Discovery of robust negative dispersion of polaritons could lead to a new nano optics probe

Brayden Lukaskawcez, third year Ph.D. student in the School of Physics and Astronomy, is a lead author on a recent paper reporting the results of an experiment that observed abundant negative dispersion of polaritons for the first time. Most waves disperse positively, which means their energy is carried in the same direction as the wave propagates. But with negative dispersion, their flow is reversed, prompting exotic phenomena like self-focusing of waves. Harnessing polaritons’ negative dispersion is key to manipulating light on the smallest possible scales. 

Lukaskawcez is a member of Professor Alex McLeod’s nano optics laboratory that specializes in the technique of scanning near-field optical microscopy in which a tiny, incredibly sharp needle probe, vibrating at high-speed close to the sample surface, is illuminated with infrared lasers. This technique allows experiments in the McLeod Lab to go beyond the diffraction limit of light, which maintains that the smallest resolvable feature in conventional optics is proportional to the light wavelength. This unique setup can generate polaritons – wave-like excitations that combine light-fields with oscillations of a material’s charge or crystal structure, forming a hybrid excitation light and matter that is compressed to nanometer scales.  The experimental setup images these polaritons as they propagate over surfaces, allowing observation of their positive or negative dispersion while they travel.

One unique aspect of this experiment was to excite polaritons in “freestanding” 2D oxide samples – “membranes” – untethered by an underlying substrate. The advantage of this freestanding platform was that it created varied shapes and edges to explore how polaritons resonate and reflect, as well as a surface that could be transferred onto different materials. Lukaskawcez explained that many of these membranes can break or shatter under strain, while others stretch like Saran wrap: “Sometimes the effect was like pressing down on a potato chip, other times the samples were larger and continuous.” 

The results of this experiment were unexpected because they observed a robust negative dispersion of polaritons that previously would demand complicated and inefficient multi-layer structures. Lukaskawcez describes negative dispersion with a simple thought experiment: you place a lamp on one side of an empty room, and you turn on the light, light moves outwards away from the bulb, that is positive dispersion. Adding negative dispersion to the empty side of the room would produce a mirror image lightbulb where the light now moves toward the source. From the outside it would look like two light bulbs in the same room, for the price of one!

Typically physicists have relied on materials like graphene that are naturally 2D in order to generate polaritons, but by using a novel hybrid Molecular Beam Epitaxy invented by Professor Bharat Jalan’s group, Jalan’s team was able to engineer artificially 2D materials made from an almost limitless library of metal oxides. These materials have several advantages, including custom thicknesses ranging from a few angstroms to tens of nanometers, bulk-like crystallinity even at small thicknesses, and the flexibility needed for wear-able electronics.  Most importantly for the future of polaritons, membranes can be easily transferred to optical environments tailored to control their polariton dispersion, allowing negative dispersion on-demand.

Not only did the experiments realize exotic negative dispersion of polaritons, they also found the dispersion was robust enough to coexist with positive dispersion within the same membrane.  Future experiments may place a top layer or phase-change material near the freestanding membrane, which provides a tool to switch the wave dispersion from negative to positive simply with an electrical signal, or even a pulse of light.  With such programmable dispersion, nanoscale light from a “polariton lightbulb” could be redirected and focused within the “empty room” of a 2D membrane through the flip of a switch.  Flexible membranes could unlock the future of circuit boards for ultra-small light waves.