Physicists create the world’s smallest pixel (so far)

Smart glasses that display information directly in the field of vision are considered a key technology of the future, but until now their use has often failed due to the cumbersome nature of the technology. However, effective light-emitting pixels are excluded by classical optics if their size is reduced to the wavelength of the emitted light.

Today, physicists at the Julius-Maximilians-University of Würzburg (JMU) have taken a decisive step towards bright miniature displays and, using optical antennas, created the world’s smallest pixel to date.

A research group led by professors Jens Pflaum and Bert Hecht was responsible for the work; the group has now published the results of its work in Scientific advances.

A display on one square millimeter

“Using a metal contact that allows current injection into an organic light-emitting diode while simultaneously amplifying and emitting the generated light, we created a pixel for orange light over an area measuring just 300 by 300 nanometers. This pixel is just as bright as a typical OLED pixel with normal dimensions of 5 by 5 micrometers,” explains Hecht, describing the main finding of the study.

To put things into perspective, a nanometer is one millionth of a millimeter. This means that a screen or projector with a resolution of 1920 x 1080 pixels could easily fit into an area of ​​just one square millimeter. This makes it possible, for example, to integrate the screen into the arms of a pair of glasses from where the light generated would be projected onto the lenses.

An OLED consists of several ultra-thin organic layers integrated between two electrodes. When the current passes through this stack, the electrons and holes recombine and electrically excite the organic molecules of the active layer, which then release this energy in the form of light quanta.

Since each pixel shines on its own, no backlight is required, enabling particularly deep blacks, vivid colors and efficient power management for wearable devices in the field of augmented and virtual reality (AR and VR).

Simple miniaturization doesn’t work

A key problem the Würzburg researchers faced in further miniaturizing their pixels was the uneven distribution of currents in these small dimensions.

“As with a lightning rod, simply reducing the size of the established OLED concept would result in currents being emitted primarily from the corners of the antenna,” says Pflaum, describing the physical context. This antenna, made of gold, would have the shape of a cuboid whose edges would measure 300 x 300 x 50 nanometers.

“The resulting electric fields would generate forces so powerful that the gold atoms becoming mobile would gradually transform into the optically active material,” Pflaum continues. These ultrafine structures, also called “filaments”, would then continue to grow until the pixel is destroyed by a short circuit.

Next step: increasing efficiency

The structure, now developed in Würzburg, contains a new, specially manufactured insulating layer above the optical antenna, which leaves only a circular opening with a diameter of 200 nanometers in the center of the antenna. This arrangement blocks currents that would be injected from edges and corners, thus allowing reliable and long-lasting operation of the light-emitting nanodiode.

Under these conditions, the filaments can no longer form. “Even the first nanopixels remained stable for two weeks under ambient conditions,” says Hecht, describing the result.

In the next steps, physicists want to further increase the efficiency from the current level of 1% and extend the color gamut to the RGB spectral range. There will then be virtually no obstacles to a new generation of miniature displays “made in Würzburg”.

With this technology, screens and projectors could become so small in the future that they could be integrated almost invisibly into body-worn devices, from eyeglass frames to contact lenses.