To gain even deeper insights into the smallest of worlds, the thresholds of microscopy must be expanded further. Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the TU Dresden, in cooperation with the Freie Universität Berlin, have succeeded in combining two established measurement techniques for the first time: near-field optical microscopy and ultra-fast spectroscopy. Computer-assisted technology developed especially for this purpose combines the advantages of both methods and suppresses unwanted noise. This makes highly precise filming of dynamic processes at the nanometer scale possible. The results were recently published in the research journal ("Optical nanoscopy of transient states in condensed matter"). 
Researchers from the HZDR and TU Dresden initially study a known thin-layer sample made of silicon and germanium using their novel nanoscope. Short Laser pulses excite the electrons in the bright stripes, which are several hundred nanometers wide, whereby the otherwise transparent sample at these locations becomes reflexive. (Image: TU Dresden) Many important but complex processes in the natural and life sciences, like, for example, photosynthesis or high-temperature superconductivity, have yet to be understood. On the one hand, this is because such processes take place on a scale of a millionth of a millimeter (nanometer) and therefore cannot be observed by conventional optical microscopic imaging. On the other hand, researchers must be able to observe precisely very rapid changes in individual stages to better understand the highly complex dynamics. The development of high-resolution temporal and spatial technologies has therefore been promoted for decades. The new camera from Dresden combines the advantages of two worlds: microscopy and ultra-fast spectroscopy. It enables unaltered optical measurements of extremely small, dynamic changes in biological, chemical or physical processes. The instrument is compact in size and can be used for spectroscopic studies in a large area of the electromagnetic spectrum. Time increments from a few quadrillionths of a second (femtoseconds) up to the second range can be selected for individual images. “This makes our nanoscope suitable for viewing ultra-fast physical processes as well as for biological process, which are often very slow,” says the HZDR’s Dr. Michael Gensch. Combining two methods guarantees high spatial and temporal resolution The nanoscope is based on the further development of near-field microscopy, in which laser light is irradiated on an ultra-thin metal point. This creates highly bundled light – a hundred times smaller than the wavelength of light, which otherwise represents the limit of "normal” optics with lenses and mirrors. “In principle, we can use the entire wavelength spectrum of near-field microscopy, from ultraviolet to the terahertz range,” says Dr. Susanne Kehr from the TU Dresden. “The focused light delivers energy to the sample, creating a special interaction between the point and the sample in what is known as the near-field. By observing the back-scattered portion of the laser light, one can achieve a spatial resolution in the order of the near-field magnitude, that is, in the nanometer range.” This technology, known as SNOM (Scanning Near-Field Optical Microscopy), is typically only utilized for imaging static conditions. Using ultra-fast spectroscopy is the crucial tool, on the other hand, enabling scientists to study dynamic processes on short timescales and with extreme sensitivity. The spatial resolution has, until now, been limited to the micrometer range however. The principle in such pump-probe experiments that function, for example, with light, pressure or electric field pulses is as follows: while a first pulse excites the sample under study, a second pulse monitors the change in the sample. If the time between them is varied, snapshots can be taken at different times, and a movie can be assembled. A clever correction of the measurement errors leads to the high sensitivity of the spectroscopic procedure. Activation by an excitation pulse means a type of disturbance for the entire sample system, which needs to be filtered out so that noise or the “background” is eliminated. This is achieved by probing the unperturbed sample with a second reference pulse directly before the excitation. This particular technology could not be combined with near-field optical microscopy until now. For the first time, the teams led by the two Dresden physicists have managed to combine all the advantages of both methods in their nanoscope. “We have developed software with a special demodulation technology with which—in addition to the outstanding resolution of near-field optical microscopy that is at least three orders of magnitude better than the resolution of common ultra-fast spectroscopy—we can now also measure dynamic changes in the sample with high sensitivity,” explains Kehr. The clever electronic method enables the nanoscope to exclusively record only the changes actually occurring in the sample's properties due to the excitation. Although other research groups have only recently reported good temporal resolution with their nanoscopes, they could not, however, obtain this important correction mode. An additional advantage to the Dresden solution is that it can easily be integrated into existing near-field microscopes. Universal in every respect “With our nanoscope’s considerable wavelength coverage, dynamic processes can be studied with the best suited wavelengths for the specific process under study. This is an important step in understanding these processes. Our colleagues at the Freie Universität Berlin have, for example, the ambitious dream of tracking structural changes during the photocycle of an individual membrane protein at specific wavelengthes in the infrared spectrum,” Gensch says. Together with his TU colleague, Susanne Kehr, he demonstrated the new method on a known sample system, a semi-conducting layer made of silicon and germanium. “Had we used an unknown sample for the demonstration, we would not have been in the position to correctly interpret the functionality of our approach,” Kehr stresses. The Dresden nanoscope is universally adaptable to respective scientific questions. The probe pulse wavelengths can, in principle, reach from the low terahertz range to the ultraviolet range. The sample can be stimulated with laser, pressure, electric field or magnetic field pulses. The principle was tested at the HZDR on a typical laboratory laser as well as on the free-electron laser FELBE. First tests on the new terahertz source TELBE, which provides extremely short electric and magnetic field pulses for excitation, are in preparation. “In the future, we will not only see how quickly a process occurs, but we can also better localize where exactly it takes place in the sample. This is especially important for our TELBE facility, which will be in operation next year,” explains Michael Gensch, head of the TELBE project at the HZDR.
