115 research outputs found
Semiconductor Bloch equation analysis of optical Stark and Bloch-Siegert shifts in monolayers WSe and MoS
We report on the theoretical and experimental investigation of
valley-selective optical Stark and Bloch-Siegert shifts of exciton resonances
in monolayers WSe and MoS induced by strong circularly polarized
nonresonant optical fields. We predict and observe transient shifts of both 1sA
and 1sB exciton transitions in the linear interaction regime. The theoretical
description is based on semiconductor Bloch equations. The solutions of the
equations are obtained with a modified perturbation technique, which takes into
account many-body Coulomb interaction effects. These solutions allow to explain
the polarization dependence of the shifts and calculate their values
analytically. We found experimentally the limits of the applicability of the
theoretical description by observing the transient exciton spectra change due
to many-body effects at high field amplitudes of the driving wave.Comment: 20 pages, 9 figures, this manuscript is related to the "Giant
valley-selective Stark and Bloch-Siegert shifts of exciton resonances in
WSe and MoS monolayers" manuscrip
Giant valley-selective Stark and Bloch-Siegert shifts of exciton resonances in WSe and MoS monolayers
In this letter we demonstrate that the valley degeneracy of exciton states in
monolayers of WSe and MoS can be lifted by the interaction with strong
circularly-polarized infrared pulses with durations of only few periods of the
electric field whose photon energy is much lower than the energy of the
excitonic transition. The observed valley-sensitive blue shifts of excitonic
absorption lines are consequences of optical Stark and Bloch-Siegert shifts
acting exclusively on the opposite valleys of the monolayer. We measured the
transient valley-selective changes of sample reflectivity for 1sA as well as
for 1sB exciton transitions corresponding to the two most intensive resonances
in the studied materials. For the studied phenomena we developed a theoretical
description based on semiconductor Bloch equations, which goes beyond the
simple two-level model used in previous investigations. The theoretical
approach takes into account Coulomb many-body effects in the monolayer and
provides a unified description of both types of shifts. The detected
room-temperature excitonic energy shifts of up to 30\,meV pave the way for
practical applications of these effects.Comment: 6 pages, 3 figures, the manuscript is related to the "Semiconductor
Bloch equation analysis of optical Stark and Bloch-Siegert shifts in
monolayers WSe and MoS" manuscrip
Polarization-controlled optimal scatter suppression in transient absorption spectroscopy
Ultrafast transient absorption spectroscopy is a powerful technique to study fast photo-induced processes, such as electron, proton and energy transfer, isomerization and molecular dynamics, in a diverse range of samples, including solid state materials and proteins. Many such experiments suffer from signal distortion by scattered excitation light, in particular close to the excitation (pump) frequency. Scattered light can be effectively suppressed by a polarizer oriented perpendicular to the excitation polarization and positioned behind the sample in the optical path of the probe beam. However, this introduces anisotropic polarization contributions into the recorded signal. We present an approach based on setting specific polarizations of the pump and probe pulses, combined with a polarizer behind the sample. Together, this controls the signal-to-scatter ratio (SSR), while maintaining isotropic signal. We present SSR for the full range of polarizations and analytically derive the optimal configuration at angles of 40.5° between probe and pump and of 66.9° between polarizer and pump polarizations. This improves SSR by 33 52 ≈. (or 3 compared to polarizer parallel to probe). The calculations are validated by transient absorption experiments on the common fluorescent dye Rhodamine B. This approach provides a simple method to considerably improve the SSR in transient absorption spectroscopy
Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As
We report on a systematic study of optical properties of (Ga,Mn)As epilayers
spanning the wide range of accessible substitutional Mn_Ga dopings. The growth
and post-growth annealing procedures were optimized for each nominal Mn doping
in order to obtain films which are as close as possible to uniform
uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the
mid-infrared absorption spectra whose position exhibits a prevailing blue-shift
for increasing Mn-doping. In the visible range, a peak in the magnetic circular
dichroism blue shifts with increasing Mn-doping. These observed trends confirm
that disorder-broadened valence band states provide a better one-particle
representation for the electronic structure of high-doped (Ga,Mn)As with
metallic conduction than an energy spectrum assuming the Fermi level pinned in
a narrow impurity band.Comment: 22 pages, 14 figure
Direct measurement of the three dimensional magnetization vector trajectory in GaMnAs by a magneto-optical pump-and-probe method
We report on a quantitative experimental determination of the
three-dimensional magnetization vector trajectory in GaMnAs by means of the
static and time-resolved pump-and-probe magneto-optical measurements. The
experiments are performed in a normal incidence geometry and the time evolution
of the magnetization vector is obtained without any numerical modeling of
magnetization dynamics. Our experimental method utilizes different polarization
dependences of the polar Kerr effect and magnetic linear dichroism to
disentangle the pump-induced out-of-plane and in-plane motions of
magnetization, respectively. We demonstrate that the method is sensitive enough
to allow for the determination of small angle excitations of the magnetization
in GaMnAs. The method is readily applicable to other magnetic materials with
sufficiently strong circular and linear magneto-optical effects.Comment: main paper: 7 pages, 3 figures; supplementary information: 11 pages,
6 figure
Exciton spin dynamics in spherical CdS quantum dots
Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in
detail experimentally and theoretically. Exciton states are calculated using
the 6-band k.p Hamiltonian. It is shown that for various sets of Luttinger
parameters, when the wurtzite lattice crystal field splitting and Coulomb
interaction between the electron-hole pair are taken into account exactly, both
the electron and hole wavefunction in the lowest exciton state are of S-type.
