The Research On Thermal-optic Characterization Of Graphene-silicon Based Devices

Silicon photonics is considered as an ideal platform for on-chip integration,owing to the advantages of low cost,compact footprint,low power consumption,and compatibility with complementary metal-oxide-semiconductor?CMOS?processes.Due to the relatively high thermo-optic?TO?coefficient of silicon?¹.44 W/m·K?,thermal tuning is often applied in various thermo-optic devices,such as optical filters,optical switches,and optical modulators.Traditionally,thermal tuning is often applied with metal heaters.To avoid the metal absorption,a thick SiO₂ layer is usually introduced between the silicon layer and the metal heater layer.Since the SiO₂ layer has a poor heat conductivity,it reduces the heating efficiency and slows down the tuning speed.Therefore,how to improve the tuning efficiency of the thermo-optic devices has become a research hotspot in academia.Graphene,a single sheet of carbon atoms in a hexagonal lattice,has generated tremendous interests due to its attractive properties,such as high carrier mobility of²00,000 cm²v^–1s^–11 at room temperature,ultra-broad absorption bandwidth,and tunable Fermi level.Thus,graphene has been incorporated to implement novel optoelectronic devices such as ultrafast optical modulators,ultra-broadband photodetectors,and ultra-sensitive optical sensors.Specifically,monolayer graphene has a low optical absorption of².3%for vertically incident light,so it can directly contact with the silicon waveguides.Graphene also has a high thermal conductivity of up to 5,300W/m·K,which is³00 times higher than that of Titanium.With these unique properties,graphene is considered as an excellent material for transparent micro-heaters integrated on optical devices.Moreover,we find that the tuning efficiency can be increased by enhancing the light-matter interaction,which can be achieved through shrinking the mode volume of the device.If the mode volume is decreased,the heated volume,needing to cover the mode volume for the effective TO tuning,is also reduced and thus the heating efficiency is increased.This paper mainly focused on the research of the thermal-optic characterization of the graphene-silicon based devices using nanobeam cavity with a small optical mode volume.Firstly,we design a photonic crystal nanobeam?PCN?cavity with ultra-small mode volume.The nanobeam waveguide etched with an array of air-holes forms a Fabry-Perot?F-P?cavity,consisting of a central-taper section and two side-reflector sections.The taper section with 11 holes is designed to reduce scattering loss and avoid the phase mismatch between the photonic crystal fundamental Bloch mode and the waveguide mode.The reflectors with 18 holes serve as two symmetrical mirrors to guarantee that the light is highly reflected.Based on finite-difference-time-domain?FDTD?simulation results,the proposed nanobeam cavity can provide a large quality factor of 5000 and a large modulation depth of 14 dB with a small mode volume?0.145?m³?.To analyze and further increase the TO tuning efficiency of the proposed structure,3D finite element method?FEM?simulations are performed to study the temperature distribution of the structure in the heating process.In the simulations,the thermal conductivities of the graphene,the silicon,and the silica are set to²,000W/m·K,⁸0 W/m·K,and¹.38 W/m·K,respectively.The heat convection coefficient of air is set to⁵ W/m²·K.And the thickness of the graphene is chosen as 0.5 nm.The temperature of the silicon in the PCN cavity increases from 300 K to 330 K when the heating power is 1 mW.Finally,based on the simulation results,the device is fabricated and tested.The TO tuning efficiency is measured to be as high as 1.5 nm/mW,which is much higher the tranditional thermal-optic devices.We also test the response time of our device,the time constants with a rise time constant of?_rise=1.11?s and a fall time constant of?_fall=1.47?s are obtained in the experiment.