4f-Less Terahertz Optical Pattern Recognition Enabled by Complex Amplitude Modulating Metasurface Through Laser Direct Writing

Optical pattern recognition (OPR) has the advantage of single intensity detection ability for low-cost terahertz (THz) systems of imaging or security checks. However, conventional 4f-system-based OPR is limited by the paraxial approximation and bulky device volumes for THz applications. Here, a full-diffraction-based 4f-less OPR method is proposed using a single complex-amplitude-modulating metasurface, which is valid for systems with large Fresnel numbers. Moreover, a laser-induced graphene technique is applied for processing the device. A 15 mm × 15 mm metasurface can be fabricated by one-step laser writing in 34 s, indicating the potential of the proposed method in developing THz OPR systems with miniaturization, fast fabrication, and low-cost.