Ultra-broadband terahertz polarization converter based on flexible metamaterial

Polarization is one of the basic properties of electromagnetic (EM) waves, and polarization conversion is essential in many terahertz (THz) applications, including communications and sensitive measurements. Due to the limitations of the materials and process, existing THz polarization converters generally have the disadvantages of narrow operating frequency band and non-flexibility. We experimentally demonstrate a flexible ultra-broadband THz polarization converter operating in reflection. It can efficiently convert a linearly polarized THz wave to its orthogonal counterpart in a broadband regime. The device consists of three layers: the lower metal film, the middle dielectric layer, and the upper metal resonance structure array. The unit cell of the upper metal resonance structure is a four-open-ring formed by subtracting two metal bars from a metal ring. The position of the opening is in the diagonal direction of 45° from the upper right corner to the lower left corner of the unit structure. The device is simulated using a full wave EM simulation software and fabricated by a surface micromachining process. The test experiment is completed using a THz-TDS spectrometer and the results show that the polarization conversion ratio is more than 0.8 in an ultra-broadband regime from 1.45 to 2.67 THz. This ultra-broadband conversion is mainly caused by the magnetic resonance of THz waves. More importantly, the convertible frequency band can be tailored for practical applications across the EM spectrum.