Using Microspectrophotometry to Tag Art for Authenticity

In art, verifying the authenticity of pieces during transactions is paramount. While functional, traditional electronic tags and labels are easily detected and counterfeited, presenting a need for a more covert and effective tagging method. Microspectrophotometry offers an innovative solution to this challenge by utilizing aluminum (Al) nanostructures resonating across ultraviolet (UV) to infrared (IR) spectra. These micro-tags are microscopic, camouflaged into their surroundings, and capable of containing multiple sets of information, making them an ideal choice for discreet art tagging.

An Introduction to Micro Tags

This technology's core lies in using native aluminum oxide (Al2O3) on Al films, which supports resonances facilitated by Al disks. These disks, with diameters significantly smaller than the wavelength of the fundamental mode, enable the micro-tags to contain two sets of information: a quick response (QR) code visible in daylight and a covert barcode extractable under 1.2 μm IR illumination or visible light darkfield imaging. Combined with their compact size, this dual-encoding capability renders these tags particularly suitable for art and high-value merchandise authentication.

Fabrication of these micro-tags involves a series of nanofabrication steps, including electron beam lithography and metal deposition, to create the Al disk-on-Al2O3 structures. The process allows for precise control over the disk dimensions, ensuring the functionality of the micro-tags across both visible and infrared spectrums. Optical and infrared microscopy techniques are then employed to verify the tags' responses to different light sources, confirming their ability to display the intended information correctly.

What Method is Used for Validation?

Reflectance spectroscopy further elucidates the optical properties of these structures, demonstrating their ability to support resonances across a wide range of wavelengths. This characteristic is crucial for the micro-tags' versatility, enabling them to display distinct visual information under different lighting conditions.

Reflectance spectra were measured using a UV-visible-NIR microphotospectrometer with a 75 W xenon lamp at normal incidence. The light was passed through a 7.1 µm × 7.1 µm aperture and an objective lens (36 × /0.5 NA) and the reflected light (0.3–1.7 µm) was collected by charge-coupled device (CCD) detectors. The two detectors are a silicon detector (working range of 200–950 nm) and an InGaAs detector (850–2300 nm). 

In application, the disparity between the visible and infrared responses of the nanodisks is exploited to design a security tag that conceals a covert infrared image within a visible one. This is achieved by selecting disk diameters and gaps that produce the same color in visible light but different reflectance in IR, and vice versa. The resulting tag can be embedded into artwork, providing a robust tool against counterfeiting while maintaining the piece's aesthetic integrity.

The potential of microspectrophotometry in tagging art for authenticity extends beyond mere identification. It offers a sophisticated method for encoding multiple layers of information onto a single tag, combining aesthetic subtlety with advanced security features. This technology represents a significant advancement in the fight against art forgery, ensuring the protection of valuable pieces with minimal intrusion.

Closing Thoughts

Microspectrophotometry presents a novel and highly effective approach to art authentication. By harnessing the unique optical properties of aluminum nanostructures, it enables the creation of covert and informative micro-tags. This breakthrough could revolutionize how artworks are tagged and authenticated, offering unparalleled security and discretion in preserving artistic heritage. If you're interested in learning more about using CRAIC systems for reflectance spectroscopy, or in other art applications, simply contact a team member today.

References & Further Reading

  1. Ray Jia Hong Ng, Ravikumar Venkat Krishnan, Zhaogang Dong, Jinfa Ho, Hailong Liu, Qifeng Ruan, Kin Leong Pey, and Joel K. W. Yang, "Micro-tags for art: covert visible and infrared images using gap plasmons in native aluminum oxide," Opt. Mater. Express 9, 788-801 (2019)

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