In 1970, Kapron et al. reported an optical fiber with a transmission loss of 20 dB/km, and a semiconductor laser continuously oscillating at room temperature was successfully demonstrated. This breakthrough led to the technological development of optical fiber communication in a big way. Optical fibers have since evolved rapidly, and are now indispensable in a wide variety of applications such as telecommunications, remote sensing, medicine, power transmission, etc. In this report, the progress of optical fibers in the last half century is reviewed, and current trends in optical fiber research are introduced.

High-resolution Rutherford backscattering spectroscopy (HR-RBS) is a powerful tool for non-destructive compositional depth profiling of nanometer-scale thin-film samples. It enables composition quantification with sub-nanometer depth resolution. Here, the analysis of insulating glass and ionic liquid solutions of alkali metal salts is presented, to demonstrate the performance of HR-RBS on various sample types.

In order to induce various electronic phase transitions in strongly correlated electron systems, lithium-ion battery-like electrochemical cells based on highly crystalline epitaxial thin films of transition-metal oxides have been developed. The electronic states of band insulators, Mott insulators, heavy-fermion metals, and superconductors are studied in detail from their transport, optical, and structural properties. This method has become a powerful tool for precise electrochemical doping of single samples and in-situ measurement of modulated electronic states. In this review, we present the results obtained in our previous studies, clarify the effectiveness and scope of application of this method, and look forward to its possible application to electrochemical devices.

The creation of a synthetic Rashba field in a Si channel of a Si spin metal-oxide-semiconductor field-effect transistor is introduced and discussed. The Rashba field is attributed to the local electric field at the Si/SiO₂ interface and can be tuned by a gate electric field. An effective magnetic field due to the Rashba field gives rise to spin-lifetime anisotropy in a Si channel, which is detected by the oblique Hanle effect. It is also found that the Rashba field is built-in even at a gate voltage of 0 V. The magnitude of the spin splitting energy is estimated to be 0.6 µeV, which is comparable to that in strained GaAs.

Ternary and multinary compounds used in polycrystalline thin-film solar cells are I–III–VI₂ semiconductors typified by CuInSe₂ (CISe), I₂–II–IV–VI₄ compounds such as Cu₂ZnSnS₄ (CZTS), and I₂–IV–VI₃ compounds such as Cu₂SnS₃ (CTS). Their crystal structures are chalcopyrite type (space group I42d (122)) for CISe, kesterite type (space group I4 (79)) for CZTS, and monoclinic system (space group Cc (9)) for CTS. In this review, we describe the characteristics of their crystal structures and electronic structures such as band-gap energy and energy levels of valence band maxima (VBM) and conduction band minima (CBM). The differences of electronic structures between ZnSe and CuGaSe₂, CuInSe₂ and Cu₂ZnSnSe₄, and Cu₂ZnSnS₄ and Cu₂SnS₃ will be discussed on the basis of the results of theoretical calculations and molecular orbital energy-level diagrams.

Injection needles used in the life sciences can be applied in the production of micrometer-sized ion beams. The needle is called tapered glass capillary optics and plays an important role in MeV-energy ion irradiation, which is difficult with samples in liquid solution due to the short range of the ions. This technique realizes MeV-ion irradiation in liquids or under atmosphere for sample analysis and modification. To understand the ion transmission characteristics, the difference between keV and MeV ion cases is shown by introducing peculiar keV ion features, such as the focusing and guiding effects as well as the delayed transmission phenomenon. Then, the latest application experiments with MeV ion microbeams with glass capillaries are reported.

The development of organic semiconductors (OSCs) with showing a very high layered crystallinity has recently prompted a considerable evolution of printed organic field-effect transistors. Herein, we first illustrate several intriguing aspects of highly layered crystalline OSCs, and discuss the molecular origin of the high layered crystallinity. We then show that it is possible to manufacture highly uniform and ultrathin crystalline semiconductor layers by utilizing the self-organized growth of the crystalline OSC layers through solution processes. Particularly, a unique method allows to produce extremely clean semiconductor interfaces that provide practical device performances with a very sharp and stable switching operation at low voltages. We report the present status of basic studies on the printed electronics technology.

