Tailoring And Applications Of Second-harmonic Generation In Single Semiconductor Nanostructures

With the developing of ultrashort and ultrafast laser,nonlinear optics has been one of the most important part of optics.Second-harmonic generation(SHG)is the fundamental second-order nonlinear optical process.Based on the SHG mechanism,SHG has many advantages,such as stable,broad-band tuning,coherent,fast and polarimetric.SHG usually can be observed from non-centrosymmetric nanostructures,and originated from crystal polarization(known as bulk SHG).SHG is very sensitive to the crystal nanostructures.Normally,the c axis of crystallographic orientation can be determined by SHG polarimetric property,namely the variation of SHG intensity with rotating the pumping laser polarization.Recently,with the further improving of the SHG polarimetric technique,all the three crystal-axis orientations can be precisely determined by the SHG method.And different crystalline structures can also be distinguished.Moreover,more complex crystal structures,such as nanotwins,can be detected.Therefore,the SHG polarimetric technique is a necessary optical method,which need to be further studied.In addition,SHG in nanomaterials is the potentially coherent light source at nanoscale.However,the low conversion efficiency of SHG in nanomaterials is the essential problem need to be solved.In this thesis,the lattice distortion of semiconductor nanowires is detected by the SHG polarimetric method.The SHG intensity is investigated in hybrid nanostructures of metal-nanocavity and two dimensional monolayer materials.The autocorrelation signals in semiconductor nanoparticles(NPs)are detected by the home-built femtosecond pump-probe system.The main contents are listed as follows:(1)We demonstrate an all-optical and highly sensitive detection of the lattice distortion in single semiconductor nanowires by SHG microscopy.By studying the SHG polarimetric response in the single bent ZnO nano wires with different curvatures,it shows a significant decrease in the SHG intensity ratio between perpendicular and parallel excitation polarization with respect to c-axis of ZnO NWs.A high detection sensitivity of 10-3 nm on the bending distortion is obtained in our experiment.Importantly,when the ZnO nanowire is twisted along its c-axis,the extraordinary non-axisymmetrical SHG polarimetric patterns are also observed,indicating the twisting distortion around c-axis of ZnO NWs.Therefore,the SHG microscopy technique can also be used to detect twisting distortion.(2)The confocal optical microscope measurement has been designed and set up by ourselves.Especially,the white light in microscope system(Kohler illumination)are described in detail.Compared with the business integration system,the system has the advantages of flexibility and high expansibility.Dark field scattering image and spectra system of single metal NPs has been designed and set up,revealing the position and spectra characterization of single metal NPs.(3)Monolayer WS2 has been successfully synthesized via chemical vapor deposition method,and we have investigated its optical properties.Metal-nanocavity consisted of metal NPs and a metal film,has been fabricated by electron-beam evaporation,atomic layer deposition and drop method.The localized surface plasmon resonance(LSPR)wavelength of the metal-nanocavity has been tailored by adjusting the spacer thickness between NPs and a metal film or choosing the NP size.The SHG in the hybrid nanostructures of metal-nanocavity and monolayer WS2 has been studied.It shows that the SHG intensity of monolayer WS2 can be tuned by metal-nanocavity.(4)Femtosecond laser pump-probe system has been designed and set up,which is the basis to study the ultrafast dynamics.The intensity and interferometric autocorrelation signals in single ZnO NPs has been investigated by femtosecond pump-probe technique.By analyzing the autocorrelation signals,the pulse width of femtosecond laser has been successfully extracted,which provides a new platform for measuring the ultrashort pulse width at nanoscale.