Optical Whispering-gallery-mode Resonances In ZnO Crystal Column

Purcell had proposed as early as 1946 that if an atom or matter was confined in a cavity in which the size is comparable to its wavelength,the nature of spontaneous emission would be changed and be regulated by the cavity.Since then,the whispering gallery mode(WGM)optical microcavity structure has gradually entered the field of view.Up to now,the optical microcavity has been a hotspot in the field of semiconductor optics.In the WGM optical microcavity composed of various materials,the light is confined in the microcavity of small volume,thus forming total internal reflection.The light field in the microcavity can form a series of standing waves or microcavity modes(cavity mode),which show properties different from that in the free space.Researchers have made great efforts to explore how to lower the threshold and improve the quality factor of optical microcavities.Most studies have focused on improving the performance of WGM microcavities by improving the two-dimensional material properties.Previous reports mainly realized the adjustment of optical mode in the microcavity by changing the laser power,but the lack of effective post-processing scheme greatly limited the optical application of the microcavity.In fact,the optical mode can be adjusted by manipulating the properties of the microcavity itself,and the polarization mode can be different at different locations in the same sample.It has been previously reported that regular polygonal microcavities have been widely used in the research of micro lasers.Photoluminescence technology is polarization sensitive to any microcavity excitation,so it is very important to study the influence of different polarization conditions on the microcavity mode.Polarized light in WGM microcavity is mainly TE polarization and TM polarization.TE polarization mode means that the electric field direction is perpendicular to the c-axis(the transverse direction of the microcavity),while TM mode means that the magnetic field direction of light is perpendicular to the c-axis.Because of the different laser irradiation locations,TM and TE modes have different polarization composite modes,so understanding the polarization mode of laser emission is a crucial step to analyze the mechanism of laser emission.In this paper,ZnO crystal column are successfully prepared by chemical vapor deposition(CVD).The ZnO crystal column has regular hexagonal prism structure,regular hexagonal cross section and smooth surface,and relatively uniform diameter,which provides necessary conditions for the study of WGM microcavity.By optical test,it is found that the PL spectrum of the ZnO crystal column produces two main resonance peaks due to the Rabi splitting,which are dependent on the spatial position and polarization to a certain extent.In the visible region,there is a wide emission band,and a series of small periodic resonance peak appear.These resonance peaks are generated by pure WGM resonance in the ZnO microcavity,and have no obvious spatial dependence on the ZnO microcavity.This paper mainly studies the following three aspects:First,through the PL test of ZnO crystal column with different morphologies and diameters prepared,it was found that the PL spectrum ultraviolet region of the regular hexagonal ZnO crystal column produced two formants due to Rabi splitting,and the intensity of the two formants changed with certain regularity.The high-order phenomenon in the visible region of PL spectrum was studied.Second,the effect of spatial position on the optical resonance mode of ZnO WGM microcavity is studied.For ZnO crystal column on the PL spectrum test of different spatial position,it is found that the two main ultraviolet regions of the PL spectrum intensity change with the changing of the spatial location,cyclical changes are simulated by using COMSOL software,which theoretically proves the periodical change of the two resonance peaks in ultraviolet region is due to the WGM caused by uneven distribution of modes.At last,the influence of polarization angle on the optical resonance mode of ZnO WGM microcavity is studied.The PL spectrum was measured at the same position of the ZnO crystal column.By changing the polarization angle,it is found that the intensity of the two formants in the ultraviolet region decreased gradually with the increase of the polarization angle,which is caused by the different exciton transitions between the valence bands,indicating that the two formants in the ultraviolet region of the ZnO crystal column are mainly the TE polarization mode.