arXiv subjects

Hugen Yan

Publications and source records attributed to Hugen Yan.

47 records · Page 3Linked to original sources

arXiv2012

Mid-infrared plasmons in scaled graphene nanostructures

Plasmonics takes advantage of the collective response of electrons to electromagnetic waves, enabling dramatic scaling of optical devices beyond the diffraction limit. Here, we demonstrate the mid-infrared (4 to 15 microns) plasmons in deeply scaled graphene nanostructures down to 50 nm, more than 100 times smaller than the on-resonance light wavelength in free space. We reveal, for the first time, the crucial damping channels of graphene plasmons via its intrinsic optical phonons and scattering from the edges. A plasmon lifetime of 20 femto-seconds and smaller is observed, when damping through the emission of an optical phonon is allowed. Furthermore, the surface polar phonons in SiO2 substrate underneath the graphene nanostructures lead to a significantly modified plasmon dispersion and damping, in contrast to a non-polar diamond-like-carbon (DLC) substrate. Much reduced damping is realized when the plasmon resonance frequencies are close to the polar phonon frequencies. Our study paves the way for applications of graphene in plasmonic waveguides, modulators and detectors in an unprecedentedly broad wavelength range from sub-terahertz to mid-infrared.

cond-mat.mes-hall↗

arXiv2012

Structure and electronic transport in graphene wrinkles

Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition, and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these collapsed wrinkle structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide folded wrinkles, we find a distinct anisotropy in their electrical resistivity, consistent with our transport simulations. These results highlight the coupling between morphology and electronic properties, which has important practical implications for large-scale high-speed graphene electronics.

cond-mat.mes-hall↗

arXiv2012

Plasmonics of coupled graphene micro-structures

The optical response of graphene micro-structures, such as micro-ribbons and disks, is dominated by the localized plasmon resonance in the far infrared (IR) spectral range. An ensemble of such structures is usually involved and the effect of the coupling between the individual structures is expected to play an important role. In this paper, the plasmonic coupling of graphene microstructures in different configurations is investigated. While a relatively weak coupling between graphene disks on the same plane is observed, the coupling between vertically stacked graphene disks is strong and a drastic increase of the resonance frequency is demonstrated. The plasmons in a more complex structure can be treated as the hybridization of plasmons from more elementary structures. As an example, the plasmon resonances of graphene micro-rings are presented, in conjunction with their response in a magnetic field. Finally, the coupling of the plasmon and the surface polar phonons of SiO2 substrate is demonstrated by the observation of a new hybrid resonance peak around 500cm-1.

cond-mat.mes-hall↗

arXiv2012

Magnetic field tuning of terahertz Dirac plasmons in graphene

Boundaries and edges of a two dimensional system lower its symmetry and are usually regarded, from the point of view of charge transport, as imperfections. Here we present a first study of the behavior of graphene plasmons in a strong magnetic field that provides a different perspective. We show that the plasmon resonance in micron size graphene disks in a strong magnetic field splits into edge and bulk plasmon modes with opposite dispersion relations, and that the edge plasmons at terahertz frequencies develop increasingly longer lifetimes with increasing magnetic field, in spite of potentially more defects close to the graphene edges. This unintuitive behavior is attributed to increasing quasi-one dimensional field-induced confinement and the resulting suppression of the back-scattering. Due to the linear band structure of graphene, the splitting rate of the edge and bulk modes develops a strong doping dependence, which differs from the behavior of traditional semiconductor two-dimensional electron gas (2DEG) systems. We also observe the appearance of a higher order mode indicating an anharmonic confinement potential even in these well-defined circular disks. Our work not only opens an avenue for studying the physics of graphene edges, but also supports the great potential of graphene for tunable terahertz magneto-optical devices.

cond-mat.mes-hall↗

arXiv2012

Manipulating infrared photons using plasmons in transparent graphene superlattices

Superlattices are artificial periodic nanostructures which can control the flow of electrons. Their operation typically relies on the periodic modulation of the electric potential in the direction of electron wave propagation. Here we demonstrate transparent graphene superlattices which can manipulate infrared photons utilizing the collective oscillations of carriers, i.e., plasmons of the ensemble of multiple graphene layers. The superlattice is formed by depositing alternating wafer-scale graphene sheets and thin insulating layers, followed by patterning them all together into 3-dimensional photonic-crystal-like structures. We demonstrate experimentally that the collective oscillation of Dirac fermions in such graphene superlattices is unambiguously nonclassical: compared to doping single layer graphene, distributing carriers into multiple graphene layers strongly enhances the plasmonic resonance frequency and magnitude, which is fundamentally different from that in a conventional semiconductor superlattice. This property allows us to construct widely tunable far-infrared notch filters with 8.2 dB rejection ratio and terahertz linear polarizers with 9.5 dB extinction ratio, using a superlattice with merely five graphene atomic layers. Moreover, an unpatterned superlattice shields up to 97.5% of the electromagnetic radiations below 1.2 terahertz. This demonstration also opens an avenue for the realization of other transparent mid- and far-infrared photonic devices such as detectors, modulators, and 3-dimensional meta-material systems.

