Laser to change the properties of materials
"These tools could allow transforming the electronic properties of materials with just the touch of a light switch, ”explained David Hsieh, a physics professor at Caltech. "But technologies have been limited by the problem of lasers creating too much heat in materials."
In a new study in Nature, Hsieh and his team, including lead author and graduate student Junyi Shan, report success in using lasers to dramatically sculpt material properties without producing any excess harmful heat. “The lasers needed for these experiments are very powerful, so it's hard not to heat and damage the materials,” says Shan. “On the one hand, we want the material to be subjected to a very intense laser light. On the other hand, we don't want the material to absorb any of that light. "
Scientists also say they have found an ideal material to demonstrate this method. The material, a semiconductor called manganese phosphorus trisulfide, naturally absorbs only a small amount of light over a wide range of infrared frequencies.
Image: Depositphotos For their experiments, Hsieh, Shan and colleagues used intense infrared laser pulses, each lasting about 10-13 seconds, to rapidly change the energy of electrons within the material. As a result, the material went from a highly opaque to a highly transparent state for certain colors of light.
Even more fundamental, according to the researchers, is that the process is reversible. When the laser turns off, the material immediately returns to its original, completely unscathed state. This would not be possible if the material absorbed the laser light and heated it because it would take a long time for the material to dissipate the heat. The heatless manipulation used in the new process is known as "coherent optical engineering."
The method works because light alters the differences between the energy levels of the electrons in the semiconductor (called band gaps) without kicking the electrons themselves in different energy levels, which is what generates heat. “It's like you have a boat, and then a big wave comes and vigorously shakes the boat up and down without dropping any of the passengers,” explains Hsieh. "Our laser is vigorously oscillating the energy levels of the material, and this alters the properties of the materials, but the electrons remain."
The results, according to Hsieh, indicate that other researchers can now potentially use light to artificially create materials, such as exotic quantum magnets, which would otherwise have been difficult or even impossible to create naturally.
