Metamaterial perfect absorbers (MPAs) often consist of metal and dielectric elements with dynamic control features of only subwavelength thicknesses. MPAs can achieve unity absorptivity of corresponding electromagnetic waves, and the sizes are much smaller than the working wavelength bands. These special absorbers with fixed absorption performance show great potentials in many important scientific and technical applications such as thermal emitters, energy harvesting, infrared detection and imaging.

The absorption bandwidth is one of the most important metrics for applications of an absorber. Traditional way to expand the absorption band is to use a multi-sized

microstructure to obtain different resonant wavelengths, and then to adjust the size of the microstructures to couple their resonances for a large absorption bandwidth. Multi-sized structures make the unit cell large and complicated, difficult to fabricate, and intolerant of polarization and incident angle. Designing a small-sized absorber is a new breakthrough. For instance, absorbers integrated onto the small pixels of the uncooled infrared detector need high and broadband absorption, ultrathin thickness, and simple structure.

In a study published in Optics Express, a research group led by Prof. LIANG Zhongzhu from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences proposed an ultra-broadband absorber with small period and thin dielectric thickness.

This novel MPA with perfect broadband absorption in the long wavelength infrared band is constructed by using a Ti/Ge/Si₃N₄/Ti four-layer configuration. The top layer is made by periodic Ti nano-structures of nano-antennas and nano-rings, the dielectric consists of two planar layers of Ge and Si3N4, and the bottom layer is made by Ti plane to inhibit the incident wave.