April 20, 2024

Metasurfaces cascaded for dynamic control THz wavefronts.

The terahertz regime (EM waves) has critical communications, security imaging, and bio-and chemical sensing applications. This wide range of applications has led to significant technological advancements. Due to the weak interaction between THz waves and natural materials, THz devices tend to be bulky and inefficient. Although there are ultra-compact THz active devices, the current approaches to dynamic control using electronic and photonic technologies could be more efficient.

Metasurfaces are a new technology that has recently opened up several possibilities in developing ultra-compact, high-performance THz devices to control dynamic wavefronts. Metasurfaces are ultrathin metamaterials made of subwavelength microstructures, i.e., meta-atoms. They enable tailored optical responses to control EM wavefronts. Scientists have created metasurfaces with pre-designed phase profiles of transmitted or reflected wavefronts. This has allowed them to demonstrate fascinating wave manipulation effects such as abnormal deflections, polarization manipulation, photonic spin Hall, and holograms.

Moreover, integrating individual meta-atoms with active elements inside passive metasurfaces enables “active” devices that dynamically manipulate EM waves. Functional features with deep subwavelengths (e.g., PIN diodes, varactors) are readily available in the microwave regime and contribute successfully to active meta devices such as beam-steering devices, programmable images, and dynamic imaging. However, at frequencies above THz, it isn’t easy to produce them. The difficulty in creating active meta devices at frequencies higher than THz is due to the size limitations and high ohmic losses of electronic circuits. While THz frequencies can control THz beams uniformly, they cannot dynamically manipulate THz wavefronts. There needs to be more than the local tuning capabilities at subwavelength depths. Therefore, developing new approaches that do not rely on local tuning is essential.

Researchers from Shanghai University, Fudan University, and other universities have developed meta devices and a general framework for dynamic control of THz waves. This was reported inĀ Advanced Photonics. Rather than controlling individual meta-atoms within a THz Metasurface via a PIN diode or varactor (e.g.), researchers from Shanghai University and Fudan University developed a general framework and meta devices for achieving dynamic control of THz wavefronts. They vary the polarization with rotating multilayer metasurfaces. The researchers show that by turning different layers in a meta device (each with a different phase profile) at different speeds, they can change the Jones-matrix effect of the device. This allows them to manipulate the wavefronts and polarizations of THz beams. Two meta devices have been demonstrated. The first meta device can efficiently redirect a normally-incident THz beam to scan a sizeable solid-angle range.

This work presents an alternative approach to low-cost dynamic control for THz waves. Researchers hope the work will inspire new applications in THz imaging, bio- and chemical sensing, and radar.

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