A manifesto for terahertz light

The story of a new alphabet: a manifesto for terahertz light

Daniele Palaferri



Light is everywhere, in the universe. From the deepest place far from the known galaxies, down to our eclectic societies. Light is technically defined by physicists as energy propagating along space and time. But more simply, light is just a language. A language we need to decode a message. This message can tell us how the universe was born, how warm the lava of an Icelandic volcano can be or how sexually attractive are mature peacocks with their open plumage. 

The only thing we need is the right dictionary. Or, just as humans see by their eyes, we need the right glasses. And such special glasses are called 'sensors'. And trust me, there is a jumbled pile of existing different sensors. Why is that? Because light is not just an ordinary language. It’s a language with many alphabets. And each alphabet can identify an animal species, a physical phenomenon, an instinct.

From the dawn of time, first forms of life developed their own alphabet. All mammals, and especially primates and human beings, use the visible light to see the world, to recognize objects, colors and discover emotions. Most of snakes use infrared light to hunt their prey, with small sensors close to the mouth. Some butterflies and bees use ultraviolet light to find nectar and distinguish males from females. The same ultraviolet we are scared of, when we are lying in summer on the beach, under a sweating sun. 

These are just well-known natural examples, if you want more just go ask a biologist. 

But if you ask a physicist, then he will tell you that there is a simple phenomenon in nature, identified by Max Planck in 1900, which correlates light, objects and observer: any thermal object emits light depending on its temperature. So we can ideally define a blackbody, an object that in principle can speak all the alphabets by just tuning its temperature. But this is not enough. Scientific curiosity has developed new systems and new materials in order to select a specific desired alphabet, i.e. in order to build a technology. Imagine exciting a physical system, like stretching a spring; and imagine that inside a solid material there is an enormous, ridiculous amount of such excited springs. Their dis-excitation can produce any alphabet you need. This is how we came up with plenty of new applications in the last century: the X-rays which corroded the hands of madame Marie Curie, the invention of several type of lasers (inspiring Star Wars weapons), the radio and micro-waves we use to communicate with antennae all over the world.

Where did we arrive? Few decades ago, scientists noticed that there was a neglected alphabet in a gap between the different technologies developed so far. An energy range not accessible from previous devices, with two unique properties: less energetic than optical systems and faster than electronic devices. The terahertz range. 

It was neglected because at that time there were no materials or instruments which enabled scientists to speak that language. But curiosity in science is like love in Shakespeare’s sonnets, it never ends. And today there are many devices developed to work at terahertz energies. And not just for fun. This weak radiation, which can penetrate different materials without deteriorating them, is interesting for a lot of applications. Think for example of the ability to detect a defect or contamination inside an object, without damaging it: this can have huge impact on the quality control branch of industrial production. Imagine the possibility to detect hidden drugs in a package, extraneous components in food or distinguish unknown chemical compounds in pharmaceutical research:  if you have the right terahertz sensor, you can unveil a substance behind any given object. These non-invasive techniques could result in delicate medical diagnosis, identifying a virus in a single cell. It will even be possible to read an ancient book without opening it! The excitement increases as the multiple possible applications come to mind. This is just the beginning. So many words you can create with this alphabet! Wake up your brain and look at the world with a new kind of glasses: the terahertz sensors