Hydrogel elicits switchable, reversible, and controllable self-trapping light beams

The next generation of optoelectronic and photonic systems — with wide-ranging potential applications in image transmission, light-guiding-light signal processing, logic gates for computing, and medicine — may be realized through the invention of circuitry-free, rapidly reconfigurable systems powered by solitons. Optical spatial solitons are self-trapped optical beams of finite spatial cross section that travel without diverging like freely diffracting beams. These nonlinear waves propagate in photoresponsive materials through self-inscribed waveguides, which are generated when the materials locally change their refractive index in response to light intensity. In conventional nonlinear materials, self-trapping requires high-powered lasers or external electric fields.

Now, a team of researchers from the University of Pittsburgh, Harvard University, and McMaster University have developed a pH-responsive poly(acrylamide-co-acrylic acid) hydrogel, a hydrophilic three-dimensionally connected polymer network, in which light self-trapping can be turned rapidly on and off many times in a controllable and reversible way using a low-intensity visible laser. They reported their work in a recent issue of Proceedings of the National Academy of Sciences.

Developed by Joanna Aizenberg’s group at Harvard University, the hydrogel contains critical covalently-tethered chromophores that absorb specific wavelengths of visible light and thereby transform their structure. In the absence of light, the gel is relaxed and the chromophores are predominantly in a ring-open merocyanine form. When the hydrogel is irradiated with visible light, the isomerization of merocyanine to its closed-ring spiropyran form triggers a local expulsion of water, a contraction of the hydrogel, and ultimately an increase in the refractive index along the irradiated path.

The novelty of this work is that this isomerization process is reversible. In the absence of light, the hydrogel reverts back to its original state.

The researchers demonstrated the reversible self-trapping process with experiments led by Kalaichelvi Saravanamuttu’s team at McMaster University—measuring the diameter and peak intensity of the beam over time using a 532 nm laser, optical lenses, neutral density filters, and a CCD camera. They also performed a series of control experiments, such as testing the hydrogel matrix without chromophores, to determine which parameters are critical for self-trapping.

“We determined it was important to have a hydrogel matrix that became more hydrophobic in the presence of light. It was important to have the chromophores covalently-tethered to the three-dimensional matrix to localize the refractive index change. And photoisomerization was critical in triggering this sequence of events,” says Saravanamuttu, an associate professor of chemistry and chemical biology and a senior author on the paper.

More surprising, when the researchers irradiated the hydrogel with two parallel lasers, the self-trapping beams interacted with each other when separated by distances up to 10 times the beam width. “They modulated each other, reducing their self-trapping efficiency, at remote distances through the interconnected and flexible network of the hydrogel,” Saravanamuttu says.

Being able to reversibly, predictably, and remotely control one self-trapped beam with another opens up the possibility of applications like all-optical computing using beams of ambient light. Traditional computations are performed using hard materials such as wires, semiconductors and photodiodes to couple electronics to light. Instead, the team hopes to control light with light. So far, they have already used the interactions of self-trapped beams to do basic binary arithmetic, says Saravanuamuttu.

These experimental results were confirmed by numerical simulations developed by senior authors Aizenberg, a professor of materials science and of chemistry and chemical biology at Harvard University, and Anna Balazs, a professor of chemical and petroleum engineering at the University of Pittsburgh, and their groups. Their model dynamically calculated the spatial and temporal evolution of the optical field as it propagated through the hydrogel, whose index of refraction was changing. Consistent with experiments, the model accurately captured the self-trapping dynamics and efficiency when using the single or double laser beams.

“This paper marks an interesting step forward that is indicative of the potential of one disruptive technology,” says John Sheridan, a professor of electrical and electronic engineering at the University College of Dublin, who was not involved in the research. “Technologies like this will provide core hardware components enabling the three-dimensional, all-optical connection and switching hardware needed for ‘Internet of things’ data integration and the 5G/6G telecommunications systems of the future.”

Currently, the speed of the waveguide formation and switching happens in seconds, though, rather than the nanoseconds typical of optoelectronic switches. So the researchers plan to investigate what parameters are slowing down the process and how to change them. For example, they will explore making the hydrogel more flexible to give the chromophores greater freedom to undergo isomerization in hopes of eliciting a faster response. They will also look at different types of isomerizable chromophores.

However, Saravanamuttu emphasizes they are not trying to replace digital computers that use conventional electronics, so speed may not be critical. Other potential applications include autonomous stimuli-responsive soft robotic systems for drug delivery or dynamic optics.

“This is particularly exciting because we see it as a material that can reciprocally interact with an environmental stimulus. It isn’t just turned on and off, but it actually changes its behavior in a dynamic way,” she says.

