Broadband Light Generation in Nonlinear Silicon Nitride Strip-Loaded Lithium Niobate Waveguides

. This work demonstrates SiN strip-loaded lithium niobate waveguides with nonlinear optical properties, focusing on their performance when pumped at telecom wavelength. Experimental results show second and third harmonic generation in periodically poled SiN/LiNbO3. Furthermore, simulations reveal that these waveguides can be dispersion engineered to generate supercontinuum. The findings highlight the potential of SiN strip-loaded lithium niobate platform in sustaining broadband nonlinear light sources.


Introduction
All-optical phenomena, relying on material nonlinearities, can enhance electronic infrastructure handling optical data, meeting the growing demand for high-speed internet and compact telecom hardware.In this context, Lithium Niobate on Insulator (LNOI) appears as an attractive platform for creating cost-effective and highly efficient photonic devices.The LNOI photonic platform provides a tightly light confining geometry that exhibits remarkable properties such as high electro-optic (r33 = 32 pm/V) and nonlinear optic (d33 = 27 pm/V) [1] coefficients.It also offers some opportunities to periodically pole the waveguide, thereby enabling quasiphase matching.
Fully etched LNOI waveguides with loss as low as 0.2 dB/cm have been achieved [2] and various linear and nonlinear devices have been demonstrated successfully [1].However, the related fabrication technology, particularly the etching of lithium niobate, can be challenging compared to the silicon-based photonic platform.One less demanding technological solution is to define the waveguide by making use of SiN optical strip loading.This approach is advantageous due to the relative ease of the deposition and etching processes of SiN, while the SiN has a slightly lower refractive index and wide transparency window, complementing LN.
In this contribution, we demonstrate how periodically poled SiN strip loaded LN waveguides can be used for creating broadband light signal via χ (2) and χ (3) nonlinear processes.We first report experimentally second-and third-harmonic generation (SHG and THG) when pumping these waveguides with near-IR pulses.We then present our simulation results of dispersion engineered SiN strip loaded LN waveguides that are capable to sustain supercontinuum generation.Both effects could be efficiently combined to achieve the generation of broadband supercontinuum down to the visible.

Waveguide design and harmonic generation measurements
The SiN strip loaded x-cut LN waveguides are designed to avoid undesirable lateral leakage at the second harmonic, which occurs when the effective index of the TE mode propagating in the SiN loaded LN waveguide is lower than the one of the LN TM slab mode.This effect can decrease the conversion efficiency of harmonic generation processes.Lateral leakage was shown to strongly depend on the dimensions of the guiding structure [3].As shown on Fig. 1 (a), most of the optical mode energy is confined in LN, and its thickness should be less than 350 nm for the second harmonic mode not to leak laterally.We thus choose the LN thickness to be 300 nm.The SiN waveguide height has a smaller impact on the leakage with the larger height being slightly more beneficial.
The waveguides were fabricated following the process described in [4].The chip contains 4.8 mm long waveguides with different poling length as well as periods (4.87 µm and 4.93 µm).The devices were characterized using an optical-fiber setup for butt-coupling a picosecond pulsed laser signal with a wavelength of 1547 nm and a coupled peak power of 53 W (Fig. 1 (a)) and 22.5 W (Fig. 1 (b)).Visible spectra were recorded, showing second and third harmonic generation at 773.5 nm and 516 nm wavelength, respectively, which exhibited the expected quadratic and cubic pump power dependences.Second harmonic is generated more efficiently than the third harmonic, and its intensity is larger for the longer poling length.Also, the third harmonic intensity is observed to be larger for a different poling period.A portion of third harmonic is most likely leaking to the slab mode as the waveguide was not engineered to guide it without leakage at these short wavelengths.

Simulations of supercontinuum generation
Broadband light signal can also be directly obtained in these waveguides by exploiting the Kerr nonlinear effect and generating supercontinuum using ultrashort intense pulses.Again, taking into account the lateral leakage limitation for our waveguide design, the thickness of LN was fixed to 300 nm.The dimensions of the SiN optical loading are engineered in such a way that we get anomalous dispersion around the pump wavelength and within as broad a range as possible towards shorter wavelengths.The resulting waveguide geometry and the related dispersion for slightly different SiN waveguide dimensions are shown in Fig. 2. The dispersion coefficients are extracted and used in the nonlinear Schrödinger equation (NLSE) to simulate the propagation of short optical pulses.The NLSE includes both third-and second-order nonlinearities and is solved numerically using the Split Step Fourier method (SSFM).
The supercontinuum spectrum shown on Fig. 3 is simulated for a 5mm long waveguide with a SiN crosssection of 1200 nm × 800 nm on top of the 300nm thick LN slab.The TE polarized pump used at the input has a 1.55 µm wavelength, a peak power of 1200 W and a pulse duration of 90 fs.We assume a waveguide loss of 2 dB/cm.On the spectrum, the effects of SHG and THG can be observed at shorter wavelengths, while the pulse significantly broadens spectrally after a propagation distance of 3.5 mm, mostly due to χ (3) based processes.Note that, as was shown in section 2, the relatively weak second harmonic signal could be enhanced using the quasi-phase matching technique induced by the periodic poling of lithium niobate.Besides, cascaded χ (2) could be harnessed in these geometries to provide even broader supercontinuum generation [5].

Conclusion
Through experimental investigation, we have demonstrated the ability of strip-loaded lithium niobate waveguides to generate second and third harmonics by effectively managing lateral leakage and employing periodic poling.Additionally, simulations have revealed that the SiN-loaded LN photonic platform can sustain the generation of supercontinuum around telecom wavelength.By synergistically combining these approaches, specifically leveraging periodic poling to enhance second and third harmonic generation, we envision the potential of this platform for creating efficient and broadband light signals that extend into the visible band.

Fig. 1 .
Fig. 1.(a) Electric field intensity for the TE mode of a striploaded LN waveguide at both 1.55 µm and 775 nm, (b) Output third harmonic signal and (c) second-harmonic signal under a coupled average pump power of 1.2 mW and 0.5 mW, respectively.The third harmonic light was recorded for 1000 times longer integration time.

Fig. 2 .
Fig. 2. (a) Dispersion curves for the waveguides with slightly different SiN dimensions, (b) spectrum at the output of the waveguide and (c) spectrum depending on the propagation distance as result of SSFM simulation.