The combination of graphene with noble-metal nanostructures is currently being explored for strong light-graphene interaction enhanced by plasmons. We introduce a novel hybrid graphene-metal system for studying light-matter interactions with gold-void nanostructures exhibiting resonances in the visible range. Strong coupling of graphene layers to the plasmon * To whom correspondence should be addressed † DTU Fotonik ‡ CNG ¶ Fudan University § DTU Nanotech CINF 1 modes of the nanovoid arrays results in significant frequency shifts of the underlying plasmon resonances, enabling more than 30% absolute light absorption in a single layer of graphene and up to 700-fold enhancement of the Raman response of the graphene. These new perspectives enable us to verify the presence of graphene on gold-void arrays and the enhancement even allows us to accurately quantify the number of layers. Experimental observations are further supported by numerical simulations and perturbation-theory analysis. The graphene gold-void platform is beneficial for sensing of molecules and placing R6G dye molecules on top of the graphene, we observe a strong enhancement of the R6G Raman fingerprints. These results pave the way toward advanced substrates for surface-enhanced Raman scattering (SERS) with potential for unambiguous single-molecule detection on the atomically well-defined layer of graphene.Graphene is an atomic monolayer formed by carbon hexagons, whose extraordinary electrical and optical properties have led to a range of promising optoelectronic devices, 1-3 such as photodetectors, 4 optical modulators, 5 and ultra-fast lasers. 6 However, all such devices suffer from the inherently weak interaction between pristine graphene and light (2.3% light absorption at normal incidence), therefore imposing substantial challenges and restrictions for many electro-optical and all-optical applications. 7,8 Doped graphene nanostructures which support surface plasmons in the teraherz and infrared regions offer an exciting route to increase the light-graphene interaction by confining the optical fields below the diffraction limit. 9-13 However, graphene is less attractive when the interband loss becomes large, and it effectively mimics a dielectric material in the visible and near-infrared frequencies. 14 One alternative way to enhance the light-graphene interaction in short wavelengths is the combination of graphene with conventional plasmonic nanostructures based on noble metals. 15 These graphene-plasmonic hybrid structures could be beneficial for both fields of investigation: first of all, graphene can influence the optical response of plasmonic structures leading to graphene-based tunable plasmonics, 16 and in turn, plasmonic nanostructures can dramatically enhance the local electric field, leading to strong light absorption and Raman signature of graphene layers.In this Letter, a novel platform based on graphene-covered gold nanovoid arrays (GNVAs) is 2 proposed to enhance the light-matter interaction in graphene-plasmonic hybrid str...

Graphene-based photodetectors, taking the advantages of high carrier mobility and broadband absorption in graphene, have recently experienced rapid development. However, their performance with respect to responsivity and bandwidth is still limited by weak light-graphene interaction and large resistance-capacitance product. Here, we demonstrate a waveguide coupled integrated graphene plasmonic photodetector on a silicon-on-insulator platform. Benefiting from plasmonic enhanced graphene-light interaction and subwavelength confinement of the optical energy, we achieve a small-footprint grapheneplasmonic photodetector working at the telecommunication window, with large bandwidth beyond 110 GHz and high intrinsic responsivity of 360 mA/W. Attributed to the unique electronic bandstructure of graphene and its ultra-broadband absorption, operational wavelength range extending beyond mid-infrared, and possibly further, can be anticipated. Our results show that the combination of graphene with plasmonic devices has great potential to realize ultra-compact and high-speed optoelectronic devices for graphene-based optical interconnects. arXiv:1808.04815v3 [physics.app-ph]

With unique possibilities for controlling light in nanoscale devices, graphene plasmonics has opened new perspectives to the nanophotonics community with potential applications in metamaterials, modulators, photodetectors, and sensors. In this paper, we briefly review the recent exciting progress in graphene plasmonics. We begin with a general description of the optical properties of graphene, particularly focusing on the dispersion of graphene-plasmon polaritons. The dispersion relation of graphene-plasmon polaritons of spatially extended graphene is expressed in terms of the local response limit with an intraband contribution. With this theoretical foundation of graphene-plasmon polaritons, we then discuss recent exciting progress, paying specific attention to the following topics: excitation of graphene plasmon polaritons, electron-phonon interactions in graphene on polar substrates, and tunable graphene plasmonics with applications in modulators and sensors. Finally, we address some of the apparent challenges and promising perspectives of graphene plasmonics.

Graphene opens up for novel optoelectronic applications thanks to its high carrier mobility, ultralarge absorption bandwidth, and extremely fast material response. In particular, the opportunity to control optoelectronic properties through tuning of Fermi level enables electro-optical modulation, optical-optical switching, and other optoelectronics applications. However, achieving a high modulation depth remains a challenge because of the modest graphene-light interaction in the graphene-silicon devices, typically, utilizing only a monolayer or few layers of graphene. Here, we comprehensively study the interaction between graphene and a microring resonator, and its influence on the optical modulation depth. We demonstrate graphene-silicon microring devices showing a high modulation depth of 12.5 dB with a relatively low bias voltage of 8.8 V. On-off electro-optical switching with an extinction ratio of 3.8 dB is successfully demonstrated by applying a square-waveform with a 4 V peak-to-peak voltage.Key words: graphene photonics, silicon microring resonator, electro-optical modulation, high modulation depth In addition to holding novel electronic properties, graphene is now also emerging as a material of interest in the area of optoelectronics. Graphene has many unique properties, such as zero-band gap and tunable Fermi level [1, 2], ultra-broad absorption bandwidth [3,4], high carrier mobility around 200000 cm 2 V -1 s -1 at room temperature [5,6], and a super high Kerr coefficient for high nonlinearity applications [7,8]. Those interesting properties give rise to many potential applications [9,10], such as wafer-scale integrated circuits [11], solar cells [12,13], high-speed graphene-silicon electro-optical modulators [14][15][16], optical-optical switches [17,18], saturation absorbers [19][20][21], photodetectors [22][23][24], and nonlinear media for four-wave mixing (FWM) [25,26].The deployment of graphene on top of a silicon waveguide is an efficient mean to make graphene-silicon hybrid devices. In order to electrostatically tune the Fermi level of graphene, there is a need to sandwich a thin layer of material with a high dielectric constant (e.g. Al 2 O 3 [27,28] or Si 3 N 4 [29]) between the silicon layer and the added layer of graphene. When this graphene-silicon capacitor is biased, carriers can be either accumulated on the graphene sheet, or swept out from the graphene sheet, resulting in a convenient tuning of the Fermi level and, thus, optical absorption [14]. This technology has enabled highspeed electro-optical modulators [14][15][16]. However, the strong light confinement in the high-index silicon gives a modest optical field overlap with the graphene layer. Hence, the graphene-light interaction is consequently too low to obtain a significant modulation depth. In order to enhance the modulation depth,