cond-mat.mes-hall↗

arXiv2011

Infrared Spectroscopy of Wafer-Scale Graphene

We report on spectroscopy results from the mid- to far-infrared on wafer-scale graphene, grown either epitaxially on silicon carbide, or by chemical vapor deposition. The free carrier absorption (Drude peak) is simultaneously obtained with the universal optical conductivity (due to interband transitions), and the wavelength at which Pauli blocking occurs due to band filling. From these the graphene layer number, doping level, sheet resistivity, carrier mobility, and scattering rate can be inferred. The mid-IR absorption of epitaxial two-layer graphene shows a less pronounced peak at 0.37\pm0.02 eV compared to that in exfoliated bilayer graphene. In heavily chemically-doped single layer graphene, a record high transmission reduction due to free carriers approaching 40% at 250 \mum (40 cm-1) is measured in this atomically thin material, supporting the great potential of graphene in far-infrared and terahertz optoelectronics.

cond-mat.mes-hall↗

arXiv2011

Temperature dependence of the anharmonic decay of optical phonons in carbon nanotubes and graphite

We report on the temperature dependence of the anharmonic decay rate of zone-center (G mode) optical phonons in both single-walled carbon nanotubes and graphite. The measurements are performed using a pump-probe Raman scattering scheme with femtosecond laser pulses. For nanotubes, measured over a temperature range of 6 K-700 K, we observe little temperature dependence of the decay rate below room temperature. Above 300 K, the decay rate increases from 0.8 to 1.7 ps-1. The decay rates observed for graphite range from 0.5 to 0.8 ps-1 for temperatures from 300 K-700 K. We compare the behavior observed in carbon nanotubes and graphite and discuss the implications of our results for the mechanism of the anharmonic decay of optical phonons in both systems.

cond-mat.mtrl-sci↗

arXiv2010

Anomalous Lattice Vibrations of Single and Few-Layer MoS2

Molybdenum disulfide (MoS2) of single and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S-Mo-S layers of the samples was independently determined by contact-mode atomic-force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the 3-dimensional to the 2-dimensional regime.

cond-mat.mtrl-sci↗

arXiv2010

Excitons and high-order optical transitions in individual carbon nanotubes

We examine the excitonic nature of high-lying optical transitions in single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A careful analysis of the principal transitions of individual semiconducting and metallic nanotubes reveals that in both cases the lineshape is consistent with an excitonic model, but not one of free-carriers. For semiconducting species, side-bands are observed at ~200 meV above the third and fourth optical transitions. These features are ascribed to exciton-phonon bound states. Such side-bands are not apparent for metallic nanotubes,as expected from the reduced strength of excitonic interactions in these systems.

cond-mat.mes-hall↗

arXiv2009

Time-resolved Raman Spectroscopy of Optical Phonons in Graphite: Phonon Anharmonic Coupling and Anomalous Stiffening

Time-resolved Raman spectroscopy has been applied to probe the anharmonic coupling and electron-phonon interaction of optical phonons in graphite. From the decay of the transient anti-Stokes scattering of the G-mode following ultrafast excitation, we measured a lifetime of 2.2+/-0.1ps for zone-center optical phonons. We also observed a transient stiffening of G-mode phonons, an effect attributed to the reduction of the electron-phonon coupling for high electronic temperatures.

cond-mat.mtrl-sci↗

arXiv2008

Raman Spectroscopy of Graphene under Uniaxial Stress: Phonon Softening and Determination of the Crystallographic Orientation

We present a systematic study of the Raman spectra of optical phonons in graphene monolayers under tunable uniaxial tensile stress. Both the G and 2D bands exhibit significant red shifts. The G band splits into two distinct sub-bands (G+, G-) because of the strain-induced symmetry breaking. Raman scattering from the G+ and G- bands shows a distinctive polarization dependence that reflects the angle between the axis of the stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene.

cond-mat.mtrl-sci↗