Research On The Key Technology For Mid-Infrared Third Harmonic Generation In Integrated Waveguide

The mid-infrared?MIR?light has significant and extensive applications in the fields of atmospheric communications,laser radar,and medical treatment due to its unique spectral characteristics.However,high-performance MIR light sources that mostly consist of discrete devices require precise alignment and adjustment.Because of the low stability and integration difficulty,they can't adapt to the development trend of the new lasers:small size,low power consumption,and large-scale integration.To overcome the drawbacks of the existing MIR lasers,an approach based on the third harmonic generation?THG?in optical waveguides is proposed to generate MIR.In this paper,two kinds of nonlinear optical waveguides with compact structure and integrating easily on a large scale are designed.Based on the theory of the harmonic generation in nonlinear optics,the derivation of the coupled-mode Schrodinger equations for THG in optical waveguides is investigated.And three key aspects that have great effects on THG conversion,namely phase-matching condition?PMC?,high third-order nonlinear susceptibility,and large modal overlap between pump and harmonic are considered.Based on the generation of surface plasmon polaritons,two optical waveguides satisfying harmonic generation conditions are proposed.Hereafter,numerical methods are employed to investigate the intrinsic relationships among the energy conversion in THG process,waveguide loss,PMC,and modal overlap,respectively.Ultimately,the waveguide structures are further optimized.According to the analyses above,a polymer-loaded surface plasma waveguide for THG from 4650 nm to 1550 nm is proposed.Based on the metallic properties and general dielectric properties of metal oxide cadmium oxide?CdO?in the MIR and near-infrared wavelength region,respectively.Thus,the perfect PMC can be satisfied between the surface plasmas modal at fundamental frequency and photonic modal at third harmonic frequency through the interaction between the surface plasmas mode and the ordinary photonic mode in the waveguide.By further optimizing the structure parameters,the THG conversion up to⁸.7×10^-5 is realized under the pump power of 100 W and pulse width of 100 fs.To generate MIR harmonic,considering the good confinement of metal materials for optical field,we propose a metal-semiconductor-metal slot waveguide?MSMSW?to realize the THG from 10.2?m to MIR.Since the permittivity of metallic silver in the MIR is very large,the optical field can be severely confined in the germanium slot region sandwiched between two silver layers.Meanwhile,large propagation loss caused by silver becomes the major factor limiting the further improvement of conversion efficiency,rather than the original PMC.Without the limitation of perfect PMC,the THG interaction mainly occurs between two fundamental modes at pump and harmonic wavelength,respectively,which leads to high nonlinear coefficient.By further optimizing the waveguide geometry,the THG efficiency up to 9.1×10^-4 is predicted for the MSMSW with 1 W pump power.Additionally,the waveguide demonstrates strong immunity to fabrication tolerance and pump wavelength drifts,and works effectively at low temperatures.Therefore,the proposed waveguides can be the promising candidates for MIR light generation with large integration capability.