The One-Beam Solution


Giving shape to an ultrafast laser pulse enables new optical methods

(l-r) Prof. Nirit Dudovich, Prof. Yaron Silberberg and Hadas Frostig                                                                                                                                                                                                                               

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 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.
Prof. Nirit Dudovich's research is supproted by the Crown Photonics Center; the Jay Smith and Laura Rapp Laboratory for Research in the Physics of Complex Systems; the Irving I Moskowitz Foundation; the Wolfson Family Charitable Trust; the Jacques and Charlotte Wolf Research Fund; and the European Research Council. Prof. Dudovich is the incumbent of the Robin Chemers Neustein Professorial Chair.
Prof. Yaron Silberberg's research is supported by the Crown Photonics Center, which he heads; and the Wolfson Family Charitable Trust. Prof. Silberberg is the incumbent of the Harry Weinrebe Professorial Chair of Laser Physics.