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 4 f ‐system‐based OPR is limited by the paraxial approximation and bulky device volumes for THz applications. Here, a full‐diffraction‐based 4 f ‐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.