A new approach to achieving broadband, nonlinear frequency conversion could be a significant advancement for quantum information systems and integrated photonics. One of the challenges in developing quantum information technology is ensuring that quantum information units (i.e., qubits) can be transferred between different wavelengths without any loss of quantum coherence or entanglement. By enabling broadband upconversion, the new approach may facilitate frequency conversion between quantum states. The approach was developed by researchers at Shanghai Jiao Tong University. The team used birefringent mode hybridization to enhance nonlinear bandwidth at the chip scale. To enable mode-hybridization-based, broadband frequency conversion, the researchers used X-cut thin film lithium niobate (TFLN), a material known for its nonlinear optical properties. TFLN has emerged as a valuable platform for manipulating nonlinear interaction at the wavelength scale. The compact optical structures at this scale can enhance conversion efficiency by tightly confining the optical field, and can provide degrees of freedom to tailor the group velocity through the structural geometry. Working with TFLN, the researchers demonstrated broadband second-harmonic generation (SHG), an important process for converting light from one wavelength to another. They achieved SHG with a 3-dB bandwidth of up to 13 nm. Using mode hybridization, a process that provides precise control over frequency conversion, the researchers realized broadband SHG in a micro-racetrack resonator. They experimentally showed that broadband SHG could also be achieved in a bent waveguide with a specific geometrical structure. The spontaneous quasi-phase matching and quasi-group velocity-matching were shown to be simultaneously satisfied in simulations. Dispersion-designed structural geometry enables group-velocity mismatch of interacting lights to be smoothed to zero, for wide-range frequency conversion. Courtesy of T. Yuan, J. Wu, et al., doi 10.1117/1.AP.6.5.056012 In general, broad, nonlinear bandwidth requires the phase-matching condition to be satisfied over a wide spectral range, which is equivalent to the simultaneous matching of both the group velocity and phase velocity of the interacting waves in the time domain. The researchers’ approach to attaining significant bandwidth in frequency conversion could increase the efficiency of quantum information transfer and integrated photonic systems. By enabling on-chip, tunable frequency conversion, this approach could improve the efficiency of quantum light source generation on integrated photonic platforms, enhancing quantum light sources and quantum information networks. This approach could also be used to expand on-chip multiplexing capacity and improve multichannel optical information processing. “Thanks to the great progress in fabrication technology on the TFLN platform, this work will pave the way to chip-scale, nonlinear frequency conversion between the ultrashort optical pulses and even the quantum states,” professor Yuping Chen said. The SHG bandwidths achieved by the researchers in the intracavity and bent waveguide could be applied to other parametric processes, such as sum-frequency generation, difference-frequency generation, and optical parametric oscillation with a femtosecond laser pulse, through additional dispersion engineering and optimization of the structure. “An efficient second-order, nonlinear process with widely-tunable pump bandwidth has been a long-pursued goal, owing to the extensive applications in wavelength division multiplexing networks, ultrashort pulse nonlinearity, quantum key distribution, and broadband single-photon source generation,” Chen said. As researchers continue to explore ways to enhance nonlinear bandwidth, their findings will continue to bring them closer to realizing the full potential of quantum information networks. The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.6.5.056012).