Research results 08｜Research Center for Materials Nanoarchitectonics

Photothermal energy conversion with novel plasmonics and metamaterials

T. Nagao , S. Ishii
Co-workers: T. D. Dao , T. Yokoyama

We developed a new class of nanomaterials/nanodevices with high photothermal conversion efficiency based on plasmonics and metamaterials. A new methodology for choosing appropriate compound materials was adopted to permit full-solar spectrum absorption for solar-heat generation. Our metamaterial perfect absorber can be applied for narrow-band infrared (IR) light sources with low energy consumption and for spectroscopic IR sensors, opening the way for new usages and demand in industrial quality control as well as in daily life.

Fig. 1(a) Schematic showing a solar water-heating system using TiN nanoparticles. (b) Spectroscopic IR sensor composed of wavelength-selective IR sensors with different wavelengths.

The technology for amplifying, confining, and scattering light at nanoscale is strongly desired as a key technology in communication, optical sensing, and energy harvesting. Plasmonics and metamaterials are now accepted as useful paradigms to achieve desirable optical properties that are not possible with natural materials. Through this approach, we have been developing various materials and devices, as briefly introduced below.

We demonstrated through numerical calculations that nanoparticles of transition metal nitrides (e.g., TiN) and carbides (e.g., TaC) absorb sunlight very efficiently and confirmed experimentally that, when dispersed in water, nanoparticles of these materials quickly raise water temperature and generate vapor^[1]. Since these nanoparticles exhibit broadband plasmon resonances that nearly overlap with the solar spectrum, their sunlight absorption efficiency is higher than that of gold and carbon nanoparticles. These nanoparticles can be applied for heating (Fig. 1(a)) and distillation of water through sunlight illumination.

IR metamaterial perfect absorbers, which can absorb 100% of IR radiation, have great potential for use in various applications such as sensing trace amounts of molecules and electrical power generation by absorbing thermal radiation. We developed IR metamaterial narrowband perfect absorbers using metal‒oxide‒metal structures in combination with alternative plasmonic materials such as Al, Mo, and TiN. Our perfect absorbers can perform as wavelength-selective IR light sources following Kirchhoff's law of thermal radiation^[2][3]. We also developed spectroscopic IR detectors using the IR metamaterial perfect absorbers for wavelength-selective IR light detection^[4] as shown in Fig. 1(b). Our IR detector can extend the usage of IR sensors to environmental sensors for robot cars, human motion sensors in hospitals, and material sensors and toxicology testing for homeland security.