The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 10 6 cm 2 V -1 s -1and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as 10 8 cm -2 , allowing a neutral state with a few charge carriers per entire micron-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity.No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap.KEYWORDS (graphene, suspended graphene, high mobility, clamped contacts) 2 Graphene has a unique conical spectrum and its electronic properties at low energies E are often described by a two-dimensional Dirac-like equation. 1,2 The resulting difference from the conventional electronic systems becomes most prominent and interesting near zero E where graphene's Fermi surface shrinks into a point (Dirac point). Unfortunately, experimental devices are always subject to disorder, finite size and other factors limiting graphene's both quality and homogeneity. In particular, local variations of chemical doping and/or strain 3 disallow the Dirac point (DP) to be achieved uniformly over the entire device area so that neutral graphene is usually split into a system of electron-hole (e-h) puddles, 4,5 a state usually referred to as the neutrality point (NP). This charge inhomogeneity impedes investigation of graphene's intrinsic properties in the immediate proximity of the DP. min is close but still notably higher than min = 4e 2 /h predicted in the ballistic limit. 22,23 No sign of diverging resistivity yields a conservative estimate on any possible bandgap as <0.5 meV. Figure 1a shows a micrograph of one of our suspended graphene devices, which was fabricated following the procedures described in refs. 8,10,11. In short, graphene was cleaved onto an oxidized Si wafer (300 nm SiO 2 ), and metallic contacts (Cr 3 nm/Au 100 nm) were deposited on top as shown in Figure 1a. Approximately a half of SiO 2 was etched away, allowing graphene to be suspended between the contacts whereas the remaining oxide served as a gate dielectric. In the second type of suspended devices, graphene was clamped between two metal pads to achieve better mechanical stability. To this end, graphene was transferred onto Au pads prefabricated on the Si wafer, and Cr/Au contacts were deposited on top of them, clamping graphene between two metal layers. This stopped graphene from moving and scrolling. Let us mention that the latter approach allowed us to fabricate a number of 4-probe devices with graphene crystals being etched into the proper Hall bar geometry. 24 Unfortunatel...
We suggest a new class of hyperbolic metamaterials for THz frequencies based on multilayer graphene structures. We calculate the dielectric permittivity tensor of the effective nonlocal medium with a periodic stack of graphene layers and demonstrate that tuning from elliptic to hyperbolic dispersion can be achieved with an external gate voltage. We reveal that such graphene structures can demonstrate a giant Purcell effect that can be used for boosting the THz emission in semiconductor devices. Tunability of these structures can be enhanced further with an external magnetic field which leads to the unconventional hybridization of the TE and TM polarized waves.PACS numbers: 78.67.Bf, 73.20.Mf A hyperbolic medium is a special class of indefinite media [1] described by the diagonal permittivity tensor with the principal components being of the opposite signs which results in a hyperbolic shape of the isofrequency contours [2,3]. Such media have a number of unique properties including negative refraction [1,4] and subwavelength imaging [5]. One of the possible realizations of hyperbolic media is a periodic metal-dielectric nanostructured metamaterial where the hyperbolic nature of the isofrequency curves appears due to the excitation of the near-field plasmon Bloch waves [6,7]. Hyperbolic metamaterials have been realized for optical, infrared, and microwave frequency ranges. Realization of the THz hyperbolic media could allow to boost otherwise slow THz radiative transitions in semiconductor devices which would lead to the development of a new class of THz sources.Graphene, a two-dimensional lattice of carbon atoms, exhibits a wide range of unique properties [8][9][10]. Surface plasmons excited in individual graphene sheets have been extensively studied, both theoretically [11][12][13][14][15][16][17] and experimentally [18,19].In this Letter, we suggest a novel class of hyperbolic metamaterials where individual graphene sheets are separated by host dielectric slabs, as shown schematically in Fig. 1. It is easy to notice an analogy between a graphene sheet placed inside a dielectric medium and a thin metal waveguide imbedded into a dielectric matrix, which also supports localized surface plasmon polaritons. Assuming this analogy, we may expect that a periodic lattice of the graphene sheets may behave like an effective hyperbolic medium due to the coupling between the surface plasmons localized at the individual graphene sheets [20]. Importantly, surface plasmons in graphene have low losses and strong localization in the THz region. Indeed, as we demonstrate below, a periodic structure of graphene layers creates a novel type of metamaterial with strong nonlocal response and hyperbolic properties of its disper- sion curves for TM-polarized waves in the THz frequency range and superior characteristics such as a giant Purcell effect and tunability by a gate voltage or magnetic field.It is important to mention that the periodic layered structure shown in Fig. 1 resembles a natural graphite which is known to exhibit m...
We propose a novel approach for efficient tuning of optical properties of a high refractive index subwavelength nanoparticle with a magnetic Mie-type resonance by means of femtosecond laser irradiation. This concept is based on ultrafast photoinjection of dense (>10(20) cm(-3)) electron-hole plasma within such nanoparticle, drastically changing its transient dielectric permittivity. This allows manipulation by both electric and magnetic nanoparticle responses, resulting in dramatic changes of its scattering diagram and scattering cross section. We experimentally demonstrate 20% tuning of reflectance of a single silicon nanoparticle by femtosecond laser pulses with wavelength in the vicinity of the magnetic dipole resonance. Such a single-particle nanodevice enables designing of fast and ultracompact optical switchers and modulators.
