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APL Materials

Engineering and polarization properties of erbium-implanted lithium niobate films for integrated quantum applications

Alex E. Kaloyeros, Souryaya Dutta, Spyros Gallis

Published Nov 1, 2024• 3 citations
Publisher

Abstract

Rare-earth-doped materials have garnered significant attention as material platforms in emerging quantum information and integrated photonic technologies. Concurrently, advances in its nanofabrication processes have unleashed thin film lithium niobate (LN) as a leading force of research in these technologies, encompassing many outstanding properties in a single material. Leveraging the scalability of ion implantation to integrate rare-earth erbium (Er3+), which emits at 1532 nm, into LN can enable a plethora of exciting photonic and quantum technologies operating in the telecom C-band. Many of these technologies rely on coupling via polarization-sensitive photonic structures such as waveguides and optical nanocavities, necessitating fundamental material studies. Toward this goal, we have conducted an extensive study on the role of implantation and post-implantation processing in minimizing implantation-induced defectivity in x-cut thin film LN on an insulator. By leveraging this, we have demonstrated a cutting-edge ensemble optical linewidth of 140 GHz for Er emission in x-cut thin-film LN at 77 K. This finding highlights the significant progress in minimizing implantation-induced defects through careful ion implantation engineering and post-implantation processing. To the best of our knowledge, this linewidth measured at a higher temperature (77 K) is the narrowest when compared to the values reported for bulk-doped and implanted LN crystals at liquid helium temperatures (∼3 K), showcasing the potential of our approach for higher-temperature operation devices. Furthermore, we show that the Er photoluminescence (PL) is highly polarized perpendicular to the x-cut LN c-axis through systematic and combinational PL and high-resolution scanning transmission electron microscopy (HRSTEM) studies. These results indicate that using Er emitters in thin film LN, along with their polarization characteristics and related ion implantation engineering, presents an opportunity to produce highly luminescent Er-doped LN photonic integrated devices and circuits for quantum and nanophotonic applications at telecom wavelengths.

