Prototype of a Solar-Driven Laser

Solar-pumped lasers convert sunlight into laser beams, which can be used to produce hydrogen. But, these lasers usually require bulky optics and solar-tracking systems, which makes them incon­venient. Now, scientists from Japan present the first fully planar solar-pumped laser. With a thickness of a few milli­meters and great scala­bility, this design break­through may herald the widespread adoption of eco-friendly solar-driven lasers.

Fully planar solar-pumped laser geometry without lens, mirror concentrator and tracking system. (Source: T. Masuda et al., Commun. Phys.)

Since the generation of laser beams requires energy, it is crucial to find responsible sources energy owing to the current environ­mental crisis. Some researchers have therefore delved into solar-driven lasers, which convert solar energy into laser radiation. In spite of some progress in this field, all existing solar-pumped laser devices require large lenses or mirrors to concen­trate as much sunlight as possible into a small area. In fact, the minimum threshold of solar light intensity that can generate a laser in existing devices is approxi­mately 10,000 times that of natural sunlight. In addition to the sheer bulkiness of the telescope-like lenses needed, complex solar-tracking mechanisms are required for these devices. To make things worse, these lasers cannot use diffuse sunlight, meaning that they would not work on cloudy days at all. These serious limi­tations have greatly hindered the adoption of solar-pumped lasers.

Fortunately, in an effort to revolutionize this field, a team of scientists from Toyota Motors and Tokai University, Japan, led by Masamori Endo have recently developed a fully planar solar-pumped laser. This solar-pumped laser does not require complex light-concen­trating systems and – by virtue of being fully planar and having a thickness of only a few milli­meters – can be easily deployed like tradi­tional solar cells. It mainly relies on a nearly flat cylindrical chamber, a lumi­nescent solar collector.

The inside of the chamber is highly reflective and contains a sensitizer. A coiled kilometer-long laser fiber is submerged in this solution, and the top of the chamber is a dichroic mirror. Solar photons, which have a wide range of freq­uencies, enter the chamber through the dichroic mirror and are absorbed by the sensitizer, which re-emits them with a specific frequency through photo­luminescence. These photons are now reflected by the dichroic mirror and are therefore trapped within the chamber. They eventually hit the laser fiber and excite it, collectively producing the required oscil­lations that generate a laser beam.

This approach, which represents a complete overhaul of existing solar-pumped laser designs, was tested experi­mentally. Although the first results were unsatis­factory as the output power of the prototype was too low, Endo and his team knew they were onto something exciting. In a subsequent study they tried to improve upon their initial design by replacing their previous sensi­tizer with cesium lead halide perovskite nano­crystals, which they chose because of their advan­tageous optical properties. By tailoring the composition of the perovskites, they tuned the energy of the photons emitted via photo­luminescence to match the optimal one required to excite the atoms in the laser fiber. This new proto­type device yielded better results and provided relevant insight into the physics at work within the planar solar-pumped laser.

Solar-pumped lasers are especially important because they can be used to produce hydrogen, a clean type of fuel that is expected to change the world in the near future. Excited about their present results and future prospects, Endo remarks, “This is a great step toward our goal, realization of a hydro­gen-society. We continue our efforts to implement our solar-pumped laser as part of a hydrogen-gene­ration platform. We believe that hydrogen is one of the best solutions to the increasing demand for renewable energy.”

Endo also says that a solar-pumped laser without concen­trator optics sounded like a very futuristic concept when he began his research. Now that it is becoming a reality through much effort, various advanced appli­cations are plausible. He concludes, “Direct energy transfers from space to remote areas could be a practical solution for power shortages. Of course, they can also be used as a power supply for outer space explo­ration.” (Source: Tokai U.)

Reference: T. Masuda et al.: All-inorganic cesium lead halide perovskite nanocrystals for solar-pumped laser application, J. Appl. Phys. 127, 243104 (2020); DOI: 10.1063/5.0011945

Link: Endo Lab, Dept. of Physics, Tokai University, Hiratsuka, Japan

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