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Background

The 21st century has been called the era of light. From optical sensing (light detection and ranging: LiDAR) enabling automated driving, autonomous robots, and smartphone facial recognition, to lasers enabling smart material processing, to (thermo-)photovoltaics for use in advanced energy production, and even to quantum information processing, the roles played by light are becoming increasingly important.

Our laboratory aims to master the control of this “light” that will illuminate the 21st century. Specifically, we are developing techniques for freely controlling light with “photonic crystals” and “photonic nanostructures.” Through our research, ranging from first principles to real-world applications, we strive to contribute to the realization of many kinds of revolutionary light-based technologies, including sustainable energy, laser material processing and manufacturing, and advanced information and communication technologies, and thereby usher the world into Society 5.0 (proposed by the Japanese government as a future society)

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Research

What are photonic crystals?

Photonic crystals are optical nanostructures possessing a periodic refractive index distribution whose period is on the order of the wavelength of light. Semiconductors with this periodic distribution feature a bandgap of photon energies (wavelengths), analogous to the bandgap of electron energies featured in materials with a periodic distribution of electric potentials. Structures resembling photonic crystals are also found in nature, a good example being the shimmering blue wings of the morpho butterfly, described as a “living gem.” Within the scales of this butterfly is a one-dimensional periodic nanostructure of equally spaced ridges. This nanostructure possesses a photonic bandgap at blue wavelengths; light of these wavelengths cannot propagate within the nanostructure and are thus strongly reflected, giving the wings their marvelous color. Photonic crystals expand upon this principle to higher dimensions, forbidding the propagation of light in as many dimensions as there exists periodicity. In our research described below, we use photonic crystals with this feature to manipulate light.
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LiDAR light source for automated driving and autonomous robots

Interest in automated driving, autonomous robots, and remote sensing is growing worldwide. For these purposes, the development of technologies known as LiDAR (light detection and ranging), whereby objects are detected by spatially scanning a pulsed laser, is being conducted all over the world. Presently, the scanning of a laser can be achieved only by mechanical means, which greatly restricts the maximum operating speed, minimum size, and reliability of LiDAR devices. In our laboratory, we have developed a laser light source that overcomes these problems: a photonic crystal laser – a type of affordable, small-scale semiconductor laser – to which we have added the capability to have its laser beam emitted at an oblique angle to the surface normal; by arranging several such lasers in an array, we can scan a laser beam across a range of angles entirely by electrical means. Recently, we have proposed and experimentally verified the concept of modulated photonic crystals, a new type of photonic crystal capable of producing laser beams over a wide range of angles in two dimensions, and we are now working to integrate an array of these photonic crystals into a chip capable of two-dimensional, wide-angle, electrically-driven beam scanning. In addition, since a single element of this array is also capable of generating a sub-nanosecond laser pulse, we expect that our photonic crystal laser will also lead to the realization of innovative light sources in sensors with, e.g., enhanced ranging resolution.

Photonic crystal laser for smart manufacturing

Smart manufacturing, in which the Internet of things (IoT) and artificial intelligence (AI) are leveraged to quickly and efficiently produce customized products to meet a diversity of needs, is attracting a great deal of interest in recent years. At the heart of this style of manufacturing is the laser. Any laser that can concentrate a large amount of energy into a small space can be expected to play an vital role in many areas of manufacturing, including cutting, micromachining, reforming, and surface treatment; however, only a laser that is not only bright and powerful, but also compact and energy-efficient will enable a “smart” manufacturing style worthy of its name. While ordinary semiconductor lasers may be compact and energy-efficient, they cannot produce high-energy beams before the area to which their energy can be concentrated spreads out due to competition between various oscillatory states. In order to solve this problem, our laboratory is working on the development of a new semiconductor laser that incorporates a photonic crystal to operate stably over a wide area. With this laser, we have recently achieved a high output beam power of 1.5 W with a diffraction-limited spot size, and we are now on the verge of achieving an even higher output power of 10 W, with a comparably high beam quality. At present, we are working toward the design and development of lasers with yet higher output powers, e.g., by visualizing photonic crystal structures using machine learning, as well as higher machining precision, e.g., by generating laser light with short, sub-nanosecond pulses and shorter, blue-violet wavelengths.

Novel green energy technology using photonic crystals

Research on renewable energy is being actively pursued as a means of solving energy and environmental problems currently facing humanity, such as the depletion of fossil fuels and anthropogenic climate change. Attracting a great deal of attention throughout the world as a particularly effective form of renewable energy is solar power, which harvests the inexhaustible energy radiating from the sun. The energy utilization efficiency of solar power has been steadily improving over the course of many years of research, but much more improvement still remains to be seen. In our laboratory, we are helping to crack the problems of energy and the environment through the use of photonic crystals, which can control the behavior of light at the fundamental level. The frequency spectrum of solar light is very wide, and ordinary solar cells can convert only a fraction of it into electricity; we expect that a complete conversion is nevertheless possible by first absorbing all of the solar energy as heat, then re-radiating this heat in a narrow frequency band onto a solar cell. In our laboratory, we are developing narrow-band thermal radiation sources by using photonic crystals to control the properties of thermal radiation, with the aim of achieving ultra-efficient solar power generation. Separately, we have succeeded in combining photonic crystals with thin-film silicon solar cells – reputed for their economy of material and low cost – to increase the absorption of light and thus the light-to-electricity conversion efficiency to values far greater than what is achievable by thin-film silicon alone. Now, we are striving to even further improve solar power generation by making use of near-field light, an endeavor which, in addition to the above, we firmly believe will play a major role in solving the problems of energy and the environment.

