A combined interferometric and microscopy technique has revealed the strange ways in which light moves when confined inside hyperbolic materials. Researchers at CIC nanoGUNE in the Basque Country and the Institute of Photonic Sciences (ICFO) in Catalunya have observed, for the first time, ultraslow pulse propagation and backward propagating waves in a subwavelength-scale (135-nm) slab of boron nitride, a natural material that exhibits hyperbolic optical properties at mid-infrared frequencies. The work lays the foundations for studying the precise manner in which light travels through complex optical systems at the subwavelength scale, the researchers said. Such a capability will be vital for verifying that future nanophotonic devices, perhaps with biosensing or optical computing applications, function as expected. Incident light pulses are converted by a gold (Au) film into slow hyperbolic polariton (HP) pulses propagating in the boron nitride (h-BN) slab. The HPs are traced in space and time by first scattering them with a nanoscale sharp scanning tip (top right), and then measuring the time delay between scattered and incident pulses as a function of tip position. Courtesy of CIC nanoGUNE. Confined within the boron nitride, light is coupled to the vibrations of the matter itself and travels in the form of phonon polaritons. Polaritons are considered a double-edged sword by the scientists trying to study them. On the one hand, they squeeze light into much smaller volumes than is normally possible, which is helpful for applications requiring the manipulation of light in tiny spaces, such as detecting and identifying individual molecules. On the other hand, ultrahigh confinement means that special techniques have to be developed to observe polariton behavior. "Because the wavelength of a polariton is so small, we cannot use conventional optical equipment, such as lenses and cameras, to image it," said former nanoGUNE postdoctoral researcher Edward Yoxall. Measured dispersion (energy versus momentum) diagram of hyperbolic phonon polaritons in boron nitride. Courtesy of CIC nanoGUNE. Instead, the researchers used a scattering-type scanning near-field IR microscope — which can visualize details just 10 nm in size — in concert with time-domain interferometry using 100-fs light pulses. This allowed them to watch polaritons passing different locations along the boron nitride slab and measure their speeds. This time- and space-resolved mapping revealed a range of intriguing polariton behaviors, including a dramatic slowing of pulse velocity — 0.002 c, or less than 1 percent of light's velocity in a vacuum — and a reversal of the direction in which polariton waves propagate in relation to the direction of the energy flow. Polariton lifetimes were shown to be in the picosecond range. "An exciting result is the speed at which the polariton moves," Yoxall said. "There's a lot of interest in slow light, and what we've shown here is a novel way of achieving this." Slow light in conventional photonic structures has potential for applications in sensing and communication technologies, owing to enhanced light-matter interactions. The deep subwavelength-scale confinement of slow polaritons in hyperbolic materials could help to miniaturize devices based on this effect. Funding came from the European Commission's Graphene Flagship program. The research was published in Nature Photonics (doi: 10.1038/nphoton.2015.166).