Supercontinuum generation in multi-layered tantalum pentoxide waveguides

Xiu-Yuan Liu^1*, Yuan-Yao Lin^1,2

¹Department of photonics, National Sun Yat-Sen University, Kaohsiung, Taiwan
²Crystal Research Center, National Sun Yat-Sen University, Kaohsiung, Taiwan

* Presenter:Xiu-Yuan Liu, email:qa30412@gmail.com

Nonlinear optical proess such as superconinuum (SC) generation, four-wave mixing (FWM) and nonlinear multimode interference (MMI) are particular useful in in spectroscopy, optical tomography and optical signall processing. Furthermore quantum light sources such as entangled photon pairs can be generated by nonlinear parametric process. Tantalum pentoxide (Ta2O5) is a crystal with large bandgap (4.2-4.4 eV), large refractive index (2.0-2.4) and possess high nonlinearity (> 10-14 cm2/W ). Its CMOS compatibility eases the device fabrication difficulty and Ta2O5 waveguide devices has successfully demonstrated SC generation FWM and MMI based alloptical signal processing. SCG is preferable in an anomalous dispersive and highly nonlinear environment. Using Ta2O5 as the core material, anomalous dispersion is achieved in air-cladded and oxide-cladded waveguides for
fundamental and higher-order guided modes, respectively. Recent research indicated the observation of higher nonlinearity in Ta2O5 thin film when its thickness is below 300 nm. Although all optical switching and potential power limiter has also been first demonstrated in Ta2O5 waveguide of 100 nm in thickness, the small waveguide thickness causes poor coupling efficiency and the filling of modal field within the waveguide is relative low. To exploit the highest possible nonlinearity, we propose multi-layered Ta2O5 waveguide for SC generation and present the structure design and simulation in this report.
The multi-layered Ta2O5 waveguide is formed by stacking 100-nm-thick Ta2O5 thin film and 20-nm-thick silicon oxide alternatively on top of the silicon dioxide substrate the structure is defined by e-beam lithography and etching process. Although the actual nonlinear coefficient of a multi-layered structure requires further justification by either Z-scan technique or self-phase modulation (SPM) spectrum broadening measurement, the dispersion relation in a multi-layered Ta2O5 waveguide should be designed and investigated. We adopted the material dispersion of 100-nm-thick Ta2O5 thin film and use the mode solver in Lumerical , a commercial software for wave optics, to obtain guided modes and their propagation constants over the visible and near infrared wavelengths. The dispersion relation of the multi-layered Ta2O5 waveguide device become anomalous when the width of the waveguide is 1μm and the thickness is 700 nm, which corresponding to the alternating staking of 6 Ta2O5 layers and 5 SiO2 layers. The SC generation spectrum is computed by nonlinear Schrodinger equation including self-phase modulation and self-steepening effect. Up to 6th order dispersion is considered for very large spectral extent of interest. When 100fs pulse at 1030 nm is launched at the soliton order of 15, over 600 nm spectral width is obtained with 6 discrete dispersive lines in shrot- and long-wavelength regime. For a comparison, the SC generation in a single layer Ta2O5 waveguide of 700 n in thickness is also performed and exhibit relative poor SC spectrum under the same pumping condition due to the stronger dispersion. The work shows the potential of multi-layered Ta2O5 waveguide for SC generation and it is a promising platform for nonlinear silicon photonic photonics.

Keywords: Supercontinuum, nonlinear optics, waveguide, multi-layer