There is a region of light that has long remained a blind spot: from 5 to 15 terahertz. It is not radio waves, nor infrared heat, but something in between, where molecules vibrate in particularly characteristic ways. Scientists from Germany have built a setup that finally peers into this 'new terahertz gap' and even beyond — past 25 terahertz.
The secret lies in a crystal with the long name BNA. When a short pulse of near-infrared light hits it, the crystal responds with a flash in that very invisible range. But to make the flash as broad as a rainbow, not as narrow as a laser pointer, the pulse is first stretched and squeezed. It is sent through a hollow glass fiber filled with gas, where the light gets slightly 'smeared' across colors, and then special optics compress it to 31 femtoseconds. That is a million times shorter than the blink between two heartbeats.
As a result, the crystal emits a broad spectrum that can be used as a ruler: point it at a substance and see which frequencies it absorbed. This reveals what molecules make up the sample. Previously, this required bulky and expensive lasers; now an ordinary ytterbium laser, already present in many labs, suffices.
It's like getting a powerful spotlight from a pocket flashlight: the light source remains the same, but now it illuminates a whole street instead of a small patch at your feet.
Terahertz radiation passes through clothing, paper, and plastic, but not through metal or water, which is why it is used in airport security scanners.
In the series 'Star Trek,' the medical tricorder scans a patient in seconds — terahertz spectroscopy brings that science fiction closer to reality.