From almost instantaneous wireless transfer of huge amounts of data and easy detection of explosives, weapons, or harmful gases, to safe 3-D medical imaging and new advances in spectroscopy —technologies based on terahertz (THz) radiation, the electro-magnetic band with wavelengths from 0.1 to 1 mm, can transform science fiction into reality. However, scientists and engineers still do not have cheap and efficient solutions for mass production of THz-based devices. For years, the THz portion of the spectrum remained unused, giving rise to the term “terahertz gap”. Research, recently published in Optics Letters by the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), suggests one possible solution for this problem: a method to increase efficiency of THz emission gallium arsenide (GaAs)-based devices. THz radiation lies between infrared and microwave radiation in the electro-magnetic spectrum. It is absorbed by water —which limits the use of THz devices in the Earth's atmosphere, laden with water vapour, to short distances—but it can penetrate fabrics, paper, cardboard, plastics, wood, and ceramics. Many materials have a unique “fingerprint”in the THz band allowing their easy identification with THz scanners. Moreover, unlike X-rays and ultraviolet light, THz radiation is safe for live tissues and DNA, due to its non-ionising properties. THz technology could be a next important breakthrough in medicine, security, chemistry, and information technology. Generation of THz waves is difficult since the frequency is too high for conventional radio transmitters, but too low for optical transmitters, like the majority of lasers. Therefore, researchers have to come up with new innovative devices. 
Athanasios Margiolakis and Bala Murali Krishna Mariserla, OIST Posdoctoral researcher, perform an experiment in Femtosecond laser laboratory. One of the most frequently used THz emitters is a photoconductive antenna, comprising two electric contacts and a thin film of semiconductor, often GaAs, between them. When the antenna is exposed to a short pulse from a laser, the photons excite electrons in the semiconductor, and a short burst of THz radiation is produced. Thus the energy of the laser beam is transformed into a THz electro-magnetic wave. OIST researchers showed that micro-structure of the semiconductor surface plays an important role in this process ("Ultrafast properties of femtosecond-laser-ablated GaAs and its application to terahertz optoelectronics"). Femtosecond-laser-ablation, in which the material is exposed to ultrashort bursts of high energy, creates micrometre-scale grooves and ripples on the surface of GaAs. “The light gets trapped in these ripples”, says Athanasios Margiolakis, a Special Research Student at OIST. Since more light is absorbed by the ablated material, the efficiency of THz emission, given a sufficiently powerful laser, increases by 65%. Other properties of the material change as well. For example, ablated GaAs shows only a third of the electrical current of non-ablated GaAs. “We observe counter-intuitive phenomena,”the researchers write, "One generally expects that the material showing the higher photocurrent would give the best THz emitter.” They explain this phenomenon by shorter carrier lifetimes. That is, electrons in ablated samples return to non-agitated states much faster than in control samples. Dr Julien Madéo, one of the OIST team members, says that “femtosecond-laser ablation allows us to engineer the properties of materials and to overcome their intrinsic limitations, leading, for example, to near 100% photon absorption as well as broader absorption bandwidth, control of the electron concentration and lifetime”. This technique is a fast, lower-cost alternative to existing methods of manufacturing materials for THz applications.
read more "Laser ablation boosts terahertz emission"
Athanasios Margiolakis and Bala Murali Krishna Mariserla, OIST Posdoctoral researcher, perform an experiment in Femtosecond laser laboratory. One of the most frequently used THz emitters is a photoconductive antenna, comprising two electric contacts and a thin film of semiconductor, often GaAs, between them. When the antenna is exposed to a short pulse from a laser, the photons excite electrons in the semiconductor, and a short burst of THz radiation is produced. Thus the energy of the laser beam is transformed into a THz electro-magnetic wave. OIST researchers showed that micro-structure of the semiconductor surface plays an important role in this process ("Ultrafast properties of femtosecond-laser-ablated GaAs and its application to terahertz optoelectronics"). Femtosecond-laser-ablation, in which the material is exposed to ultrashort bursts of high energy, creates micrometre-scale grooves and ripples on the surface of GaAs. “The light gets trapped in these ripples”, says Athanasios Margiolakis, a Special Research Student at OIST. Since more light is absorbed by the ablated material, the efficiency of THz emission, given a sufficiently powerful laser, increases by 65%. Other properties of the material change as well. For example, ablated GaAs shows only a third of the electrical current of non-ablated GaAs. “We observe counter-intuitive phenomena,”the researchers write, "One generally expects that the material showing the higher photocurrent would give the best THz emitter.” They explain this phenomenon by shorter carrier lifetimes. That is, electrons in ablated samples return to non-agitated states much faster than in control samples. Dr Julien Madéo, one of the OIST team members, says that “femtosecond-laser ablation allows us to engineer the properties of materials and to overcome their intrinsic limitations, leading, for example, to near 100% photon absorption as well as broader absorption bandwidth, control of the electron concentration and lifetime”. This technique is a fast, lower-cost alternative to existing methods of manufacturing materials for THz applications.