In this review, I will discuss the improvement of photovoltaic parameters, such as short-circuit current density (J_SC), open-circuit voltage (V_OC), and fill factor (FF), in terms of photophysical elementary processes of photovoltaic conversion in polymer solar cells. These elementary processes can be directly observed using time-resolved spectroscopic measurements. Thus, I will introduce the latest research topics, focusing on these spectroscopic analyses. Finally, I will mention future prospects for further improvements in the power conversion efficiency of polymer solar cells.

Perovskite solar cells have recently attracted great interest as next-generation solar cells owing to their potential for application as low-cost, light-weight, and/or flexible solar cells. While their power-conversion efficiency matches that of conventional widely used crystal Si solar cells, their long-term stability and large-area module production are required to be improved for practical use. We found that the addition of a small amount of alkaline metals such as Cs and Rb improves the stability and crystal quality of perovskite films, which facilitates the realization of highly stable and large-area perovskite solar cells. In this study, we review the effect of alkaline metal additives for lead–halide perovskite materials.

We have been developing a novel hydrogen sensor using sintered ceria (CeO_2−σ) nanoparticles in a collaboration with the Japan Aerospace Exploration Agency for application in spacecraft. The sensor can rapidly detect hydrogen in vacuum or oxygen-free conditions. This property originates from the fact that ceria is an oxide ion–electron mixed conductor. In this paper, the adsorption state of gases on the ceria surface is discussed based on the relationship between the gas partial pressure and carrier concentration. In addition, the sensing mechanism is clarified by combining the results of the in-situ X-ray absorption fine structure analysis of the valence change of ceria before and after hydrogen adsorption.

Metallic delafossites ABO₂ (PdCoO₂, PdCrO₂, PdRhO₂, and PtCoO₂) are among the most conductive metals, with electrical conductivity comparable to that of elemental Au. This remarkable conductivity resides in a quasi-two-dimensional layered crystal structure consisting of alternating A⁺ and [BO₂]⁻ layers. In this article, I introduce our recent research on thin-film growth of metallic delafossites. I also briefly overview the physical properties of metallic delafossites, with a focus on the surface/interface phenomena originating from their polar layered crystal structure.

Nanodevices with nanopores and nanogaps can be used to measure a single biomolecule via an electric current. Nanopores thus enable the analysis of a wide range of targets, including bacteria, DNA, proteins, RNA, and viruses. A nanogap is typically the core unit in a single-molecule quantum sequencer, and these are used for the determination of small biomolecules, such as DNA, peptides, and RNA. The application of nanopores and nanogaps has promoted new discoveries in the fields of biology, pharmacy, and medicine. In particular, they have been widely used in the development of innovative methods for the diagnosis of disease and digital platforms to detect and identify a wide variety of biomolecules by optimizing nanostructures for the measurement of specific targets.

Cryogenic-CMOS (cryo-CMOS) technology has attracted considerable attention for realizing large-scale quantum computers. To develop cryo-CMOS circuits, a compact model that reproduces cryogenic MOSFET operations is required. Therefore, understanding the metal–oxide–semiconductor field-effect transistor (MOSFET) performance at cryogenic temperatures is critically important. However, the cryogenic operation of MOSFET is quite different from that at room temperature, and device physics is not fully understood. This study provides an overview of cryo-CMOS technology with a particular focus on device performance and presents future outlooks.

Machine learning is increasingly attracting attention in the research field of novel material development. Among the various material preparation methods, thin-film technology is actively used in various fundamental researches, industries, and applications for preparing thin films of various types of materials and nanostructured materials. In this article, our recent researches concerning novel material development using a combination of thin-film technologies and machine-learning approaches are presented. Our material development methods with the use of machine-learning approaches achieved more efficient material development compared to previous methods and can be used for the exploration of novel thermoelectric materials.

As a new target for photovoltaic (PV) power generation, a vehicle-integrated PV is being promoted. Because the bodies of vehicles are composed of smooth curved surfaces from the viewpoint of aerodynamics and design, solar cells must be applied to curved surfaces. However, all the currently available solar cells are flat, and the most popular Si solar cell is made of a brittle material that is easily broken. Because the vehicle body has a three-dimensional (3D) shape (such as a spherical surface) rather than a two-dimensional shape with unidirectional bending (such as a cylindrical surface), a different development approach is required. This article presents bending tests of solar cells, a mechanical-stress analysis, and prototyping and outdoor tests of 3D curved-surface modules.

