When it comes to luminescent materials that emit light, most follow what is known as Stokes’ Law. This law, named after George Gabriel Stokes who first described the phenomenon in 1852, states that the wavelength of the emitted fluorescent light is always longer (and therefore less energetic) than the wavelength of the absorbed exciting radiation. While Stokes’ Law holds true for the majority of luminescent materials, there are some instances where it gets turned on its head. In rare cases, the emitted light actually has a shorter wavelength and higher energy than the absorbed radiation. This counterintuitive phenomenon is called anti-Stokes emission.
How is Anti-Stokes Emission Possible? At first glance, anti-Stokes emission seems to violate the law of conservation of energy. How can a material emit light with more energy than it absorbed? The answer lies in the fact that the absorbing molecule or atom receives additional energy from its surroundings via collisions with other particles.
Here’s how it works: When a photon is absorbed by a molecule, it gets promoted to an excited state. Under normal circumstances, it would simply decay back down and emit a photon with slightly less energy than the one it absorbed, in accordance with Stokes’ Law.
However, if the excited molecule experiences collisions with other molecules or atoms in the surrounding medium, it can gain extra kinetic energy on top of the energy from the absorbed photon. This excess kinetic energy gets transformed into additional potential energy, promoting the molecule to an even higher excited state.
When this super-excited molecule relaxes and emits a photon, the emitted light has a higher energy (shorter wavelength) than the originally absorbed light. This anti-Stokes photon carries not only the energy from the initial absorption event, but also the surplus kinetic energy picked up from collisions.
Applications of Anti-Stokes Materials While anti-Stokes emission was theorized in the late 19th century, the first anti-Stokes luminescent materials weren’t reported until the 1970s. Since then, researchers and companies have been working to develop and optimize these unique pigments for various applications.
One of the most promising uses for anti-Stokes pigments is in security inks and document coding. These inks, when exposed to infrared or near-infrared light, emit light in the visible spectrum that is easily detected by standard camera sensors and the human eye. However, the inks remain invisible under normal lighting conditions.
This unique property makes anti-Stokes security inks extremely difficult to counterfeit or reproduce without access to the specialized pigments and formulations. They can be used to mark currency, important documents, product packaging, and more with covert codes that are easily verified but hard to replicate.
The First Anti-Stokes Pigments The first anti-Stokes materials proposed were oxysulfide compounds like Y2O2S doped with certain rare earth elements. While these early oxysulfides exhibited anti-Stokes properties, they left much to be desired in terms of efficiency and brightness.
It wasn’t until the early 2000s that a new class of anti-Stokes pigments based on gadolinium oxysulfide (Gd2O2S) took the spotlight. My company was at the forefront of developing and commercializing these new ultra-bright anti-Stokes pigments for use in security inks.
The Magic of Gadolinium Oxysulfide What makes gadolinium oxysulfide so special? It all comes down to the unique electronic structure of gadolinium and how it interacts with different dopants like praseodymium, erbium, and other rare earth elements.
When gadolinium oxysulfide is doped with the right combination of rare earth ions, it exhibits exceptional anti-Stokes properties. It can absorb infrared light around 1000nm and emit extremely bright green, red, or other visible luminescence, with anti-Stokes shift values of over 10,000 cm^-1.
This massive anti-Stokes shift and bright visible emission cannot be matched by other host materials like the early oxysulfides. It allows gadolinium oxysulfide pigments to be easily visualized with standard camera sensors and imaging equipment, while remaining completely hidden under normal lighting conditions.
Furthermore, these pigments are highly stable, durable, and can be easily incorporated into a wide range of ink formulations for printing on various substrates.
Setting the New Standard Thanks to their incredible brightness, massive anti-Stokes shift, and easy detectability, gadolinium oxysulfide pigments have become the gold standard for anti-Stokes security inks over the past 20 years.
While early anti-Stokes materials showed promise, it was the development of these rare earth-doped gadolinium oxysulfide pigments that truly unlocked the potential of anti-Stokes technology for real-world security and authentication applications.
As counterfeiters continue to develop more sophisticated techniques, innovators are constantly working to raise the bar and develop even more advanced anti-counterfeiting solutions. But for now, brilliant anti-Stokes pigments reign supreme when it comes to marking secured items with an invisible but easily verifiable code.
The anti-Stokes world is full of fascinating physics and chemistry that seems to defy conventional wisdom. Yet it’s this mind-bending behavior that has opened up incredible opportunities in security, authentication, and beyond for those who can truly master these unique luminescent materials.
