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Self-assembling colloids show new route to photonic crystals

14 Mar 2023

University of Birmingham investigates manufacture of bioinspired structure to manipulate light.

A project at the University of Birmingham has devised a way of fabricating a complex optically active structure usually found in nature, to open up new ways for manipulating and controlling light.

Published in Advanced Materials, the work could provide a new approach to the manufacture of photonic crystals, optical nanostructures with a periodically changing refractive index in which the propagation of photons can be controlled.

Such materials have long been investigated as potential routes to improved performance in thin-film applications and solar cells, optical sensors, or in integrated photonic circuits. But the techniques used to make them can be challenging.

Researchers have investigated a number of manufacturing approaches, such as the one developed in 2020 by a Japanese-Dutch collaboration in which thin photonic crystals were built from stacked arrays of rods in two perpendicular directions, in a semiconductor wafer.

The Birmingham project aimed to show that a structural arrangement called a gyroid, found in nature in places such as the wings of some butterflies and known to create optically active materials, could be produced. In 2017 a project at the University of Surrey investigated the use of 3D ceramic printing to create gyroid structures for photonic materials.

The new study found that complex gyroid structures could, according to calculations, self-assemble from colloidal particles to create nanosale structures, potentially a new route to producing the photonic crystal materials.

Novel photonic materials for light-harvesting and sensing

"This is a new and exciting way to fabricate nanophotonic media with exceptional and tailored chiro-optical properties, with immense control over their properties," commented Angela Demetriadou from the University of Birmingham.

Most work on self-assembling colloidal photonic crystals has focused on creating diamond structures, according to the Birmingham team, with much less attention paid to "single gyroid" arrangements - chiral versions of the gyroid structure able to act as chiral photonic crystals.

The new approach envisages using "patchy spheres," nanoscale colloidal particles with surfaces deliberately patterned in a manner that controls the subsequent self-assembly operation, effectively encoding information about the target structure in the constituent building blocks.

By using patchy spheres which are themselves chiral, a gyroid with chiroptical properties can be assembled, and the procedure could be a more versatile route to self-assembled photonic crystals than using colloidal diamond. Gyroids made this way can "exhibit rich chiropical effects, opening the door for developing novel photonic materials with applications in light-harvesting technology and sensing among others," said the project in its published paper.

"To the best of our knowledge, this is the first report of direct self-assembly of single colloidal gyroid structures from designer building blocks," commented Birmingham's Dwaipayan Chakrabarti. "We hope that our novel approach will stimulate further investigations in the field of colloidal self-assembly, especially experimental efforts to build on this exciting development."

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