NEW YORK, Feb. 11, 2021 — A team at Columbia University has introduced a way to program a layered crystal in such a way that it is able to open doors to imaging capabilities beyond common limits, on demand. The technique exerts control over nanolight — light that is able to access the nanoscale — providing insight into the field of optical quantum information processing.
Light can only be focused down to a certain level, referred to as the Abbe limit. At this point, to a traditional optical microscope, objects that are closer than this limit would appear as one. Under certain conditions, however, these rules can be broken, using van der Waals crystals; these crystals are layered and possess semiconductor properties that allow for on-demand optical carrier injection.
In these specific instances, light can be confined without limit, enabling imaging of even the smallest of objects.
An optically excited gas of electronic carriers confined to the planes of the layered van der Waals semiconductor tungsten diselenide is shown. The consequent hyperbolic response permits passage of nanolight. Courtesy of Ella Maru Studio.
The researchers, who have been studying and working with van der Waals crystals for the past few years, used tungsten diselenide, a compound that garners considerable interest for potential use in numerous applications due to its electric and photonic properties.
In terms of the crystal’s layered structure, the consequent strong anisotropy of the electronic structure is conducive for electronic hyperbolicity, or the ability to pass nanolight through its structure, said Aaron Sternbach, a graduate student in Columbia’s Department of Physics.
By illuminating the crystal with a pulse of light, the team changed the crystal’s electronic structure.
“The absorption process promotes electronic charge carriers, or electron-hole pairs, to an excited state. In the excited state, the charge carriers can reflect mid-infrared light, but since this is an anisotropic layered crystal, only light incident from the ‘top down’ is reflected while the crystal remains transmissive for light of the same frequency which is incident from the ‘side’ — which satisfies the precondition for hyperbolicity,” Sternbach told Photonics Media. “The best name I can think of for this process is ‘optical carrier injection.’”
The new structure arising from this reorganization enables superfine nanoscale details to be transported through the crystal where they could be imaged on the surface.
However, the technology isn’t without its drawbacks.
“A big challenge with hyperbolicity rooted in electronic processes is the relatively high loss within the crystals. Losses can be mitigated at low temperatures, or potentially in other crystals that are less sensitive to temperature-driven fluctuations,” Sternbach said. “Mitigating losses and thereby improving the photonic quality factors in hyperbolic crystals rooted in electronic processes are likely to be subjects of next-generation research.”
As far as next steps, because the discovery shows a method of accessing hyperbolic properties on demand and on ultrafast timescales, Sternbach believes that there will be more research efforts focusing on areas where light plays a dual role in programming quantum matter and in investigating emergent optically driven phases with nano-optics.
“We are also likely to see further reports where electronic forms of hyperbolicity are uncovered,” Sternbach said.
Dmitri Basov, the Higgins Professor of Physics at Columbia University and senior author on the research paper, expressed similar sentiment. “Laser pulses allowed us to create a new electronic state in this prototypical semiconductor, if only for a few picoseconds,” he said. “This discovery puts us on track toward optically programmable quantum phases in new materials.”
The research was published in Science (www.doi.org/10.1126/science.abe9163).