Terahertz radiation sits between microwaves and infrared light. It passes through clothing, paper, and plastic but is blocked by water and metal. That's why these waves are used in security scanners and for quality control of materials. But this radiation has another property — polarization, the direction of wave oscillation. Many materials transmit waves with different polarizations differently, and this can be used to study them.
The new system works like polarized sunglasses that cut out glare. But instead of lenses, it uses frequency-selective surfaces — thin gratings that respond differently to waves depending on their polarization and frequency. The device quickly measures the intensity of transmitted radiation and from that data computes the Stokes parameters — four numbers that fully describe polarization. All processing happens in real time, with no moving parts or complex optics.
For testing, they took a quartz plate 1 mm thick. Quartz is an anisotropic crystal: light travels at different speeds along different axes, so polarization changes. The device measured this change and obtained a birefringence value (the difference in speed for two polarizations) that matched the results of the traditional method. Then the system was used to scan a sample, rotating the gratings to 0°, 45°, 90°, and 135°. This produced polarization maps — images showing how the material affects waves at each point.
Such maps help find defects in crystals, analyze biological tissues, or inspect composite materials. The device is compact and does not require expensive femtosecond lasers, making it accessible to laboratories and industry.
Terahertz radiation is sometimes called the "terahertz gap" because for a long time there were neither good sources nor detectors for this range: it lies between optics and electronics.