Proceedings - Viện Vật lý

_________________________________________________________________________________Những tiến bộ trong Quang học, Quang tử, Quang phổ và Ứng dụng. 8/2006, Cần Thơ, Việt NamRANDOM LASER IN ZNO POWDERNguyen The Binh, Nguyen Van Hao, Nguyen Thi Hang,Duong Vu Truong, Pham Van ThinHanoi University of Science, VNU, 334 Nguyen Trai Road, Thanh Xuan , HanoiAbstract: The microlasers have potential applications in integrated photoniccircuits. The fabrication of the conventional microlasers including microdisk laser,microsphere laser…requires expensive crystal growth and micro-fabricationfacilities. The demonstration of random lasers opens up the possibility of usingdisordered medium as alternative sources of microlasers. The physical mechanismof optical confinement is based on the Anderson localization of light in a randommedium of micrometer-scale. An active random medium can be obtained in formsof thin film, powder or cluster. We studied to use ZnO powder as an amplifyingrandom medium. The stimulated emission and lasing from ZnO powder pumpedby Nitrogen laser was observed. The results and discussions will be represented inthis report.1. IntroductionIn recent years, there have been substantial efforts to study random lasers inform of semiconductor thin films, clusters and powders. It has been demonstratedthat in a disordered medium, strong and recurrent scattering helps in formingclosed-loop-feed back paths which results in lasing instead of traditional laser welldefinedcavities. If the amplification along such a loop path exceeds the loss, lasersoscillation could occur in the loop which serves as a laser resonator. Therequirement of the phase shift along the loop being equal a multiple of 2determines the oscillation frequencies.[1,2] The main requirement to observe thisrandom laser is that the scattering mean free path should be equal or less than theemission wavelength. The photon localization in this case is similar to Andersonlocalization of electron in disordered systems. The key condition to realize arandom laser is the simultaneous realization of strong scattering and high gain.Among several semiconductor materials, ZnO is a wide-gap material which hasattracted great interest because of its potential to produce efficient blue and ultraviolet-light-emittinglasers. Optically pumped stimulated emission (SE) has beenobserved by many researchers from ZnO epitaxial layers grown by a variety ofmethods. The demonstration of SE with excitonic origin paves the way for therealization of blue-violet laser diodes based on ZnO. Excitonic emission may berealized for ZnO due to its large exciton binding energy (~ 60 meV). Excitonexcitonscattering-induced stimulated emission is very important for the realizationof low- threshold lasers [1,5,6]. ZnO material has several helpful properties forlasing in blue-violet region such as band gap of 3.3eV, tunable from 2.8 to3.3eV298