This rules out the spatial-symmetry-induced origin of the dark exciton in CdS
quantum dots. The exciton bleaching dynamics is studied using time- and
polarization-resolved transient absorption technique of ultrafast laser
spectroscopy. Several samples with a different mean size of CdS quantum dots in
different glass matrices were investigated. This enabled the separation of
effects that are typical for one particular sample from those that are general
for this type of material. The experimentally determined dependence of the
electron spin relaxation rate on the radius of quantum dots agrees well with
that computed theoretically.Comment: 24 pages, 10 figure
Experimental observation of the optical spin transfer torque
The spin transfer torque is a phenomenon in which angular momentum of a spin
polarized electrical current entering a ferromagnet is transferred to the
magnetization. The effect has opened a new research field of electrically
driven magnetization dynamics in magnetic nanostructures and plays an important
role in the development of a new generation of memory devices and tunable
oscillators. Optical excitations of magnetic systems by laser pulses have been
a separate research field whose aim is to explore magnetization dynamics at
short time scales and enable ultrafast spintronic devices. We report the
experimental observation of the optical spin transfer torque, predicted
theoretically several years ago building the bridge between these two fields of
spintronics research. In a pump-and-probe optical experiment we measure
coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by
circularly polarized laser pulses. During the pump pulse, the spin angular
momentum of photo-carriers generated by the absorbed light is transferred to
the collective magnetization of the ferromagnet. We interpret the observed
optical spin transfer torque and the magnetization precession it triggers on a
quantitative microscopic level. Bringing the spin transfer physics into optics
introduces a fundamentally distinct mechanism from the previously reported
thermal and non-thermal laser excitations of magnets. Bringing optics into the
field of spin transfer torques decreases by several orders of magnitude the
timescales at which these phenomena are explored and utilized.Comment: 11 pages, 4 figure
Experimental observation of the optical spin-orbit torque
Spin polarized carriers electrically injected into a magnet from an external
polarizer can exert a spin transfer torque (STT) on the magnetization. The phe-
nomenon belongs to the area of spintronics research focusing on manipulating
magnetic moments by electric fields and is the basis of the emerging
technologies for scalable magnetoresistive random access memories. In our
previous work we have reported experimental observation of the optical
counterpart of STT in which a circularly polarized pump laser pulse acts as the
external polarizer, allowing to study and utilize the phenomenon on several
orders of magnitude shorter timescales than in the electric current induced
STT. Recently it has been theoretically proposed and experimentally
demonstrated that in the absence of an external polarizer, carriers in a magnet
under applied electric field can develop a non-equilibrium spin polarization
due to the relativistic spin-orbit coupling, resulting in a current induced
spin-orbit torque (SOT) acting on the magnetization. In this paper we report
the observation of the optical counterpart of SOT. At picosecond time-scales,
we detect excitations of magnetization of a ferromagnetic semiconductor
(Ga,Mn)As which are independent of the polarization of the pump laser pulses
and are induced by non-equilibrium spin-orbit coupled photo-holes.Comment: 4 figure, supplementary information. arXiv admin note: text overlap
with arXiv:1101.104
Comparison of micromagnetic parameters of the ferromagnetic semiconductors (Ga,Mn)(As,P) and (Ga,Mn)As
We report on the determination of micromagnetic parameters of epilayers of the ferromagnetic semiconductor (Ga,Mn)As, which has an easy axis in the sample plane, and (Ga,Mn)(As,P), which has an easy axis perpendicular to the sample plane.We use an optical analog of ferromagnetic resonancewhere the laser-pulse-induced precession of magnetization is measured directly in the time domain. By the analysis of a single set of pump-and-probe magneto-optical data, we determined the magnetic anisotropy fields, the spin stiffness, and the Gilbert damping constant in these two materials. We show that incorporation of 10% of phosphorus in (Ga,Mn)As with 6% of manganese leads not only to the expected sign change of the perpendicular-to-plane anisotropy field but also to an increase of the Gilbert damping and to a reduction of the spin stiffness. The observed changes in the micromagnetic parameters upon incorporating P in (Ga,Mn)As are consistent with the reduced hole density, conductivity, and Curie temperature of the (Ga,Mn)(As,P) material.We also show that the apparent magnetization precession damping is stronger for the n=1 spinwave resonance mode than for the n=0 uniform magnetization precession mode
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