Even though we may “see things differently,” the process of seeing is the same for each of us: Light strikes an object and we then perceive the light waves that are bounced off that object in our direction. This is a description of basic, everyday “linear optics.” A new method (, "Single-beam spectrally controlled two-dimensional Raman spectroscopy") developed at the Weizmann Institute of Science enables researchers to truly see things differently. Their method, which is based on optical phenomena known as “nonlinear optics,” enables them to uncover new and interesting details about the lives of molecules. This method may be particularly useful for learning about the dynamics of the large molecules that operate within biological systems. The method is a type of spectroscopy – the study of properties of materials by the light scattered from them. The spectroscopy method the Weizmann researchers developed is two-dimensional: It measures the response of the molecule to simultaneous illumination with two different wavelengths. Hadas Frostig, a research student in the group of Prof. Yaron Silberberg of the Institute’s Physics of Complex Systems Department, explains that such two-dimensional methods can reveal subtle, dynamic changes in a molecule’s structure: “The structure of a molecule influences how it will interact with other molecules in its environment. For example, the particular way in which a protein is folded determines its function in the human body,” she says. The Weizmann Institute study was not the first to attempt to develop this type of two-dimensional spectroscopic method, which is based on a nonlinear phenomenon called Raman scattering. Raman scattering spectroscopy, originally reported in 1928, involves measuring the difference between the wavelength of light aimed at an object and the wavelength that is returned, or scattered, from an object. Research first conducted in the 1990s led to the development of several Raman-based two-dimensional methods. 
Two-dimensional spectroscopy setup: A unique method of shaping the ultrashort pulses enables them to act as “many beams in one”. Yet there were problems with these methods. For example, they required the use of multiple beams traveling in different directions, and these had to be precisely matched to one another. Stability was thus a serious issue. But one complication seemed to be inherently unsolvable: Two-dimensional Raman spectroscopy returns two signals – one that carries useful information and one that is simply “there.” The second, useless signal was not only hard to separate from the useful one; it was often the stronger of the two. Despite many attempts to weaken or eliminate the second signal, the method remained complex and it eventually fell into disuse. The research group – including Silberberg and Frostig, Prof. Nirit Dudovich of the same department, Dr. Tim Bayer, a former postdoctoral fellow in Silberberg’s group, and Prof. Yonina Eldar of the Technion – Israel Institute of Technology – used a method that is based on extremely short flashes of light. Indeed, they are so short that they are to a minute as a minute is to the age of the Universe. The team’s innovation was to give that tiny fraction of a blip a shape in time. Shaping the pulses enabled them to obtain a two-dimensional measurement with just one laser beam. This single-beam approach addressed two of the problems with previous Raman spectroscopy methods: It circumvented the need to coordinate the different light sources, and having all the light travel in the same path eliminated the useless signal. Silberberg explains that the new method does not require any special filtering of the light, since it makes use of both the light that scatters from the sample and the light that does not scatter. “The light that has not scattered both eliminates the useless signal and helps amplify the one we want to see,” he says. Although this new method is still in its early stages, the scientists are hopeful that it will develop into a system that will shed light on the dynamics of large molecules such as proteins, helping us understand how the structural changes they undergo lead to complex biological processes.
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Stick and slip
What do the sounds of a creaky old hinge and a cello have in common? Both rely on the same kind of friction: two surfaces that alternately stick and slide against one another. This physical phenomenon is called stick-slip and, in the case of the creaky hinge, it is often mitigated by the application of a lubricant between the surfaces. It has long been accepted that such a thin layer of lubrication between sliding surfaces alternates along with the cycles of sticking and slipping; it starts as a solid, turns to liquid in the slipping phase and then back to a solid when the surfaces stick once again. But a recent paper in the ("On the question of whether lubricants fluidize in stick–slip friction") suggests this model is incorrect. The findings arose out of research by the group of Prof. Jacob Klein of the Weizmann Institute’s Materials and Interfaces Department, with the collaboration of Prof. Arie Yeredor of Tel Aviv University. 