Thermoelectric (TE) power generation, a potential clean power technology, has attracted significant attention owing to the recent development of high-performance TE materials. We focused on Mg₂Sn and aimed to improve its TE performance by preparing single crystals (SCs) and introducing lattice defects. In this paper, experimental results on crystal structure analysis, microstructures, and TE properties are presented. It is demonstrated that the TE performance of Mg₂Sn SCs can be superior to that of Mg₂Sn polycrystals via lattice-defect engineering.

Intermetallic compounds can be used as novel catalysts because of their unique electronic structures and ordered surface structures. From Heusler alloys (X₂YZ), novel catalysts can be discovered among many possible sets of X, Y, and Z. The component elements can be substituted by others, in many cases, which enables a fine control of catalysis. We have investigated the catalytic properties of Heusler alloys that were unknown as catalysts. A discovery of good catalysts for selective hydrogenation of alkynes and a systematic control of catalysis were achieved using powder catalysts synthesized metallurgically. The durability and roles of X, Y, and Z were also investigated. Recently, nanoparticle catalysts were successfully synthesized.

In this article, we review our recent studies on the excitonic optical properties of deep-ultraviolet (UV) luminescent AlGaN. The characteristics of optically pumped stimulated emission from AlGaN-based UV-C multiple quantum wells were studied with respect to the temperature. A change in the mechanism of optical gain formation from excitonic transition to degenerated electron–hole plasma was observed with an increase in the temperature. Excitonic stimulated emission was observed up to 450 K, indicating a low threshold carrier density and high thermal stability at room temperature. Furthermore, lasing spectra with fine structures due to the longitudinal cavity mode were clearly observed, confirming excitonic lasing at room temperature.

Semiconductor devices, which have become indispensable in our daily lives, operate based on the movement of charge carriers (electrons and holes) in real and energy spaces. We developed a photoelectron emission microscope (PEEM) with femtosecond laser pulses to evaluate semiconductor materials and devices. In addition to spatial resolution using a PEEM and temporal resolution using laser pulses, the energy resolution is accompanied by continuously varying the photon energy of the laser pulses in the ultraviolet region. As a result, the dynamics of conducting electrons are imaged with a high signal-to-noise ratio. In this paper, we describe the details of the system and present two results on advanced materials.

In the field of quantum electron optics, the quantum states of finite-size wave packets of single electrons should be manipulated. Among the various types of wave packets, a single electron confined in the moving potential of a surface acoustic wave (SAW) serves as the smallest wave packet. In this article, we discuss the transfer of single electrons via SAWs and the control of SAW-driven quantum transport.

NbN is a superconducting material used in single-photon detectors and quantum bits. Because NbN is lattice-matched to the wide-gap semiconductor AlN, it is possible to integrate the functions of nitride semiconductors and superconductors via epitaxial growth. However, the basic properties of NbN thin films epitaxially grown on nitride semiconductors are still unclear. In this study, we show the structural and electrical properties of NbN thin films grown on AlN by sputtering. We also discuss how the difference in the crystal structure between AlN and NbN leads to the formation of NbN twins.

Considering that advanced autonomous robots and autonomous driving systems also have numerous weaknesses and imperfections, why not expose such weaknesses moderately instead of hiding them? In this article, we will discuss how humans and robots may coexist in the future, introducing the concept of “weak robots,” such as Sociable Trash Box that cannot pick up trash by themselves, but do so with the help of children, and conversational robots that sometimes forget important words when telling children old stories.

Scanning probe microscopy has been used to characterize structures and electronic properties of surfaces as well as to construct nanostructures via atom-by atom manipulation. Recent advances in tip-functionalized scanning probe microscopy allow us to measure the inner structures of molecules. This bond-resolved imaging technique is of particular importance in the investigation of precursors and products during on-surface reactions. In this article, tip-induced structural isomerization and additional reactions will be discussed.

In this article, we introduce our studies on the application of chalcopyrite phosphides to solar cells, including bulk crystal growth based on the phase diagram, and thin film deposition through chemical potential diagram. A phase diagram is a useful tool for the fabrication of multi-component materials, while the chemical potential diagram, which is a kind of phase diagram with chemical potentials as axes, is well suited for discussing vapor growth. As another example of using the chemical potential diagram, the stability of a hetero-interface is presented. In addition, bandgap control through order-disorder phenomena is described.

