In the segment 'A Look Around Innovation,' we introduce research and development (R&D) sites that have led to social implementation. In the 22nd installment of this series, we introduce Assistant Professor Yutaka Okazaki from the Graduate School of Energy Science at Kyoto University, who has developed a "circularly polarized light conversion film" based on a new concept. The film has potential applications in the real world such as improving the conversion efficiency of plant photosynthesis and solar cells, and 3D liquid crystal displays for telemedicine.
Orange light when UV light is applied — Vibration rotates like a screw thread
JST News visited Okazaki, who conducts research at Kyoto University's Yoshida Campus, where the vast grounds are dotted with the Clock Tower Centennial Hall and the important cultural property Seifu Kaikan, combining history with cutting-edge research. Okazaki presented a small orange film. When ultraviolet light invisible to the naked eye is applied to this film, it glows brightly (Figure 1). The wavelength of the light changes, converting ultraviolet light into light visible to the naked eye. However, the true essence of this film is not merely its wavelength conversion function. By passing general unpolarized light such as sunlight or LED light through this film, it can be converted into circularly polarized light.
Waves can be broadly classified into transverse waves and longitudinal waves. Transverse waves are waves which oscillate perpendicular to the direction of energy propagation, while longitudinal waves oscillate in the same direction as the wave propagation. Light is a type of electromagnetic wave, and electromagnetic waves are transverse waves that propagate with electric and magnetic fields alternately vibrating in directions perpendicular to each other with respect to the direction of propagation. Ordinary light has random vibration directions and is not polarized, so it is called unpolarized light.
In contrast, light with aligned vibration directions is called polarized light. Light that propagates without the vibration plane moving is called linearly polarized light, and light that propagates with the vibration plane rotating like a screw thread is called circularly polarized light. Linearly polarized light is used in various situations such as liquid crystal displays, sunglasses, and filters. Recent research has revealed that circularly polarized light is not only advantageous for accurate transmission of optical information but also accelerates plant growth and improves solar cell conversion efficiency when irradiated.
Okazaki explained the mechanism by which circularly polarized light improves the plant growth rate as follows: "It is said that circularly polarized light improves the efficiency of photosynthesis. Since chlorophyll, which plays an important role in photosynthesis, has left-right asymmetry, right circularly polarized light and left circularly polarized light have different absorption rates. However, I don't think that alone creates enough difference to improve growth rate, so there is thought to be some amplification mechanism there. The fact that we still don't understand it is the interesting part."
Devising a "luminescence-based Convertor": Achieving both polarization purity and light intensity
Previously, three main methods had been proposed as technologies for generating circularly polarized light from unpolarized light: the "filter method," "selective reflection method," and "circularly polarized luminescence method." However, these methods could not achieve both circular polarization purity and light intensity, which are important for circularly polarized light applications. The filter method and selective reflection method can achieve high circular polarization purity but results in low light intensity. Conversely, the circularly polarized luminescence method can achieve high light intensity but results in low circular polarization purity (Figure 2).
Okazaki brilliantly solved this difficult problem of achieving both circular polarization purity and light intensity by focusing on linearly polarized luminescence (LPL), which can achieve both polarization degree and brightness. LPL is a phenomenon where when unpolarized light is applied, linearly polarized light is emitted with a different wavelength from that of the unpolarized light. He devised a "luminescence-based circular polarization converter," combining an LPL film that realizes this LPL with a lambda (λ)/4 retardation film that converts linearly polarized light to circularly polarized light (Figure 3). Although both LPL films and λ/4 retardation films had existed before, combining both to achieve circular polarization purity, light intensity, and wavelength conversion all at a high level was Okazaki's own original idea.
Success in optical information multiplexing through laminating — Applications to security printing also possible
Through three years of JST's A-STEP program, Okazaki succeeded in further evolving the circularly polarized light conversion film. The luminescence-based circular polarization converter is characterized by being able to freely adjust the wavelength and direction of the circularly polarized light obtained by selecting the luminescent material and controlling the angle at which the retardation films are attached. Okazaki developed LPL films incorporating various luminescent materials, including those that are red, blue, and yellow. Additionally, he realized optical information multiplexing based on light addition, which was impossible with existing circularly polarized filters that utilize light absorption characteristics (light subtraction), using a "Multi-layered luminescence-based circular polarization converter" that laminates multiple LPL films. Specifically, this involves simply laminating two types of LPL films showing different emission spectra on a λ/4 retardation film.
From just two types of LPL films, it becomes possible to provide four types of polarization information: 2 waveforms × 2 types of left or right circularly polarized light = 4 types. Furthermore, by using LPL films with different fluorescence lifetimes, he demonstrated that circularly polarized light with time-varying waveforms can be easily generated. Optical information multiplexing can also be considered for applications such as security printing to improve confidentiality.
Regarding his future research goals, Okazaki stated: "If we can increase the growth rate of trees using circularly polarized light, this could lead to promoting the conversion and fixation of carbon dioxide, a greenhouse gas, into substances, and promoting the use of woody biomass, which is a carbon-neutral resource replacing fossil resources. Also, by targeting agricultural crops, this might lead to improving food self-sufficiency rates. Furthermore, since we can use circularly polarized light to save power in 3D liquid crystal displays compatible with stereoscopic vision, there are discussions about applying this to surgery in telemedicine. I hope that the results of this research will lead to accelerating various studies related to circularly polarized light applications." The circularly polarized light conversion film can be said to be an innovative new material with great potential.
(Article: Hideo Ishii, Photography: Hiroshi Matsui)
