Indium Tin Oxide Might Be the Material Photonics Has Been Waiting For

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There are plenty of reasons why it’s useful to transfer information through photons or use light particles to carry out tasks within a system or device, speed chief among them. But in order to use photons with even greater dexterity in the future, researchers will need to control the way light behaves as it passes through a material.

One way to do this is by adjusting the material’s refractive index to cause light to travel faster or slower through it. This is a particularly good option for materials that naturally alter their refractive index according to the intensity of light to which they are exposed.

Such materials behave differently depending on whether the light passing through comes from a low-power source or a high-powered laser. These materials are known as optically nonlinear. In the world of photonics, having a higher degree of optical nonlinearity is considered an attractive trait.

Now a team led by Robert Boyd, a physicist at the University of Ottawa and the University of Rochester, has found that a transparent metal called indium tin oxide (ITO), which is often used in touchscreens and on airplane windows, can achieve a particularly high degree of optical nonlinearity—making it a good candidate for future photonics applications.

This flexibility in the refractive index offers a wider range of potential photon speeds and therefore, a greater degree of control over the photon's functions. It could enable researchers to more easily manipulate photons for a wide range of applications, including microscopy and data processing.

Their sample achieved a degree of nonlinearity that beat that of other materials by factors of 100. For example, ITO’s nonlinearity is about 10,000 times larger than carbon disulfide, a popular reference material, and several hundred times larger than gallium arsenide, a compound semiconductor frequently used in light emitting diodes.

“I think this is an exciting and important finding that will undoubtedly impact photonics in general and silicon nanophotonics in particular,” says Sadik Esener, a photonics expert at the University of California San Diego who was not involved in the research.

Some materials also snap quickly back to their original refractive index once photons have passed through, while others linger in their new state. For most applications, it’s helpful if a material can make this adjustment faster rather than slower. In their experiment, ITO recovered in just 360 femtoseconds—a few millionths of a billionth of a second.

“The nonlinearity that we measured was extremely large and extremely fast,” says Israel De Leon, a co-author and electrical engineer at Tecnologico de Monterrey in Mexico.

In addition to measuring nonlinearity, the group also showed that there is a large variation in how the material absorbs light at different intensities, which can also be a useful property for photonics.

The group used a laser to test each of these properties in the indium tin oxide. By positioning their sample in front of the beam, they measured the refractive index and absorption close to the beam’s focus where intensity was highest and further away. They published their work on Thursday in Science.  

The group’s findings also turned a bit of conventional photonics wisdom on its head. For a lot of materials, the nonlinear changes that occur under certain intensities of light are thought to represent a shift of just a small percentage compared to the standard value of the refractive index that the material maintains for most light. However, the nonlinear changes detected by Boyd’s group were 170 percent greater than the value of indium tin oxide’s standard refractive index—far greater than the group expected when they began.

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