read more "Camera for the nano-cosmos"
Researchers from the HZDR and TU Dresden initially study a known thin-layer sample made of silicon and germanium using their novel nanoscope. Short Laser pulses excite the electrons in the bright stripes, which are several hundred nanometers wide, whereby the otherwise transparent sample at these locations becomes reflexive. (Image: TU Dresden) Many important but complex processes in the natural and life sciences, like, for example, photosynthesis or high-temperature superconductivity, have yet to be understood. On the one hand, this is because such processes take place on a scale of a millionth of a millimeter (nanometer) and therefore cannot be observed by conventional optical microscopic imaging. On the other hand, researchers must be able to observe precisely very rapid changes in individual stages to better understand the highly complex dynamics. The development of high-resolution temporal and spatial technologies has therefore been promoted for decades. The new camera from Dresden combines the advantages of two worlds: microscopy and ultra-fast spectroscopy. It enables unaltered optical measurements of extremely small, dynamic changes in biological, chemical or physical processes. The instrument is compact in size and can be used for spectroscopic studies in a large area of the electromagnetic spectrum. Time increments from a few quadrillionths of a second (femtoseconds) up to the second range can be selected for individual images. “This makes our nanoscope suitable for viewing ultra-fast physical processes as well as for biological process, which are often very slow,” says the HZDR’s Dr. Michael Gensch. Combining two methods guarantees high spatial and temporal resolution The nanoscope is based on the further development of near-field microscopy, in which laser light is irradiated on an ultra-thin metal point. This creates highly bundled light – a hundred times smaller than the wavelength of light, which otherwise represents the limit of "normal” optics with lenses and mirrors. “In principle, we can use the entire wavelength spectrum of near-field microscopy, from ultraviolet to the terahertz range,” says Dr. Susanne Kehr from the TU Dresden. “The focused light delivers energy to the sample, creating a special interaction between the point and the sample in what is known as the near-field. By observing the back-scattered portion of the laser light, one can achieve a spatial resolution in the order of the near-field magnitude, that is, in the nanometer range.” This technology, known as SNOM (Scanning Near-Field Optical Microscopy), is typically only utilized for imaging static conditions. Using ultra-fast spectroscopy is the crucial tool, on the other hand, enabling scientists to study dynamic processes on short timescales and with extreme sensitivity. The spatial resolution has, until now, been limited to the micrometer range however. The principle in such pump-probe experiments that function, for example, with light, pressure or electric field pulses is as follows: while a first pulse excites the sample under study, a second pulse monitors the change in the sample. If the time between them is varied, snapshots can be taken at different times, and a movie can be assembled. A clever correction of the measurement errors leads to the high sensitivity of the spectroscopic procedure. Activation by an excitation pulse means a type of disturbance for the entire sample system, which needs to be filtered out so that noise or the “background” is eliminated. This is achieved by probing the unperturbed sample with a second reference pulse directly before the excitation. This particular technology could not be combined with near-field optical microscopy until now. For the first time, the teams led by the two Dresden physicists have managed to combine all the advantages of both methods in their nanoscope. “We have developed software with a special demodulation technology with which—in addition to the outstanding resolution of near-field optical microscopy that is at least three orders of magnitude better than the resolution of common ultra-fast spectroscopy—we can now also measure dynamic changes in the sample with high sensitivity,” explains Kehr. The clever electronic method enables the nanoscope to exclusively record only the changes actually occurring in the sample's properties due to the excitation. Although other research groups have only recently reported good temporal resolution with their nanoscopes, they could not, however, obtain this important correction mode. An additional advantage to the Dresden solution is that it can easily be integrated into existing near-field microscopes. Universal in every respect “With our nanoscope’s considerable wavelength coverage, dynamic processes can be studied with the best suited wavelengths for the specific process under study. This is an important step in understanding these processes. Our colleagues at the Freie Universität Berlin have, for example, the ambitious dream of tracking structural changes during the photocycle of an individual membrane protein at specific wavelengthes in the infrared spectrum,” Gensch says. Together with his TU colleague, Susanne Kehr, he demonstrated the new method on a known sample system, a semi-conducting layer made of silicon and germanium. “Had we used an unknown sample for the demonstration, we would not have been in the position to correctly interpret the functionality of our approach,” Kehr stresses. The Dresden nanoscope is universally adaptable to respective scientific questions. The probe pulse wavelengths can, in principle, reach from the low terahertz range to the ultraviolet range. The sample can be stimulated with laser, pressure, electric field or magnetic field pulses. The principle was tested at the HZDR on a typical laboratory laser as well as on the free-electron laser FELBE. First tests on the new terahertz source TELBE, which provides extremely short electric and magnetic field pulses for excitation, are in preparation. “In the future, we will not only see how quickly a process occurs, but we can also better localize where exactly it takes place in the sample. This is especially important for our TELBE facility, which will be in operation next year,” explains Michael Gensch, head of the TELBE project at the HZDR.
