Dual-color center diamond for concealable physically unclonable functions

Physical unclonable functions (PUFs) offer a promising defensive measure against the escalating challenges posed by the increasingly rampant counterfeit products. Conventional PUF materials with a singular physical property encounter limitations in encoding flexibility and capacity. Here, we propose a dual-color center diamond-based PUF (D-PUF) ink that exploits four diverse optical characteristics of dual-color center in diamond to design a concealable multi-level cryptographic authentication protocol. Through simple writing, stamping, or spraying, intricate covert random patterns can be directly generated on the objects, which are imperceptible under visible light. When challenged by a 532 nm laser, the D-PUF exhibits four distinct optical responses, including Raman, zero phonon line (ZPL) of germanium vacancies (GeV), ZPL of silicon vacancies (SiV), and the intensity ratios of these ZPLs. These responses were harvested simultaneously to construct the four-level separate encodable matrices. Furthermore, M-ary encoding algorithms were implemented to encrypt PUFs with flexibility. The resulting multi-level PUF system attains notable uniqueness, repeatability, extensive encoding capacity (> 10⁴⁸¹⁶⁴/(100 pixels)²), and ultra-high information entropy (6 bits/pixel). This study inspires designing new generations of multi-level PUFs with enhanced coding flexibility and holds significant promise for applications in print security.