Investigation On All-optical Photodetection Based On The Coupling Between ZnO Nanowire And Optical Microfiber

Photodetection has shown broad application prospects in the fields of aerospace technology,military activities,and daily life.Research on high performance detectors never stops,from the synthesis and selection of detector materials to the design of structures and the fabrication of devices.For the miniaturization and integration of devices,researchers have gradually put more sight into the world of nanomaterials.Among the numerous nanomaterial systems,ZnO has exhibited broad applications in nano-optoelectronic devices,nano-bioprobes,nanosensors and nanomachines,due to the excellent structural characteristics and optoelectronic properties.As one of the typical representatives of II-VI semiconductors,ZnO has a band gap of 3.37 eV and an exciton binding energy of up to 60 meV,which is an important material for realizing high-efficiency ultraviolet optoelectronic devices at room temperature.Ultraviolet photodetectors based on ZnO semiconductor materials have been extensively investigated however,these detectors generally suffers slow response and low signal-to-noise ratio due to intrinsic electrical limitations.In this thesis,a new type of ultraviolet all-optical detector is designed to overcome the problems existed in traditional ZnO-based photodetectors,and some innovative results have been obtained.The main contents are as follows:1.The hydrothermal reaction and chemical vapor deposition method were used to fabricate the ZnO nanowires on the silicon substrate.When employing hydrothermal reaction,the seed layer of ZnO was prepared on the silicon wafer,and then the ZnO nanowires were successfully realized at a precise reaction concentration with reaction temperature of 80°C and reaction time of about 12 hours.When chemical vapor deposition was conducted,high-purity carbon powder and ZnO nanoparticles were used as raw materials,and the relationship between growth time and product size were explored at a high temperature of 1120°C.The synthesized nanowires were characterized by XRD,SEM and STEM,respectively,and results showed that the obtained ZnO nanowires exhibited high quality of crystalline.2.The ZnO nanowire/microfiber coupler structures were fabricated successfully.Firstly,the method of microfiber fabrication using oxyhydrogen flame heating was studied and improved,and microfibers with adiabatic transition region that support the single-mode operation were fabricated.Secondly,a micro-manipulation system was built up.The manipulation of ZnO nanowires can be realized by using the tungsten probe in the system,and single ZnO nanowire/microfiber directional couplers were fabricated with different diameters.The relationship between the resonance dip and the diameter of ZnO nanowires was explored,and results showed that the resonant dip redshifted as the diameter of ZnO nanowires increasing,within a precise range.3.An ultraviolet all-optical detection system for ZnO nanowire/microfiber coupler was built up to characterize the device performance.A 266-nm single-frequency deep-ultraviolet laser was used as the excitation light,and the response of devices operated at LP₁₁ mode and LP₀₁ mode were tested,respectively.It was found that the resonant dip redshifted with the increasing of UV intensity.The photodetection scheme is free of dark current and photocurrent due to the employment of optical signal demodulation,and thus exhibits very high signal-to-noise ratio.Experimental results showed that the sensitivity of the device was up to 1.657 nm/（W/cm²）,and the rise time and recovery time were 0.43ms and 0.47ms,respectively,which were faster than that of traditional electrical photodetectors.4.The physical process for all-optical photodetection of the device was explored and discussed in detail.The light injection effect leads to an increase in the concentration of unbalanced carriers,resulting in a significant bandgap shrinkage effect and leading to the increase of ZnO refractive index,which changes the phase matching condition with the microfiber and results in a red-shift of the resonance dip of the device.