Terahertz imaging for noninvasive measurements.

Surface plasmons in graphene o er a compelling route to many useful photonic technologies [1][2][3] . As a plasmonic material, graphene o ers several intriguing properties, such as excellent electro-optic tunability 4 , crystalline stability, large optical nonlinearities 5 and extremely high electromagnetic field concentration 6 . As such, recent demonstrations of surface plasmon excitation in graphene using near-field scattering of infrared light 7,8 have received intense interest. Here we present an all-optical plasmon coupling scheme which takes advantage of the intrinsic nonlinear optical response of graphene. Free-space, visible light pulses are used to generate surface plasmons in a planar graphene sheet using di erence frequency wave mixing to match both the wavevector and energy of the surface wave. By carefully controlling the phase matching conditions, we show that one can excite surface plasmons with a defined wavevector and direction across a large frequency range, with an estimated photon e ciency in our experiments approaching 10 −5 . Graphene has attracted significant interest recently as a unique optical material. In particular, it has been predicted and experimentally shown that graphene can support highly confined surface plasmons 1,9 , with electrically tunable dispersion 7,8 . Despite these promising discoveries, the burgeoning field of graphene plasmonics has some serious obstacles to overcome if it is to progress from the proof-of-principle stage. Problems arise due to the small wavelength of the surface plasmons, two orders of magnitude smaller than light of the same frequency. This has led to the development of specialized measurement techniques, most of which use infrared light and geometries with scattering resonances 10-12 or near-field sources 7,8 to excite graphene surface plasmons. However, the far-infrared regime, in which graphene plasmons are predicted to have long lifetimes, lacks developed sources and detectors compared to the visible regime. Alternative approaches, such as the manipulation of surface acoustic waves to couple to the graphene surface plasmons 13,14 , therefore hold promise. Particularly desirable is the potential to excite a plasmon eigenstate with a singular energy, momentum and direction, vital for many future applications, including plasmonic circuits. In this respect, very recent progress has been made, with the development of carefully designed nanoantennas which can locally excite and direct surface plasmons in graphene 11 . Here, the combination of infrared source frequency and nanoantenna dimensions determine the frequency, wavevector and direction of the surface plasmons generated. In this letter, we investigate a competing approach that embodies many of these desirable aspects of directivity without requiring careful nanofabrication of antennas. This all-optical approach can access a distinctly broad frequency range, even down to the far infrared. We coherently excite surface plasmons using two visible frequency free-space beams via difference frequenc...

The ultrafast dynamics and conductivity of photoexcited graphene can be explained using solely electronic effects.

Near infrared pump-probe spectroscopy has been used to measure the ultrafast dynamics of photoexcited charge carriers in monolayer and multilayer graphene. We observe two decay processes occurring on 100 fs and 2 ps timescales. The first is attributed to the rapid electron-phonon thermalisation in the system. The second timescale is found to be due to the slow decay of hot phonons. Using a simple theoretical model we calculate the hot phonon decay rate and show that it is significantly faster in monolayer flakes than in multilayer ones. In contrast to recent claims, we show that this enhanced decay rate is not due to the coupling to substrate phonons, since we have also seen the same effect in suspended flakes. Possible intrinsic decay mechanisms that could cause such an effect are discussed. The symmetric, linear electronic band structure of graphene gives rise to some very unusual physical properties, such as quantised transmission 1 , extremely high thermal conductivity 2 and high carrier mobility.3 An important underlying feature is the very strong electronphonon coupling that exists in graphene, which is revealed by the presence of Kohn anomalies.4 In graphite, it is known that strongly coupled optical phonons have high quantum energies of up to 0.2 eV and are excited only by electrons of elevated energy.4-6 To progress towards applications in real (high-current) circuits and devices, it is crucial to understand how graphene behaves under such high energy, non-equilibrium conditions. Despite the surge of interest in this material and its potential applications, investigations into the kinetic properties of "hot" charge carriers remain rather limited. 5,7Hot electron relaxation in large area, epitaxially grown graphene layers using pump-probe spectroscopy has been studied previously. [8][9][10][11][12][13][14][15][16] These measurements point to biexponential decay dynamics characterised by a fast ∼100 fs component and a slow ∼2 ps component. There is, however, significant variation in the reported timescales. Epitaxial graphene exhibits inhomogeneity in layer thickness on the micron scale 12 and can result in significant variability in relaxation dynamics from sample to sample.8 Pump-probe measurements have also been performed on mechanically exfoliated graphene, 17,18 which is homogeneous over much greater length scales. It was concluded that the slow relaxation process was caused by the coupling to phonons in the substrate. 17In this paper we use near infrared pump-probe spectroscopy to investigate the relaxation dynamics of hot carriers in mechanically exfoliated graphene. Similar to previous results we find that the relaxation occurs on two timescales, one fast (∼100 fs) and the other slow (∼2 ps). By measuring the relaxation in monolayer and multilayer graphene flakes we show a clear correlation between the slow decay rate and flake thickness, with the fastest rate observed for monolayer graphene. This slow decay rate is found to occur in both supported and suspended flakes. Therefore, in contrast to r...

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