In a major leap forward in quantum nanophotonics, a team of European and Israeli physicists has introduced a new type of polarizing cavity and redefined the limits of light confinement. The groundbreaking work, detailed in a study published today (February 6) in the journal Nature Materials, demonstrates an unconventional approach to photon confinement that overcomes traditional limitations of nanophotonics.
Physicists have long looked for ways to force photons into smaller and smaller cavities. A photon's natural length scale is wavelength, and when a photon is forced into a cavity that is much smaller than the wavelength, it actually becomes more "concentrated."
This concentration enhances interactions with electrons, amplifying quantum processes within the cavity. However, despite great success in confining light to deep subwavelength volumes, dissipation (light absorption) effects remain a major obstacle.
Photons in nanocavities are absorbed very rapidly, much faster than the wavelength, and this dissipation limits the suitability of nanocavities for some of the most exciting quantum applications.
Innovative nanocavity design
Professor Frank Koppens' research group from ICFO in Barcelona, Spain, has solved this conundrum by creating nanocavities with unparalleled subwavelength volumes and extended lifetimes.
These nanocavities are less than 100x100nm² in area and only 3nm thick, but they can confine light for a longer period of time. The key lies in the use of hyperbolic-phonon-polarons, unique electromagnetic excitations that occur in two-dimensional materials that form cavities.
Different from previous cavity studies based on phonon polaritons, this study utilizes a new indirect constraint mechanism. The nanocavities are created by drilling nanoscale holes into the gold substrate using the extremely high precision (2-3 nanometers) of a helium focused ion beam microscope.
After drilling the hole, the two-dimensional material hexagonal boron nitride (hBN) is transferred on top. Hexagonal boron nitride supports electromagnetic excitons called hyperbolic photon polaritons, which are similar to ordinary light but can be confined to extremely small volumes.
When polarons pass over the edge of a metal, they are strongly reflected by the metal and become trapped. Therefore, this method avoids directly shaping boron hydride and maintains its original mass, making the photons in the cavity highly concentrated and long-lived.
Unexpected experimental success
The discovery resulted from a chance observation while using near-field optical microscopy to scan two-dimensional material structures in another project. Near-field microscopy can excite and measure polarons in the mid-infrared range of the spectrum, and the researchers noticed that these polarons reflected unusually strongly at the edges of the metal. This unexpected observation led to further research, which led to the discovery of the unique confinement mechanism and its relationship to nanorelay formation.
However, after making and measuring the cavities, the team discovered a huge surprise. First author Dr. Hanan Herzig Sheinfux from the Department of Physics at Bar-Ilan University said: "Experimental measurements are often worse than predicted by theory, but in this case we found that the experimental results exceeded optimistic simplified theoretical predictions. This unexpected success opens the door to novel applications and advances in quantum photonics, pushing the limits of what we thought was possible."
Dr. Herzig Sheinfux conducted this research together with Professor Koppens while working as a postdoctoral fellow at ICFO. He plans to use these cavities to observe quantum effects previously thought impossible and to further investigate the fascinating counter-intuitive physics of hyperbolic phonon polariton behavior.
Compiled source: ScitechDaily