Whispering gallery mode(WGM)-enhanced nonlinear optical phenomena from crown-like nanostructure zinc oxide(ZnO)samples are observed.The samples are synthesized by vapor-phase transport method.The morphology and crystal structure are examined and characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD)and high-resolution transmission electron microscopy(HRTEM),and they are excited by femtosecond laser pulses with central wavelengths of 355 nm,800 nm and 1150 nm,respectively.The typical stimulated emission presents a red shift compared with spontaneous emission,which is observed under the excitation of 355 nm with a relatively low threshold.The ultraviolet(UV)frequency up-conversion emission is obtained when the excitation pulse wavelengths are selected as 800nm and 1150 nm,respectively.The peak position and the relationship between the emission intensity and excitation intensity demonstrate that the UV up-conversion photoluminescence(PL)is induced by two-and three-photon absorptions.The PL characteristics and their WGM-enhanced mechanism are investigated.
