Holographic Color Prints Combine Phase and Amplitude Control of Light for Better Optical Security

A new optical anticounterfeiting device, called “holographic color prints” by its developers at the Singapore University of Technology and Design (SUTD), provides a dual function to increase security and deter counterfeiting. The device creates images that appear as a regular color print under white light. But under red, green, or blue laser illumination, the device can project up to three different holograms. It is able to modulate both the phase and amplitude of light.

Enhanced optical security provided by a six-color holographic color print. Courtesy of SUTD.

Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature. Microscopic color prints modulate the amplitude, whereas holograms typically modulate the phase of light. This relatively simple structure can be easily imitated, the researchers believe.

To provide its device with dual capabilities, the SUTD team designed a pixel that overlays a structural color element onto a phase plate to control both the phase and amplitude of light, and arrayed these pixels into monolithic prints. Each pixel, strategically arranged on a plane, acts as a “speed bump” to provide the phase control and as a “roadblock” to provide the amplitude control for light. Nanostructured posts of different heights are used as structural colored filters to modulate the amplitude of light. The team developed a computer algorithm that takes multiple images as its input and generates an output file to determine the positions of different phase and colored filter elements.

A holoscopic print was then sculpted with the aid of a nanoscale 3D printer. The team used Luigi Russolo’s painting "Perfume" (1910) as a color print that is viewable under ambient white light. Different thicknesses of polymerized cuboid were used to modulate the phase plates and form three multiplexed holograms, projected as a red thumbprint, a green key, and blue lettering that reads “SECURITY.” All three images were embedded within the single print.

According to the team, the holographic color prints can be readily verified but are challenging to emulate. The new holographic color printing device could provide an enhanced, fresh approach to securing documents and deterring counterfeiters. “The relationship of holograms in combating counterfeiting is analogous to antibiotics against infections,” said professor Joel Yang. “Every so often, new technology is needed to deter counterfeiters as the old-fashioned holograms become easier to copy.”

The prints, which consist of nano-3D-printed polymer structures, could find particular use in optical document security. Information in the prints is encoded only in the surface relief of a single polymeric material, so nanoscale 3D printing of customized masters that could then be mass-manufactured by nanoimprint lithography is possible.

“For the first time, multiple holograms that are color selective are ‘woven’ into a colorful image using advanced nanofabrication techniques,” Yang said. “We are hopeful that these new holographic color prints are user friendly but counterfeiter unfriendly: They are readily verified but challenging to copy, and can provide enhanced security in anticounterfeiting applications.”

The research was published in Nature Communications (https://doi.org/10.1038/s41467-018-07808-4).