It is mandatory to survey the output performances of photovoltaic (PV) modules to appropriately operate a PV system. This article introduces an outdoor electroluminescence (EL) method. Synchronized operation of the image sensor and injection current involving an appropriate bandpass optical filter allows to obtain a clear EL image even in a high-light-intensity environment. The developed system also provides an accurate estimation of the open-circuit voltage from each PV cell using the EL image.

In recent years, to advance particle beam cancer treatment, there has been fundamental research to understand the biological effects of radiation at the atomic level. Here, we show some recent results of the basic process of biomolecular damage in liquid water caused by ion beams. In the experiments of this research, we used a biomolecular solution target introduced to the vacuum by a liquid molecular beam or microdroplet method. Secondary ion mass spectrometry is applied to measure the fragments of biomolecules emitted from a target irradiated with an ion beam. The experimental and simulation research determined the energy range of secondary electrons involved in damaging biomolecules in liquid water caused by ion beams. The damage process for secondary electrons near the ion track is described.

Direct writing of metal/metal oxide microstructures using femtosecond laser pulse-induced photothermochemical reduction is demonstrated. Two-dimensional/three-dimensional (3D) microstructures were fabricated using linear optical absorption-induced thermochemical reduction of CuO, NiO, and their mixed nanoparticles. Cu-rich and Cu₂O-rich patterns were selectively formed by controlling the degree of reduction of CuO nanoparticles by changing the writing speed. Thermosensors such as a thermistor-type Cu-rich/Cu₂O-rich sensor and thermoelectric-type Cu₂O/NiO (p-type) and Cu–Ni-rich (n-type) sensor are demonstrated. In addition, Cu-based 3D microstructures were fabricated using nonlinear optical absorption-induced thermochemical reduction of Cu₂O nanospheres. The thermochemical reduction was induced around the focal spot inside the nanosphere ink. Such direct writing technique is useful for fabrication of microdevices consisting of various functional materials.

In recent years, research on the application of artificial neural networks to the modeling and simulation of physical phenomena has been attracting significant attention. In addition to modeling phenomena without known governing equations, such research is expected to accelerate and improve physical simulations. In this paper, we first explain Hamiltonian neural networks, which is a representative example of such research. Then, two improved models, the neural symplectic form and DGNet, are explained.

Materials informatics is a fusion field of materials and information sciences, and has recently been attracting attention from both industry and academia because it has the potential to accelerate materials development beyond conventional materials science by incorporating the developments in information science into materials science. In this article, we first consider materials informatics from the historical paradigm, and then describe the expectations and significance of materials informatics. Then, we show that information technology does not solve a narrowly defined objective in materials science only, such as increasing simulation speed, but that concepts that have not existed in materials science so far are being produced through an integration with information science.

For changing and directing cell fate, various external stimuli have been used. Plasma, the fourth state of matter, has become available for those applications by recent technological advancement. In addition to selected cell killing and promotion of cell proliferation, plasma can be used to affect cell differentiation. To elucidate how plasma affects cell fate, a microdevice referred to as Plasma-on-Chip was developed. Cells in liquid medium can be located in the vicinity of gaseous plasma. The microdevice achieves direct plasma exposure of cultured cells by taking advantage of micro air–liquid interface for changing and directing cell fate. Here, results and future prospects of the Plasma-on-Chip are described.

Control and switching of physical properties, such as electronic conductivity, in semiconductors are important technology to develop high performance semiconductor devices. In this review, we introduce a new concept to induce giant electronic conductivity switching driven by direct phase transition from three-dimensional (3D) to 2D structures in (Pb_1−xSn_x)Se, a solid solution of 3D PbSe and 2D SnSe semiconductors. We induced the direct phase boundary between 3D and 2D crystal structures in (Pb_1−xSn_x)Se epitaxial films by using a nonequilibrium growth technique. Reversible giant electronic conductivity change was attained at x∼0.5 originating in the abrupt band structure switch from a gapless Dirac-like state to a semiconducting state. The present idea using crystal structure dimensionality modulation would lead to further functional property switching in semiconductors.