Simultaneous atomic-scale AFM (a) and STM (b) images of the (101) surface of anatase titanium dioxide. The parallelograms indicate the same surface area in (a) and (b). The positions of maximum signal (bright spots) in the AFM and STM images clearly differ. By using single water molecules as atomic markers and combining simultaneous AFM and STM measurements with first-principles calculations, the authors demonstrated that the AFM images the first atomic layer of oxygen atoms -pink spheres in the model of the anatase (101) surface depicted in (c)- and the STM images the titanium atoms at the third atomic layer -dark gray spheres in (c). The research team consisting of Oscar Custance and Tomoko Shimizu, group leader and senior scientist, respectively, at the Atomic Force Probe Group, NIMS, Daisuke Fujita and Keisuke Sagisaka, group leader and senior researcher, respectively, at the Surface Characterization Group, NIMS, and scientists at Charles University in the Czech Republic, Autonomous University of Madrid in Spain, and other organizations combined simultaneous atomic force microscopy (AFM) and scanning tunneling microscopy (STM) measurements with first-principles calculations for the unambiguous identification of the atomic species at the most stable surface of the anatase form of titanium dioxide (hereinafter referred to as anatase) and its most common defects. In recent years, anatase has attracted considerable attention, as it has become a pivotal material in devices for photocatalysis and for the conversion of solar energy into electricity. It is extremely challenging to grow large single crystals of anatase, and most of the applications of this material are in the form of nanocrystals. To enhance the catalytic reactivity of anatase and the efficiency of devices for solar energy conversion based on anatase, it is critical to gain in-depth understanding and control of the reactions taking place at the surface of this material down to the atomic level. Only a few research groups worldwide possess the technology to create proper test samples and to make in-situ atomic-level observations of anatase surfaces. In this study, the research team used samples obtained from anatase natural single crystals extracted from naturally occurring anatase rocks. The team characterized the (101) surface of anatase at the atomic level by means of simultaneous AFM and STM. Using single water molecules as atomic markers, the team successfully identified the atomic species of this surface, a result that was additionally confirmed by the comparison of simultaneous AFM and STM measurements with the outcomes of first-principles calculations. In regular STM (in which an atomically sharp probe is scanned over the surface by keeping constant an electrical current flowing between them), it is difficult to stably image anatase surfaces as this material presents poor electrical conductivity over some of the atomic positions of the surface. However, simultaneous operation of AFM and STM allowed imaging the surface with atomic resolution even within the material’s band gap (the region where the flow of current between the probe and the surface is, in principle, prohibited). Here, the detection of inter-atomic forces between the last atom of the atomically sharp probe and the atoms of the surface by AFM was of crucial importance. By regulating the probe-surface distance using AFM, it was possible to image the surface at the atomic scale while collecting STM data over both conductive and nonconductive areas of the surface. By comparing simultaneous AFM and STM measurements with theoretical simulations, the team was able not only to discern which atomic species were contributing to the AFM and the STM images but also to identify the most common defects found at the surface. In the future, based on the information gained from this study, the NIMS research team will conduct research on molecules of technological relevance that adsorb on anatase and characterize these hybrid systems by using simultaneous AFM and STM. Their ultimate goal is to formulate novel approaches for the development of photocatalysts and solar cell materials and devices.
