As electronic devices and circuits shrink into the nanoscale, the ability to transfer data on a chip at low power with little energy loss is becoming a critical challenge. Over the past decade, squeezing light into tiny devices and circuits has been a major goal of nanophotonics researchers. Electronic oscillations at the surface of metals, known as surface plasmon polaritons or plasmons, have become an intense area of focus. Plasmons are hybrids of light (photons) and electrons in a metal. If this nanolight can be harnessed, researchers can improve sensing, subwave-length waveguiding, and optical transmission of signals.
A novel cryogenic near-field optical microscope was developed that directly images the propagation and dynamics of graphene plasmons at variable temperatures down to -250 °C. The temperature-dependent results provide direct physical insight into the fundamental physics of plasmon propagation in graphene.
Compact nanolight has the capability to travel along the surface of graphene for distances of many tens of microns without unwanted scattering. The physics limiting the travel range of nanolight is a fundamental finding and may lead to new applications in sensors, imaging, and signal processing.
Graphene, a one-atom-thick material that is one of the most promising candidates for novel photonic materials, has optical properties that are readily tunable and can be altered at ultrafast time scales; however, implementing nanolight without introducing unwanted dissipation in graphene has been very difficult to achieve.
The new method confines light to the nanoscale. Plasmon-polaritons, or resonant modes, could be formed in the graphene and propagate through the material as hybrid excitations of light and mobile electrons. These plasmon-polariton modes can confine the energy of electromagnetic radiation, or light, down to the nanoscale. The challenge was how to visualize these waves with ultra-high spatial resolution to study the performance of plasmonic modes at varying temperatures.
A microscope was built to explore the plasmon-polariton waves at high resolution while the graphene was cooled to cryogenic temperatures. Lowering the temperatures allowed researchers to “turn off” various scattering, or dissipation, mechanisms as they cooled down the samples and learned which mechanisms were relevant.
Plasmons in graphene can be tuned and controlled via an external electric field that gives graphene an advantage over conventional plasmonic media such as metal surfaces, which are inherently non-tunable. Moreover, the lifetimes of plasmon waves in graphene are now found to exceed those in metals by a factor of 10 to 100, while propagating over comparably longer distances. These features offer advantages for graphene as a plasmonic medium in next-generation optoelectronic circuits.