  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 on insulator. The thin film is composed of lithium
niobate doped with Tm3+. 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 way for
scalable on-chip lasers, optical signal processing devices, and integrated
optical quantum memories.
