Gilles Rosolen

Nonuniform doping of graphene for plasmonic tapers

A numerical study of graphene plasmons (GPs) propagating along various doping profiles of the graphene sheet is proposed. We show that the GP reflection at an abrupt doping interface is a simple function of the doping difference, consistent with a Fresnel model. In addition, improved transmission is obtained with a tapered interface and we find an optimal size for the transition zone. By considering localized inhomogeneities we describe a Fabry–Perot type cavity leading to an absorption above 60%. Furthermore, an alternative design of a 6 nm length cavity is proposed achieving 100% absorption. These results may lead to novel optical switches, circuits or nano-sensors.

Nonlinear optical enhancement caused by a higher order multipole mode of metallic triangles

We describe a nonlinear optical study of gold triangles that exploits a higher order plasmonic resonance. A comprehensive nonlinear optical characterisation was performed both by second harmonic generation (SHG) and two photon fluorescence spectroscopy (2PF). We demonstrate and explain the enhancement of the coherent and incoherent nonlinear optical emission by a higher order multipolar mode of the plasmonic structure. The peculiarities of the mode shape and its influence on intensity and polarisation of the nonlinear signal are experimentally and numerically confirmed.

Graphene as a transparent electrode for amorphous silicon-based solar cells

The properties of graphene in terms of transparency and conductivity make it an ideal candidate to replace indium tin oxide (ITO) in a transparent conducting electrode. However, graphene is not always as good as ITO for some applications, due to a non-negligible absorption. For amorphous silicon photovoltaics, we have identified a useful case with a graphene-silica front electrode that improves upon ITO. For both electrode technologies, we simulate the weighted absorption in the active layer of planar amorphous silicon-based solar cells with a silver back-reflector. The graphene device shows a significantly increased absorbance compared to ITO-based cells for a large range of silicon thicknesses (34.4% versus 30.9% for a 300 nm thick silicon layer), and this result persists over a wide range of incidence angles.

Making two-photon processes dominate one-photon processes using mid-IR phonon polaritons

Significance The recent discovery of nanoscale-confined phonon polaritons in polar dielectric materials has generated vigorous interest because it provides a path to low-loss nanoscale photonics at technologically important mid-IR and terahertz frequencies. In this work, we show that these polar dielectrics can be used to develop a bright and efficient spontaneous emitter of photon pairs. The two-photon emission can completely dominate the total emission for realistic electronic systems, even when competing single-photon emission channels exist. We believe this work acts as a starting point for the development of sources of entangled nano-confined photons at frequency ranges where photon sources are generally considered lacking. Additionally, we believe that these results add a dimension to the great promise of phonon polaritonics. Phonon polaritons are guided hybrid modes of photons and optical phonons that can propagate on the surface of a polar dielectric. In this work, we show that the precise combination of confinement and bandwidth offered by phonon polaritons allows for the ability to create highly efficient sources of polariton pairs in the mid-IR/terahertz frequency ranges. Specifically, these polar dielectrics can cause emitters to preferentially decay by the emission of pairs of phonon polaritons, instead of the previously dominant single-photon emission. We show that such two-photon emission processes can occur on nanosecond time scales and can be nearly 2 orders of magnitude faster than competing single-photon transitions, as opposed to being as much as 8–10 orders of magnitude slower in free space. These results are robust to the choice of polar dielectric, allowing potentially versatile implementation in a host of materials such as hexagonal boron nitride, silicon carbide, and others. Our results suggest a design strategy for quantum light sources in the mid-IR/terahertz: ones that prefer to emit a relatively broad spectrum of photon pairs, potentially allowing for new sources of both single and multiple photons.

Patterned graphene edges for tailored reflection of plasmonic modes.

Combining graphene with plasmonics is expected to lead to new nanoscale applications such as sensors, photodetectors, and optical circuits, since graphene plasmons in the infrared have relatively low losses and are easily tunable. It was shown that the edges of a graphene sheet completely reflect these plasmons with negligible radiation losses. Here, however, we examine structured graphene edges, which provide the ability to tailor and even completely cancel the reflection. These properties depend on the suitable dimensions of the edge grating. We explain the reflection modulation via the appearance of longitudinal Fabry-Perot type modes. Interesting phase changes and resonances appear when the longitudinal modes interact with lateral modes mediated by edge plasmons.

Metasurface-based multi-harmonic free-electron light source

Metasurfaces are subwavelength spatial variations in geometry and material where the structures are of negligible thickness compared to the wavelength of light and are optimized for far-field applications, such as controlling the wavefronts of electromagnetic waves. Here, we investigate the potential of the metasurface near-field profile, generated by an incident few-cycle pulse laser, to facilitate the generation of high-frequency light from free electrons. In particular, the metasurface near-field contains higher-order spatial harmonics that can be leveraged to generate multiple higher-harmonic X-ray frequency peaks. We show that the X-ray spectral profile can be arbitrarily shaped by controlling the metasurface geometry, the electron energy, and the incidence angle of the laser input. Using ab initio simulations, we predict bright and monoenergetic X-rays, achieving energies of 30 keV (with harmonics spaced by 3 keV) from 5-MeV electrons using 3.4-eV plasmon polaritons on a metasurface with a period of 85 nm. As an example, we present the design of a four-color X-ray source, a potential candidate for tabletop multicolor hard X-ray spectroscopy. Our developments could help pave the way for compact multi-harmonic sources of high-energy photons, which have potential applications in industry, medicine, and the fundamental sciences.Metasurfaces: X-rays from electrons hit by laser‘Metasurfaces’, whose composition and surface geometry vary at scales smaller than the wavelength of light, could become the new tunable sources of high-energy photons such as X-rays and gamma rays. Gilles Rosolen and colleagues at the Massachusetts Institute of Technology, USA, with co-workers in Belgium and Singapore, used simulation studies to explore the possibilities. Their work demonstrates how a pulsed laser could stimulate free electrons traveling close to a metasurface to generate photons with much higher energies than the incident laser light. The emitted frequency spectrum profile could be controlled by changing the metasurface geometry, the energy of the free electrons, and the angle of the applied laser light. The researchers say that the procedure could be developed to build novel tabletop sources of high-energy photons with many potential applications in industry, medicine and fundamental science.

Graphene plasmons embedded in a gain medium: layer and ribbon plasmons

Graphene plasmonics has attracted much attention due to its remarkable properties such as tunable conductivity and extreme confinement. However, losses remain one of the major drawbacks to developing more efficient devices based on graphene plasmons. Here we show that when a gain medium is introduced around a 1D graphene sheet, lossless propagation can be achieved for a critical gain value. Both numerics and analytics are employed; and with the Drude approximation the analytical expression for this critical gain becomes remarkably simple. Furthermore, we examine a single 2D graphene nanoribbon within a gain environment. We report that the plasmonic resonant modes exhibit a spasing effect for a specific value of the surrounding gain. This feature is indicated by an absorption cross section that strongly increases and narrows. Finally, we manage to connect the ribbon results to the 1D sheet critical gain, by taking external coupling into account.