The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

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Graphene photonics and optoelectronics

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

The optical conductance of monolayer graphene is defined solely by the fine structure constant, α =  (where e is the electron charge, is Dirac's constant and c is the speed of light). The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero-gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band is demonstrated. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the graphene thickness. These results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth, and wideband tunability.

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Atomic‐Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers

The optical conductance of monolayer graphene is defined solely by the fine structure constant. The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero-gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, we demonstrate the use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the thickness of graphene. Our results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth and wideband tuneability.

Atomic layer graphene as saturable absorber for ultrafast pulsed lasers

Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solutions are becoming indispensable. The recent realization that 2D layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this Review, we cover the state of the art of optical modulators based on 2D materials, including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as 2D heterostructures, plasmonic structures, and silicon and fibre integrated structures. We also take a look at the future perspectives and discuss the potential of yet relatively unexplored mechanisms, such as magneto-optic and acousto-optic modulation.

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Optical modulators with 2D layered materials

Time-resolved transmissivity and reflectivity of exfoliated graphene and thin graphite films on a 295 K SiO(2)/Si substrate are measured at 1300 nm following excitation by 150 fs, 800 nm pump pulses. From the extracted transient optical conductivity we identify a fast recovery time constant which increases from approximately 200 to 300 fs and a longer one which increases from 2.5 to 5 ps as the number of atomic layers increases from 1 to approximately 260. We attribute the temporal recovery to carrier cooling and recombination with the layer dependence related to substrate coupling. Results are compared with related measurements for epitaxial, multilayer graphene.

Ultrafast carrier kinetics in exfoliated graphene and thin graphite films.

Optical second harmonic generation (SHG) of 800 nm, 150 fs fundamental pulses is observed from exfoliated graphene and multilayer graphitic films mounted on an oxidized silicon (001) substrate. The SHG anisotropy is observed as a sample is rotated about the surface normal. For p-polarized fundamental and SHG light, the isotropic SHG from a graphene layer only slightly interferes with the fourfold symmetric response of the underlying substrate, while other samples show a threefold symmetry characteristic of significant SHG in the multilayer graphitic films. The dominance of the threefold anisotropy is maintained from bilayer graphene to bulk graphite.

Second harmonic generation from graphene and graphitic films

We have measured second-harmonic generation (SHG) from graphene and other graphitic films, from two layers to bulk graphite, at room temperature; all samples are mounted on a 300 nm oxide layer of a Si(001) substrate. With 800 nm, 150 fs fundamental pulses, the anisotropic response was recorded for combinations of $p$-, $s$-, and diagonally polarized fundamental and second-harmonic beams as the samples were rotated about their normal. Graphene samples display SHG signatures only slightly different from that of the bare substrate which shows SHG with fourfold rotational symmetry. All other layered systems show threefold symmetry, although the ratio of isotropic to anisotropic response varies with the number of layers. A model based on linear light propagation in layered media with interface dipole and bulk quadrupole SHG sources is presented for the analysis. We show that data from all layered samples can be understood in terms of well-known linear optical properties, the SHG response of the bare substrate and four independent, complex nonlinear dipole susceptibility tensor elements of the graphene/air interface.

https://www.ee.t.u-tokyo.ac.jp/~syama/NL/CNTnonlinear/dean2.pdf

Graphene and few-layer graphite probed by second-harmonic generation: Theory and experiment

Optical second harmonic generation (SHG) is used to probe surface strain in 150 and 190-nm thin films of MnAs grown epitaxially on GaAs(001). The $p$-polarized SHG signal produced by $p$-polarized 775-nm, 200-fs pulses is theoretically and experimentally shown to be sensitive to the normal component of surface strain from $\ensuremath{-}20$ to 70 ${}^{\ensuremath{\circ}}$C, which includes the ferromagnetic/paramagnetic striped coexistence phase region that exists from $\ensuremath{\sim}$10 to 40 ${}^{\ensuremath{\circ}}$C. We use this dependence to time-resolve the surface strain dynamics in MnAs following pumping with 200-fs pulses of 1.0 or 2.0 mJ cm${}^{\ensuremath{-}2}$ that raise the surface temperature by tens of degrees. For a film at $\ensuremath{-}20{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C the strain reaches a minimum value in $\ensuremath{\sim}$10 ps, indicative of electron-lattice thermalization, before recovering on a 500-ps time scale consistent with a one-dimensional heat diffusion model. For a film at 20 ${}^{\ensuremath{\circ}}$C the minimum strain is reached only after $\ensuremath{\sim}$200 ps and attains a value higher than predicted by the heat diffusion model; recovery, however, still occurs in $\ensuremath{\sim}$500 ps. The long strain fall time possibly reflects the influence of latent heat and stripe dynamics in the coexistence phase. The larger calculated drop in surface strain may be due to deficiencies in the one-dimensional heat diffusion model. The nonequilibrium surface strain also may not be determined by the local temperature alone but by the constraints throughout the film/substrate system, which are certainly known to govern the strain and stripe characteristics under equilibrium conditions.

Ultrafast surface strain dynamics in MnAs thin films observed with second harmonic generation

When laser-etching channels through solid targets, the etch-rate is known to decrease with increasing depth, partly because of absorption at the sides of the channel. For ultrafast-laser pulses at repetition rates >100MHz, we show that the etch-rate is also affected by optical properties of the beam: the channel acts as a waveguide, and so the pulses will decompose into dispersive normal modes. Additionally, plasma on the inner surface of the channel will cause scattering of the beam. These effects will cause a loss of spatial coherence in the pulse, which will affect the propagated intensity distribution and ultimately the etch-rate. We have characterized this effect for various foil thicknesses to determine the evolution of the beam while drilling through metal.

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Jesse Dean

Papers

Ultrafast carrier kinetics in exfoliated graphene and thin graphite films.

Second harmonic generation from graphene and graphitic films

Graphene and few-layer graphite probed by second-harmonic generation: Theory and experiment

Ultrafast surface strain dynamics in MnAs thin films observed with second harmonic generation

The effects of degraded spatial coherence on ultrafast-laser channel etching.