Structured light makes circular holes distinguish between left and right

Researchers from Macquarie University have shown that circular dichroism - the differential absorption of a system to left and right circularly polarised light - can be induced in mirror symmetric samples such as circular holes or spheres, so long as structured light is used.

Circular dichroism is a common technique used in biomolecular sciences to distinguish between molecules which are chemically identical but whose atoms are configured in mirror symmetric dispositions. Most of the life on Earth is built from ‘left’ molecules, while ‘right’ molecules sometimes are responsible of some health problems, eg, Alzheimer’s disease.

Writing in the journal Nature Communications, the researchers explained, “Typically, circular dichroism can only be observed in chiral objects. Here we present experimental results showing that a non-chiral sample such as a subwavelength circular nanoaperture can produce giant circular dichroism when a vortex beam is used to excite it.”

The team measured the transmission through very small circular holes (around 10,000 times smaller than a millimetre). Using a typical light beam, the electric field in a right circularly polarised beam rotates clockwise, while the left circular polarisation counterpart will rotate anticlockwise. However, in the vortex beams, the electric field in the left circular polarisation rotated clockwise, but the one in the right circular polarisation did not rotate at all.

Image credit: Xavier Vidal and Alexander Buese.

“The ratios between the transmission of the two polarisations in some holes was almost 100 to one,” said postdoctoral research Dr Xavier Vidal. “This is a case of extreme circular dichroism, and we observed it in a completely mirror symmetric structure, which defied our intuition.”

The team realised that the properties of the structured light, combined with circular polarisation, completely change the electromagnetic structure of light. In this sense, while a circular hole cannot distinguish between the electromagnetic field of a ‘normal’ light beam, which is either right or left circularly polarised, when using a vortex beam, the same hole is very sensitive to the sense of rotation of polarisation.

“What these experiments clearly show is that there is a wealth of information that can be retrieved from the circular dichroism measurements, which are routinely done in chemical laboratories, by cleverly patterning the spatial structure of the light beams used,” said lead researcher Associate Professor Gabriel Molina-Terriza.

Potential applications include optical processors using metallic circuits, extremely sensitive sensors for biomolecular detection and controlling the quantum properties of macroscopic objects.

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