&Bullet; physics 14, 97
A new waveguide design uses a series of openings or slots to confine the waves to a narrow path.
Waves that pass a crack usually propagate. But new experiments with plasmonic waves – combinations of light and electron oscillations – and with water waves show that a series of slits can trap waves on a narrow path  . The slots block the outer edges of the waves, while the unblocked sections concentrate in the direction of the central axis of the waveguide due to interference effects. The simple slot-based design could be useful in situations where traditional waveguides are difficult to fabricate, such as experiments with terahertz (far infrared) light waves.
Waveguides are available in different designs. A common shape is a hollow metal tube that can transmit radio waves or microwaves. At shorter wavelengths, a more effective approach is to use a fiber of dielectric material whose index of refraction is such that waves are reflected off the outer cladding of the fiber rather than escaping. In some cases, however, it is difficult to find a dielectric material with the required index of refraction.
PhD student Dror Weisman from Tel Aviv University in Israel and his colleagues have developed a waveguide based on slits. “We don’t need a special material with a certain refractive index,” says Weisman. “The only thing we need is a material that blocks waves.” The idea is based on what is known as diffractive focusing, in which waves passing through a gap form a narrow beam before they diverge. This focusing is difficult to observe because it takes place close to the slit: with a slit about five wavelengths wide, the focusing occurs at a point about ten wavelengths away from the slit.
In a previous study of diffractive focusing, Weisman and his colleagues examined waves propagating through two slits in series, with the second slit near the focal position of the first  . They found that the diffraction pattern when passing through both slits was similar to the diffraction pattern for one slit, suggesting that the process could be repeated with a third slit, a fourth slit, and so on. The team predicted that if each crevice focused its output on the following crevice, the series would act as a waveguide.
The researchers have now confirmed this prediction with two experiments. The first was plasmonic waves, which are electromagnetic waves that travel along a metal surface and are coupled to electrons in the metal. To create a gap-based plasmonic waveguide on a silver plate, the team fabricated pairs of thin, silver walls, with each pair separated by a 14 micrometer gap. They tested waveguides with 2, 3, or 4 slots with 40 micrometers between consecutive slots.
Using a laser, the team generated plasmonic beams with a wavelength of about 1 micrometer and examined the output of each waveguide with a near-field scanning optical microscope. They estimated that each gap caused about 10% loss of plasmonic beam energy due to the cutting of the sidewalls through the walls. With regard to the loss per distance traveled, the slotted waveguide was comparable to conventional plasmonic waveguides.
In a second experiment, the team tested the slot-based waveguide with water waves generated in a tank. Instead of directing the waves through a series of barriers (spatial slots), the researchers repeatedly operated their wave generator over short time windows (temporal slots). Each outbreak of waves “focused” into a narrow wave packet before it spread out in space and time. The team measured the shape of the wave packet near the focal point and fed the amplitude and phase data back into the wave generator to generate the next burst, removing the data that fell outside the time window. This iterative process kept the wave packet constrained.
The gap-based waveguide is a general solution that should be applicable to all waveforms, says Weisman. The technique could be practical for terahertz light, for which the existing waveguides often require complex structures to be fabricated. A gap-based terahertz waveguide would be easier to manufacture, which could be beneficial for long waveguides, says Weisman. Another benefit is that the slot sizes and positions are adjustable. For example, the team showed that decreasing the slot width allows the waveguide to taper a beam, while shifting the slot positions can change the direction of the beam.
“This is a new element in the toolbox of photonic device design,” says optics expert John Dudley of the University of Franche-Comté in France. “It’s one of those wonderful papers that takes a seemingly familiar and fundamental concept of physics and applies it to create a new functionality,” says Dudley.
Michael Schirber is the corresponding editor for physics based in Lyon, France.
- D. Weisman et al., “Diffractive guidance of waves through a periodic arrangement of slots”, Phys. Rev. Lett.127, 014303 (2021).
- D. Weisman et al., “Diffractive focusing of waves in time and space”, Phys. Rev. Lett.118, 154301 (2017).