Abstract
Total internal reflection is one of the most important phenomena when a propagated wave strikes a medium boundary, which possesses a wide range of applications spanning from optical communication to a fluorescence microscope. It has also been widely used to demonstrate conventional laser actions with resonant cavities. Recently, cavity-free stimulated emission of radiation has attracted great attention in disordered media because of several exciting physical phenomena, ranging from Anderson localization of light to speckle-free imaging. However, unlike conventional laser systems, the total internal reflection has never been implemented in the study of laser actions derived from randomly distributed media. Herein, we demonstrate an ultra-low threshold cavity-free laser system using air bubbles as scattering centers in which the total internal reflection from the surface of air bubbles can greatly reduce the leakage of the scattered beam energy and then enhance light amplification within a coherent closed loop. Our approach provides an excellent alternative for the manipulation of optical energy flow to achieve ultra-low threshold cavity-free laser systems, which should be very useful for the development of high performance optoelectronic devices.
Highlights
The random multiple scattering of light is a ubiquitous phenomenon in nature, for instance, the clouds, sugar, and salt become transparent if only single scattering happens
The interference effect among the multiple scattering in a disorder medium, which may result in an enhanced backscattering and localization of light, has received intense interest.[1−5] As compared with the localization of electrons,[6] the localization of photons provides a unique and fascinating phenomenon, which can induce laser actions without conventional cavity such as Fabry−Perot resonance.[7−11] The resonators of conventional laser devices have to be designed precisely and arduously, while for a cavity-free lasing system, it possesses two amazing features simultaneously, namely, laser-level intensity and angle-free emission, which are mutually exclusive in incandescent bulbs, light-emitting diodes (LEDs), and conventional lasers
The as-grown film consists of semiconductor quantum dots (QDs) as well as air bubbles embedded in the polymer film
Summary
The random multiple scattering of light is a ubiquitous phenomenon in nature, for instance, the clouds, sugar, and salt become transparent if only single scattering happens. To the best of our knowledge, it is the lowest threshold pumping energy density ever reported among all random lasing systems based on the colloidal QDs as the gain media.[34−38] The low threshold value is a result of the formation of coherent closed loops because of multiple scatterings in which the total internal reflection of the emitted light results in almost no energy loss during the multiple scattering processes.
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