Novel all-silicon metamaterials enhances control of terahertz polarization

(Nanowerk News) Researchers are working to unlock the immense potential of terahertz waves for applications ranging from medical imaging to wireless communications. However, efficiently controlling the polarization state of these high-frequency electromagnetic waves has remained an enduring challenge.
Conventional approaches relying on natural birefringent crystals or dielectric waveplates are hampered by narrow operational bandwidths, bulky hardware, and susceptibility to damage. These limitations have throttled progress towards commercially viable terahertz systems that fully exploit the information encoded in electromagnetic wave polarization.
Recent advances in metamaterials – artificial structures engineered with properties unattainable in nature – have brought fresh hope. Carefully designed metamaterial arrays allow researchers to overcome the constraints of natural materials and exercise unprecedented control over terahertz wave propagation.
Building on this momentum, researchers at Tianjin University have now demonstrated an all-silicon metamaterial polarization converter with record-high performance across an exceptionally broad frequency range.
As detailed in their paper published in Frontiers of Optoelectronics ("An all‑silicon design of a high‑efficiency broadband transmissive terahertz polarization convertor"), the team’s ingenious design achieves over 80% polarization conversion efficiency across the vast 1.00-2.32 THz bandwidth. For context, this range represents over double the operational width of the most advanced prior demonstrators.
cross-shaped microstructure
Schematic diagram of the designed cross-shaped microstructure. a Schematic of proposed convertor. b Stereograph of microstructure. Here, h represents height; H is substrate thickness; a–d show length and width; P is period; θis the included angle between the cross-shaped secondary axis and the x-axis; t is the length of high-resistance silicon. (© Frontiers of Optoelectronics)
At the heart of this breakthrough lies a deceptively simple cross-shaped silicon microstructure patterned onto a dielectric substrate. By tuning the dimensions and periodic arrangement of these sub-wavelength elements, the researchers induce strong artificial birefringence, causing different polarizations of the impinging terahertz waves to accumulate different phase shifts as they pass through.
Meticulous parameter optimization allowed the team to reach the sweet spot where orthogonal polarizations develop exactly the required phase offset for efficient polarization rotation. With all parts fabricated from silicon using standard lithographic techniques, the devices are eminently manufacturable.
Remarkably, by dynamically rotating the orientation of the cross-shaped metamolecules, the researchers can actively switch between linear-to-linear and linear-to-circular polarization conversion on demand. Even wide-angle and strongly oblique terahertz illumination barely degrades performance, evidencing the robustness of the team’s sound design principles.
The dramatically expanded functionality and bandwidth of these all-silicon metamaterial polarization manipulators promise to breathe new life into terahertz technology commercialization efforts. Integrating such devices into chip-scale terahertz spectrometers and imaging systems could prove truly transformational, unleashing applications from condensed matter research to pharmaceutical quality control.
With further development, the unprecedented polarization handling capabilities may also open new horizons in long-range wireless communications. By multiplexing multiple data streams onto orthogonal polarizations, metamaterial-enabled terahertz links could drastically augment channel capacities to help satisfy the world’s seemingly inexorable connectivity appetite.
Of course, real-world deployment remains a distant goal given the early development stage. But by overcoming longstanding challenges related to efficient broadband terahertz polarization handling, this advance represents a major leap towards technologies that once seemed firmly confined to the realms of science fiction.
Source: Nanowerk (Note: Content may be edited for style and length)
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