A new mechanism uses photonic crystals to concentrate light on the chip

Focusing light on a volume as small as the wavelength itself is a critical challenge for many applications. Researchers at Amolf, Tu Delft and Cornell University in the US have demonstrated new ways to concentrate light on a very small scale. These methods take advantage of the special properties of photonic crystals and work for a wider range of wavelengths than alternatives. Researchers published the findings on science advances on April 18th.

Focused light is important for various technical applications of photonic chips, such as quantum communication, optical sensors, and on-chip lasers. “We’ve known two common strategies for focusing light, either using optical cavity or in waveguides that compress light like funnels,” says Ewold Verhagen, leader of Amolf Group.

“The first method uses resonance. This limits the focus or density of light to a specific wavelength. The second method works only with devices that are much larger than the wavelength of light used, just like traditional lenses.”

Block light

Theoretical ideas from researchers at Cornell University led by Gennady Shvets pointed out new ways in which the PhD. Candidate Daniel Muis and his colleagues held the first demonstration. An important aspect of this method is the so-called topology of physical systems.

Muis says, “In principle, we use photonic crystals, a silicon slab with a regular pattern of very small holes that prohibit light propagation in silicon slabs. However, if we place two of these crystals in adjacent patterns, a waveguide is created along the boundary. Light scattering or reflection due to crystal defects is suppressed.”

Researchers wondered what would happen if such a waveguide was suddenly terminated with a “wall” of material that could not pass through. “They can’t go anywhere to light and they should accumulate in front of that wall because they suppress reflections,” Muis says. “The light will eventually bounce through the waveguide, but only after the delay. This will amplify the light field locally.”

Light density

Amolf’s Verhagen and Tu Delft’s Kobus Kuipers group decided to validate the predictions in an experiment along with Cornell researchers. The topological waveguides were made of amorphous silicon chips. To visualize the predicted accumulation of light within the photonic crystal, Muis used a unique microscope with Tu Delft to scan the light field via ultra-thin needles on the surface of the crystal. This microscope can localize light intensity on a scale of about 1,000 times the thickness of human hair.

“We actually saw a clear amplification of the optical field at the edge of the topological waveguide. Interestingly, this only happened when the ‘wall’ ending the waveguide was positioned at a certain angle. This was exactly what Cornell partners predicted,” Muis said.

“It demonstrates that light amplification is associated with topological suppression of back reflection. Light amplification is concentrated on very small volumes, as small as the wavelength of the light itself. The main advantage of this method is that it is essentially broadband.

Articles on Science Advancements with Equal Contributions from Muis and his Cornell colleague Yandong Li can be read as recipes for further research or application of light amplification in this form. The demonstrated mechanisms should also be applied to other types of waves in a structured medium, including the sound waves and even electrons of a particular crystal.

Muis said, “The next step would be to use a pulsed laser to examine the time interval at which light continues to accumulate, to see how much field amplification can be maximized, and to use it for applications of light manipulation on optical chips.”