Photoluminescence spectra of zno microspheres: effects of exciton-polariton and Purcell factor
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.295
Photoluminescence spectra of zno microspheres: effects of exciton-polariton and Purcell factor Ching-Hang Chien1,2,3 and Yia-Chung Chang1 1
Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan
2
Nano Science and Technology Program, TIGP, Academia Sinica, Taipei 115, Taiwan
3
Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan
ABSTRACT
We present theoretical calculations of the line shapes of emission spectra of ZnO micro spheres (MSs), including the exciton-polariton and Purcell effect. Our calculation explains the red shift of emission peaks of whispering gallery modes (WGMs) in UV range commonly observed in ZnO MSs. We show that the red shift of the UV emission peak is caused by the combination of cavity effect and the polariton dispersion. The positions and relative strengths of sharp peaks arising from WGMs are also simulated by our calculation, and theoretical predictions match well with experimental data. Our calculation provides useful guide lines for the design of MS cavities for applications in white-light illumination, optical communication, and biosensing.
INTRODUCTION Whispering galley modes (WGMs) in micro cavities have attracted a great deal of interest in recent years due to the high quality factor (Q) associated with WGMs, which makes them attractive for applications in light-emitting devices and biosensors [1,2]. The interest in WGMs can be traced back to a century ago when Rayleigh studied the propagation of sound in a gallery with round surface [3]. Recently, WGMs in resonators of various shapes such as micodisks [4], ZnO microrods with hexagonal crosssection [5,6], and microspheres (MSs) [7,8] were investigated experimentally and theoretically.
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Braginsky et al. [1] showed WGMs of dielectric silica microspheres have very high quality factor (Q). The high-Q properties make WGM a good candidate for highsensitivity sensor. To evaluate the cavity loss, one can seek the analytic continuation of the mode frequency in complex plane and find the complex frequencies (with the imaginary parts describing the loss) in a micro cavity by solving a transcendental equation obtained by matching boundary conditions for the electromagnetic fields [9] within the framework of Mie-scattering theory [10]. Previous analyses of WGMs focused mostly on the mode frequencies and Q factors of a cavity [7-9]. The WGM peak positions in a MS are very sensitive to such physical parameters as the diameter and refractive index. A proper refractive index formula is not easily measured in a micro cavity. Furthermore, the crystalline quality of ZnO MSs may depend on the fabrication process. ZnO MSs fabricated by hydrothermal method [7] tend to contain porous Z
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