Lasering in on material changes

Imagine a future where computers run on light and windows can change into mirrors in the blink of an eye. All this may be possible with the right lasers.

It might sound like something from sci-fi, but optical engineering is an expanding field of work that aims to develop lasers that can change the properties of materials on the fly.

“These tools could let you transform the electronic properties of materials at the flick of a light switch,” says David Hsieh, a professor of physics at the California Institute of Technology. “But the technologies have been limited by the problem of the lasers creating too much heat in the materials.”

Lasers that transform matter

Now, Hsieh and colleagues have found a way to make lasers sculpt the properties of materials without any excess damage from laser-made heat.

“The lasers required for these experiments are very powerful, so it’s hard to not heat up and damage the materials,” says lead author Junyi 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 what Shan calls a “sweet spot”, where the frequency of the laser is fine-tuned in such a way that it alters the material’s properties without imparting any unwanted heat.

The researchers found the perfect material on which to test their method – a semiconductor called manganese phosphor trisulphide, which naturally absorbs only a tiny amount of light over a broad range of infrared frequencies.

The team used intense infrared lasers, each lasting just one-tenth of a picosecond (less than a trillionth of a second), to rapidly change the energy of electrons inside the material, forcing it to change from opaque to completely transparent.

If that wasn’t spectacular enough, the whole effect was fully reversable, simply by turning the laser off.

How does it work?

Normally, this instantaneous change would be impossible because the material would heat up too much and it would take a long time for the heat to dissipate and allow the material to revert to opaque.

However, this method works because the light alters the distances between electron energy levels – called band gaps – without knocking the electrons onto other levels themselves. Electrons jumping between levels is what causes heat, so this method prevents too much heat build-up.

“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.” 

The researchers say this shows the great potential of using light to artificially create materials, such as exotic quantum magnets, which would be 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 realise different properties, we can take just one material and ultimately give it a broad range of useful properties.”

The study was published in Nature.