An ordinary laser emits narrow lines, like a string that sounds only one note. For spectroscopy, this is inconvenient: if a substance absorbs light between the lines, you might miss it. A terahertz quantum cascade laser is a semiconductor chip the size of a fingernail that emits waves with frequencies from 1 to 6 terahertz (trillions of oscillations per second). Such waves pass through paper, plastic, and clothing, but not through metal or water.
The authors of the study applied an additional low-frequency signal to the laser, as if gently shaking the string so it sounds with all overtones at once. At a modulation frequency below 1 gigahertz (a billion oscillations per second), the laser produced a spectrum from 1.9 to 4.1 terahertz — more than an octave, with no dips. When the modulation frequency was increased, the individual lines merged into a continuous glow from 3 to 4 terahertz. This mode is called superluminescent: the light remains directional but loses sharp peaks, like the noise of rain instead of individual drops.
Calculations and computer models showed what happens inside the laser: light pulses race around the resonator and collide, smearing each other out. The practical benefit is in absorption spectroscopy: you can check a sample for many substances at once without retuning the laser. Such sources will be useful for drug quality control, explosives detection, or studying gases.
Terahertz radiation is sometimes called a "see-through beam": it sees through clothing and packaging but does not harm tissues, unlike X-rays.
In the series "Star Trek," the medical tricorder uses something similar for instant analysis of substances — now a laser without gaps makes this a step more real.