In the electromagnetic spectrum, T-rays inhabit prime real estate between the shorter wavelengths of infrared light and the longer wavelengths of microwaves. But T-rays have not been as commercially successful as these neighbors because T-rays are difficult to produce and transmit. Now, in the 8 June and 28 September Physical Review Letters, researchers describe generating T-rays in a hair-thin plasma filament made by a laser pulse. The technique could create high-intensity radiation at distant locations, to probe beneath surfaces for security or military applications.
T-rays–light waves with a frequency in the terahertz range–are ideal for sensing tooth decay and skin cancer, as well as for airport security sensors that peer through fabric to spot concealed weapons. These devices exist but are expensive, in part because of the lack of cheap, convenient T-ray sources. For one thing, existing sources must be within a few meters of their targets because the rays are absorbed by humidity in the air. A beam that could travel hundreds or thousands of meters through air could be used by spy planes to see through tents or by security officers to spot concealed weapons from far away. Another problem is the relatively low power of T-ray sources.
André Mysyrowicz of the Polytechnic University and the National University for Advanced Techniques (ENSTA) in Palaiseau, France, and his colleagues reported their new method in Physical Review Letters in June. It forms T-rays out of thin air, so they can be created far from the equipment and close to where they are needed. The researchers start with a femtosecond pulse of laser light, whose finger-width cross-section is brighter in the center than at the edges. This intense light creates an effect called self-focusing, in which the beam grows ever narrower and brighter as it travels because the speed of light in air depends somewhat on the intensity at each point in the beam. The air acts like a focusing lens. Once the beam collapses to about the width of a hair, the beam is intense enough to ionize the air, preventing further narrowing. This ionization creates a centimeters-long, string-like plasma of ions and electrons called a filament.
“The plasma that is created is rather peculiar,” says Mysyrowicz. “You have this plasma moving just behind the ionization front.” The intense, oscillating electric field in the laser light vibrates electrons in the plasma, causing them to emit T-rays, he says. Because this method generates T-rays in the air near where they are needed, it avoids the absorption problem that plagued previous production methods. The researchers speculate that it will work even kilometers away from the laser source.
In their latest paper, Mysyrowicz’s team describes a further enhancement of the technique. Before the self-focusing, they split the laser beam into two parallel beams, which then form two long plasma filaments with a few hairs-worth of space between them. The two conductive filaments act like parallel wires, and the voltage between them drives electrons across the gap, which generates T-rays. The new arrangement produces radiation ten times brighter than the single filament method. “The twin filaments produce a maximum intensity in the direction of propagation,” rather than spreading out in a cone pattern, as with the single filament, says team member Yi Liu, also of the Polytechnic University. “And with most experiments, that is what you need.”
“The light string is basically a focused light channel,” says Jerome Moloney, an applied mathematician at the University of Arizona in Tucson, referring to the new bi-filamentation method. “The novelty of this research is that you can create a plasma at a distance and that can create the terahertz radiation for remote imaging.”