Nanocavity Approach Enables Unprecedented Confinement, Lifetime

Researchers from The Institute of Photonic Sciences (ICFO) have introduced a type of polaritonic cavity that redefines the limits of light confinement. The work demonstrates an unconventional way of confining photons, overcoming traditional limits in nanophotonics.

Confining photons to increasingly small volumes has long been an area of interest for physicists. The natural length scale of the photon is the wavelength and when a photon is forced into a cavity much smaller than the wavelength, it effectively becomes more concentrated. This concentration enhances interactions with electrons, amplifying quantum processes within the cavity.

Researchers have developed nanocavities that enable deep subwavelength volume and extended lifetime using an unconventional method. The technique has potential applications in a variety of quantum technologies. Courtesy of Matteo Ceccanti/ICFO.

However, despite significant success in confining light into deep subwavelength volumes, the effect of dissipation, or optical absorption, remains a major obstacle. Photons in nanocavities are absorbed very quickly, much faster than the wavelength, and this dissipation limits the applicability of nanocavities to major applications in quantum technology.

Researchers in the quantum nano-optoelectronics group led by ICFO professor Frank Koppens addressed this challenge by creating nanocavities with an unparalleled combination of subwavelength volume and extended lifetime.

The discovery began with a chance observation made during a different project while using a nearfield optical microscope to scan 2D material structures. The nearfield microscope enables the exciting and measuring of polaritons in the mid-infrared range of the spectrum; the researchers noticed an unusually strong reflection of these polaritons from the metallic edge. This unexpected observation sparked a deeper investigation, leading to the realization of the unique confinement mechanism and its relation to nanoray formation.

The nanocavities, measuring smaller than 100 × 100 nm in area and only 3-nm thin, confine light for significantly longer durations. The key lies in the use of hyperbolic-phonon-polaritons, unique electromagnetic excitations occurring in the 2D material forming the cavity.

Unlike previous studies on phonon polariton-based cavities, this work uses a new and indirect confinement mechanism. The nanocavities are crafted by drilling nanoscale holes in a gold substrate with the extreme (2 to 3 nm) precision of a helium-focused ion beam microscope.

Hexagonal boron nitride (hBN), the 2D material at the heart of the research advancement, supports the production of hyperbolic photon polaritons that can be confined to extremely small volumes. Courtesy of Matteo Ceccanti/ICFO.

After making the holes, hexagonal boron nitride (hBN), a 2D material, is transferred on top of it. The hBN supports electromagnetic excitations called hyperbolic photon polaritons, which are similar to ordinary light, except that they can be confined to extremely small volumes.

When the polaritons pass above the edge of the metal, they experience a strong reflection, which allows them to be confined. This method thus avoids shaping the hBN directly and preserves its pristine quality, enabling highly confined photons in the cavity with long lifetimes.

“Experimental measurements are usually worse than theory would suggest, but in this case, we found the experiments outperformed the optimistic simplified theoretical predictions,” said first author and researcher in Bar-Ilan University’s Department of Physics, Hanan Herzig Sheinfux. “This unexpected success opens doors to novel applications and advancements in quantum photonics, pushing the boundaries of what we thought was possible.”

Herzig Sheinfux conducted the research in Koppens’ group as a postdoctoral researcher at ICFO. He intends to use these cavities to see quantum effects that were previously thought impossible, as well as to further study the intriguing and counterintuitive physics of hyperbolic phonon polariton behavior.

The research was published in Nature Materials (www.doi.org/10.1038/s41563-023-01785-w).