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Cavity-enhanced Narrowband Spectral Filters in Rare-earth Ion-doped Thin-film Lithium Niobate

Yuqi Zhao, Dylan L. Renaud, Demitry Farfurnik, Marko Loňar, Neil E. Sinclair, Edo Waks A

Published Jan 1, 2024• 0 citations
Publisher

Abstract

We demonstrate cavity-enhanced narrow bandpass filtering based on rare-earth-ion-doped thin-film lithium niobate by spectral hole burning in a critically-coupled resonance mode. Furthermore, we show that bandstop filtering can also be realized utilizing an under-couped cavity.
DOI
10.1364/cleo_si.2024.stu4p.1

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Abstract On-chip optical filters are fundamental components in optical signal processing. While rare-earth ion-doped crystals offer ultra-narrow optical filtering via spectral hole burning, their applications have primarily been limited to those using bulk crystals, restricting their utility. In this work, we demonstrate cavity-enhanced spectral filtering based on rare-earth ions in an integrated nonlinear optical platform. We incorporate rare-earth ions into high quality-factor ring resonators patterned in thin-film lithium niobate. By spectral hole burning at 4 K in a critically coupled resonance mode, we achieve bandpass filters ranging from 7 MHz linewidth, with 13.0 dB of extinction, to 24 MHz linewidth, with 20.4 dB of extinction. By reducing the temperature to 100 mK to eliminate phonon broadening, we achieve an even narrower linewidth of 681 kHz, which is comparable to the narrowest filter linewidth demonstrated in an integrated photonic device, while only requiring a small device footprint. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. For instance, as opposed to the bandpass filter, we demonstrate a bandstop filter utilizing an under-coupled ring resonator. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental components for optical signal processing and optical memories on-a-chip.
We propose using thin-film lithium niobate on insulator (LNOI) doped with Erbium (Er3+) as a promising solution for implementing large-scale quantum memory. However, the transition from bulk crystals to thin films poses challenges, notably reduced optical depth, which is critical for broad atomic frequency comb memory. To address this, we plan to utilize impedance-matched cavities, increasing the effective optical depth. Furthermore, the cavity would boost the rate of spontaneous emission, increasing the efficiency of spectral hole burning. Our preliminary results reveal high-Q micro-ring resonators (Q≈190k) on Er3+: LNOI, demonstrating a nearly 3.5-fold reduction in the optical lifetime due to cavity resonance.
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.
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.