Singapore group claims breakthrough in printing 3D photonic crystals…

17 Sep 2024

…while Oak Ridge Lab uses AI to improve analysis of plasma plumes during pulsed laser deposition.

Photonic crystals are materials with repeating internal structures that interact with light in unique ways – such as in opals and the brightly colored shells of certain insects. Although these crystals are made of transparent materials, they exhibit a “photonic bandgap” that blocks light at certain wavelengths and directions.

One type of this effect is a complete photonic bandgap, which blocks light from all directions. This effect can enable precise control of light, opening up possibilities for advances in telecommunications, sensing, and quantum technologies.

Researchers have been working on different methods to create these advanced photonic crystals. While 1D and 2D photonic crystals have been used in various applications, creating 3D photonic crystals with a complete photonic bandgap in the visible range has been challenging due to the need to achieve nanoscale precise control of all three dimensions during fabrication.

But now researchers in Singapore and China have achieved an unprecedented feat. Led by Professor Joel Yang from Singapore University of Technology and Design (SUTD), the team has developed a revolutionary method to print 3D photonic crystals using a customised titanium resin.

The achievement is described in Printing of 3D photonic crystals in titania with complete bandgap across the visible spectrum, published in Nature Nanotechnology.

Complete bandgap across visible range

Unlike in previous attempts, the new method has yielded crystals that are of high resolution, possess a high refractive index, and feature a complete bandgap across the wavelength range of visible light. The developers say that their innovation “holds immense potential for transforming industries”.

Dr. Zhang Wang, SUTD research fellow and first author of the paper, said, “For decades, researchers have been trying to produce photonic crystals that completely block light in the visible range. These crystals will have potential use in the elaborate 3D control of light flow, the behaviour of single-photon emitters, and quantum information processing.”

The SUTD team fabricated their 3D photonic crystal by using two-photon polymerisation lithography (TPL), a technique used in additive manufacturing. Commercially available resins used in TPL printing are made of organic materials that have a low refractive index. This meant that it would be impossible for any printed structure to block the complete spectrum of visible light.

Titanium dioxide, on the other hand, is an inorganic material with a high refractive index. The team first developed a custom-made titanium resin, then printed photonic crystals using standard TPL before heating them in air to remove organic components. The heating process also oxidised the titanium ions within the crystals, turning the ions into titanium dioxide.

“The structure of the crystals shrinks by a factor of six during the heating process, and its pitch can become as small as 180 nm after shrinkage,” said Dr Zhang. The pitch refers to the distance between different layers within the printed crystal. After successfully fabricating the photonic crystals to a high resolution, the team observed a complete photonic bandgap across the visible range in these 3D structures. The customisability inherent to TPL means that the printed crystals can be modified for specific purposes, with defects.

Dr. Zhang said the process holds promise as a versatile platform for fabricating diverse materials—including glass, ceramics, and metals—at the nanoscale. This versatility is expected to create new avenues of exploration as researchers experiment with different materials and nanostructure configurations.

…while AI improves analysis of plasmas during PLD

In another study, published in Nature, scientists from Oak Ridge National Laboratory, Oak Ridge, TN, have developed a deep learning model to analyze high-speed videos of plasma plumes during pulsed laser deposition (PLD).

PLD employs powerful laser pulses to vaporize a target material, creating a plasma plume, which then settles onto a target surface to form ultrathin films. This method is crucial for creating advanced materials used in electronics and energy technologies.

“We have taught AI to do what expert scientists have always done intuitively — assess a plasma plume to check if the color, shape, size and brightness look the same as they did the last time a good sample was made,” said ORNL’s Sumner Harris, the lead author of the study. “This not only automates quality control but also reveals unexpected insights into how these microscopic particles behave during film formation.”

This achievment builds on ORNL’s previous development of an autonomous PLD system, which accelerated materials discovery tenfold, promising to transform materials synthesis monitoring and further streamline the creation of next-generation materials.