Electromagnetic (EM) waves in the terahertz (THz) regime contribute to important applications in communications, security imaging, and bio and chemical detection. This wide applicability has resulted in significant technological advancements. However, due to the weak interactions between natural materials and THz waves, conventional THz devices are generally bulky and inefficient. Although there are ultracompact active THz devices, current electronic and photonic approaches to dynamic control have been ineffective.
Recently, rapid developments in metasurfaces have opened up new possibilities for the creation of ultra-compact and high-efficiency THz devices for dynamic wavefront control. Ultrafine metamaterials formed by lower wavelength planar microstructures (i.e. meta-atoms), metasurfaces enable customized optical responses for EM wavefront control. By building metasurfaces that have certain predefined phase profiles for transmitted or reflected waves, scientists have demonstrated fascinating effects of wave manipulation, such as abnormal light deflection, polarization manipulation, photonic spin-hall and holograms.
In addition, the integration of active elements with individual meta-atoms within passive metasurfaces enables “active” metadispositives capable of dynamically manipulating EM wavefronts. While elements active in the deep subwavelengths are easy to find in the microwave regime (eg, PIN diodes and varactors), and successfully contribute to active meta-devices for beam steering, Programmable holograms and dynamic imaging make them difficult to create at frequencies above THz. This difficulty is due to size restrictions and significant ohmic losses in electronic circuits. Although THz frequencies can control THz beams uniformly, they are generally unable to dynamically manipulate THz wavefronts. This is ultimately due to gaps in local tuning capabilities at deep subwavelength scales in this frequency domain. Therefore, the development of new approaches that circumvent the need for local tuning is a priority.
As stated in Advanced photonics, researchers from Shanghai University and Fudan University have developed a general framework and metadispositives to achieve dynamic control of THz wavefronts. Instead of locally controlling individual meta-atoms in a THz metasurface (e.g., via a PIN diode, varactor, etc.), they vary the polarization of a light beam with rotating multi-layered cascading metasurfaces. They demonstrate that rotating different layers (each with a particular phase profile) in a cascading meta-device at different speeds can dynamically change the effective property of the Jones matrix of the entire device, achieving extraordinary front-d manipulation. wave and polarization characteristics of THz beams. Two metadispositives are demonstrated: the first metadispositive can efficiently redirect a normally incident THz beam to scan over a wide solid angle range, while the second can dynamically manipulate both the wavefront and the polarization of a THz beam. .
This work proposes an interesting alternative to realize a dynamic control at low cost of THz waves. The researchers hope the work will inspire future applications in THz radar, as well as bio and chemical detection and imaging.
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