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A simple and elegant pathway to attain a soft tunable 3D photonic crystal with complete photonic band gap

New Delhi :- 

A new pathway to attain soft tunable 3D photonic crystal that can control light in all directions by introducing nanoparticles of appropriate shape and size in the blue phase liquid crystal can pave the way for sophisticated photonic devices.

Photonic crystals like wings of a butterfly, peacock feathers, etc are fascinating optical materials owing to their ability to manipulate light. They are optical analogues of semiconductors with a refractive index contrast along the light propagation direction. Photonic crystals can control light either along a particular direction (incomplete photonic band gap — PBG) or in all directions (complete PBG).

Fabrication of three-dimensional (3D) photonic crystals that can operate in the visible spectrum is challenging due to the nanometer length scales requiring sophisticated techniques.

The blue phases (BPs) exhibited by highly chiral liquid crystals are increasingly being explored as cost-effective alternatives to conventional 3D photonic crystals, as the photonic property is realised by the self-assembly of LC molecules.

However, the refractive index contrast in BPs is very small, and hence they belong to the class of incomplete PBG materials. A 3D photonic crystal with tunable and complete photonic bandgap opens up the possibilities to be applied in sophisticated photonic devices such as lossless optical waveguides utilized for guiding optical signals from one point to another without any significant loss of signal intensity,

The research team from the Centre for Nano and Soft Matter Sciences, Bengaluru, an autonomous institution of Department of Science and Technology (DST) has shown an elegant pathway to achieve complete photonic bandgap in BPs. The simple and cost-effective methodology developed by the team led by Dr Geetha Nair involves ingenious ways of incorporating high refractive index nanoparticles of appropriate shape and size in the blue phase liquid crystal.

The spherical-shaped, Selenium nanoparticles with a high refractive index that get confined inside the defect cores of the BP effectively increased the refractive index contrast. This led to PBG width getting enhanced, a clear indication that the BP is driven towards a complete PBG system. The results were published in       the Journal Nanoscale,

“Extensive FEM (Finite Element Method — a numerical method in electromagnetics) simulations carried out on such systems clearly indicate that getting a complete PBG system depends not just on the magnitude of refractive index contrast, but also on the symmetry of the photonic crystal, or blue phase in this case,” said Nurjahan Khatun, the PhD student who worked on the project. She also mentioned that experiments are underway to realise a room temperature stable and tunable complete PBG blue phase that may find applications in emerging field of photonic integrated circuits.

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Figure: The pristine BP shows a PBG width of 2.2% which increases to 5.6% on addition of high-index Se nanoparticles (Se-doped BP) clearly indicating that BP is driven towards a complete PBG system.