DOI
10.1063/5.0220106

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Rare-earth-doped materials have garnered significant attention as material platforms in emerging quantum information and integrated photonic technologies. Concurrently, advances in its nanofabrication processes have unleashed thin film lithium niobate (LN) as a leading force of research in these technologies, encompassing many outstanding properties in a single material. Leveraging the scalability of ion implantation to integrate rare-earth erbium (Er3+), which emits at 1532 nm, into LN can enable a plethora of exciting photonic and quantum technologies operating in the telecom C-band. Many of these technologies rely on coupling via polarization-sensitive photonic structures such as waveguides and optical nanocavities, necessitating fundamental material studies. Toward this goal, we have conducted an extensive study on the role of implantation and post-implantation processing in minimizing implantation-induced defectivity in x-cut thin film LN on an insulator. By leveraging this, we have demonstrated a cutting-edge ensemble optical linewidth of 140 GHz for Er emission in x-cut thin-film LN at 77 K. This finding highlights the significant progress in minimizing implantation-induced defects through careful ion implantation engineering and post-implantation processing. To the best of our knowledge, this linewidth measured at a higher temperature (77 K) is the narrowest when compared to the values reported for bulk-doped and implanted LN crystals at liquid helium temperatures (∼3 K), showcasing the potential of our approach for higher-temperature operation devices. Furthermore, we show that the Er photoluminescence (PL) is highly polarized perpendicular to the x-cut LN c-axis through systematic and combinational PL and high-resolution scanning transmission electron microscopy (HRSTEM) studies. These results indicate that using Er emitters in thin film LN, along with their polarization characteristics and related ion implantation engineering, presents an opportunity to produce highly luminescent Er-doped LN photonic integrated devices and circuits for quantum and nanophotonic applications at telecom wavelengths.
Lithium niobate on insulator (LNOI), thanks to its electro-optic properties and second order nonlinearity, is one of the most promising photonic materials for on-chip implementation of a complex photonic integrated circuit (PIC) [1]. Integration of rare earth ion emitters (RIE), characterized by high coherent transitions in both optical and microwave domains, into LNOI is a very attractive perspective to fully exploit the potential of this material in quantum optics applications and for on chip light generation and amplification. By choosing Erbium ions these functionalities can be implemented at telecom wavelengths (~1550 nm). Erbium integration in LNOI can be achieved using the smart cut technique [2]. However, this approach implies heating the material up to ~1100 °C, approaching the Curie temperature of lithium niobate (~1200 °C). Ion implantation also permits the incorporation of RIE into the lithium niobate (LN) crystal structure, operating at lower temperature with high spatial precision of the doped region in a complex PIC.
ABSTRACT Lithium niobate on insulator (LNOI) has emerged as a promising platform for photonic integrated circuits due to its strong electro‐optic, piezoelectric and nonlinear properties. The incorporation of rare‐earth ions, particularly erbium (Er 3+ ), enables potential functionalities such as quantum memory and single‐photon emission in the telecom band. This study presents a method for the Er 3+ doping of X‐cut LNOI using a focused ion beam (FIB) implantation system with sub‐100 nm spatial precision, demonstrating the ability to dope optically emitting rare‐earth ions into this technologically relevant platform. Photoluminescence (PL) studies of doped regions reveal characteristic emission consistent with bulk Er‐doped lithium niobate. The PL temperature dependence is studied from 300 to 5 K. The emission exhibits conventional behavior down to approximately 50 K, followed by a marked decrease in the emission intensity and lifetime at lower temperatures. This anomaly is suggested to relate to a suppression of the pyroelectric response in LiNbO 3 below 50 K, which affects the local electric field and consequently could modify the Er 3 + emission. The results demonstrate a method for targeted Er 3+ doping into the most widely used cut of LNOI for integrated photonic devices and provide further important considerations for their exploitation in cryogenic quantum devices.
As an active material with favorable linear and nonlinear optical properties, thin-film lithium niobate has demonstrated its potential in integrated photonics. Integration with rare-earth ions, which are promising candidates for quantum memories and transducers, will enrich the system with new applications in quantum information processing. Here, we investigate the optical properties at 1.5 μm wavelengths of rare-earth ions (Er3+) implanted in thin-film lithium niobate waveguides and micro-ring resonators. Optical quality factors near a million after post-annealing show that ion implantation damage can be repaired. The transition linewidth and fluorescence lifetime of erbium ions are characterized. The ion-cavity coupling is observed through a Purcell enhanced fluorescence from which a Purcell factor of 3.8 ± 0.5, compared with waveguide lifetime, is extracted. This platform is compatible with top-down lithography processes and leads to a scalable path for controlling spin-photon interfaces in photonic circuits.
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.
Integrated photonics capable of incorporating rare-earth ions with high optical coherence is desirable for realizing efficient quantum transducers, compact quantum memories, and hybrid quantum systems. Here we describe a photonic platform based on the SmartCut erbium-doped lithium niobate thin film, and explore its stable optical transitions at telecom wavelength in a dilution refrigerator. Optical coherence time of up to $180\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$, rivaling the value of bulk crystals, is achieved in optical ridge waveguides and ring resonators. With this integrated platform, we demonstrate tunable light-ion interaction and flexible control of optical thickness by exploiting long waveguides, whose lengths are in principle variable. This unique ability to obtain high optical density using low-concentration ions further leads to the observation of multiecho pulse trains in centimeter-long waveguides. Our results establish a promising photonic platform for quantum information processing with rare-earth ions.
Rare earth ions are known as promising candidates for building quantum light-matter interface. However, tunable photonic cavity access to rare earth ions in their desired host crystal remains challenging. Here, we demonstrate the integration of erbium doped yttrium orthosilicate (Er3+:Y2SiO5) with thin-film lithium niobate photonic circuit by plasma-activated direct flip chip bonding. Resonant coupling to erbium ions is realized by on-chip electro-optically tuned high Q lithium niobate micro-ring resonators. Fluorescence and absorption of erbium ions at 1536.48 nm are measured in the waveguides, while the collective ion-cavity cooperativity with micro-ring resonators is assessed to be 0.36. This work presents a versatile scheme for future rare earth ion integrated quantum devices.