In the field of integrated photonics, a longstanding goal has been to minimize the size of systems. This work by Hwang et al. challenges this conventional wisdom when it comes to achieving efficient chip-based ultraviolet (UV) light generation.

The focus of this study is on lithium niobate waveguides, a material extensively studied in the past for its bulk electro-optic properties, that is experiencing a renewed interest in the realm of integrated photonics. In comparison to other materials commonly used in integrated photonics, such as silicon, lithium niobate offers unique advantages: a transparency window encompassing both visible and a portion of UV wavelengths, a significant second-order nonlinearity, ferroelectric properties, and active tunability. Leveraging these distinctive features, UV light can be generated through frequency doubling of a near-infrared pump. This work shows that achieving efficient second harmonic generation (SHG) requires a counterintuitive approach.

In this study, a comprehensive range of waveguides was fabricated by etching thin films of lithium niobate-on-insulator (LNOI) with two different thicknesses: 200 nm and 600 nm. Variations were made in both the depth and lateral size of the etched regions. Since the waveguides are considerably longer than the coherence length of lithium niobate in the UV region, about 2 microns, periodic poling is applied along the propagation direction. This process involves reversing the ferroelectric orientation of lithium niobate over domains of one coherent length, but it can be quite challenging at this scale, susceptible to imperfections that can significantly reduce nonlinear conversion efficiency. Furthermore, the effective refractive index, and consequently the coherent length of the waveguide, depends on its cross-sectional dimensions, making thickness imperfections an additional hindrance to conversion efficiency.

Through a rigorous experimental and numerical characterization, Hwang et al. discover that nonlinear UV light generation is notably enhanced in thicker waveguides—those etched into the 600 nm thick LNOI films. They conclude that the detrimental effects of fabrication and poling imperfections on SHG efficiency are considerably mitigated in larger waveguides. This benefit conveniently compensates for the reduced conversion efficiency and increased mode leakage inherent in larger systems. By sweeping the pump wavelength and adjusting the waveguide temperature to preserve phase matching, they successfully generated light at a record-breaking wavelength of 355 nm. This marks the shortest UV wavelength achieved through SHG in lithium niobate and represents a significant step toward the realization of UV-integrated photonic devices with promising applications in protein sensing, optogenetics, and information processing.

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