Enhancing the graphene photocurrent using surface plasmons and a p-n junction:

Compared to conventional photodetectors, graphene photodetectors (GPDs) have unprecedented compactness, ultra-broad detection bandwidth, and ultra-fast response speed. However, since graphene is only a single layer of carbon atoms, it absorbs light very weakly, resulting in a very low responsivity in GPDs. We developed a novel GPD architecture which hybridizes plasmonic and electrical enhancements. Specifically, the plasmonic structure increases light-to-electricity conversion in GPD, and a split gate creates a p-n junction in graphene to augment the photo-electron harvesting. We demonstrate a 25-fold increase in photocurrent generation with our proposed architecture.

Spatial and temporal nanoscale plasmonic heating quantified by thermoreflectance:

Thermoplasmonics (optical/plasmonic heating systems) is a burgeoning field because it provides remotely controllable nanoscale heat sources. We characterize the optical heating of gap plasmon structures (with varying disk radius) with both high spatial resolution (200 nm) and temporal resolution (1 ps), using optical thermoreflectance imaging (OTI) and time-domain thermoreflectance (TDTR). Both methods rely on the temperature-dependent surface reflectivity (ΔR = ΔT × C_th) of materials to probe temperature changes. With OTI, we obtain the thermal maps and reveal the correlation between temperature rise and optical absorption; with TDTR, we measure the dynamic temperature decay in response to impulse heating. By fitting the TDTR result with numerical simulation, we quantitatively retrieve the temperature decay time constant, and show that it varies with the optical absorption of the plasmonic structure.

Synchrotron radiation from an accelerating light pulse

Synchrotron radiation is generated by charged particles traveling in curved trajectory at relativistic speeds, and is usually observed in large scale facilities such as particle accelerators. We demonstrate synchrotron radiation using a light pulse, a metasurface, and nonlinear crystal, on the scale of 100 microns. The metasurface, patterned on the nonlinear crystal lithium tantalate (LiTaO3), forces the light pulse to propagate in curved trajectory in LiTaO3, therefore the light pulse mimics a spiraling charged particle that emits synchrotron radiation. In our system, the synchrotron radiation from the light pulse is generated by the nonlinear polarization induced by the pulse. We detect the propagating light pulse and its synchrotron radiation using a spatially resolved pump-probe setup. Since the light pulse propagates at a speed three times that of its nonlinear polarization, the detected radiation pattern represents that of superluminal synchrotron radiation (i.e. radiation emitted by charged particle traveling in curved trajectory at speeds greater than speed of light). We demonstrate the world's smallest synchrotron radiation system, and our findings hold promise for the development of powerful on-chip THz light sources.

Accelerating light with metasurfaces

It is possible to create light beams that do not propagate in a straight line. Such beams are called accelerating beams, and are commonly generated using experimental setups that require length scales of at least several centimeters and have a limited ability to accelerate light inside a solid state material. However, with a carefully designed and fabricated 33-nanometer-thick planar device known as a metasurface, light can be guided along a hundred-micron-scale circular arc inside a glass chip. Moreover, the experimentally realized metasurfaces can bend light to much steeper angles than can conventional bulky setups, and precisely control the trajectory of light propagation. Such metasurfaces open up new applications for accelerating beams.

Plasmon resonance in multilayer graphene nanoribbons:

Despite being capable of achieving the highest confinement among all existing conductive materials, graphene does suffer from poor plasmonic response because the single atomic sheet only interacts weakly with incident EM wave. Multilayer graphene sheets have been theoretically and experimentally demonstrated to exhibit higher conductivity than single layer graphene due to strong interlayer coupling, but carrier mobility is expected to decrease as the number of graphene layer increases. In this work, we investigate how the graphene plasmon resonance is affected by the interplay between these two competing effects.

We experimentally studied the plasmon resonance in single- double- and triple layer graphene nanoribbons (GNRs). We observed an increase in the resonance intensity from single layer GNRs to double layer GNRs, but the trend was not obvious from double layer to triple layer. We also developed a numerical model in which graphene was modeled as a surface current, the new model greatly reduces the computational cost and instability compared to conventionally used finite-thickness model, while maintaining a good match with experimental results.