Enhancement of optical response with high-index dielectric nanoparticles is attributed to the excitation of their Mie-type magnetic and electric resonances. Here we study Raman scattering from crystalline silicon nanoparticles and reveal that magnetic dipole modes have much stronger effect on the scattering than electric modes of the same order. We demonstrate experimentally a 140−fold enhancement of Raman signal from individual silicon spherical nanoparticles at the magnetic dipole resonance. Our results confirm the importance of the optically-induced magnetic response of subwavelength dielectric nanoparticles for enhancing light-matter interactions.
Recent trends to employ high-index dielectric particles in nanophotonics are motivated by their reduced dissipative losses and large resonant enhancement of nonlinear effects at the nanoscale. Because silicon is a centrosymmetric material, the studies of nonlinear optical properties of silicon nanoparticles have been targeting primarily the third-harmonic generation effects. Here we demonstrate, both experimentally and theoretically, that resonantly excited nanocrystalline silicon nanoparticles fabricated by an optimized laser printing technique can exhibit strong second-harmonic generation (SHG) effects. We attribute an unexpectedly high yield of the nonlinear conversion to a nanocrystalline structure of nanoparticles supporting the Mie resonances. The demonstrated efficient SHG at green light from a single silicon nanoparticle is 2 orders of magnitude higher than that from unstructured silicon films. This efficiency is significantly higher than that of many plasmonic nanostructures and small silicon nanoparticles in the visible range, and it can be useful for a design of nonlinear nanoantennas and silicon-based integrated light sources.
Optical bound states in the continuum (BICs) provide a way to engineer resonant response in photonic crystals with giant quality factors. The extended interaction time in such systems is particularly promising for enhancement of nonlinear optical processes and development of a new generation of active optical devices. However, the achievable interaction strength is limited by the purely photonic character of optical BICs. Here, 1 arXiv:1905.13505v1 [cond-mat.mes-hall] 31 May 2019 we mix optical BIC in a photonic crystal slab with excitons in atomically thin transition metal dichalcogenide MoSe 2 via strong coupling to form exciton-polaritons with Rabi splitting exceeding 27 meV. We experimentally show BIC-like behavior of both upper and lower polariton branches, with complete suppression of radiation into far-field at the BIC wavevector and strongly varying Q-factor in its vicinity. Owing to an effective disorder averaging through motional narrowing, we achieve small polariton linewidth of 2 meV and demonstrate linewidth control via angle and temperature tuning. Our results pave the way towards developing tunable BIC-based polaritonic devices for sensing, lasing, and nonlinear optics. Optical bound states in the continuum (BICs), supported by photonic crystal structures of certain geometries, have received much attention recently as a novel approach to generating extremely spectrally narrow resonant responses. 1,2 Since BICs are uncoupled from the radiation continuum through symmetry protection 3 or resonance trapping, 4 their high quality factors, while reaching 10 5 − 10 6 , can be robust to perturbations of photonic crystal geometric parameters. This enables a broad range of practical applications, including recently demonstrated spectral filtering, 5 chemical and biological sensing, 6,7 and lasing. 4Providing an efficient light-trapping mechanism, optical BICs are particularly attractive for enhancing nonlinear optical effects, with recent theoretical proposals discussing enhanced bistability 8 and Kerr-type focusing nonlinearity. 9 However, for practical realization of these proposals, a significantly stronger material nonlinear susceptibility is needed than generally available in dielectric-based photonic crystals.An attractive approach to the enhancement of effective nonlinearity is through the use of exciton-polaritons -hybrid quasi-particles that inherit both the coherent properties of photonic modes and interaction strength of excitons. 10,11 Hybrid nanophotonic systems incorporating atomically thin transition metal dichalcogenides (TMDs) have proven to be a particularly promising platform owing to their ease of fabrication and possibility of room temperature operation. [12][13][14] In addition to conventional microcavity-based designs, TMD
Ordered hybrid nanostructures for nanophotonics applications are fabricated by a novel approach via femtosecond laser melting of asymmetric metal-dielectric (Au/Si) nanoparticles created by lithographical methods. The approach allows selective reshaping of the metal components of the hybrid nanoparticles without affecting the dielectric ones and is applied for tuning of the scattering properties of the hybrid nanostructures in the visible range.
We reveal unusually strong polarization sensitivity of electric and magnetic dipole resonances of high‐index dielectric nanoparticles placed on a metallic film. By employing dark‐field spectroscopy, we observe the polarization‐controlled transformation from high‐Q magnetic‐dipole scattering to broadband suppression of scattering associated with the electric dipole mode, and show numerically that it is accompanied by a strong enhancement of the respective fields by the nanoparticle. Our experimental data for silicon nanospheres are in an excellent agreement with both analytical calculations based on Green's function approach and the full‐wave numerical simulations. Our findings further substantiate dielectric nanoparticles as strong candidates for many applications in enhanced sensing, spectroscopy and nonlinear processes at the nanoscale.
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