48 research outputs found
A- and B-Exciton Photoluminescence Intensity Ratio as a Measure of Sample Quality for Transition Metal Dichalcogenide Monolayers
The photoluminescence (PL) in monolayer transition metal dichalcogenides
(TMDs) is dominated by recombination of electrons in the conduction band with
holes in the spin-orbit split valence bands, and there are two distinct
emission features referred to as the A-peak (ground state exciton) and B-peak
(higher spin-orbit split state). The intensity ratio of these two features
varies widely and several contradictory interpretations have been reported. We
analyze the room temperature PL from MoS2, MoSe2, WS2, and WSe2 monolayers and
show that these variations arise from differences in the non-radiative
recombination associated with defect densities. Hence, the relative intensities
of the A- and B-emission features can be used to qualitatively asses the
non-radiative recombination, and thus the quality of the sample. A low B/A
ratio is indicative of low defect density and high sample quality. Emission
from TMD monolayers is governed by unique optical selection rules which make
them promising materials for valleytronic operations. We observe a notably
higher valley polarization in the B-exciton relative to the A-exciton. The high
polarization is a consequence of the shorter B-exciton lifetime resulting from
rapid relaxation of excitons from the B-exciton to the A-exciton of the valence
band.Comment: Final version is published online at APL Material
Understanding Variations in Circularly Polarized Photoluminescence in Monolayer Transition Metal Dichalcogenides
Monolayer transition metal dichalcogenides are promising materials for
valleytronic operations. They exhibit two inequivalent valleys in the Brillouin
zone, and the valley populations can be directly controlled and determined
using circularly polarized optical excitation and emission. The
photoluminescence polarization reflects the ratio of the two valley
populations. A wide range of values for the degree of circularly polarized
emission, Pcirc, has been reported for monolayer WS2, although the reasons for
the disparity are unclear. Here we optically populate one valley, and measure
Pcirc to explore the valley population dynamics at room temperature in a large
number of monolayer WS2 samples synthesized via chemical vapor deposition.
Under resonant excitation, Pcirc ranges from 2% to 32%, and we observe a
pronounced inverse relationship between photoluminescence (PL) intensity and
Pcirc. High quality samples exhibiting strong PL and long exciton relaxation
time exhibit a low degree of valley polarization, and vice versa. This behavior
is also demonstrated in monolayer WSe2 samples and transferred WS2, indicating
that this correlation may be more generally observed and account for the wide
variations reported for Pcirc. Time resolved PL provides insight into the role
of radiative and non-radiative contributions to the observed polarization.
Short non-radiative lifetimes result in a higher measured polarization by
limiting opportunity for depolarizing scattering events
Magneto-reflection spectroscopy of monolayer transition-metal dichalcogenide semiconductors in pulsed magnetic fields
We describe recent experimental efforts to perform polarization-resolved
optical spectroscopy of monolayer transition-metal dichalcogenide
semiconductors in very large pulsed magnetic fields to 65 tesla. The
experimental setup and technical challenges are discussed in detail, and
temperature-dependent magneto-reflection spectra from atomically thin tungsten
disulphide (WS) are presented. The data clearly reveal not only the valley
Zeeman effect in these 2D semiconductors, but also the small quadratic exciton
diamagnetic shift from which the very small exciton size can be directly
inferred. Finally, we present model calculations that demonstrate how the
measured diamagnetic shifts can be used to constrain estimates of the exciton
binding energy in this new family of monolayer semiconductors.Comment: PCSI-43 conference (Jan. 2016; Palm Springs, CA
Exciton Diamagnetic Shifts and Valley Zeeman Effects in Monolayer WS and MoS to 65 Tesla
We report circularly-polarized optical reflection spectroscopy of monolayer
WS and MoS at low temperatures (4~K) and in high magnetic fields to
65~T. Both the A and the B exciton transitions exhibit a clear and very similar
Zeeman splitting of approximately 230~eV/T (), providing
the first measurements of the valley Zeeman effect and associated -factors
in monolayer transition-metal disulphides. These results complement and are
compared with recent low-field photoluminescence measurements of valley
degeneracy breaking in the monolayer diselenides MoSe and WSe. Further,
the very large magnetic fields used in our studies allows us to observe the
small quadratic diamagnetic shifts of the A and B excitons in monolayer WS
(0.32 and 0.11~eV/T, respectively), from which we calculate exciton
radii of 1.53~nm and 1.16~nm. When analyzed within a model of non-local
dielectric screening in monolayer semiconductors, these diamagnetic shifts also
constrain and provide estimates of the exciton binding energies (410~meV and
470~meV for the A and B excitons, respectively), further highlighting the
utility of high magnetic fields for understanding new 2D materials.Comment: 9 pages, 5 figure
Control of magnetic contrast with nonlinear magneto-plasmonics
The interaction between surface plasmons (SP) and magnetic behavior has generated great research interest due to its potential for future magneto-optical devices with ultra-high sensitivity and ultra-fast switching. Here we combine two surface sensitive effects: magnetic second-harmonic generation (MSHG) and SP to enhance the detection sensitivity of the surface magnetization in a single-crystal iron film. We show that the MSHG signal can be significantly enhanced by SP in an attenuated total reflection (ATR) condition, and that the magnetic contrast can be varied over a wide range by the angle-of-incidence. Furthermore, the magnetic contrast of transverse and longitudinal MSHG display opposite trends, which originates from the change of relative phase between MSHG components. This new effect enhances the sensing of magnetic switching, which has potential usage in quaternary magnetic storage systems and bio-chemical sensors due to its very high surface sensitivity and simple structure
Nonlinear magneto-plasmonics
Nonlinear magneto-plasmonics (NMP) describes systems where nonlinear optics, magnetics and plasmonics are all involved. In such systems, nonlinear magneto-optical Kerr effect (nonlinear MOKE) plays an important role as a characterization method, and Surface Plasmons (SPs) work as catalyst to induce many new effects. Magnetization-induced second-harmonic generation (MSHG) is the major nonlinear magneto-optical process involved. The new effects include enhanced MSHG, controlled and enhanced magnetic contrast, etc. Nanostructures such as thin films, nanoparticles, nanogratings, and nanoarrays are critical for the excitation of SPs, which makes NMP an interdisciplinary research field in nanoscience and nanotechnology. In this review article, we organize recent work in this field into two categories: surface plasmon polaritons (SPPs) representing propagating surface plasmons, and localized surface plasmons (LSPs), also called particle plasmons. We review the structures, experiments, findings, and the applications of NMP from various groups. (C) 2015 Optical Society of Americ