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I think everyone is aware of the trick with invisible ink. Write your message in lemon juice on paper, and when the juice dries it cannot be seen. But if you heat the paper, the lemon juice reacts with it and turns brown, bringing forth your shining prose for all to read.
That’s so old school—I want laser powered invisible writing (and, no, I am not paid to make sense. Why do you ask?). Since lasers are what make life worthwhile, others evidently felt the same. Lo and behold, there has now been laser-powered invisible writing.
Watching glass glow
To create laser powered invisible writing, we need to delve into how light interacts with matter. Imagine a glass plate. If I shine a laser through the glass plate, pretty much nothing seems to happen. But internally, there is a whole lot going on. The electric field from the laser beam grabs hold of the electrons surrounding the atoms in the glass and gives them a good shaking. As the electrons shake up and down, they absorb and re-emit light from the laser. The color doesn’t change, but the light slows down a little.
These electrons are bound to atoms, so they are not completely free to move around. As long as the laser light is rather puny, the light doesn’t try to move the electrons very far. However, as we ramp the laser power up, the light tries to move the electrons further and further. The electrons don’t appreciate this, and at some point they stop responding to the driving electric field.
For a weak laser, the electrons move with the electric field perfectly, while for strong fields they don’t. And if they don’t, the light the electrons emit does not have the same color as the light they absorbed. So, we can shine red light into our glass and get blue light out. Or for extreme cases, white light comes out because the electrons are oscillating at many, many different frequencies.
The color(s) emitted by a material depends on its structure—more precisely on its symmetry. Particular crystal structures will emit light with half the wavelength, while glasses (which are not crystalline) will emit light with a third the wavelength.
Those are the two properties that the researchers make use of: a material can emit light with a different color than it absorbs, and its structure determines that color.
Artificial molecules and really tiny writing
Nature hasn’t provided the structures that the researchers wanted, so they created them. The structures are very simple: little (about 150nm wide) gold Y-shapes on a glass plate. The Y-shape has the right symmetry to absorb radiation at one wavelength and emit it at half the wavelength. By choosing the dimensions of the Y-shape, the researchers can choose the wavelength it works most efficiently for. The researchers chose a Y-shape that emits best at about 600nm (red light), which means it needs to be illuminated with light at 1200nm wavelength (near infrared) to work.
An array of these will evenly absorb a little bit of the input wavelength and evenly emit red light. But there is a trick here: the orientation of the Y-shape matters. If one Y-shape is rotated 15 degrees counter-clockwise and a neighbor is rotated 15 degrees clockwise, they will emit out of phase with each other, and the light from the two destructively interferes. In other words, if your array of Y-shapes consisted of pairs with a 30 degree rotation between them, no red light would be emitted.
But this only happens for the infrared photons that are converted to red—the rest of the infrared light doesn’t change at all. And if you shine red light directly on it, you can’t tell the difference either. The Ys are invisible, and it appears to be a uniform surface.
So, now you can write invisibly. To demonstrate this, the researchers printed arrays of Y-shaped pairs. Where they wanted emission, the pairs were aligned to each other; where they wanted darkness, the Y-shapes were mutually rotated to have a 30-degree angular offset. For all normal illumination, the surface appeared a uniform shade. But when illuminated with the right color of infrared laser light, the letters shone out in a lovely red.
Even better, by choosing different mutual rotation angles, the intensity of the red light can be varied. So, a pair with a mutual rotation of 15 degrees has half the intensity of two that are aligned.
Fun and all, but what is it for?
I must admit that I fell in love with the idea, but I don’t really buy the researchers’ justification for it: security. Somehow, somewhere, they’ve gotten the idea that this is a form of encryption. As they’ve demonstrated it, I guess you could call it encryption, but only in the weakest possible form. I see it more as a form of advanced water marking.
But I don’t really care. I am happy with the control and precision of the experiment. These are the kinds of developments that are required to build the next generation of optical devices for computing and communications. And, frankly, the coolness is reason enough.
Nano Letters, 2017, DOI: 10.1021/acs.nanolett.7b00676