16 research outputs found
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
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
Thermal and electrostatic tuning of surface phonon-polaritons in LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterostructures
Phonon polaritons are promising for infrared applications due to a strong light-matter coupling and subwavelength energy confinement they offer. Yet, the spectral narrowness of the phonon bands and difficulty to tune the phonon polariton properties hinder further progress in this field. SrTiO3 â a prototype perovskite oxide - has recently attracted attention due to two prominent far-infrared phonon polaritons bands, albeit without any tuning reported so far. Here we show, using cryogenic infrared near-field microscopy, that long-propagating surface phonon polaritons are present both in bare SrTiO3 and in LaAlO3/SrTiO3 heterostructures hosting a two-dimensional electron gas. The presence of the two-dimensional electron gas increases dramatically the thermal variation of the upper limit of the surface phonon polariton band due to temperature dependent polaronic screening of the surface charge carriers. Furthermore, we demonstrate a tunability of the upper surface phonon polariton frequency in LaAlO3/SrTiO3 via electrostatic gating. Our results suggest that oxide interfaces are a new platform bridging unconventional electronics and long-wavelength nanophotonics.</p
Giant Faraday rotation in single- and multilayer graphene
Optical Faraday rotation is one of the most direct and practically important
manifestations of magnetically broken time-reversal symmetry. The rotation
angle is proportional to the distance traveled by the light, and up to now
sizeable effects were observed only in macroscopically thick samples and in
two-dimensional electron gases with effective thicknesses of several
nanometers. Here we demonstrate that a single atomic layer of carbon - graphene
- turns the polarization by several degrees in modest magnetic fields. The
rotation is found to be strongly enhanced by resonances originating from the
cyclotron effect in the classical regime and the inter-Landau-level transitions
in the quantum regime. Combined with the possibility of ambipolar doping, this
opens pathways to use graphene in fast tunable ultrathin infrared
magneto-optical devices
Two-dimensional conical dispersion in evidenced by optical spectroscopy
Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. The coneâs lack of protection by the lattice symmetry immediately sparked vast discussions about the size and topological or trivial nature of a possible gap opening. Here, we report on a combined optical and transport study of ZrTe5, which reveals an alternative view of electronic bands in this material. We conclude that the dispersion is approximately linear only in the a-c plane, while remaining relatively flat and parabolic in the third direction (along the b axis). Therefore, the electronic states in ZrTe5 cannot be described using the model of 3D Dirac massless electrons, even when staying at energies well above the band gap 2Î ÂŒ 6 meV found in our experiments at low temperatures
Magneto-optical spectroscopy of epitaxial graphene
In this thesis we experimentally study the infrared magneto-optical properties of single and multilayer epitaxial graphene grown on a SiC substrate. Which is a promising material due to the scalability of the production method. However, graphene grown on SiC is also very complex, due to grain boundaries, wrinkles and in the case of multilayer graphene a twisted stacking. The optical experiments reveal a giant Faraday rotation in highly doped single layer graphene of several degrees. The spectra also show clear evidence for plasmonic and magnetoplasmonic excitations. From the transmission and Faraday rotation spectra the optical conductivity and a.c. Hall conductivity â the optical analogue of the d.c. Hall conductivity â are obtained, respectively
Determination of the gate-tunable band gap and tight-binding parameters in bilayer graphene using infrared spectroscopy
We present a compelling evidence for the opening of a bandgap in exfoliated
bottom-gated bilayer graphene by fitting the gate-voltage modulated infrared
reflectivity spectra in a large range of doping levels with a tight-binding
model and the Kubo formula. A close quantitative agreement between the
experimental and calculated spectra is achieved, allowing us to determine
self-consistently the full set of Slonczewski-Weiss-McClure tight-binding
parameters together with the gate-voltage dependent bandgap. The doping
dependence of the bandgap shows a good agreement with the existing calculations
that take the effects of self-screening into account. We also identify certain
mismatches between the tight-binding model and the data, which can be related
to electron-electron and electron-phonon interactions.Comment: 13 pages, 10 figure
Open Data corresponding to the publication âRaman spectroscopic evidence for multiferroicity in rare earth nickelate single crystalsâ
AbstractExperimental Raman spectra of RNiO3 (R=Y, er, Ho, Dy, Sm, Nd) at different temperatures
Spectroscopic Determination of the Electronic Structure of a Uranium Single-Ion Magnet
International audienc
Strong field transient manipulation of electronic states and bands
In the present review, laser fields are so strong that they become part of the electronic potential, and sometimes even dominate the Coulomb contribution. This manipulation of atomic potentials and of the associated states and bands finds fascinating applications in gases and solids, both in the bulk and at the surface. We present some recent spectacular examples obtained within the NCCR MUST in Switzerland