Figure 1: The revolution in superconducting transition temperatures. Superconductivity that occurs via a completely new mechanism was discovered in copper oxide compounds, and the critical temperature—the lowest temperature at which superconductivity is manifested—was raised dramatically. (Image: The University of Tokyo) Tokura began his research into high-temperature superconductors in the latter half of the 1980s when he spent a year abroad at an IBM research center. The prevailing view at the time was that superconductivity was an extreme form of heightened electrical conductivity in metals. However, the state of superconductivity in the copper oxide superconductors that Tokura studied seemed to be an emergent phenomenon in a material that immediately beforehand was an electrical insulator and did not pass any electricity at all. Let’s examine this a little more closely. The strong interactions among electrons in copper oxide compounds mean that the electrons are firmly locked into the atomic lattice structure. The electrons in such a material are described as being “strongly correlated.” In this state, the electrons are pinned and cannot move within the compound. As a result the material acts as an electrical insulator, with properties completely opposite to those of a superconductor. However, when some electrons are pulled out of one of these copper oxide compounds, the electrons arranged systematically in the lattice structure “melt” as one, and the copper oxide insulator immediately becomes a superconductor. Removing just a few percent of the electrons from an insulator changes its physical properties completely. This is precisely what we mean by an emergent phenomenon; one that cannot be explained by reductionist thinking about individual electrons. Colossal magnetoresistance and multiferroics Working his way along the periodic table from titanium to copper, Tokura conducted a series of experiments adding or removing some electrons to or from a variety of compounds of each element. In experiments conducted in the 1990s, Tokura discovered an even more interesting phenomenon, that of colossal magnetoresistance. He found that the electrical resistance of an oxide compound with a structure known as perovskite changes by more than 1,000-fold in the presence of a magnetic field. Apply an electric field to any solid and it becomes electrically charged overall and electrically polarized, with the ends of the material becoming positively and negatively charged. Apply a magnetic field to any solid and it becomes magnetically charged overall and magnetized, with the ends of the material becoming south and north poles. 
Figure 2: Conceptual diagram of a multiferroic material. The material becomes electrically polarized when a magnetic field is applied (left) and magnetized when an electric field is applied (right). (Image: The University of Tokyo) The scientist Pierre Curie, husband of the renowned French physicist Marie Curie, hypothesized the existence of materials that would become electrically polarized in the presence of a magnetic field and magnetized in the presence of an electric field. The direction of magnetization in ferromagnetic materials such as permanent magnets, which can become magnetized in the absence of a magnetic field, and the direction of the polarization in ferroelectric materials, that can become polarized in the absence of an electric field, can each be reversed by tiny magnetic fields or electric charges. If a material is both ferromagnetic and ferroelectric, and if the magnetization and electrical polarization are interrelated, the material of Pierre Curie’s imagination will become a reality. Materials that concurrently have such properties—ferromagnetism, ferroelectricity, ferroelasticity, etc.—are known as multiferroics (figure 2). Altering magnetization by the application of an electric field, non-obvious input-output relationships and the like are examples of emergent phenomena that result when multiple electrons demonstrate as a whole properties beyond those of individual electrons. The skyrmion: a particle generated from multiple electron spins In 2010, a completely new particle known as the skyrmion was observed, identified as an emergent phenomenon of electron spins (figure 3). Tokura is vigorously pursuing this field of emergent matter science. 
Figure 3. Direct observation of skyrmions.In a world-first, Tokura’s research group successfully observed individual skyrmions using a specialized electron microscope. (Image: X.Z. Yu and Y. Tokura) A skyrmion consists of a whirlpool-like collection of thousands of electron spins, but behaves as if it were a single particle. As you might expect, this too is an emergent phenomenon that cannot be explained by reduction to individual electron spins. The skyrmion “particle” can be moved with minimal electrical energy consumption, it can affect the trajectories of electrons as if it were a giant magnetic field, and even has the potential to act like a magnetic monopole. Tokura is not only fascinated by the interesting physical properties of skyrmions, but also thinks about the significant potential for next-generation electronics, saying “In this day and age, experiments with practical applications are vitally important.” A non-dissipative quantum-electrical circuit that operates with negligible energy consumption could be the ultimate environmentally friendly device. Fantastic scenarios abound, but given the time taken for physics to yield innovations to date, Tokura believes that it may take a few decades to centuries for them to become a reality. The future of emergent matter science research Research into emergent matter science has yielded high-temperature superconductors, multiferroics, and skyrmions. Because they are all difficult to visualize in concrete terms, Tokura says “I am used to hearing that emergent matter science is difficult and incomprehensible.” On the other hand, it may look even to researchers in the same field that he is jumping from project to project. “It may appear to lack consistency, but it does have an internal logic that makes sense,” stresses Tokura. “Because it is impossible to understand the world on the basis of a single principle,” Tokura’s motive is to pursue new phenomena that cannot be reduced to a single logic, instead weaving a new logic to explain them. Emergent phenomena are those for which the whole cannot be explained by recourse to the individual parts. Because it is difficult to predict the phenomena that will emerge from the individual parts, phenomena that no one anticipated and innovations in the fields of science and technology may yet remain hidden. Emergent matter science is shaping the future by identifying emergent phenomena in the field of condensed matter physics.