Novel thermal light source for environmental and medical sensing

The sensing of various gases and liquids is important for environmental monitoring and health examinations. Such sensing applications use infrared light covering a range of wavelengths within which the substances under investigation possess characteristic absorption peaks known as spectral fingerprints. Until now, this infrared light was generated by simply heating a material until it emits thermal radiation. However, these sources generate a broad spectrum of infrared light, most of which is unnecessary, and thus the energy utilization efficiency of these sources is extremely low. Furthermore, these sources are generally switched on and off by merely raising and lowering their operating temperature, a process which is extremely slow. In our laboratory, we make use of photonic crystals to develop next-generation thermal radiation sources that can efficiently emit light in a narrow range of wavelengths of our choosing and also have their emission intensity switched at high speeds. So far, we have successfully realized the emission of infrared light within a frequency band over 70 times narrower and with a switching speed over 6000 times faster than what is possible with conventional sources, as well as high-speed switching between multiple emission wavelengths. Our work is attracting interest for reducing the size and power consumption of infrared sensors used in environmental, medical, and industrial fields of science.

2D photonic crystals as a platform for light-based quantum information processing

High-speed, stable, and secure communication and information processing using light are two of the most important technical challenges of our time. In our laboratory, we are investigating the application of two-dimensional photonic crystals formed by various materials such as silicon – which is environmentally friendly and highly compatible with electronic devices – and silicon carbide – whose wide electronic band gap permits operation at visible wavelengths – to the development of devices that can manipulate the flow of light. So far, we have successfully realized an optical nanocavity that can strongly confine light into a tiny space surrounded by a photonic crystal, and we have proposed and demonstrated the integration of many such nanocavities onto a small chip for the purposes of optical buffering and wavelength-division multiplexing. With applications to quantum information processing in mind, we have also been striving to achieve external arbitrary control of optical states confined within these chip-integrated nanocavities. In fact, we have already succeeded in the control of optical coupling and decoupling, and recently also transmission, between separate nanocavities with arbitrary timing. We believe that this work, which permits the control of light confined to extremely small spaces, will contribute to the realization of unprecedentedly small-scale, on-chip, light-based quantum information processing devices. Separately, we are performing research into the use of nanocavities for the realization of silicon Raman lasers, as well as various functional silicon-carbide devices, which can be stably operated at visible wavelengths and strong optical intensities owing to the wide band gap of silicon carbide. We are also looking into the use of machine learning to improve the performance of these devices.

Design and visualization of photonic crystals with machine learning

Photonic crystals have the potential to control light in a variety of ways by the appropriate design of their periodic structure; indeed, various functionalities have already been realized. Until now, photonic crystal design has been done manually by the researcher according to physical intuitions derived from personal experience. However, these designs are inevitably sub-optimal, as incorporating in this way every single factor that can affect the photonic crystal characteristics is extremely tedious. In recent years, machine learning has been attracting attention as a method for processing large amounts of data to automatically formulate new knowledge and rules; here we believe it can be applied to transcend the limitations of manual design. As an example of its utility, we have used machine learning to optimize the light confinement properties of a photonic crystal nanocavity, wherein an algorithm was trained to adjust the position of countless lattice points surrounding the nanocavity without direct human intervention, yielding the design of a nanocavity with a light confinement factor greater than ever before. In addition to design, we are also using machine learning to assist in the automated visualization and characterization of fabricated devices. We expect this approach will lead to an even higher degree of light control in the future, including intelligent operation control of photonic crystal devices.

3D photonic crystals for the ultimate control of light

When it comes to manipulating light, one needs look no further than three-dimensional (3D) photonic crystals. With a periodic refractive index in all three dimensions, 3D photonic crystals are capable of completely controlling the flow of light in every single direction. Unfortunately, actually fabricating a 3D structure with wavelength-scale periodicity has proven to be exceptionally difficult. In our laboratory, after many years of effort, we have become able to fabricate 3D photonic crystals with unparalleled high quality, and within these crystals we have successfully observed physically significant phenomena, such as the enhancement and suppression of luminescence. More recently, we have realized new functionalities, such as ultra-compact optical waveguiding in three dimensions and optical confinement in photonic crystal surface states. We continue to develop these fully 3D crystals with simple fabrication techniques involving multi-directional simultaneous etching. Through this research we aim to challenge physical limits, master the control of light, and create new concepts.