Transforming materials with light
New successes in optical engineering could lead to ultrafast light-based computers and more

Date:
December 8, 2021
Source:
California Institute of Technology
Summary:
Researchers have figured out a way to use lasers to alter materials
without the creation of damaging heat.



FULL STORY ========================================================================== Imagine windows that can easily transform into mirrors, or super
high-speed computers that run not on electrons but light. These are just
some of the potential applications that could one day emerge from optical engineering, the practice of using lasers to rapidly and temporarily
change the properties of materials.


========================================================================== "These tools could let you transform the electronic properties of
materials at the flick of a light switch," says Caltech Professor of
Physics David Hsieh.

"But the technologies have been limited by the problem of the lasers
creating too much heat in the materials." In a new study in Nature,
Hsieh and his team, including lead author and graduate student Junyi Shan, report success at using lasers to dramatically sculpt the properties of materials without the production of any excess damaging heat.

"The lasers required for these experiments are very powerful so it's hard
to not heat up and damage the materials," says 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." To get around this the team found a "sweet spot," Shan says,
where the frequency of the laser is fine-tuned in such a way to markedly
change the material's properties without imparting any unwanted heat.

The scientists also say they found an ideal material to demonstrate
this method. The material, a semiconductor called manganese phosphor trisulphide, naturally absorbs only a small amount of light over a
broad range of infrared frequencies. 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 inside the
material. As a result, the material shifted from a highly opaque state,
for certain colors of light, to becoming highly transparent.

Even more critical, the researchers say, is the fact that the process is reversible. When the laser turns off, the material instantly goes back
to its original state completely unscathed. This would not be possible
if the material had absorbed the laser light and heated up, because
it would take a long time for the material to dissipate the heat. The
heat-free manipulation used in the new process is known as "coherent
optical engineering." The method works because the light alters the differences between the energy levels of electrons in the semiconductor
(called band gaps) withoutkicking the electrons themselves into different energy levels, which is what generates heat.

"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," explains Hsieh. "Our laser is vigorously
rocking the energy levels of the material, and that alters the materials' properties, but the electrons stay put." Researchers have previously
theorized how this method would work. For example, in the 1960s, Caltech alumnus Jon H. Shirley (PhD '63), put forth mathematical ideas about
how to solve for electron-energy levels in a material in the presence
of light. Building on this work, Hsieh's Caltech team collaborated
with theorists Mengxing Ye and Leon Balents from UC Santa Barbara
to calculate the expected effects of laser illumination in manganese
phosphor trisulphide.

The theory matched the experiments with "remarkable" accuracy, says Hsieh.

The findings, Hsieh says, mean that other researchers can now potentially
use light to artificially create materials, such as exotic quantum
magnets, which would have been otherwise difficult or even impossible
to create naturally.

"In principle, this method can change optical, magnetic and
many other properties of materials," says Shan. "This is an
alternative way of doing materials science. 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." ========================================================================== Story Source: Materials provided by
California_Institute_of_Technology. Original written by Whitney
Clavin. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jun-Yi Shan, M. Ye, H. Chu, Sungmin Lee, Je-Geun Park, L. Balents,
D.

Hsieh. Giant modulation of optical nonlinearity by Floquet
engineering.

Nature, 2021; 600 (7888): 235 DOI: 10.1038/s41586-021-04051-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211208123351.htm

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