Nonlinear Frequency Conversion in Lithium Niobate Nanophotonic Circuits for Quantum Spectroscopy

Quantum light sources are becoming an increasingly popular alternative to pulsed lasers for spectroscopy, microscopy, and sensing. The inherent quantum correlations of entangled photons present unique advantages in spectroscopy, enabling high signal-to-noise ratios, low excitation fluxes, and time-resolved measurements without requiring a pulsed laser. Entangled photon sources for spectroscopic measurements typically consist of bulk crystals or ion-diffused waveguides. Integrated platforms such as thin-film lithium niobate have potential for highly efficient, tailored, and compact entangled photon sources through periodically poled nanophotonic waveguides. The advantageous nonlinear optical properties of lithium niobate coupled with the nanophotonic thin film platform allows for frequency conversion, quantum state generation, state manipulation, and sample interaction all on a single compact chip, demonstrating thin-film lithium niobate's potential for compact and portable integrated spectrometers. Here, we present our work in frequency conversion and sample interactions in thin-film lithium niobate. Most of the previous demonstrations of nanophotonic lithium niobate waveguides have focused on infrared wavelengths for applications in quantum communication and computing, leaving the shorter wavelengths that are of interest for spectroscopy still a largely unexplored space. In this work, frequency conversion in thin-film lithium niobate is investigated from ultraviolet through telecom wavelengths. Periodically poled lithium niobate nanophotonic waveguides are fabricated for second harmonic generation in the ultraviolet-A region and entangled photon generation at visible and near-infrared wavelengths. Using a violet continuous wave laser, a waveguide with a fluorescent dye-doped polymer cladding layer is investigated for sample interactions. Finally, preliminary work in entangled photon triplet generation down to telecom wavelengths is explored. This work represents a step towards compact, on-chip spectrometers and sensors through lithium niobate photonic integrated circuits.