Tiny, flexible lasers detect counterfeits

We’re all familiar with fingerprint scanning or voice recognition to unlock phones. But imagine having a mini laser built onto a contact lens to confirm your identity. No, this isn’t a new technology from Infinity War, but a recent development from the University of St. Andrews where scientists have created ultrathin lasers from flexible materials, that can stick onto banknotes and contact lenses to be used as wearable security tags. Izzie Clarke spoke to Malte Gather.

Malte - A laser is a device that produces a very special type of light. It generally comprises of three main components. The most important component is a so-called ‘gain medium’ which is a material that can literally photocopy the particles of which light is made - the so called ‘photons’. Then the second component is a resonator: that’s a structure that confines the light within the gain medium for a long enough time for the photocopying process to work. Then the third component we need is a source of energy that drives the gain medium, that drives our photocopying machine so that the laser can operate.

Our laser, like all lasers comprises of these three components, but we have literally stripped it down. In particular, we have removed the rigid substrate, which is a structure onto which normally the laser is bound or confined. So we end up with just an ultrathin film that’s less than a thousandth of a millimetre thick, which contains the gain material and the resonator structure to confine the light.

Izzie - So basically this is an incredibly thin laser?

Malte - Yes, correct. Incredibly thin, approaching the ultimate weight limit of a laser.

Izzie - How do you make these?

Malte - Like all other lasers, we start by have a planar sheet of glass or silicon wafer, which we call the substrate, and then we produce our laster on top. But the trick here really is that we initially put down what we call ‘a sacrificial layer,’ which is a layer of a material that can later on be dissolved away, so that then the laser that has been built onto the substrate, detaches from the substrate and then we have our laser membrane or a sticker that we can put on other things.

Izzie - In my mind, I’m sort of imagining just one of those transfer tattoos.

Malte - It’s a little bit like that, I guess. This is the first time someone’s mentioned this but it’s a little bit like that, I agree.

Izzie - Why would we need something like this?

Malte - We have a number of applications in mind. Most importantly because the laser is so thin and because it’s mechanically flexible, we can now take the laser membrane stick it onto a variety of different objects. This then allows applications, for example in authentication control, where we can use the laser to tell us whether an object is real or fake.

Izzie - Okay. Like what?

Malte - All our lasers are extremely efficient in turning energy into laser light, which means it can produce laser light but very low intensity laser light. So low that it’s safe to produce it in your own eye. One of the most important things, of course, to verify sometimes is the human being itself. Is the person who they claim to be? We have put our lasers on contact lenses that a person can then wear and, essentially, they then start shooting a little laser beam out of their eye, very much like Superman does. But we don’t use this a weapon but we really just use it as an authentication device.

Izzie - How does that authentication work? How is it that by looking at a little beam from one of these flexible lasers that that then gives enough information to say: okay yes, this is the real deal, continue as you want?

Malte - Lasers emit light of a very specific colour or we also say of a very specific wavelength. We can measure the wavelengths of the light that’s being emitted and, in our membrane lasers, that becomes a unique feature of each laser or each set of laser devices that we make. We can increase complexity a little bit here and combine several lasers on one contact lens, each of them having a slightly different wavelength. And this becomes what we call a ‘laser barcode’ or an ‘optical barcode’ that really uniquely identifies one contact lens, one laser membrane, and then also the person wearing it.