The sound of a violin arises from a type of friction known as "stick-slip" The group – including research student Irit Rosenhek-Goldian and associate staff scientist Dr. Nir Kampf, both members of Klein’s lab – tested this assumption in an ideal system: two perfectly smooth surfaces separated by a thin layer of lubricating material. The material in question contains molecules organized in layers, totaling around four to five nanometers in thickness. The idea was that as the surfaces stuck and then slipped, the lubricant molecules would also stick – as a solid – and then liquefy and slip over one another fluidly. The shift from solid to liquid should entail another change: “When a material is solid, it is generally denser than its liquid form,” says Klein. “Thus, when the lubricating material turns liquid, it should physically expand by around ten percent; that is, the thin lubricant layer should expand by around 0.5 nanometers.” But where there should have been a bit more space between the surfaces in the slip stage, there was none. The measurements, which were accurate down to 0.1 nanometers, revealed no change at all in the volume of the lubricant as it went from stick to slip and back again. The conclusion: The lubricant does not change from solid to liquid after all. Since friction and wear account for billions of dollars of loss annually, a better understanding of the basic science underlying lubrication may lead to significant improvements in both biomedical and industrial applications.
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Light-emitting metallic gels
Researchers at MIT have developed a family of materials that can emit light of precisely controlled colors — even pure white light — and whose output can be tuned to respond to a wide variety of external conditions. The materials could find a variety of uses in detecting chemical and biological compounds, or mechanical and thermal conditions. The material, a metallic polymer gel made using rare-earth elements, is described in a paper in the by assistant professor of materials science and engineering Niels Holten-Andersen, postdoc Pangkuan Chen, and graduate students Qiaochu Li and Scott Grindy ("White-Light-Emitting Lanthanide Metallogels with Tunable Luminescence and Reversible Stimuli-Responsive Properties"). 
Luminescent materials produced by the MIT team are shown under ultraviolet light, emitting different colors of light that can be modified by their environmental conditions. (Photo: Tara Fadenrecht) The material, a light-emitting lanthanide metallogel, can be chemically tuned to emit light in response to chemical, mechanical, or thermal stimuli — potentially providing a visible output to indicate the presence of a particular substance or condition. The new material is an example of work with biologically inspired materials, Holten-Andersen explains. “My niche is biomimetics — using nature’s tricks to design bio-inspired polymers,” he says. There are an amazing variety of “really funky” organisms in the oceans, he says, adding: “We’ve barely scratched the surface of trying to understand how they’re put together, from a chemical and mechanical standpoint.” Studying such natural materials, evolved over millions of years to adapt to challenging environmental conditions, “allows us as engineers to derive design principles” that can be applied to other kinds of materials, he adds. Holten-Andersen’s own research has examined a particular kind of crosslinking in the threads mussels use to anchor themselves to rocks, called metal-coordination bonds. These bonds, he adds, also play an important role in many biological functions, such as binding oxygen to hemoglobin in red blood cells. He emphasizes that the idea is not to copy nature, but to understand and apply some of the underlying principles of natural materials; in some cases, these principles can be applied in materials that are simpler in structure and easier to produce than their natural counterparts. In this case, the use of a metal from the lanthanide group, also known as rare-earth elements, combined with a widely used polymer called polyethylene glycol, or PEG, results in a material that produces tunable, multicolored light emissions. The light emission can then reflect very subtle changes in the environment, providing a color-coded output that reveals details of those conditions. “It’s super-sensitive to external parameters,” Holten-Andersen says. “Whatever you do will change the bond dynamics, which will change the color.” So, for example, the materials could be engineered to detect specific pollutants, toxins, or pathogens, with the results instantly visible just through color emission. The materials can also detect mechanical changes, and could be used to detect stresses in mechanical systems, Holten-Andersen says. For example, it’s difficult to measure forces in fluids, he says, but this approach could provide a sensitive means of doing so. The material can be made in a gel, a thin film, or a coating that could be applied to structures, potentially indicating the development of a failure before it happens. Metal-coordination bonds in polymers have been the subject of other work by Holten-Andersen: In a separate paper he published Aug. 31 in the journal Nature Materials, he reported making polymers with tunable mechanical properties, including stiffness. These materials are naturally self-assembling and self-healing, he says, and could be useful as energy-absorbing materials or in biological implants that need to be able to absorb impacts without breaking, he says. “What’s nice here is that the materials change color in response to such a wide and rich set of stimuli,” says Stephen Craig, a professor of chemistry at Duke University who was not involved in this research. He adds, “The fact that the reference state can be made white is quite useful; it’s often easier to detect by eye that something has a faint shade of green, for example, than that it is one shade of green as opposed to another.” Craig sees a variety of potential uses for such materials. “I can imagine using these or similar materials as in situ monitors of a wide range of conditions,” he says, adding that practical deployment of the technology could be facilitated by the fact that “the core hydrogel scaffold used here is so prevalent in studies of both biological and fundamental polymer physics questions.”
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