Multi-octave frequency comb from an ultra-low-threshold nanophotonic parametric oscillator

This article presents the development of a multi-octave frequency comb generated by an ultra-low-threshold nanophotonic optical parametric oscillator (OPO) using thin-film lithium niobate (LN) as the nonlinear medium. The key innovation is achieving multi-octave spectral broadening with femtojoule-level pump energies, which significantly reduces the energy requirements compared to previous frequency comb generation techniques.

Key Findings:

1. Ultra-Low Threshold and Dispersion Engineering: The OPO operates with an ultra-low threshold of ~18 fJ, enabled by near-zero dispersion engineering in the pump and signal. This low-energy operation is a significant advancement in generating broadband frequency combs, which have traditionally required high pump energies.

2. Coherent Multi-Octave Frequency Comb: The OPO demonstrated a 2.6-octave coherent frequency comb at 121 fJ of pump energy. This is the first demonstration of multi-octave comb generation using femtojoule-level energy, which is orders of magnitude more energy-efficient than previous methods.

3. Coherence and Recoherence: The system transitions between incoherent and coherent operation regimes as the pump pulse energy increases. At lower energies, the OPO operates in an incoherent regime, but as the energy increases above the threshold, coherence is re-established, confirming the effectiveness of the design.

4. Phase-Sensitive Amplification: The key mechanism for achieving such broadband generation is phase-sensitive amplification. The OPO system uses a low-finesse resonator, which selectively resonates with frequencies around the half-harmonic of the pump, leaving higher harmonics non-resonant, thereby minimizing the energy losses typically associated with spectral broadening.

5. Simulation and Design: Numerical simulations were used to model the nonlinear interactions in the OPO and predict the spectral broadening behavior. The design of the OPO, including the waveguide parameters and poling period, was optimized to achieve the desired low threshold and broad spectrum.

6. Path Towards Three-Octave Comb: By modifying the device's design (e.g., using a chirped poling period), the system was shown to potentially achieve a three-octave coherent frequency comb with pump energies around 250 fJ, extending the spectral output even further.

7. Applications: This breakthrough in energy-efficient, broadband frequency comb generation opens up numerous applications in integrated photonics, including wavelength-division multiplexing, dual-comb spectroscopy, and frequency synthesis.

Conclusion: The work demonstrates a highly efficient and scalable approach to generating multi-octave frequency combs in an integrated nanophotonic platform. This advancement could significantly impact fields requiring broadband, coherent light sources, making the technology suitable for a wide range of applications in science and industry.