The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on sub-picosecond time scales and that interband recombination times are carrier density dependent.

Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene

We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

Applying time-dependent photoemission we unravel the graphene growth process on a metallic surface by chemical vapor deposition (CVD). Graphene CVD growth is in stark contrast to the standard growth process of two--dimensional films because it is self-limiting and stops as soon as a monolayer graphene has been synthesized. Most importantly, a novel phase of metastable graphene was discovered that is characterized by permanent and simultaneous construction and deconstruction. The high quality and large area graphene flakes are characterized by angle-resolved photoemission proofing that they are indeed monolayer and cover the whole 1$\times$1 cm Nickel substrate. These findings are of high relevance to the intensive search for reliable synthesis methods for large graphene flakes of controlled layer number.

Dynamics of graphene growth on a metal surface: a time-dependent photoemission study

This article gives a brief overview of the linear and nonlinear optical response of graphene and its applications. Graphene exhibits unique physical properties. The charge carriers in graphene follow the relativistic massless Dirac equation; thus, they are known as the Dirac fermions. These Dirac fermions in graphene exhibit peculiar linear band structure at the Dirac point in the first Brillouin zone. These carriers exhibit novel linear and nonlinear optical properties in the presence of an intense electromagnetic field. The unique optical properties (linear and nonlinear) are a consequence of the unusual low-energy electronic band structure of graphene. The linear and nonlinear optical response of graphene can be tuned in the terahertz regime by the application of magnetic field. The uniqueness of graphene is shown by different findings in nonlinear optics such as four-wave mixing, saturation and reverse saturation absorption, two-photon absorption and optical limiting, among others. Therefore, the present review article describes the recent development in the field of linear and nonlinear optics of graphene. The different applications of graphene in the context of optics have been highlighted.

Linear and Nonlinear Optical Properties of Graphene: A Review

Graphene has been theoretically shown to exhibit anomalous Rabi oscillations (AROs) far from resonance in addition to conventional Rabi oscillations close to resonance (classical light frequency matches particle-hole frequency). The ARO has been attributed to the pseudospin degree of freedom that is unique to graphene-like systems. In this work, we show the same phenomenon also occurs in the single-photon limit or even in a vacuum. This is to be expected for conventional Rabi oscillations; however, the prediction that AROs also occur in the single-photon situation means that this notion of ARO is robust and not an artifact of approximations. We also study collapse and revival of both conventional and AROs in response to a coherent radiation field and extract the collapse and revival times in both cases.

Quantum Rabi oscillations in graphene

We study the phenomenon of crossover of Rabi oscillations in graphene as a function of detuning—the difference between the frequency of the incident wave and interband energy ( 2 v F | k | ). It is shown by comparison with an exactly solved model with bands having linear dispersion but lacking pseudospin that this crossover is unique to graphene, attributable to the pseudospin character of the graphene hamiltonian. A group theoretic argument for why this model is solvable is given. We compute the nonlinear current using our formalism, the main prediction being the threshold behavior (with exponent equal to 1/2) of the slowly varying part of the current in frequency domain with threshold frequency being 2 ω R 2 / ω (‘anomalous’ Rabi frequency) where ω R is the Rabi frequency for zero detuning. The novelty of our approach is the introduction of an alternative to the rotating wave approximation (RWA) (called asymptotic RWA here) which is argued to be important in demonstrating this crossover. We provide an interpolation method between these two regimes, that shows novel phenomena attributable to harmonic generation. A fully numerical solution to the Bloch equations verifies the analytical results and the various approximation schemes.

Crossover of coherent Rabi oscillations in graphene

In this work, we study the well-known phenomenon of the Bloch–Siegert shift, paying special attention to the change in the resonance condition of anomalous Rabi oscillations that are peculiar to graphene. The inclusion of frequency doubling (counter-rotating terms) in the conventional rotating wave approximation leads to a rather small shift in the resonance condition which forms the conventional Bloch–Siegert shift. In case of anomalous Rabi oscillations, however, band anisotropy caused by the inclusion of next-nearest neighbor hopping together with frequency doubling or the Rashba effect leads to a Bloch–Siegert shift which is quite substantial (in percentage terms, actually infinite), lending credence to the claim that the anomalous Rabi oscillation is sensitive to qualitative changes in the low energy band structure of graphene and as such is a useful indicator of the chiral massless Dirac fermion nature of quasiparticles in graphene.

Band-anisotropy induced Bloch–Siegert shift in graphene

The pump–probe experiment is typically used to study relaxation phenomena in nonlinear optical systems. Here we use it as a tool to study the phenomenon of anomalous Rabi oscillations in graphene that was predicted recently in single-layer graphene. Unlike conventional Rabi oscillations, anomalous Rabi oscillations are unique to graphene (and possibly to surface states of topological insulators (TIs)), attributable to the pseudospin (conventional spin for TI) degree of freedom and Dirac-fermion character of the graphene system. A pump pulse of a finite duration long enough to contain a large number of cycles induces a current density that oscillates with the frequency of the pump pulse. The amplitude associated with these fast oscillations is seen to exhibit much slower oscillations with a frequency given by ${ 2 \omega ^2_{\mathrm {R}} }/{ \omega } $
 – the anomalous Rabi frequency, where ω
 R is the conventional Rabi frequency and ω is the frequency of the external pump field. This effect is easily probed by a probe pulse subsequent to the pump, where it manifests itself as periodic oscillations of the probe susceptibility as a function of pump duration at each probe frequency. Alternatively, it is also seen as an oscillatory function of the pump–probe delay with other variables remaining fixed. This period corresponds to the anomalous Rabi frequency. An analysis of the previously reported experimental data confirms the presence of anomalous Rabi oscillations in graphene.

A theoretical study of pump–probe experiment in single-layer, bilayer and multilayer graphene

Strain Effect on the Nonlinear Electromagnetic Response of 2D Carbon Based Material

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