A team of European and Israeli physicists has made a breakthrough in quantum nanophotonics. They have introduced a new type of polariton cavity and redefined the limits of photon confinement. The pioneering work, detailed in a paper published in Nature Materials on 6 June, demonstrates an unconventional way of confining photons that overcomes the conventional limits of nanophotonics.
Physicists have long been looking for ways to compress photons smaller and smaller. The spatial scale of a photon is the wavelength. When a photon is forced into a cavity much smaller than its wavelength, it becomes more “concentrated”. This enhances the photon’s interaction with electrons and amplifies quantum processes in the cavity. However, despite the great success of scientists in limiting the photon volume to the deep sub-wavelength range, the effect of dissipation remains a major obstacle. Photons in nanocavities are absorbed very quickly, and this dissipation limits the suitability of nanocavities for some quantum applications.
The research team overcame these limitations this time by creating nanocavities with breakthrough sub-wavelength volumes and lifetimes. These nanocavities have an area of less than 100 x 100 square nanometres and a thickness of only 3 nanometres, limiting light for much longer. The key to this is using hyperbolic phonon polarisation excitons, a unique electromagnetic excitation that occurs in the two-dimensional material in which the cavities are formed.
Unlike before, this study utilized a new indirect confinement mechanism. The researchers drilled nanocavities in a gold substrate. After punching the holes, they transferred the two-dimensional material hexagonal boron nitride over the gold substrate. Hexagonal boron nitride can help to enable the electromagnetic excitation process of hyperbolic phonon polarised excitons. When polaritons pass above the edge of the gold substrate, they are strongly reflected and thus confined. This approach therefore avoids direct shaping of the hexagonal boron nitride while preserving its pristine quality, resulting in highly confined and long-lived photons in the cavity.
This result opens the door to new applications and advances in quantum photonics, breaking previously thought limits on photon confinement. As a next step, the researchers intend to use these cavities to observe previously considered impossible quantum effects and further investigate the intriguing and counter-intuitive physics of hyperbolic phonon polarisation exciton behavior.
Light is an unruly being for which scientists seek to build cages. While photonic devices are limited in size by the unavoidable diffraction limit, breakthroughs in materials science have led to new types of nanocavities that confine light beyond the diffraction limit and are the cornerstone of future optoelectronic manipulation – not only suitable for manipulating individual photons but also for helping optical pathways to replace electronic ones, thereby reducing power consumption. Gold thin film has been chosen as the substrate material for the nanocavity due to its excellent mirror optical properties, while hexagonal boron nitride is a popular two-dimensional material after graphene. They will join hands to open up new dimensions of semiconductor applications and bring us more surprises.