Random lasing in uniform perovskite thin films

Lasers represent a key enabling technology that drive many applications e.g. in communications, consumer electronics and healthcare. Their use is ever increasing and there is a great need for lasers that are simple and cheap to manufacture; the ultimate in simplicity would be the ability to produce lasing action from a simple continuous thin film and significant improvements have already been made towards this goal. An excellent example for this development is the construction of self-assembled resonators or the observation of random lasing using thin films of semiconductor nano- and microparticles.

A material that has recently inspired many researchers is the family of metal halide perovskites. These materials exhibit excellent absorption and high open-circuit voltages properties, which is particularly attractive for solar cell applications. Conversely, the light emission properties of these perovskite materials have also been studied, with reports of laser operation across a wide range of wavelengths and using a variety of resonator structures. By controlling the conditions of the solution-process appropriately, however, the crystallisation properties of halide perovskite materials can also be adjusted to make random lasers, that is, lasers capable of manipulating their feedback process by using scattering alone. Such a device would meet the goal of realising a laser in a simple thin film alone.

PhD student Amma Safdar, Dr. Yue Wang and Professor Thomas Krauss from the University of York, Department of Physics in the United Kingdom have now demonstrated a simple and cost-effective technology for creating random lasing from perovskite film materials. The study was based on using methylammonium lead triiodide as an example of a perovskite material. The team investigated the various conditions required for random lasing and was able to explain the different laser operation methods through measurements of the incurred losses and optical gain in the system. The research work is published in the highly respected Photonics journal Optics Express.

The thin films used in the experiment were produced on a glass substrate by wet chemical synthesis and were characterised using X-ray diffraction and scanning electron microscopy.

From the study, the authors successfully demonstrated the simplicity and high performance of their random lasing technology. They achieved a threshold value of about 10µJ/cm², which is remarkably low compared to the threshold values obtained with other laser configurations.

The study uncovered a unique role of the thin film roughness. The authors recognised that the roughness produced its own random lasing actions and hence the cause of the low threshold value and high amplification obtained. Some of the lasing action observed include dual and single mode operation which is unique for random lasing and the standard features of random lasers action, such as line-width narrowing and nonlinear output curve were also observed. The beauty of the approach described by the authors is that determining whether lasing actions or not is simply determined by controlling the film crystallization nature. This simple control mechanism is also useful for optimising the operation characteristics of the devices.

The fact that this random lasing method can be performed at room temperature means it is readily compatible with consumer applications according to the authors, it will help advancing the promising field of perovskite light emitters.