42 research outputs found
Decrypting the cyclotron effect in graphite using Kerr rotation spectroscopy
We measure the far-infrared magneto-optical Kerr rotation and reflectivity
spectra in graphite and achieve a highly accurate unified microscopic
description of all data in a broad range of magnetic fields by taking
rigorously the c-axis band dispersion and the trigonal warping into account. We
find that the second- and the forth-order cyclotron harmonics are optically
almost as strong as the fundamental resonance even at high fields. They must
play, therefore, a major role in magneto-optical and magneto-plasmonic
applications based on Bernal stacked graphite and multilayer graphene.Comment: 4 pages, 3 figures + Supplemental Materia
Magneto-optical Kramers-Kronig analysis
We describe a simple magneto-optical experiment and introduce a
magneto-optical Kramers-Kronig analysis (MOKKA) that together allow extracting
the complex dielectric function for left- and right-handed circular
polarizations in a broad range of frequencies without actually generating
circularly polarized light. The experiment consists of measuring reflectivity
and Kerr rotation, or alternatively transmission and Faraday rotation, at
normal incidence using only standard broadband polarizers without retarders or
quarter-wave plates. In a common case, where the magneto-optical rotation is
small (below 0.2 rad), a fast measurement protocol can be realized,
where the polarizers are fixed at 45 with respect to each other. Apart
from the time-effectiveness, the advantage of this protocol is that it can be
implemented at ultra-high magnetic fields and in other situations, where an
\emph{in-situ} polarizer rotation is difficult. Overall, the proposed technique
can be regarded as a magneto-optical generalization of the conventional
Kramers-Kronig analysis of reflectivity on bulk samples and the Kramers-Kronig
constrained variational analysis of more complex types of spectral data. We
demonstrate the application of this method to the textbook semimetals bismuth
and graphite and also use it to obtain handedness-resolved magneto-absorption
spectra of graphene on SiC.Comment: 11 pages, 4 figur
Colossal infrared and terahertz magneto-optical activity in a two-dimensional Dirac material
When two-dimensional electron gases (2DEGs) are exposed to magnetic field,
they resonantly absorb electromagnetic radiation via electronic transitions
between Landau levels (LLs). In 2DEGs with a Dirac spectrum, such as graphene,
theory predicts an exceptionally high infrared magneto-absorption, even at zero
doping. However, the measured LL magneto-optical effects in graphene have been
much weaker than expected because of imperfections in the samples available so
far for such experiments. Here we measure magneto-transmission and Faraday
rotation in high-mobility encapsulated monolayer graphene using a custom
designed setup for magneto-infrared microspectroscopy. Our results show a
strongly enhanced magneto-optical activity in the infrared and terahertz ranges
characterized by a maximum allowed (50%) absorption of light, a 100% magnetic
circular dichroism as well as a record high Faraday rotation. Considering that
sizeable effects have been already observed at routinely achievable magnetic
fields, our findings demonstrate a new potential of magnetic tuning in 2D Dirac
materials for long-wavelength optoelectronics and plasmonics.Comment: 14 pages, 4 figure
Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors
We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field
effect transistors exhibiting high-quality ambipolar transport. By properly
biasing the gate electrode we can invert the sign of the photocurrent showing
that the minority photocarriers are either electrons or holes. Both in the
electron- and the hole-doping regimes the photocurrent decays exponentially as
a function of the distance between the illumination spot and the nearest
contact, in agreement with a two-terminal Schottky-barrier device model. This
allows us to compare the value and the doping dependence of the diffusion
length of the minority electrons and holes on a same sample. Interestingly, the
diffusion length of the minority carriers is several times larger in the hole
accumulation regime than in the electron accumulation regime, pointing out an
electron-hole asymmetry in WS2
Mono- and Bilayer WS2 Light-Emitting Transistors
We have realized ambipolar ionic liquid gated field-effect transistors based
on WS2 mono- and bilayers, and investigated their opto-electronic response. A
thorough characterization of the transport properties demonstrates the high
quality of these devices for both electron and hole accumulation, which enables
the quantitative determination of the band gap ({\Delta}1L = 2.14 eV for
monolayers and {\Delta}2L = 1.82 eV for bilayers). It also enables the
operation of the transistors in the ambipolar injection regime with electrons
and holes injected simultaneously at the two opposite contacts of the devices
in which we observe light emission from the FET channel. A quantitative
analysis of the spectral properties of the emitted light, together with a
comparison with the band gap values obtained from transport, show the internal
consistency of our results and allow a quantitative estimate of the excitonic
binding energies to be made. Our results demonstrate the power of ionic liquid
gating in combination with nanoelectronic systems, as well as the compatibility
of this technique with optical measurements on semiconducting transition metal
