Yunyun Dai

Transfer matrix method for optics in graphene layers

A transfer matrix method is developed for optical calculations of non-interacting graphene layers. Within the framework of this method, optical properties such as reflection, transmission and absorption for single-, double- and multi-layer graphene are studied. We also apply the method to structures consisting of periodically arranged graphene layers, revealing well-defined photonic band structures and even photonic bandgaps. Finally, we discuss graphene plasmons and introduce a simple way to tune the plasmon dispersion.

Plasmonic analog of electromagnetically induced transparency in nanostructure graphene

Graphene has shown intriguing optical properties as a new class of plasmonic material in the terahertz regime. In particular, plasmonic modes in graphene nanostructures can be confined to a spatial size that is hundreds of times smaller than their corresponding wavelengths in vacuum. Here, we show numerically that by designing graphene nanostructures in such deep-subwavelength scales, one can obtain plasmonic modes with the desired radiative properties such as radiative and dark modes. By placing the radiative and dark modes in the vicinity of each other, we further demonstrate electromagnetically induced transparency (EIT), analogous to the atomic EIT. At the transparent window, there exist very large group delays, one order of magnitude larger than those offered by metal structures. The EIT spectrum can be further tuned electrically by applying a gate voltage. Our results suggest that the demonstrated EIT based on graphene plasmonics may offer new possibilities for applications in photonics.

Nanofocusing in a tapered graphene plasmonic waveguide

Gated or doped graphene can support plasmons making it a promising plasmonic material in the terahertz regime. Here, we show numerically that in a tapered graphene plasmonic waveguide mid- and far-infrared light can be focused in nanometer scales, far beyond the diffraction limit. The underlying physics lies in that when propagating along the direction towards the tip both the group and phase velocities of the plasmons supported by the tapered graphene waveguide are reduced accordingly, eventually leading to nanofocusing at the tip with a huge enhancement of optical fields. The nanofocusing of optical fields in tapered graphene plasmonic waveguides could be potentially exploited in the enhancement of light–matter interactions.

Symmetry breaking induced excitations of dark plasmonic modes in multilayer graphene ribbons

Multilayer graphene can support multiple plasmon bands. If structured into graphene ribbons, they can support multiple localized plasmonic modes with interesting optical properties. However, not all such plasmonic modes can be excited directly due to the constrains of the structural symmetry. We show by numerical simulations that by breaking the symmetry all plasmonic modes can be excited. We discuss the general principles and properties of two-layer graphene ribbons and then extend to multilayer graphene ribbons. In multilayer graphene ribbons with different ribbon widths, a tunable broadband absorption can be attained due to the excitations of all plasmonic modes. Our results suggest that these symmetry-broken multilayer graphene ribbons could offer more degrees of freedom in designing photonic devices.

Gate-tunable third-order nonlinear optical response of massless Dirac fermions in graphene

Graphene with massless Dirac fermions can have exceptionally strong third-order optical nonlinearities. Yet reported values of nonlinear optical susceptibilities for third-harmonic generation (THG), four-wave mixing (FWM) and self-phase modulation vary over six orders of magnitude. Such variation likely arises from frequency-dependent resonance effects of different processes in graphene under different doping. Here, we report an experimental study of THG and FWM in graphene using gate tuning to adjust the doping level and vary the resonant condition. We find that THG and sum-frequency FWM are strongly enhanced in heavily doped graphene, while the difference-frequency FWM appears just the opposite. Difference-frequency FWM exhibited a novel divergence towards the degenerate case in undoped graphene, leading to a giant enhancement of the nonlinearity. The results are well supported by theory. Our full understanding of the diverse nonlinearity of graphene paves the way towards future design of graphene-based nonlinear optoelectronic devices.Third-harmonic generation and four-wave mixing of light can be enhanced in graphene with gate tuning to adjust the doping level. The findings may lead to new graphene-based nonlinear optoelectronic devices.

Enhanced light-matter interactions in graphene-covered dielectric magnetic mirrors.

Enhanced interactions of light with graphene on the surface of a lossless dielectric magnetic mirror (DMM) are studied theoretically and experimentally in the visible range, where the DMM is composed of truncated dielectric photonic crystals (PCs). The absorption of graphene on the DMM was enhanced by about 4-fold for the spectral range within the forbidden gap of PCs over a wide range of incidence angles for both transverse electric and transverse magnetic polarizations compared with that of free-standing graphene. Moreover, the enhanced local electric field on the DMM surface led to much better detection efficiencies of the photocurrent, Raman spectroscopy and enhanced third-harmonic generation of graphene. These results offer a new way to achieve an enhanced interaction of light with graphene and develop new compact graphene-based devices.