Study On Property Of Optical Modulation Based On Electron Concentration Distribution In A Plasmon Hybrid Waveguide

The optical modulator is a device that modify the polarization,frequency,amplitude and phase of light electrically or optically,which plays a pivotal role in the high-speed communication systems.The traditional optical modulator mainly controls the light through modifying the optical properties of the medium.Surface plasmon polaritons（SPPs）are modes of electromagnetic waves propagating along a metal surface by the interaction of light and surface charges.In this mode,the light is localized at the interface between metal and dielectric,and there is a strong coupling between the light field and the free electrons on the metal surface,which offers a unique way to concentrate optical fields down to nanometer-sized regions and shrink the photonic circuits below the diffraction limit.It provides a feasible way to further reduce the footprint of optoelectronic devices.The light-matter interaction can be further enhanced through combining such strongly localized SPPs with the conventional optical modulation effects,thereby improve the modulation depth.Thanks to the dependence of SPPs on electron concentration,our paper has realized the modulation on the frequency,amplitude and phase of SPPs by controlling the means of electron concentration distribution.As a result of electron concentration in metal is too high,it is difficult to be modulated.The research of this paper is mainly focused on the graphene and indium tin oxide（ITO）,which is of great significance in the fields of photoelectric detection,optical communication and sensing.The main contents of this paper include:（1）The research on the frequency modulation characteristics of SPPs by direct controlling the spatial distribution of electron concentration based on an interdigital electrode.A hybrid graphene-dielectric-interdigital electrode structure is designed,the spatial distribution of electron concentration in graphene is directly controlled by the interdigital electrode.Firstly,the characteristics of electron concentration distribution are simulated with the COMSOL Multiphysics software under the electrostatic field.Secondly,the sample is prepared by the traditional lithography method and the transmission spectra at different voltages are measured with a Fourier infrared spectrometer.In the case of different voltages,the confined area of electrons and the spatial distribution of electron concentration are changed at the same time,which leads to the shift of the resonance peak in experiment.The measured shift of resonance peak has a maximum distance of 33 cm^-1 and the transmission changed about 8%.Finally,the reason for the shift of the resonance peak is further verified by utilizing the graphene nano-ribbons model.The experimental results show that this method can improve the modulation depth of SPPs to some extent and paved a new way for modulating the frequency of SPPs.（2）The research on the amplitude modulation characteristics of SPPs by controlling the spatial distribution of electron concentration based on a grating.In this paper,a method to modulate the amplitude of SPPs using the periodic spatial distribution of electron concentration is proposed and demonstrated by a hybrid grating-dielectric-graphene structure.The electron concentration on the graphene surface exhibits a periodic spatial distribution when a voltage is applied between the grating and the graphene,resulting in a periodic spatial distribution of the Fermi level and the propagating SPPs is modulated due to scattering effects.The effects of different voltages,periods and wavelengths on the propagating SPPs have been studied and analyzed using the COMSOL Multiphysics software.The numerical results show that both the voltage and the period can be utilized to control the amplitude of SPPs effectively.Compared with the method by controlling the whole distribution of electron concentration,the modulation depth is improved about 18.6 d B.In addition,the modulation depth is about 15.2 d B when the method is applied to the ITO,which further proves the universality of spatial distributed electron concentration for controlling the amplitude of SPPs.Since this scattering effect is independent of the wavelength,an ultra-broadband modulation can be achieved theoretically.（3）The research on the phase modulation characteristics of SPPs by controlling the whole electron concentration distribution based on Mach-Zehnder（M-Z）Interferometer.In this paper,an M-Z interferometer embedded a small modulation region composed of ITO in its arm is designed and the interaction between the SPPs and ITO in the modulation region is enhanced.Even a small change of electron concentration distribution in ITO has a great influence on the phase of the SPPs.A good switching effect is achieved when the modulated SPPs interfere at the output port.The effects of different electron concentrations,lengths of modulation region,widths of modulation region and wavelengths on the propagating SPPs are analyzed using COMSOL Multiphysics simulation software.The simulation results show that the modulation depth can reach 15.2 d B when the electron concentration in the modulation region changes by4×10²⁰ cm^-3,and the size of modulation region is merely 4μm×0.5μm.Compared with the traditional M-Z interferometer modulator based on SPPs,this structure has higher modulation depth and smaller structure size,which is of great significance for the integration and application of optoelectronic devices.