ABSTRACT
In recent years, enormous attention has been paid to optical microresonators, which hold a great promise for microlasers as well as fundamental studies in cavity quantum electrodynamics. Among various kinds of optical microresonators, whispering gallery mode (WGM) cavity is one of the most-interesting configurations because of its intrinsically high quality factor, low lasing threshold, and relatively simple fabrication. In this chapter, we will describe the WGM resonators in ZnO nano/microsturcutres with hexagonal cross section. The content of this chapter are arranged as following. Firstly, a general introduction of the basic properties of ZnO and WGM will be presented. Secondly, the experimental techniques about different methods to synthesize ZnO and knowledge about optical characterizations will be introduced. In the third section, some recent key research findings about the modification of spontaneous emission, different kinds of WGM lasing as well as exciton polariton in hexagonal ZnO will be summarized and
discussed. Finally, the potential device application and future directions of ZnO WGM will be drawn. At the end of the chapter, a selection of references will be given for further reading that penetrate deeper into the topic or give more detail description. 12.1 IntroductionThe development of optical microresonators have attracted extensive research interest [1-4], not only because of their small mode volume and high quality (Q) factor for fundamental physics investigations [5,6], but also for potential applications in such as laser sources, active filters, and photonic sensing devices for label-free detection [7,8]. With the rapid development of nanoscience and nanotechnology, different kinds of structures and fabrication techniques have been adopted to construct optical microresonators. Laser emissions have been observed from resonators with various geometries, such as Fabry-Pérot (F-P) cavity [9,10], photonic crystal [11,12], random cavity [13,14], plasmonic cavity [15,16], distributed feedback resonator [17], as well as whispering gallery modes (WGM) [7,18]. In the case of WGM resonator, the light is trapped inside the cavity by multiple total internal reflections at the interface near the structure and the surrounding medium [19]. Compared to other resonators, WGM exhibits much lower loss and thus is expected to lead to low lasing threshold and high Q lasers.Till date, fabrication of solid-state microresonators is primarily based on either top-down or bottom-up technique [20]. In the top-down approach, a microresonator can be obtained by etching an epitaxially grown planar cavity [9]. This approach critically relies on epitaxial growth, thus requires costly apparatus and sophisticated grow techniques. Meanwhile, the strict requirement on lattice matching between the epitaxial layers and the substrate severely limits material choices. Moreover, this method introduces optical losses at the boundary due to the imperfection of the etching process, leading to lower Q factor of the microresonators. In comparison, for the bottom-up approach, the microcavities are directly formed by self-assembled crystallisation of nanostructures, which shows inherent advantages such as high material quality and high throughput assembly. By controlling the experimental parameters, different shapes and sizes of the microresonators can
be obtained [18,21-23]. Meanwhile, the approach takes advantage of both the radiative recombination in the material as internal light source and the resonator formed by the structure itself.During the past decade, low-dimensional semiconductor structures have attracted extensive research interest and various notable achievements have been achieved [24-30]. Among various kinds of semiconductors, ZnO is recognized as potential building blocks for nanometer-scale electronic and optoelectronic devices operated in the blue and ultraviolet (UV) spectral regions due to its wide band gap (~3.37 eV) and large exciton binding energy (~60 meV) at room temperature [25,31]. Moreover, the richness of nanostructures of ZnO facilitates the realization of various interesting optical confinement and laser cavities. The resonant process in ZnO nano/microstructures can be classified into three different mechanisms: (1) Random lasing from randomly distributed ZnO powders or
nanowires. In the case of random lasing, the light is amplified along the closed-loop feedback paths by the recurrent scattering at the boundaries. Generally speaking, random lasing presents indefinite lasing modes with a very high threshold due to high loss during the random scattering path [14,32]. (2) F-P lasing from a single ZnO structure. In the case of F-P lasing, the mirrors are formed by two end facets. For ZnO, the F-P modes suffer strongly from the mirror losses at the interface between ZnO and air due to a reflectance of about 20%. This is why it is difficult to obtain lasing from F-P cavity in ZnO [33]. (3) WGM lasing from hexagonal ZnO structures. In the case of WGM, light propagates circularly in the cavity due to multiple total internal reflections at the inner walls of ZnO hexagon. Due to a lower loss and better confinement, ZnO WGM cavity is expected to demonstrate a high Q factor as well as a low threshold compared to random cavity and F-P cavity [18,34].The WGM resonate phenomena have been investigated in ZnO nanodisks [21], nanonails [35,36], nano/microwires [37,38], and microdisks [18]. However, it is only until recently that distinct WGM
modes have been identified from individual ZnO microwire laser at low temperature [37]. After that, various kinds of WGM lasing
and potential device application using hexagonal ZnO have been demonstrated. Since this is a rapidly expanding field in which some unique properties have been discovered as well as some potential applications have been achieved, this chapter intends to provide the state-of-the-art research activities about the WGM in ZnO with hexagonal cross section. 12.2 Experimental and Instrumental
MethodologyIn this section, the main experimental means used for ZnO nano/ microstructures synthesis and optical characterization will be briefly described. The principle of the experimental techniques will be introduced. The purpose of this section is to provide a fundamental description of the experimental and instrumental methodology that is necessary for understanding the experimental results to be presented later. 12.2.1 Material GrowthThe most popular approach for ZnO nano/microstructure growth is vapor phase transport (VPT) method [39-42]. Generally speaking, the mechanism of the VPT approach is the introduction of a catalytic liquid alloy phase that can rapidly adsorb a vapor to super saturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid-solid interface. In the ZnO growth process, substrates such as silicon wafer or c-plane sapphire was pre-coated with a ~20 nm Au film. The source materials of the mixture of high purity ZnO (99.99%) and graphite powder (99.99%) with a 1:1 weight ratio were placed at the end of a slender one-end sealed quartz tube. The source was placed in the center of a horizontal tube furnace maintained at around 950-1000°C and the substrate temperature at about 600°C. After growth for 40 min, the ZnO nano/microstructures can be obtained. More detailed description of the growth process can be found in [39,43] and the references therein.FESEM images of ZnO microstructures were shown in Fig. 12.1. The wires or the disks can be scratched from the substrate and isolated on other substrates. This preparation technique enables
one to investigate the optical properties from a single structure. It is interesting to note that most of the nanowires and microdisks have a perfect hexagonal shape and the trace with boundaries are regularly arrayed with rotation angles of 60°, which confirms the high quality of the ZnO structures.
