Optics, Chemistry, Materials Sciences Use Light to Manipulate Light

Researchers from the University of Pittsburgh’s Swanson School of Engineering, Harvard University, and McMaster University have revealed a hydrogel that can respond to optical stimuli and modify the stimuli in response.

A convergence of optical, chemical, and materials sciences has revealed a way to use light to control the local dynamic behavior within a material. The illuminated material mimics the ability of the iris and pupil to dynamically respond to incoming light. Once the light enters the sample, the material itself modifies the behavior of the light, trapping it within regions of the sample.

“Until only a decade or so ago, the preferred state for materials was static. If you built something, the preference was that a material be predictable and unchanging,” said Anna Balazs, an author of the study and a professor of chemical and petroleum engineering at the University of Pittsburgh (Pitt). “However, as technology evolves, we are thinking about materials in new ways and how we can exploit their dynamic properties to make them responsive to external stimuli. For example, rather than programming a computer to make a device perform a function, how can we combine chemistry, optics, and materials to mimic biological processes without the need for hard-wired processors and complex algorithms?”

The findings follow Balazs’ continued research with spiropyran (SP)-functionalized hydrogels and the material’s photosensitive chromophores. Although the SP gel resembles gelatin, it is distinctive in its ability to contain beams of light and not disperse them, similar to the way fiber optics passively control light for communication. However, unlike a simple polymer, the water-filled hydrogel reacts to the light and can trap the photons within its molecular structure.

“The chromophore in the hydrogel plays an important role,” Balazs said. “In the absence of light, the gel is swollen and relaxed. But when exposed to light from a laser beam about the width of a human hair, it changes its structure, shrinks, and becomes hydrophobic. This increases the polymer density and changes the hydrogel’s index of refraction and traps the light within regions that are denser than others. When the laser is removed from the source, the gel returns to its normal state. The ability of the light to affect the gel and the gel in turn to affect the propagating light creates a beautiful feedback loop that is unique in synthetic materials.”

The group found that the introduction of a second, parallel beam of light creates a type of communication within the hydrogel. One of the self-trapped beams not only controls a second beam, but it can occur despite significant distance between the two, due to the response of the hydrogel medium. Victor Yashin, a visiting research assistant professor at Pitt and co-author of the study, said that this type of control is possible because of the evolution of materials and not necessarily advances in laser technology.

“The first observation of self-trapping of light occurred in 1964, but with very large, powerful lasers in controlled conditions,” Yashin said. “We can now more easily achieve these behaviors in ambient environments with far less energy, and thus greatly expand the potential use for nonlinear optics in applications.”

The group believes that opto-chemo-mechanical responses present a potential sandbox for exploration into soft robotics, optical computing, and adaptive optics.

“There are few materials designed with a built-in feedback loop,” Balazs said. “The simplicity of the responses provides an exciting way to mimic biological processes such as movement and communication, and open new pathways toward creating devices that aren’t reliant on human control.”

The research was published in the Proceedings of the National Academy of Sciences this month.