Read the article in Proceedings of the National Academy of Sciences

Figure caption: (a) Schematic illustration of the experimental setup used to probe laser self-trapping due to photoinduced local contraction of the hydrogel. A 532 nm laser beam is focused onto the entrance face of the hydrogel, propagated through the material, and imaged onto a CCD camera. (b) Illustration of beam-induced contraction of the hydrogel when continuously irradiated with a 532 nm laser beam. Credit: Saravanamuttu group, McMaster University, Aizenberg Group, Harvard University, Balazs Group, University of Pittsburgh; PNAS doi.org/10.1073/pnas.1902872117

This is a reposting of my MRS news brief, courtesy of the Materials Research Society.

Floppy vibration modes explain negative thermal expansion in solids

Animation showing how solid crystals of ScF3 shrink upon heating. While the bonds between scandium (green) and fluorine (blue) remain relatively rigid, the fluorine atoms along the sides of the cubic crystals oscillate independently, resulting in a wide range of distances between neighboring fluorine atoms. The higher the temperature, the greater the buckling in the sides of the crystals leading to the overall contraction (negative thermal expansion) effect. Credit: Brookhaven National Laboratory

Matching the thermal expansion values of materials in contact is essential when manufacturing precision tools, engines, and medical devices. For example, a dental filling would cause a toothache if it expanded a different amount than the surrounding tooth when drinking a hot beverage. Fillings are therefore comprised of a composite of materials with positive and negative thermal expansion, creating an overall expansion tailored to the tooth enamel.

The underlying mechanisms of why crystalline materials with negative thermal expansion (NTE) shrink when heated have been a matter of scientific debate. Now, a multi-institutional research team led by Igor Zaliznyak, a physicist at Brookhaven National Laboratory, believes it has the answer.

As recently reported in Science Advances, the scientists measured the distance between atoms in scandium trifluoride powder, a cubic NTE material—at temperatures ranging from 2 K to 1099 K—using total neutron diffraction. The research team determined the probability that two particular atomic species would be found at a given distance. They studied scandium trifluoride because it has a simple atomic structure in which each scandium atom is surrounded by an octahedron of fluorine atoms. According to the prevailing rigid-unit-mode (RUM) theory, each fluorine octahedron should vibrate and move as a rigid unit when heated — but that is not what they observed.

“We found that the distances between scandium and fluorine were pretty rigidly-defined until a temperature of about 700 K, but the distances between the nearest-neighbor fluorines became ill-defined at temperatures above 300 K,” says Zaliznyak. “Their probability distributions became very broad, which is basically a direct manifestation of the fact that the shape of the octahedron is not preserved. If the fluorine octahedral had been rigid, the fluorine-fluorine distance would have been as well defined as scandium-fluorine.”

With the help of high school researcher David Wendt and condensed matter theorist Alexei Tkachenko, Zaliznyak developed a simple model to explain these experimental results. The team went back to the basics—the fundamental laws of physics.

“When we removed the ill-controlled constraint that there must be these rigid units, then we could explain the fundamental interactions that govern the atomic positions in the [ScF3] solid using just Coulomb interactions.”

The team developed a negative thermal expansion model that treats each Sc-F bond as a rigid monomer link and the entire ScF3 crystal structure as a floppy, under-constrained network of freely jointed monomers. Each scandium ion is constrained by rigid bonds in all three directions, whereas each fluorine ion is free to vibrate and displace orthogonally to its Sc-F bonds. This is a direct three-dimensional analogy of the well-established behavior of chainlike polymers. And their simple theory agreed remarkably well with their experiments, accurately predicting the distribution of distances between the nearest-neighbor fluorine pairs for all temperatures where NTE was observed.   

“Basically we figured out how these ceramic materials contract on warming and how to make a simple calculation that describes this phenomenon,” Zaliznyak says.

Angus Wilkinson, an expert on negative thermal expansion materials at the Georgia Institute of Technology who is not involved in the project, agrees that Zaliznyak’s work will change the way people think about negative thermal expansion in solids.

“While the RUM picture of NTE has been questioned for some time, the experimental data in this paper, along with the floppy network (FN) analysis, provide a compelling alternative view,” says Wilkinson. “I very much like the way the FN approach is applicable to both soft matter systems and crystalline materials. The floppy network analysis is novel and gives gratifyingly good agreement with a wide variety of experimental data.”

According to Zaliznyak, the next major step of their work will be to study more complex materials that exhibit NTE behavior now that they know what to look for.

Read the article in Science Advances.

This is a reposting of my news brief, courtesy of MRS Bulletin.