Photosensitive PbTe Dielectric Films as Universal Functional Modules for Optically Tunable Terahertz Metamaterials

Abstract

Optically tunable dynamic terahertz (THz) metamaterials hold significant application prospects in real-time THz modulation, high-speed communications, intelligent sensing, and nondestructive testing. However, existing optically modulated THz metamaterials commonly face critical bottlenecks such as limited photosensitive material systems, complex fabrication processes, stringent high-intensity excitation conditions, and poor universality of device architectures. To address these issues, we propose the narrow-bandgap PbTe semiconductor as a photosensitive dielectric layer for constructing optically tunable THz metamaterials. The PbTe thin film was fabricated via facile and cost-effctive electron beam evaporation and incorporated as dielectric layers, with quartz substrates underneath and metallic resonator arrays of various materials and geometries constructed on top, thereby forming complete THz metamaterial architectures. Experimental results show that owing to the narrow bandgap of PbTe at approximately 1.12 eV, significant THz modulation can be achieved with extremely low optical excitation density of 12.2 mW/cm2, which is far below the natural solar intensity. Combined photocurrent spectroscopy, Mott Schottky analysis, and Drude–Smith model fitting elucidate that the increase in photoinduced carrier concentration and the decrease in relaxation time jointly induce an increase in the real part and a decrease in the imaginary part of the complex permittivity of PbTe, which constitute the microscopic physical mechanism underlying the optically induced impedance mismatch and enhanced phase delay. More importantly, regardless of the material composition (Au/Cu) or geometry (square-shape/cross-shape) of the metallic resonators, the same optical modulation trend remains robust, consistently exhibiting enhanced impedance mismatch, increased reflectivity, reduced absorptance, and improved phase-delay capability. Furthermore, increasing the thickness of the PbTe dielectric layer can also further enhances the modulation depth of these effects, fully demonstrating its excellent universality and design flexibility. This work presents an innovative strategy of employing PbTe semiconductors to construct optically tunable terahertz metamaterials, which can serve as an optical functional module that imparts dynamic tunability to arbitrary THz metamaterials, featuring excellent universality, low energy consumption, and high modulation depth, holding significant potential for applications in photoresponsive terahertz devices.