An angle- and polarization-selective dual-wavelength narrowband thermal emitter for infrared multilevel encryption

Background

The explosive growth of data has exacerbated information security challenges, driving widespread attention to encryption technology as a critical component for information security. To enhance the security of data, researchers have explored various digital cryptographic techniques. However, relying solely on digital encryption methods is insufficient to completely prevent information interception and leakage during transmission. This limitation has led to the emergence of joint encryption techniques that combine both the digital algorithms and physical keys to further enhance security. The optical key is one of the important physical keys, serving as a pivotal component in advanced encryption systems. Previous studies have shown that certain smart luminescent compounds, metasurface technology and multifunctional deep-subwavelength thin films have significantly broadened the scope of encryption technologies.

While extensive research has been conducted in the visible light spectrum, new areas of optical information encryption remain a challenge, particularly in the infrared spectrum. Thermal radiation-based encryption in the infrared range has emerged as an effective alternative, leveraging the advantage of infrared data visualization. With advancements in thermal sensing equipment, temperature-responsive infrared thermal radiation can be detected using infrared cameras, allowing for the direct storage of digital information. Information encoding is realized by modulating thermal radiation intensity through precise control of radiation polarization. Moreover, the integration of multichannel independent optical recording techniques can substantially improve both encryption robustness and storage capacity.

Research Progress

In this work, we present a cost-effective, lithography-free, wafer-scale thermal emitter with angle- and polarization-selective dual-wavelength narrowband characteristics enabling infrared information encryption and decryption. This emitter is composed of an ENZ material upon a metallic layer, and it exhibits angle and polarization selectivity which are based on absorption enhancement near the longitudinal-optical (LO) wavelength (attributed to the Berreman mode) and the transverse optical (TO) wavelength (resulting from asymmetric Fabry-Pérot (FP) resonance). Despite their structural simplicity, the proposed emitters demonstrate versatile wavelength-, angle-, and polarization-selective optical functionalities across the LWIR spectrum. We select a 1000 nm SiO2/100 nm Al structure to construct the deep-subwavelength thermal emitter, which simultaneously generates two absorption peaks within the 7.5-14 µm range, corresponding to the working wavelength of the LWIR camera. Utilizing the polarization and direction selectivity, we schematically illustrates a novel multi-channel detection scheme. It consists of a LWIR camera (7.5-14 µm), an IR bandpass filter (wideband filters with 500 nm bandwidth and 8 µm central wavelength), and an IR polarizer (KRS-5 holographic wire grid polarizer). The infrared image, whether containing Quick Response (QR) Code information or not, can be acquired through precise control of three independent dimensions (directionality θ , polarization  and wavelength ). To improve the security of information transmission, we present a high-security cryptographic communication scheme based on the proposed thermal emitter. It can realize eight independent conditions with different incident angle, polarization angle and wavelength characteristics. By using thermal emitter as the physical layer key, an encrypted communication scheme is established. With the thermal emitter serving as a physical-layer key, we build a high-security cryptographic communication scheme. The encrypted information is directly transmitted to the recipient, who can decrypt the message using a preset code and a physical-layer key.

Future Prospects

In the foreseeable future, our work enables the transmission of encoded information, as the receiver deciphers the desired message by processing the sequence to obtain the corresponding key. While maintaining effective, the current encryption approach shows susceptibility to frequency-based cryptanalysis. Future developments will incorporate advanced cryptographic protocols to strengthen system security. Notably, the encryption system is capable of generating non-unique ciphertexts from identical plaintexts through physical key modification, a characteristic that substantially enhances the security level of the cryptographic system and demonstrates significant potential for propelling the advancement of next generation encryption technologies. By integrating digital and physical layer encryption, the security of information transmission is significantly enhanced, establishing a blueprint for encryption systems and optically reconfigurable security frameworks in infrared communication networks.

Sources: https://spj.science.org/doi/10.34133/research.0719