Lasing in low-dimensional ZnO objects: Interrelation between the crystallite morphology and feedback formation mechanism
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ICAL PROPERTIES OF CRYSTALS
Lasing in Low-Dimensional ZnO Objects: Interrelation between the Crystallite Morphology and Feedback Formation Mechanisms L. E. Li and L. N. Demyanets Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiœ pr. 59, Moscow, 119333 Russia e-mail: [email protected] Received November 16, 2007
Abstract—Specific features of UV lasing in low-dimensional zinc oxide materials have been investigated. Two main types of lasers, differing in the feedback formation mechanism—classical random lasers and microlasers—have been selected in active disordered zinc oxide materials. In the first case, feedback is provided by strong light backscattering, and a large number of scattering particles are involved in the cavity formation. In the second case, each crystallite operates as an individual microlaser. The main lasing spectral parameters are reported for the distinguished types of lasers. PACS numbers: 78.55.Et, 78.45.+h, 42.70.Hj DOI: 10.1134/S1063774508040196
INTRODUCTION Zinc oxide, a wide-gap semiconductor of the AIIBVI group, is characterized by a set of unique physical and optical properties, facilitating its use in various fields of modern technology; it has been an object of intense study in the last decade [1–3]. ZnO has a wide range of potential applications, including piezoelectric devices, gas sensors [4], electrodes for solar cells, varistors, lasers, planar field emission displays, superthin flexible TFT screens [5], light-emitting diodes [6, 7], ultrafast scintillators [8, 9], and substrates for GaN [10]. The extremely high exciton binding energy (60, 26, 22, and 40 meV for ZnO, GaN, ZnSe, and ZnS, respectively) makes it possible to obtain high-efficiency UV emission at room temperature, which is related to the direct exciton recombination. It was noted that ZnO is maximally adapted for UV lasing at room temperature [11]. The report about low-threshold effective lasing at room temperature in polycrystalline zinc oxide films (with the microcrystallite size in the range from 50 to 150 nm) [12] caused great interest in the optical and lasing characteristics of disordered zinc oxide materials. Currently, the interest in ZnO is revived and, according to the estimations of [13], more than 2000 papers devoted to this compound are published annually. At the same time, analysis of the literature showed that only about a hundred papers devoted to the lasing in disordered media based on zinc oxide were published from 1996 to 2007; such a small number of publications reflects both the complexity of synthesis of nanomaterials of high optical quality and the specificity of experimental observation of lasing in nanoobjects.
Lasing in a material is determined by two factors: optical gain, which is generally provided by inverted population in the excited material, and feedback. Micro(nano)crystalline ZnO makes it possible to implement simultaneously both of these functions, which are necessary for lasing: effective optical gain and feedback formation. Measurements of the optical
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