Design of mid-Infrared supercontinuum generation in lithium niobate on sapphire waveguide through lateral leakage engineering

. We study the lateral leakage of a silicon nitride loaded lithium niobate on sapphire waveguide in the mid-infrared regime. We then combine lateral leakage and dispersion engineering to numerically demonstrate mid-infrared supercontinuum generation extending from 2400nm to 5000nm


Introduction
Molecules detection have a large number of applications in bio-imaging, food industry, environmental monitoring and security [1,2].To obtain high sensitivity, the molecules have to be located by measuring their fundamental vibration transitions which fingerprints are strong in the mid-infrared (mid-IR, 2-20μm) regime.Broadband sources with high spectral brightness in mid-IR are important for such technologies.Despite the great potential that mid-IR technologies offer, its range of applications is still limited, mostly because of the size and cost of mid-IR sensing devices [2,3].Therefore, there is a great need to develop compact and cost effective integrated mid-IR platforms to access these applications.In this contest, mid-IR supercontinuum generation (SCG) in an integrated platform has been developed to access these applications.Mid-IR SCG has been demonstrated using several group-IV platforms [4,5].In our group, SCG has been achieved using platform like silicongermanium on silicon [6] and germanium on silicon waveguides [7].
Another promising nonlinear material that can be used for realizing SCG is lithium niobate (LN).LN platform has been widely used in the past for optoelectronics applications, and more recently, for photonic integrated circuits [8].LN has a large transparency window (400nm-5000nm), strong second-order nonlinearity, and thirdorder nonlinearity.Loncar et al. [9] have demonstrated SCG in the near-infrared (near-IR) in LN on an insulator waveguide by directly etching LN thin films.Direct etching of LN has been widely known to be difficult [9].One can overcome that by optically loading the thin film of LN with a strip of silicon nitride (SiN).This has been already demonstrated in several integrated photonic components in LN on an insulator (LNOI) platform [8].Silica substrate in the mid-IR regime can lead to higher absorption loss since its optical transparency window is up to 3500nm.Instead, we use sapphire which is transparency window is from 150nm 5500nm.
Here, we design a SiN-loaded LN on a sapphire waveguide through a lateral leakage study in the mid-IR regime.We numerically analyze the generation of supercontinuum generation covering a spectral band of 2400nm to 5000nm, where many molecules in the atmosphere have their fingerprints

Design and simulations
Lateral leakage describes effects that can occur in a rib or ridge waveguide when the effective refractive index of the guided mode is lower than the effective refractive index of the orthogonally polarized slab mode.It was first observed in silicon on an insulator rib/ridge waveguide [10].LN being a birefringence material, we found that ridge waveguide in LN on sapphire as shown in the inset of Fig1 can lead to a similar effect.For x-cut LN on sapphire waveguides, the TE mode will experience an extraordinary refractive index which is lower than the ordinary refractive index of LN for the TE mode.However, since LN is birefringent, the effective refractive index of TE guided mode can be lower than that of the TM slab mode which will result in the leakage of the TE waveguide mode.Therefore, our first goal is to achieve low lateral leakage of the waveguide mode in the mid-IR regime while designing the waveguide.Fig. 1 shows the effective refractive index difference between the TE waveguide mode and the TM slab mode (Δneff = Δneff,TE − Δneff,TM ) as a function of LN thickness and wavelength in the mid-IR regime.The parameters used are as follows: silicon thickness (hSiN) was 500nm and the waveguide width (w) was 4000nm.In the graph, the black line indicates an effective refractive index difference between the modes of zero (Δneff =0), which means that a waveguide with LN thickness above the black line can suffer from lateral leakage.One can observe that for waveguides that are designed for shorter wavelengths, the thickness of lithium niobate needs to be thinner.
For our design, we consider this trade-off between the LN thickness and the wavelength, so we chose a lithium niobate thin-film thickness of 700nm which should support mode without leakage down to 1300nm wavelength.We now investigate the dispersion engineering which is required for SCG.To maximize the bandwidth of the SC, we need to achieve a low anomalous dispersion.Fig 2 shows the simulated results of the dispersion for the TE waveguide mode using 700nm LN thickness, 500nm SiN thickness, and three different waveguide widths of 3000nm, 4000nm, and 5000nm.

Fig. 2. Simulated Dispersion for selected waveguide widths
We simulate SCG in a 5 cm long SiN-loaded LN waveguide with 4000nm width, 700nm LN thickness, and 500nm SiN thickness by numerically solving the nonlinear Schrodinger equation using the split-step Fourier method.Our model includes third-order nonlinearity, higher-order dispersion, self-steepening, and losses.The simulated supercontinuum shown in Fig. 3 is obtained using a tunable OPA laser source (MIROPA-fs, Hotlight Systems) that delivered ∼200 fs pulses centered at 3400nm at a 63 MHz repetition rate.A relatively broad SC was generated spanning from 2400nm to 5000nm.Fig. 3. Simulated spectral pulse evolution along the waveguide length of a 5 cm for a peak power of 1.25kW.

Conclusion
In conclusion, we numerically demonstrated mid-IR SC generation in a strip-loaded Lithium niobate on sapphire waveguide.We designed the waveguide to support mode without leakage and to generate a broadband supercontinuum suitable for spectroscopy applications.
We acknowledge the support of the International Associated Laboratory in Photonics between France and Australia (LIA ALPhFA) and The I3E ECLAUSion project between France and Australia, H2020 EU framework, Marie Skłodowska-Curie grant (No 801512).This work was also supported by RMIT AWS Cloud Supercomputing (RACE.

Fig. 1 .
Fig. 1.Calculated effective index difference between TE waveguide mode and the TM slab mode.Waveguide with Δneff <0 can suffer from lateral leakage