In-depth studies have been performed on the response of simple plasmonic nanorods to polarized illumination. A chiral reaction can occur depending on how that polarization is oriented to the nanostructure’s axes of symmetry. This chiral response can be investigated using a second polarizer to regulate the spectrum response of the system.

In a new approach to security that unites technology and art, EPFL researchers have combined silver nanostructures with polarized light to yield an array of brilliant colors, which can be used to encode messages.

Polarized light shining through the nanostructures from specific directions reflects different and easily recognizable colors. Numbers can be assigned to these other colors, which can then be used to represent letters using the electronic communication standard code ASCII.

The researchers used a quaternary code containing the numbers 0, 1, 2 and 3 to encrypt a message (as opposed to the more commonly used binary codes 0 and 1). The process of chromo coding is made as a sequence of four-digit sequences composed of different color combinations that can be used to spell out a message.

Using their system, the color sequence of orange, yellow, red, and white represented the numbers 1, 0, 2, 0, respectively; a series of numbers that in turn code for the letter ‘H’ in the secret test message ‘Hello!’.

top row represents the message "Hello!"
The top row represents the message “Hello!” in quaternary code, which is revealed when the silver nanostructures are illuminated with the correct polarization. The bottom two rows display incorrect sequences when incorrect polarization keys are used. © NAM EPFL

Olivier Martin, who has studied nanostructure optics for many years as head of the Nanophotonics and Metrology Lab EPFL’s School of Engineering, said: “Each color code is not unique, meaning that the same number – 0, 1, 2 or 3 – can represent a different color. This makes the encryption system even more secure because the chance of guessing the correct code sequence is smaller.”

The key to this new method is the unusual response of the silver nanostructures to polarized light. Variations in the length and position of the nanostructures were first used to create the different colors the researchers saw. They were then exposed to polarized light, causing the light waves to oscillate in predetermined directions (vertical, horizontal or diagonal).

The light reflected from the nanostructures ranged in polarization from dull to brilliant, producing strong colors that were then passed through a second polarizer for study.

Martin explains, “To our surprise, the nanostructures showed a so-called chiral response, because they reflected the polarized light in a different direction than the excitation itself. In physics and chemistry, chirality – or the properties of a material that arise from its geometric asymmetry – is an important and well-studied functional aspect of molecules such as proteins. But it was not expected to be seen in the symmetrical silver nanostructures.”

“Chirality is a concept that is often misused and difficult to grasp. The fundamental aspect of chirality in simple geometries as exhibited by our nanostructures is an important finding of this study.”

In addition to encoding messages, this method can also be used to reproduce a painting. To test, scientists produced Picasso’s Mediterranean landscape – at the nanometer scale. They replaced the pixels of a digital reproduction of the painting with their silver nanostructures. As with the chromo encryption method, the artwork was revealed only when light polarized in the correct direction was shone on the “nano-painting”.

That’s what Martin says the combination of nanotechnology with human visual perception of the method has great potential for artistic applications and encryption techniques, such as more secure banknotes.

“Nanomaterials and color are at the intersection of high-tech and artistry, and that really appeals to me. Using nanostructures you can encode a huge amount of information in an extremely small area, so there is the potential for very high information density. At the same time, an approach to encryption that can be read and interpreted by the naked human eye, rather than a computer, could be beneficial.”

Magazine reference:

  1. Wang, H.-C., Martin, OJF, Polarization-controlled chromo encryption. Adv. Optical Matter. 2023, 2202165. DOI: 10.1002/adom.202202165