dichalcogenides. These findings further open the way to the investigation of
the optical properties of these systems in a carrier density range much broader
than that explored until now.Comment: 22 pages, 6 figures, Nano Letters (2014
Magnetoplasmon resonances in polycrystalline bismuth as seen via terahertz spectroscopy
We report the magnetic field-dependent far-infrared reflectivity of
polycrystalline bismuth. We observe four distinct absorptions that we attribute
to magnetoplasmon resonances, which are collective modes of an electron-hole
liquid in magnetic field and become optical and acoustic resonances of the
electron-hole system in the small-field limit. The acoustic mode is expected
only when the masses of distinct components are very different, which is the
case in bismuth. In a polycrystal, where the translational symmetry is broken,
a big shift of spectral weight to acoustic plasmon is possible. This enables us
to detect an associated plasma edge. Although the polycrystal sample has grains
of randomly distributed orientations, our reflectivity results can be explained
by invoking only two, clearly distinct, series of resonances. In the limit of
zero field, the optical modes of these two series converge onto plasma
frequencies measured in monocrystal along the main optical axes.Comment: Accepted in PR
Microscopic Origin of the Valley Hall Effect in Transition Metal Dichalcogenides Revealed by Wavelength Dependent Mapping
The band structure of many semiconducting monolayer transition metal
dichalcogenides (TMDs) possesses two degenerate valleys, with equal and
opposite Berry curvature. It has been predicted that, when illuminated with
circularly polarized light, interband transitions generate an unbalanced
non-equilibrium population of electrons and holes in these valleys, resulting
in a finite Hall voltage at zero magnetic field when a current flows through
the system. This is the so-called valley Hall effect that has recently been
observed experimentally. Here, we show that this effect is mediated by
photo-generated neutral excitons and charged trions, and not by inter-band
transitions generating independent electrons and holes. We further demonstrate
an experimental strategy, based on wavelength dependent spatial mapping of the
Hall voltage, which allows the exciton and trion contributions to the valley
Hall effect to be discriminated in the measurement. These results represent a
significant step forward in our understanding of the microscopic origin of
photo-induced valley Hall effect in semiconducting transition metal
dichalcogenides, and demonstrate experimentally that composite quasi-particles,
such as trions, can also possess a finite Berry curvature.Comment: accepted for publication in Nano Letter
Topological valley plasmons in twisted monolayer-double graphene moir\'e superlattices
In topological photonics, artificial photonic structures are constructed for
realizing nontrivial unidirectional propagation of photonic information. On the
other hand, moir\'e superlattices are emerging as an important avenue for
engineering quantum materials with novel properties. In this paper, we combine
these two aspects and demonstrate theoretically that moir\'e superlattices of
small-angle twisted monolayer-bilayer graphene provide a natural platform for
valley protected plasmons. Particularly, a complete plasmonic bandgap appears
stemming from the distinct optical conductivities of the ABA and ABC stacked
triangular domains. Moreover, the plasmonic crystals exhibit nonzero valley
Chern numbers and unidirectional transport of plasmonic edge states protected
from inter-valley scattering is presented
Fabry-Perot enhanced Faraday rotation in graphene
We demonstrate that giant Faraday rotation in graphene in the terahertz range
due to the cyclotron resonance is further increased by constructive Fabry-Perot
interference in the supporting substrate. Simultaneously, an enhanced total
transmission is achieved, making this effect doubly advantageous for
graphene-based magneto-optical applications. As an example, we present
far-infrared spectra of epitaxial multilayer graphene grown on the C-face of
6H-SiC, where the interference fringes are spectrally resolved and a Faraday
rotation up to 0.15 radians (9{\deg}) is attained. Further, we discuss and
compare other ways to increase the Faraday rotation using the principle of an
optical cavity
High sensitivity variable-temperature infrared nanoscopy of conducting oxide interfaces
Probing the local transport properties of two-dimensional electron systems
(2DES) confined at buried interfaces requires a non-invasive technique with a
high spatial resolution operating in a broad temperature range. In this paper,
we investigate the scattering-type scanning near field optical microscopy as a
tool for studying the conducting LaAlO3/SrTiO3 interface from room temperature
down to 6 K. We show that the near-field optical signal, in particular its
phase component, is highly sensitive to the transport properties of the
electron system present at the interface. Our modelling reveals that such
sensitivity originates from the interaction of the AFM tip with coupled
plasmon-phonon modes with a small penetration depth. The model allows us to
quantitatively correlate changes in the optical signal with the variation of
the 2DES transport properties induced by cooling and by electrostatic gating.
To probe the spatial resolution of the technique, we image conducting
nano-channels written in insulating heterostructures with a voltage-biased tip
of an atomic force microscope.Comment: 19 pages, 5 figure