All-dielectric bifocal metalens with diffraction-limited focusing and polarization-dependent characteristics

Terahertz (THz) metalenses have attracted significant attention due to their potentials in advanced imaging, sensing, and communication applications, offering compact, lightweight designs and superior focusing capabilities compared to the conventional lenses. However, few studies have focused on polarization-independent bifocal metalenses in this frequency range. In this work, we design and demonstrate a transmissive all-dielectric bifocal metalens (ADBM), studied through simulation, optimization, fabrication, testing, and imaging methods. The metalens, composed of cross-shaped microrod phase elements made of high-resistivity silicon (Si), features low absorption, high transmission, and a simplified fabrication process. It generates distinct focal lengths for incident waves in transverse electric (TE) and transverse magnetic (TM) polarization states. They are 19.93 mm and 37.98 mm in TE and TM modes, respectively, enabling bifocal functionality. To enhance focusing performances, an error evaluation function is introduced to reduce phase errors during polarization multiplexing. The impact of different parameters in the error evaluation function on focusing performances is also discussed. ADBM achieves high Strehl ratios of 0.87 and 0.92 in TE and TM modes, respectively, indicating diffraction-limited focusing performances. Additionally, ADBM exhibits broadband focusing performances across the 0.70 THz to 1.10 THz spectra range for both polarizations. The fabricated ADBM exhibits high-resolution imaging capabilities, whihc are in agreement with simulations. This innovative design provides a new strategy for controlling orthogonal polarization states, promising broad applications in advanced optical imaging and communication.