So far, Modern Sciences has discussed the several uses of lasers mainly within the nuclear energy research sector, such as the use of 192 of them to approach nuclear fusion “ignition,” and explorations into their future in the field using argon fluoride (ArF) lasers. And nuclear energy isn’t alone; several fields are constantly exploring how to utilize laser for the benefit of their research.
One such field is the field of materials science, where changing a material’s properties to adapt it to new use cases is par for the course. This, however, brings scientists to a problem that, at first glance, seems inevitable.
As some would expect, lasers are pretty powerful sources of energy; these lasers stream photons of light—as well as heat. And in the case of material modifications, the extra heat is pretty much unwelcome, as any additional heat imparted into a material can change its resulting properties due to unexpected chemical or physical changes.
This very conundrum was at the heart of some new methodology of using lasers to change material properties, the results of which was published in the journal Nature. The research team, led by California Institute of Technology graduate student Junyi Shan, instead devised a clever workaround to avoid the unwanted heat problem altogether: they instead fine-tuned the laser’s frequency.
“The lasers required for these experiments are very powerful so it’s hard to not heat up and damage the materials,” said Shan. “On the one hand, we want the material to be subjected to very intense laser light. On the other hand, we don’t want the material to absorb any of that light at all.”
According to Shan, this is why they explored the possibility of changing the infrared laser’s frequency, in the hopes of finding a so-called “sweet spot” where they can safely blast a material with lasers without imparting heat onto the target itself—a process they called coherent optical engineering.
The target in question is a material called manganese phosphorus trisulfide (MnPS3), a semiconductor that in its base form absorbs little infrared light. Using the infrared laser, Shan and team proceeded to “flash” the MnPS3 with laser pulse, with each pulse lasting in the range of a measly 10-13 seconds.
In essence, the laser changes the gaps in the energy levels of electrons in the semiconductor without actually kicking them off their place—a process that, according to the team, would have generated excess heat should it have taken place.
These infrared laser flashes caused the MnPS3 to morph from being highly opaque to certain colors of light, to becoming highly transparent. The process is also reversible, as the MnPS3 simply reverts back to its opaque form by simply turning off the laser.
“It’s as if you have a boat, and then a big wave comes along and vigorously rocks the boat up and down without causing any of the passengers to fall down,” said Caltech Professor of Physics David Hsieh. “Our laser is vigorously rocking the energy levels of the material, and that alters the materials’ properties, but the electrons stay put.”
Hsieh continued: “These tools could let you transform the electronic properties of materials at the flick of a light switch, [but] the technologies have been limited by the problem of the lasers creating too much heat in the materials.”
Finally, Shan followed: “In principle, this method can change optical, magnetic and many other properties of materials. […] Rather than making new materials to realize different properties, we can take just one material and ultimately give it a broad range of useful properties.”
References
- California Institute of Technology. (2021, December 8). Transforming materials with light. EurekAlert! https://www.eurekalert.org/news-releases/937064
- Shan, J.-Y., Ye, M., Chu, H., Lee, S., Park, J.-G., Balents, L., & Hsieh, D. (2021). Giant modulation of optical nonlinearity by Floquet engineering. Nature, 600(7888), 235–239. https://doi.org/10.1038/s41586-021-04051-8
