30 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
Surface plasmon-enhanced transverse magnetic second-harmonic generation
We present experimental studies on surface plasmon (SP) enhanced transverse magnetic second-harmonic generation (T-MSHG) in single-crystal iron films grown by molecular beam epitaxy at room temperature on MgO (001) substrates. We show that it is possible to achieve both strongly enhanced T-MSHG intensity and high magnetic contrast ratio under attenuated total reflection configuration without using complex heterostructures because MSHG is generated directly at the iron surface where SPs are present. The T-MSHG has a much larger contrast ratio than transverse magneto-optical Kerr effect (T-MOKE) and shows great potential for a new generation of bio-chemical sensors due to its very high surface sensitivity. In addition, by analyzing the experimental results and the simulations based on SP field-enhancement theory, we demonstrate that the second-order susceptibility of MSHG shows great anisotropy and the tensor odd chi(odd)(xzz). is dominant in our sample. (C) 2013 Optical Society of Americ
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
Determination of Interface Atomic Structure and Its Impact on Spin Transport Using Z-Contrast Microscopy and Density-Functional Theory
We combine Z-contrast scanning transmission electron microscopy with
density-functional-theory calculations to determine the atomic structure of the
Fe/AlGaAs interface in spin-polarized light-emitting diodes. A 44% increase in
spin-injection efficiency occurs after a low-temperature anneal, which produces
an ordered, coherent interface consisting of a single atomic plane of
alternating Fe and As atoms. First-principles transport calculations indicate
that the increase in spin-injection efficiency is due to the abruptness and
coherency of the annealed interface.Comment: 16 pages (including cover), 4 figure