Nanostructured graphene on SiO2 substrates pave the way for enhanced light-matter interactions and explorations of strong plasmon-phonon hybridization in the mid-infrared regime. Unprecedented large-area graphene nanodot and antidot optical arrays are fabricated by nanosphere lithography, with structural control down to the sub-100 nanometer regime. The interaction between graphene plasmon modes and the substrate phonons is experimentally demonstrated and structural control is used to map out the hybridization of plasmons and phonons, showing coupling energies of the order 20 meV. Our findings are further supported by theoretical calculations and numerical simulations.Plasmon polaritons, the light-driven collective oscillation of electrons, provide the foundation for various applications ranging from metamaterials, plasmonics, photocatalysis to biological sensing [1]. Owing to the two dimensional (2D) feature of the excitations, plasmon polaritons supported by graphene are of particular interest [2][3][4]. Tight mode confinement, long propagation distance, and remarkable electrostatic tunability of graphene plasmon polaritons lead to new applications for waveguides, modulators and super-lenses [5] [6] [7]. Plasmon polaritons in a graphene sheet can be excited by advanced near-field scattering microscopy [3, 4], dielectric subwavelength grating coupler [8,9], or nanoscale patterning [10][11][12][13][14], while the dispersion of graphene plasmons may be influenced by the interaction of electrons and the surface optical phonons of the polar substrate [15][16][17]. Using angle-resolved reflection electronenergy-loss spectroscopy, strong plasmon-phonon coupling has been confirmed in epitaxial graphene placed on the silicon carbide substrate [18]. The plasmonphonon interaction leads to additional damping channel for graphene plasmons by the mid-infrared transmission measurement assisted by an infrared microscope coupled to a Fourier-transform infrared spectrometer [19]. More recently, phonon-induced transparency in bilayer graphene nanoribbon has been demonstrated, resulting in a maximum slow light factor of around 500 [20].In this paper, we demonstrate an effective approach for patterning graphene sheets into large-area ordered graphene nanostructures by combining nanosphere lithography (NSL) with O 2 reactive ion etching (RIE). Without high-cost and low-throughput lithographic patterning and sophisticated instruments, we realize nanoscale graphene dot and antidot arrays with dimensions down to 100 nm via a single-step fabrication process. Mid-infrared (5 -15 µm) plasmons in graphene dot and antidot arrays can be controlled through either the structure size or plasmon-phonon hybridization (when using a polar substrate). Measured reflection spectra illustrate the excitation of plasmons in the graphene nanostructures and their coupling with surface polar phonons in the SiO 2 substrate. These results are further supported by theoretical predictions and numerical simulations. Our study has enabled large-area fabrication ...

Slow light has been widely utilized to obtain enhanced nonlinearities, enhanced spontaneous emissions and increased phase shifts owing to its ability to promote light–matter interactions. By incorporating a graphene on a slow-light silicon photonic crystal waveguide, here we experimentally demonstrate an energy-efficient graphene microheater with a tuning efficiency of 1.07 nmmW−1 and power consumption per free spectral range of 3.99 mW. The rise and decay times (10–90%) are only 750 and 525 ns, which, to the best of our knowledge, are the fastest reported response times for microheaters in silicon photonics. The corresponding figure of merit of the device is 2.543 nW s, one order of magnitude better than results reported in previous studies. The influence of the length and shape of the graphene heater to the tuning efficiency is further investigated, providing valuable guidelines for enhancing the tuning efficiency of the graphene microheater.

We experimentally demonstrate graphene-plasmon polariton excitation in a continuous graphene monolayer resting on a two-dimensional subwavelength silicon grating. The subwavelength silicon grating is fabricated by a nanosphere lithography technique with a self-assembled nanosphere array as a template. Measured transmission spectra illustrate the excitation of graphene-plasmon polaritons, which is further supported by numerical simulations and theoretical prediction of plasmonband diagrams. Our grating-assisted coupling to graphene-plasmon polaritons forms an important platform for graphene-based opto-electronics applications. * saxi@fotonik.dtu.dk 1

Abstract:We investigate the performance of bends and splitters in graphene nanoribbon waveguides. Although the graphene waveguides are lossy themselves, we show that bends and splitters do not induce any additional loss provided that the nanoribbon width is sub-wavelength. We use transmission line theory to qualitatively interpret the behavior observed in our simulation. Our results pave a promising way to realize ultra-compact devices operating in the terahertz region.