Directional Coupling And Frequency Conversion In Subwavelength Optical Waveguides

With the development of science and technology,traditional electrical integrated circuits are approaching the limit in speed and bandwidth.Encoded photons can be used in communication applications with a high speed,low loss,and a bandwidth of hundreds of THz,so optical integrated circuits are expected to break the bottleneck of the traditional circuits.The micro-nano waveguides are the basic element in the integrated optical chip,which confines the free space light to the micro-nano size in the form of evanescent waves and realizes the transmission and manipulation of light.Different from light in free space,the strong electromagnetic field enhancement of the propagating surface wave can significantly enhance the time or strength of the interaction between light and matter,thus significantly improving the conversion efficiency and reducing the power consumption in the optoelectric devices.In addition,the high confinement of the structured optical field also results in some strange physical phenomena.For example,the locking effect of the transverse spin and the propagation direction of the guided wave.On the other hand,the advanced energy materials,which drive the optical integrated circuit,determine the evolvability,expansibility,and practicability of the photoelectric devices.The recent discovery of layered transition metal dichalcogenides has aroused a wide range of research interests,including valleytronics,chiral Raman or phonon,and second-order nonlinear effects.The excellent optical properties of these materials make them have great potentials in the photoelectric integrated devices.So,in this paper,the sub-wavelength optical waveguides are integrated with and twodimensional transition metal dichalcogenides,and the directional coupling between the guided waves and the chiral Raman signals,as well as the enhancement and manipulation of the second-order nonlinear processes.The main research results are as follows:1.One-dimensional surface plasmon polariton waveguide can significantly enhance the spin-orbit interaction of light.Taking a single silver nanowire waveguide as an example,our theoretical simulation shows that the propagation direction of the surface plasmon polariton has a one-to-one relation with the near-field chirality.Specifically,the chirality of the electric field on both sides of the waveguide is opposite,and the chirality on one side of the waveguide gradually decreases and even reverses when away from the nanowire.Further simulations reveal that when a high refractive index substrate exists,the initial surface plasmon polariton mode hybridized and formed a new mode.Among them,the hybridization of TM₀,HE_-1,and HE_-2 forms the H₀ mode,and the reversed near-field chirality of H₀ mode mainly comes from the participation of HE_-2 mode.In addition,the reversal position of H₀ mode depends on the refractive index of the substrate and the distance between the substrate and the silver nanowire.This work deeply studies the influence of the substrate on the near field of the surface plasmon polariton,which is instructive for the research and development of the surface plasmon polariton devices.2.Based on the spin-momentum locking effect of plasmon,we use a single silver nanowire to achieve directional separation of chiral Raman signals in tungsten disulfide.By scanning the excitation position relative to the nanowire waveguide,we experimentally proved that the directional coupling efficiency between the chiral Raman signals and the plasmon achieves up to 91.5 ± 0.5%,approaching the theoretical limit.The results show that the directivity is highly dependent on the spin density of the local field and the degree of the circular polarization of the Raman signals.Different from the previous reports about the manipulations of the elastic scattered light or fluorescence,Raman scattering is a nonelastic scattering process,with a slower decoherent rate and wider working ranges.This is the first report about extending the spin-momentum locking effect of light to the region of the inelastic Raman scattering.The extension and application of the spin-orbit interaction of light enrich the content of traditional chiral photonics and provides a new scheme for micro-nano optical devices such as optical information processing and manipulation based on the inelastic scattering.3.In the hybrid system of one-dimensional photonic crystal and monolayer tungsten disulfide,the coupling between the propagating Bloch surface wave and tungsten disulfide can prolong the interaction length between light and material（tungsten disulfide）and thus increases the nonlinear conversion efficiency of second harmonic generation.In this experiment,the Bloch surface waves are excited simultaneously and reach the tungsten disulfide region at the same time to participate in the three-wave mixing process.This strategy is not limited by the phase-matching conditions in the nonlinear process,and the nonlinear conversion efficiency is as high as 8.26×10^-5/W,which is at least 3 orders of magnitude higher than the initial nonlinear efficiency of tungsten disulfide.In addition,the spatial distributions of the second harmonic generation and sum-frequency generation are systematically studied by Fourier imaging technique.The results show that the radiation direction of the second harmonic is perpendicular to the propagation direction of the Bloch surface wave,and the divergence angle of the signals is as small as 3 degrees.However,when two Bloch surface waves with different energies are excited,a deflected angle always exists compared to the direction of the second harmonic generation,which depends on the wave vector difference of Bloch surface wave.Finally,we also study the relationship between the polarization direction of the nonlinear signals and the crystal axis of tungsten disulfide.When the crystal axis is rotated with respect to the wave vector of Bloch surface waves,the emission polarizations of second harmonic generation and sum-frequency generation are always consistent and change with the twist angle.This work contributes to the nonlinear application of two-dimensional material in integrated optical devices and provides a versatile platform for the generation of coherent optical signals.