Combining a free-electron laser and a photonic crystal makes it easier to identify a drug than using chemical analysis. A terahertz laser can show the molecular structure of drugs because the beam it produces is at a wavelength suitable for examining molecular and atomic bonds. But it’s restricted to particular wavelengths because its light source is a semiconductor, limiting the number of colors that can be produced. Photonic crystals for a 6-GHz laser. The regular arrangement of the rods provides the unique properties that manipulate the laser beam. Courtesy of the University of Twente. To overcome this obstacle, doctoral student Thomas Denis of the University of Twente decided to use a free-electron laser, which is not restricted to a fixed state. When combined with photonic crystals, the resulting device – called a photonic free-electron laser, or pFEL – was compact and highly flexible. The photonic crystals can be created at the micro level or on a much larger scale. Their dimensions and shape determine the rough wavelength region, and the precise wavelength can be set and adjusted by changing the speed of the electrons being fired at it. While current terahertz lasers are very large, Denis’ pFEL is not much bigger than a domestic microwave oven, thanks to the use of photonic crystals. Despite its small size, the source can still provide high power. Denis also discovered a way of “looking” inside a photonic crystal, something not typically possible. By interfering slightly with the wavelength pattern in the crystal using a tiny metal ball, the actual pattern can be measured.