Dual Encryption of Terahertz Metasurface by Frequency-Temperature Co-Determination

In recent years, terahertz metasurfaces have become powerful tools for manipulating terahertz waves. The rapid advancement of intelligence and information technology creates a growing demand for tunable and reconfigurable devices in engineering applications to adapt to various scenarios and conditions. Dynamic metasurfaces, with their tunable properties, enable the creation of reconfigurable and programmable electromagnetic devices, thus advancing the development of sophisticated multifunctional systems. Apart from techniques including optical pumping of semiconductor materials and microelectromechanical systems, phase-change materials have also emerged as a promising approach for the dynamic modulation of terahertz metasurfaces. In this letter, vanadium dioxide with two different phase transition temperatures was integrated into a single unit. By utilizing the effect of electromagnetically induced transparency and prearranging metasurface units according to a specific hologram, an image requiring encryption was encoded into the metasurface, achieving optical encryption through temperature variation. When the temperature is below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$60~^{\circ }$ </tex-math></inline-formula> C, the transmitted light field appears as random spots, devoid of any useful information. However, when the temperature rises to between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$60~^{\circ }$ </tex-math></inline-formula> C and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$72~^{\circ }$ </tex-math></inline-formula> C and terahertz waves at a frequency of 0.631 THz are incident, the encrypted letter “C” can be reconstructed. When the temperature exceeds <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$72~^{\circ }$ </tex-math></inline-formula> C and terahertz waves at a frequency of 0.43 THz are incident, the encrypted letter “E” can be reconstructed. If the conditions of temperature and frequency are not simultaneously satisfied, no useful information can be obtained. The proposed method provides a novel approach for the design of reconfigurable terahertz metasurfaces, with substantial potential for applications in optical encryption and anti-counterfeiting technologies.