Abstract
Two different types of lasing modes, vertical Fabry-Perot cavity and random lasing, were observed in ZnO epi-films of different thicknesses grown on Si (111) substrates. Under optical excitation at room temperature by a frequency tripled Nd:YVO₄ laser with wavelength of 355 nm, the lasing thresholds are low due to high crystalline quality of the ZnO epitaxial films, which act as microresonators. For the thick ZnO layer (1,200 nm), its lasing action is originated from the random scattering due to the high density of crack networks developed in the thick ZnO film. However, the low crack density of the thin film (555 nm) fails to provide feedback loops essential for random scattering. Nevertheless, even the lower threshold lasing is achieved by the Fabry-Perot cavity formed by two interfaces of the thin ZnO film. The associated lasing modes of the thin ZnO film can be characterized as the transverse Gaussian modes attributed to the smooth curved surfaces.
Highlights
In the past decades, random lasing (RL) and Fabry-Perot (FP) lasing of semiconductor nanostructures have received extensive investigations by both experiment and theory for their importance and promising applications in solid-state laser
in ZnO epi-films of different thicknesses grown on Si
which act as microresonators
Summary
Random lasing (RL) and Fabry-Perot (FP) lasing of semiconductor nanostructures have received extensive investigations by both experiment and theory for their importance and promising applications in solid-state laser. The nano-sized materials function as both gain mediums and optical resonant cavities without external mirrors for supporting essential feedback. For nano-sized FP optical cavities, the well-defined two-end faces of the gain medium act as two reflecting mirrors and provide the photon feedback required for the realization of lasing. FP lasing had been successfully achieved in many semiconductor nanostructures such as GaN, CdS, and ZnO nanowires [1,2,3,4,5]. RL action is not achieved by an external resonator but by multiple scatterers distributed randomly in a gain medium or which by themselves act as optical amplifiers. The lasing mechanisms of the RL and FP lasing employ the gain material and the self-contained optical cavity via interface scattering or reflection, and the two kinds of lasing actions should happen simultaneously in the nanostructures
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