| Since the discovery of graphene,its two-dimensional atomic structure and unique physical properties have received continuous attention from researchers.There is an intersection between the conduction band and valence band in the energy band structure of monolayer graphene,which is called the Dirac point.The energy dispersion relationship is a linear distribution near the Dirac point,leading to that the carriers in graphene exhibit the nature of massless Dirac Fermion.The unique electronic structure of graphene determines its excellent electrical properties,such as the extremely high carrier mobility at room temperature,which is very attractive for the applications in the field of micro-nano electronics.Graphene also has the special optical properties,for example,the transmittance is very high and constant in the wide range from visible to infrared wavelength;and the graphene surface plasmons(SPs)can be excited in the infrared and even terahertz wavelengths,which has the characteristics of low loss,flexible tuning and high localization.Therefore,the applications of graphene are highly valued in micro-nano optoelectronic devices.In recent years,the research on graphene SPs devices has been one of the hottest topic in the exploration of graphene optoelectronic devices.The latest studies have shown that the combination of graphene with different substrates is likely to further enhance its performance and even generate unexpected new physical properties.However,the current researches are mostly focused on the exploration of combining graphene with silicon-based materials in micro-nano electronic or optoelectronic devices and the researches on the combinations of graphene with optoelectronic insulating substrates are very limited.The optoelectronic insulating materials have excellent optical or optoelectronic properties themselves,especially that LiNbO3 crystal with perovskite-like structure has been widely used in the field of optics and is called optical "silicon" material due to its key role in the marerials for optoelectronic devices.Some of the theoretical findings have demonstrated that the combination of graphene and LiNbO3 crystal affects the carrier properties in graphene,thereby changing the electrostatic surrounding of graphene to open the zero band gap,which may generate new optoelectronic properties to give promise of the great potential in the fields of photodetection,sensing and photomemory.The previous researches on the combination of graphene and LiNbO3 are mainly confined to the theoretical studies,but rarely confirmed by the experiments in the corresponding reports.Based on this understanding,a new method of combining graphene with LiNbO3 crystal is explored,and the optical properties of graphene-LiNbO3 structure are investigated by the experimental measurement and simulation analysis in this paper,which provide the theoretical fundamental and experimental basis for achieving the wide applications of graphene-LiNbO3 structure in optoelectronic devices.In this paper,the research can be divided into two parts,which are the new method for preparing graphene on the surface of LiNbO3 crystal and the study on the interfacial optical properties of graphene-LiNbO3 structure,respectively.In the first part of the research,the direct combination of micron-scale graphene and LiNbO3 is realized and the physical mechanism of graphene synthesis is analyzed.In the second part,it is found that the coupling of graphene SPs with the waveguide modes and the Fabry-Perot(F-P)cavity modes in graphene-LiNbO3 structure can realize the infrared optical modulation for LiNbO3 waveguide and the significant absorption enhancement in the wide infrared band.The coupling effect is also shown in the modulation of graphene interlayer twisted angle on the dielectric and optical properties in composite structure.The main contents and experimental results of this paper are as follows:1.Turbostratic graphene was directly synthesized on the surface of LiNbO3 crystals via C ion implantation.C ions were implanted into LiNbO3 film and bulk crystals at an energy of 30 keV with fluences of(2～10)×1015/cm2.At the annealing of 500℃～700℃,C atoms were diffused and segregined in LiNbO3 crystals,then precipitated and bonded at the surface of LiNbO3 to form graphene structure with sp2-hybridization.Raman spectroscopy was used to characterize the synthesis of turbostratic graphene in different implantation fluences and annealing conditions.Scanning electron microscopy and energy dispersive spectrometer were used to observe the morphology of turbostratic graphene and its distribution at the surface of LiNbO3.The synthesis mechanism of turbostratic graphene is closely relevant to the diffusion of C atoms and the lattice defects of LiNbO3 crystals,both of which are extremely sensitive to the annealing condition.The direct C implantation into LiNbO3 provides a promising way for integrating graphene-LiNbO3 structure in microelectronics devices.2.A graphene-LiNbO3 composite structure was fabricated by combining the large-sized graphene and LiNbO3 film crystal,which was characterized by Raman spectroscopy,scanning electron microscopy and atomic force microscopy.The optical waveguide characteristics of the composite structure were investigated by the prism coupling system.Compared with the pure LiNbO3 film,the reflectivity of graphene-LiNbO3 structure shows an overall decrease in the reflection spectrum due to the lower transmittance of graphene on LiNbO3 substrate.The effective refractive index of planar waveguide mode has a significant increase of 0.061 in the composite structure at the excitation of 1540 nm,which may be attributed to the generation of SPs at the graphene-LiNbO3 interface excited by infrared radiation,excluding the impact of graphene material itself.Based on the effect of graphene on waveguide index,a branched graphene waveguide structure was designed and simulated,showing the good performance of beam splitting and light guiding.The preparation of the graphene-LiNbO3 composite structure is compatible with the optoelectronic integration processes,which is of importance to the construction of noval optoelectronic integrated devices.3.An absorption enhancement structure was demonstrated based on the SPs effect at the graphene-LiNbO3 interface by combining graphene with a LiNbO3-SiO2 substrate.Graphene was characterized by atomic force microscopy and Raman spectroscopy.The infrared spectra of graphene-LiNbO3-SiO2 composite structure was investigated by the attenuated total reflection(ATR)technique,the results of which has shown an extraordinary absorption enhancement with the maximum absorption rate of more than 90%at a wide frequencies of 500～4000 cm-1.The analysis reveals that the absorption enhancement can be explained by the coupling resonance of the LiNbO3-SiO2 F-P cavity modes and the LiNbO3 planar waveguide modes with SPs at the graphene-LiNbO3 interface.The coupling resonance can be enhanced with the ambient temperature,leading to the further improvement of absorption rate.Compared with the general absorption enhancement structure related to graphene SPs,the graphene-LiNbO3-SiO2 structure has excellent advantages of simple design and easy fabrication,providing the experimental and theoretical supports for the development of novel graphene SPs devices.4.The physical properties of bilayer graphene are significantly different from those of monolayer graphene,particularly when the two graphene layers are rotated to each other by a certain angle,completely changing the atomic arrangement and electronic band of the intrinsic bilayer graphene.Bilayer graphene without interlayer rotation and twisted bilayer graphene with a rotation angle of 90°were fabricated on LiNbO3 film substrates,which were characterized by optical microscopy and Raman spectroscopy.The optical waveguide properties of the structures were measured by the prism coupling technique,to investigate the interaction of different graphene structures with visible and near-infrared radiation.It is found that the structures composed of bilayer graphene or twisted bilayer graphene respectively with LiNbO3 exhibit different guiding modes and polarization response to the incident light,which is related to the dielectric properties of the two bilayer graphene structure.The energy bands of two graphene structure are calculated in the ideal state,the results of which are consistent with the infrared optical absorption of the two composite structures,providing the theoretical basis for further analyzing the experimental results.In addition,under the near-infrared excitation,the coupling between SPs excited at the graphene-LiNbO3 interface and the waveguide modes enhances the transmission efficiency of guiding modes in the composite structures.In this paper,it is found from the study of large-angle twisted bilayer graphene that the large-angle interlayer rotation can achieve the effective modulation on the dielectric and optical properties,which is a crucial supplement to the current research focuses in twisted graphene and of importance to expand the graphene application in the field of optoelectronic integrated devices. |
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