-doped polycrystalline and epitaxial TiO2 thin films on Si (100), r-sapphire, and SrTiO3 (100). We observe that the inhomogeneous optical linewidth of the Er photoluminescence is comparable or better for polycrystalline Er:TiO2(grown on Si) in comparison to single crystal epitaxial films on sapphire or SrTiO3, implying a relative insensitivity to extended defects. We investigated the effect of the film/substrate and film/air interface and found that the inhomogeneous linewidth and spectral diffusion can be significantly improved via bottom buffer and top capping layers of undoped TiO2. Using such approaches, we obtain inhomogeneous linewidths of 5.2 GHz and spectral diffusion of 180 MHz in Er:TiO2 /Si(100) films and have demonstrated the engineerability of quantum-relevant properties in these thin films.