Controlling light at the nanoscale – Fellows’ seminar by Magnus Jonsson

18 April 2023

Harry Potter’s famous invisibility cloak may not be as fictional as once assumed. In fact, it may be achievable. Making materials that change colour, enhancing energy efficient reflective displays and even using the coldness of outer space to replace air-conditioners on earth – these are some of the potential outcomes of the work of the Organic Photonics and Nano-Optics group, headed by Magnus Jonsson at the Laboratory of Organic Electronics, Linköping University, Sweden.

STIAS Fellow Magnus Jonsson during his seminar on11 April 2023.

And it’s all about controlling light and heat at the minuscule nanoscale level. A nanometre equals one billionth of a metre. “So it’s like comparing a tennis ball to the size of the earth” explained Jonsson. “This is all about light which has been important to society since we started making fires and developing light sources. It’s important to control light in various ways. We are using conducting polymers with the aim of controlling light and heat at the nanoscale. We hope to show many application possibilities.”

Optical nanoantennas

Jonsson explained that we have actually used nanostructures as far back as the Roman Empire when stained glass first appeared. Metallic nanoparticles were embedded in the glass to create beautiful colours. The colours appear because light at certain frequencies transforms into plasmons, which are collective charge oscillations in the metallic nanoparticles. So the nanoparticles basically act as antennas for light making them useful in many areas.

A limitation, however, has been that optical nanoantennas made from metals like gold or silver have fixed properties that cannot be tuned after they are made, limiting their use to static functions. Tuneability is about changing the parameters and properties within a structure by applying electric potential.

Enter conducting polymers – organic polymers that can conduct electricity. (Their discovery and development led to the awarding of the 2000 Nobel Prize in Chemistry jointly to Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa). Jonsson’s research group has shown that using conducting polymers enable a new type of dynamically tuneable antennas for light. The optical response of conducting polymer nanoantennas and metasurfaces can be tuned by varying the oxidation state of the polymer, which reversibly switches the material between optically metallic and dielectric.

“Such optical nanoantennas are important for many applications, not least as the building blocks for a next generation of optical components that are flat and ultrathin (optical metasurfaces),” explained Jonsson.

Colourful displays

Turning to colour, Jonsson explained that we know from countless examples in nature that some objects (like bird feathers) present beautiful colours due to their structure – structural colours arise when light is internally reflected inside the material at the nanometre scale and is often referred to as an interference effect. The group has found that conducting polymers (such as PEDOT, poly[3,4-ethylenedioxythiophene]) offer a new way of forming materials with colours that can be controlled by applied electrical potential.

“We have been able to develop materials with such properties by combining electroactive functions of conducting polymers with structural coloration effects in thin films. Such systems are important for applications such as reflective displays in colour,” said Jonsson.

This method could enable manufacturing of thin and lightweight displays with high energy-efficiency for a broad range of applications. So your e-reader might be clearer, in colour, power saving and kinder on the eyes. Even in daylight.

Passive radiative cooling – can we use the coldness of outer space to cool objects on earth?

In our ever-warming world we need more environmentally sustainable options for cooling.

“Cooling (via air-conditioners and fans) is estimated to consume 10% of global electricity usage,” said Jonsson. ‘We need to reduce the need for air-conditioning in the world.”

All objects and materials on earth (including humans) emit heat as infrared light (at much longer wavelengths than the visible range). (This is why thermal cameras can detect people because they are generally warmer than the surrounding objects.)

Jonsson explained that as the earth’s atmosphere is able to transmit light in the infrared wavelength range, coldness in outer space, where the temperature is about –270 degrees Celsius, can be used to remove heat from objects on earth. The massive temperature difference causes a net heat transport out. The temperature of an object can therefore be lowered to below the ambient temperature with the help of passive radiative cooling.

“We’ve known about this since ancient times – it’s been used to make ice in warm climates – but there is now renewed interest to do it during daytime,” he said.

Using a sky simulator as well as wood-based cellulose materials, the researchers have shown that the temperature of a device can be regulated by electrically tuning the extent to which it emits heat. The concept uses a conducting polymer to electrochemically tune the emissivity of the device. “We’ve shown a small yet clear temperature regulation of objects at ambient conditions merely by tuning their ability to radiate heat.”

“It’s about trying to harvest the coldness of space – if possible, using materials that don’t absorb sunlight,” said Jonsson.

“We have managed to demonstrate the electrical tuning in the laboratory but, of course, there’s lots of work still to do to make it work on a practical level.”

He explained that in the long term, it might be possible to create systems that can be placed on a roof, like solar panels, controlling the infrared thermal radiation from the house and cooling when needed. The method requires very little energy consumption and causes minimal pollution. Other applications could include tuneable clothing and wallpaper to control thermal flows and improve thermal comfort indoors at lowered energy consumption.”

He also mentioned that other groups are looking at the possibility of making paint with similar properties which can be used on roofs.

“Such radiative cooling, combined with suppressed solar heating, should definitely be used more,” he added.

“While the three examples in my seminar have in common that they use conducting polymers for tuneability, they relate to different optical phenomena: from resonant excitations in nanostructures to thermal emission from large surfaces. The examples also differ in terms of spectral ranges, together covering the range from the visible to the mid infrared around 10 micrometre wavelengths.”

Generally, work in this field has been used for energy-conversion systems, biosensors for medical diagnosis and drug screening, photo detectors, solar fuels, water purification and biomolecular mapping.

‘Our work is focusing on tuneable organic nanoproperties, tuneable coloration and tuneable radiative cooling,” said Jonsson.

“More exotic optical properties, including invisibility and video holograms, are already possible on a small scale. I believe they will be fully possible in future,” he added.

In addition to being a STIAS fellow, Jonsson is a Wallenberg Academy Fellow, and the Knut and Alice Wallenberg Foundation is a major funder of his group’s work.


Michelle Galloway: Part-time media officer at STIAS
Photograph: Noloyiso Mtembu

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