Mechanism Research And Design Of Mid-infrared Photonic Functional Devices Based On Sub-wavelength Periodic Nanostructures

The mid-infrared(MIR)spectra encompass two important atmospheric windows(3 ?m-5 ?m and 8 ?m-14 ?m)and covers the primary absorption vibration bands of most biological and chemical molecules,which is of interest to sensitive chemical analysis.Therefore,MIR light has shown great potential within various applications,such as infrared counteraction,free space optical communication,biomedical analysis and atmospheric environment monitoring.Compared with the relatively mature nearinfrared photonic functional devices,their MIR counterparts are still at the initial stage of study,which cannot meet the increasing demand for practical applications.The problem of MIR sources has been resolved with the introduction and development of quantum cascade lasers,which has promoted the study of MIR photonic functional devices.However,it is vital for promoting the development of MIR application that the integrated design and performance improvement of linear and nonlinear optical functional devices.Based on the development status and application requirements of MIR photonic functional devices,this paper has carried out the design and analysis of four kinds of photonic functional devices.They are optical modulator and circularly polarized controller for transmission control,and two kinds of wavelength converters for light detection.The specific contents of the research are as follows:1.The principle of interaction between light and sub-wavelength periodic nanostructures is systematically analyzed.The results show that sub-wavelength periodic nanostructures can significantly modulate the phase or amplitude of incident light.The optical-electrical characteristics of graphene and the principle of excited surface plasmon polariton in graphene are discussed in detail.According to the modulation requirements of optical characteristics,different forms of sub-wavelength periodic nanostructures are designed.They can be used for the structural designs of optical modulator,circularly polarized light controller and wavelength converters to improve the key performance of MIR photonic functional devices.2.The voltage modulation advantages of the conductivity of the double-layer graphene are demonstrated theoretically.Based on advantages of double-layer graphene,the design of a tunable MIR optical amplitude modulator with high modulation depth and low insertion loss is proposed.The significantly enhanced local fields generated by excited strong surface plasmon polariton can improve the interaction between the matter with incident light in proposed modulator.Compared with single-layer graphene,the equivalent carrier concentration of double-layer graphene has a larger variation and achieves a higher modulation depth with the same bias voltage,but the insertion loss is almost unchanged.The designed modulator exhibits a modulation depth up to 21 d B over a wide range of wavelength(3.17 ?m-4.4 ?m).Moreover,the minimum insertion loss is about 0.1 d B and 3 d B bandwidth is 47.4 GHz.This modulator with combined advantages of high modulation depth,low insertion loss and ultrafast modulation speed,holds potential in efficient manipulating MIR lights in free space communication.3.The design of a MIR circularly polarized controller is proposed based on rotated double-layer graphene Moiré patterns.The Moiré patterns have structural chirality,which are formed by stacking double-layer graphene nanoribbons with a relative in-plane rotation angle.The incident light with different circularly polarization excites surface plasmon polariton with different intensity,resulting in a strong optical chirality.It is demonstrated that the circular dichroism is tunable and the strongest circular dichroism can reach 5.94 deg at 13.6 ?m.The proposed structure with graphene-based Moiré patterns has some advantage of rotation-dependent chirality,flexible tunability and cost-effective fabrication.It solves the problems of complex structure and weak chiral response existing in the current MIR chiral structure.It will be used for many essential MIR applications,such as chiral sensors,chiroptical detectors and the chirality analyzation of biochemical molecules.4.The design of a MIR wavelength convertor with high conversion efficiency is proposed,which combines sub-wavelength silicon scatterer arrays with lithium niobate waveguide.With the help of phase-gradient metasurface generated by sub-wavelength periodic silicon nanoscatterer array,the asymmetric coupling of waveguide mode is adjusted,and reduce the inverse up-conversion process.Thus,the conversion efficiency from the optical power of signal light to sum-frequency light is obviously improved.When the signal light wavelength range is 3.25 ?m-3.50 ?m,the normalized optical power of the sum-frequency light is always increased by about 19 times,compared with no-metasurface waveguide.The convertor with strong structural robustness and high conversion efficiency,provides a method for the improvement of efficiency of the detectors in MIR integrated photonic platform.5.The design of a novel plasmon-enhanced MIR wavelength convertor with arrays of graphene nanoribbons is proposed.Different from the normal research on graphene-based nonlinear optical device,this work puts the emphasis of research on the enhancement effect of graphene plasmon on the third-order nonlinearity susceptibility of graphene nanoribbons.It is demonstrated that the third-order nonlinearity susceptibility of graphene nanoribbons with excited graphene surface plasmon polariton can be an order of magnitude larger than the intrinsic susceptibility of a continuous graphene sheet.The enhanced nonlinear response of graphene nanoribbons,as well as enhanced electric field of signal light,serve as an excellent platform to boost the conversion efficiency of four-wave mixing effect in this convertor.When the wavelength of the incident signal is 3.07 ?m,the conversion efficiency of four-wave mixing effect can be remarkably improved by 60 times compared with the graphene sheet without graphene plasmon resonance.The proposed compact and tunable MIR convertor provides a more highly efficient solution for MIR wavelength detection with a poor efficiency and more ideas for playing the role of graphene in the nonlinear MIR photonic devices.