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Register a new userHelicity sensitive plasmonic teraherts interferometer
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Plasmonic interferometry is a rapidly growing area of research with a huge potential for applications in terahertz frequency range. In this Letter, we explore a plasmonic interferometer based on graphene Field Effect Transistor connected to specially designed antennas. As a key result, we observe helicity- and phase-sensitive conversion of circularly-polarized radiation into dc photovoltage caused by the plasmon-interference mechanism: two plasma waves, excited at the source and drain part of the transistor interfere inside the channel. The helicity sensitive phase shift between these waves is achieved by using an asymmetric antenna configuration. The dc signal changes sign with inversion of the helicity. Suggested plasmonic interferometer is capable for measuring of phase difference between two arbitrary phase-shifted optical signals. The observed effect opens a wide avenue for phase-sensisitve probing of plasma wave excitations in two-dimensional materials.
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We develop a theory of the helicity driven nolinear dc response of gated two-dimensional electron gas to the terahertz radiation. We demonstrate that the helicity-sensitive part of the response dramatically increases in the vicinity of the plasmonic resonances and oscillates with the phase shift between excitation signals on the source and drain. The resonance line shape is an asymmetric function of the frequency deviation from the resonance. In contrast, the helicity-insensitive part of the response is symmetrical. These properties yield significant advantage for using plasmonic detectors as terahertz and far infrared spectrometers and interferometers.
Collective modes of doped two-dimensional crystalline materials, namely graphene, MoS$_2$ and phosphorene, both monolayer and bilayer structures, are explored using the density functional theory simulations together with the random phase approximation. The many-body dielectric functions of the materials are calculated using an {it ab initio} based model involving material-realistic physical properties. Having calculated the electron energy-loss, we calculate the collective modes of each material considering the in-phase and out-of-phase modes for bilayer structures. Furthermore, owing to many band structures and intreband transitions, we also find high-energy excitations in the systems. We explain that the material-specific dielectric function considering the polarizability of the crystalline material such as MoS$_2$ are needed to obtain realistic plasmon dispersions. For each material studied here, we find different collective modes and describe their physical origins.
We discuss plasmons of biased twisted bilayer graphene when the Fermi level lies inside the gap. The collective excitations are a network of chiral edge plasmons (CEP) entirely composed of excitations in the topological electronic edge states (EES) that appear at the AB-BA interfaces. The CEP form an hexagonal network with an unique energy scale $epsilon_p=frac{e^2}{epsilon_0epsilon t_0}$ with $t_0$ the moire lattice constant and $epsilon$ the dielectric constant. From the dielectric matrix we obtain the plasmon spectra that has two main characteristics: (i) a diverging density of states at zero energy, and (ii) the presence of a plasmonic Dirac cone at $hbaromegasimepsilon_p/2$ with sound velocity $v_D=0.0075c$, which is formed by zigzag and armchair current oscillations. A network model reveals that the antisymmetry of the plasmon bands implies that CEP scatter at the hexagon vertices maximally in the deflected chiral outgoing directions, with a current ratio of 4/9 into each of the deflected directions and 1/9 into the forward one. We show that scanning near-field microscopy should be able to observe the predicted plasmonic Dirac cone and its broken symmetry phases.
Non-centrosymmetric transition metal monopnictides, including TaAs, TaP, NbAs, and NbP, are emergent topological Weyl semimetals (WSMs) hosting exotic relativistic Weyl fermions. In this letter, we elucidate the physical origin of the unprecedented charge carrier mobility of NbP, which can reach $1times10^{7}$ cm $^{2}$V$^{-1}$s$^{-1}$ at 1.5 K. Angle- and temperature-dependent quantum oscillations, supported by density function theory calculations, reveal that NbP has the coexistence of p- and n-type WSM pockets in the $k_{z}$=1.16$pi$/c plane (W1-WSM) and in the $k_{z}$=0 plane near the high symmetry points $Sigma$ (W2-WSM), respectively. Uniquely, each W2-WSM pocket forms a large dumbbell-shaped Fermi surface (FS) enclosing two neighboring Weyl nodes with the opposite chirality. The magneto-transport in NbP is dominated by these highly anisotropic W2-WSM pockets, in which Weyl fermions are well protected from defect backscattering by real spin conservation associated to the chiral nodes. However, with a minimal doping of $sim$1% Cr, the mobility of NbP is degraded by more than two order of magnitude, due to the invalid of helicity protection to magnetic impurities. Helicity protected Weyl fermion transport is also manifested in chiral anomaly induced negative magnetoresistance, controlled by the W1-WSM states. In the quantum regime below 10 K, the intervalley scattering time by impurities becomes a large constant, producing the sharp and nearly identical conductivity enhancement at low magnetic field.
Within the two antenna model, we develop a theory of the recently observed helicity-sensitive detection of terahertz radiation by FETs. The effect arises because of the mixing of the ac signals produced in the channel by the two antennas. We calculate the helicity-dependent part of the photoresponse and its dependence on the antenna impedance, gate length, and gate voltage.
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