Terahertz (THz) waves, situated between the microwave and infrared regions of the electromagnetic spectrum, have drawn considerable attention over the past two decades. Researchers from the Aerospace Information Research Institute (AIR) of the Chinese Academy of Sciences, in collaboration with Nanjing University, have achieved a groundbreaking milestone in this field. Their innovative method, published in Optica on January 18, introduces unprecedented control over the polarization of THz waves, marking a transformative breakthrough in optics and photonics.

Polarization—the orientation of light waves—is a cornerstone of modern technology, critical in everything from next-generation wireless communications to biomedical imaging. However, manipulating the polarization of THz waves has long been a formidable challenge.

This difficulty arises primarily due to two key features of THz waves. First, the mesoscale wavelength, which ranges from micrometers to millimeters, is about three orders of magnitude larger than that of visible light, resulting in inefficient light-matter interactions. Second, the extremely broad bandwidth, spanning from 0.1 to 10 THz, demands highly achromatic responses, which are difficult to achieve.

The research team has overcome these barriers with a novel device called the Phase-Compensated Mirror Total Internal Reflection (PCMT) system. By adjusting two key parameters—mirror-prism distance and liquid crystal birefringence—the researchers achieved achromatic phase control, enabling precise manipulation of polarization states across a wide and tunable bandwidth with minimal intensity change.

This device has demonstrated the ability to generate and actively switch between orthogonal linear polarizations and left- or right-handed circular polarizations over a broad frequency range of 1.6 to 3.4 THz. It achieves exceptional degrees of polarization, with both linear (DoLP) and circular (DoCP) values exceeding 0.996. Furthermore, it enables the realization of arbitrary polarization states at any center frequency, with relative bandwidths surpassing 90%.

Prof. CHEN Xuequan at AIR, one of the lead researchers, remarked, "Our work demonstrates that THz waves, often considered unruly, can now be tamed and orchestrated with unprecedented precision. This breakthrough opens the door to a multitude of applications, making THz technologies more accessible and versatile than ever before."

This innovation has the potential to redefine industries reliant on polarization-sensitive technologies, offering a versatile and efficient solution for communication, sensing, material characterization, and more. The team is optimistic about integrating their device with other THz systems, paving the way for compact, multifunctional solutions to meet the demands of next-generation technologies.

The proposed THz polarization controlling device. (a) schematic of the device and (b) measured polarization outputs. (Image by AIR)