Citation: Lin, J.-H.; Chen, G.-R.; Li, S.-J.; Song, Y.-F.; Liu,W.-R. Gain-Guiding Anisotropic Polarized Amplified Spontaneous Emissions from C-Plane ZnO/ZnMgO Multiple Quantum Wells. Materials 2022, 15, 6668. https://doi.org/10.3390/ ma15196668 Academic Editor: Giovanni Onida Received: 20 August 2022 Accepted: 20 September 2022 Published: 26 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Gain-Guiding Anisotropic Polarized Amplified Spontaneous Emissions from C-Plane ZnO/ZnMgO Multiple Quantum Wells Ja-Hon Lin 1,* , Gung-Rong Chen 1 , Sheng-Jie Li 1 , Yu-Feng Song 2,* and Wei-Rein Liu 3,* 1 Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan 2 Intelligent Internet of Things and Intelligent Manufacturing Center, College of Electronics and Information Engineering, Shenzhen University, Shenzhen 518060, China 3 National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan * Correspondence: jhlin@ntut.edu.tw (J.-H.L.); yfsong@szu.edu.cn (Y.-F.S.); weireinliu@gmail.com (W.-R.L.) Abstract: A microcavity laser with linear polarization finds practical applications in metrology and biomedical imaging. Through a pulsed light excitation, the polarization characteristics of amplified spontaneous emissions (ASEs) from ten-period ZnO/Zn 0.8 Mg 0.2 O multiple quantum wells (MQWs) on a C-Plane sapphire substrate were investigated at room temperature. Unlike unpolarized spontaneous emissions, with 35 meV of energy differences between the C and AB bands, the ASE of MQWs revealed transverse-electric (TE) polarization under the edge emission configuration. The excited ASE from the surface normal of the polar ZnO/Zn 0.8 Mg 0.2 O MQWs with hexagonal symmetry revealed linear polarization under the pump of the stripe line through the focusing by using a cylindrical lens. The polarization direction of ASE is independent of the pump polarization but always perpendicular to the pump stripe, even if the cylindrical lens is rotated 90 degrees because of the gain-guiding effect. Keywords: stimulated emission; multiple-quantum well; exciton–exciton scattering; waveguide effect 1. Introduction Zinc oxide (ZnO), a II-VI compound semiconductor with a hexagonal wurtzite struc- ture, reference [1] has attracted considerable attention in the past several decades due to its promising applications in ultraviolet (UV) optoelectronic devices. ZnO has become one of the most promising materials in solar cells, light-emitting diodes (LEDs), laser diodes (LDs), etc., due to its direct wide bandgap of about 3.37 eV and large exciton binding energy of about 60 meV (RT). ZnO/ZnMgO multiple quantum wells (MQWs), in contrast to the intrinsic ZnO, possess superior advantages, such as tunable band-gaps, large ex- citon binding energy [2], and great improvements in radiative efficiency [3]. Thus, ZnO heterostructures [4,5] or ZnO-based quantum wells (QWs) [6] have been well-designed or actively developed to produce practical optoelectronic devices, such as highly efficient LEDs or LDs in the blue/UV spectrum. Scientists are also interested in carrier dynam- ics [7], acoustic phonon generation [8], and room temperature amplification spontaneous emission (RT-ASE) [2,9–11] from ZnO/ZnMgO MQWs. In previous reports, low-threshold RT-ASE from ZnO bulk, thin film [12,13], and MQWs [9] has been produced based on the exciton–exciton scattering (ex–ex scattering) instead of the electron-hole plasma re- combination. Furthermore, the excitation threshold of ASE and the radiation lifetime of ZnO/ZnMgO MQWs have been investigated in relation to the good thickness [10] and Mg concentration [11]. On the other hand, the aluminum gallium nitride (AlGaN) MQW is a potential device used to produce a deep UV laser for application in high-density optical storage, water purification, and biomedical detection. In the analysis on the effect of a crystal-field split-off hole (CH) and heavy-hole (HH) band crossover on the gain characteristics of AlGaN QW with AlN barriers, a large TM-polarized material gain was achievable for high Al-content, Materials 2022, 15, 6668. https://doi.org/10.3390/ma15196668 https://www.mdpi.com/journal/materials