A wave penetrates a material: usually this leads to wave interference, to darker and brighter areas. Obstacles Change the Wave When a light wave travels through free space, its intensity can be the same everywhere. But as soon as it hits an obstacle, the wave is diffracted. At some points in space, the wave becomes brighter, in other places it becomes darker than it would have been without hitting the object. This is the reason we can see objects that do not emit light by themselves. In recent years, however, experiments have been carried out with new materials which have the ability to modify light in a special way: they can locally amplify light, similar to a laser, or absorb light, like sunglasses do. “When such processes are possible, we have to employ a mathematical description of the light wave which is quite different from the one we use for normal, transparent materials,” says Professor Stefan Rotter (TU Wien). “In this case we speak of non-hermitian media.” New Solutions for the Wave Equation Konstantinos Makris and Stefan Rotter from TU Wien, together with Ziad Musslimani and Demetrios Christodoulides from Florida (USA), discovered that this alternative description allows new kinds of solutions for the wave equation. 
Specially designed non-hermitian materials remain completely unperturbed. “The result is a light wave with the same brightness at each point in space, just like a wave in free space, even though it travels through a complex, highly structured material”, says Konstantinos Makris. “In some sense, the material is completely invisible to the wave, even though the light passes through the material and interacts with it.” The new concept is reminiscent of so-called “meta-materials”, which have been created in recent years. These materials have a special structure, which allows them to diffract light in unusual ways. In certain cases the structure can bend the light around the object, so that the object becomes invisible, much like Harry Potter’s invisibility cloak. “The principle of our non-hermitian materials, however, is quite different”, says Stefan Rotter. “The light wave is not bent around the object, but fully penetrates it. The way the material influences the wave is, however, fully cancelled by a carefully tuned interplay of amplification and absorption.” In the end, the light wave is exactly as bright as it would have been without the object – at each and every point in space. Several technical problems still have to be solved until such materials can be routinely fabricated, but scientists are already working on that. The theoretical work now published, however, shows that besides meta-materials there is another, extremely promising way to manipulate waves in unconventional ways. “With our work we have opened a door, behind which we expect to find a multitude of exciting new insights”, says Konstantinos Makris.
This is an ultra-soft elastomer fabricated by crosslinking bottlebrush polymers contains only crosslinks (red chains) and no entanglements. (Image courtesy of Li-Heng Cai, Harvard SEAS) The material is featured on the cover of the journal ("Soft Poly(dimethylsiloxane) Elastomers from Architecture-Driven Entanglement Free Design"). "Conventional elastomers are intrinsically stiff because of how they are made," said lead author Li-Heng Cai, a postdoctoral fellow at SEAS. "The network strands are very long and are entangled, similar to a bunch of Christmas lights, in which the cords are entangled and form knots. These fixed entanglements set up an intrinsic lower limit for the softness of conventional elastomers." In order to fabricate a soft elastomer, the team needed to eliminate the entanglements from the beginning by developing a new type of polymer that was fatter and less prone to entanglement than linear polymers. The polymers, nicknamed bottlebrushes, are easily synthesized by mixing three types of commercially available linear silicone polymers. "Typically the fabrication of such bottlebrush molecules requires complex chemical synthesis," said co-first author Thomas E. Kodger, Ph.D.' 2015, now a postdoctoral fellow at University of Amsterdam. "But we found a very simple strategy by carefully designing the chemistry. This system creates soft elastomers as easily as silicone kits sold commercially." The softness of the elastomers can be precisely controlled by adjusting the amount of cross-linked polymers to mimic everything from soft brain tissue and relatively stiff cells. "If there are no crosslinks, all the bottlebrush molecules are mobile and the material will flow like a viscous liquid such as honey," said Cai. "Adding crosslinks connects the bottlebrush molecules and solidifies the liquid, increasing the material stiffness." In addition to controlling the softness, the team also found a way to independently control the liquid-like behavior of the elastomer. "To make the conventional elastomer softer, one needs to swell it in a liquid," said coauthor Michael Rubinstein, John P. Barker Distinguished Professor in Chemistry at the University of North Carolina at Chapel Hill. "But now we can adjust the length of 'hairy' polymers on the bottlebrush molecules to tune the liquid-like behavior of soft elastomers -- without swelling -- allowing us to make these elastomers exceptionally non-adhesive yet ultra-soft." These qualities make the material not only ideal for medical devices, such as implants, but also for commercial products such as cosmetics. "The independent control over both softness and liquid-like behavior of the soft elastomers will also enable us to answer fundamental questions in biomedical research," said Weitz. "For example, stem cell differentiation not only depends on the softness of materials with which they are in contact, but recent findings suggest that it is also affected by how liquid-like the materials are. This discovery will provide entirely new materials to study the cell behavior on soft substrates." "The exceptional combination of softness and negligible adhesiveness will greatly broaden the application of silicon-based elastomers in both industry and research," said Weitz.