9 research outputs found
Dynamical properties of excitonic quasi-particles in II-VI semiconductors
This work focuses on the dynamics of exciton polaritons and exciton magnetic polarons in II-VI semiconductors.
The propagation dynamics of exciton polaritons, formed between photons and the fundamental exciton resonance, are investigated in (Cd,Zn)Te crystal.
Due to the polariton dispersion a sub-mm thick crystal can delay the propagation of light on a sub-ns scale, with the delay increasing as the photon energy approaches the energy of the exciton resonance.
The Zeeman splitting induced by the application of an external magnetic field gives rise to polarization effects resulting in oscillations of the polarization of transmitted light in the time domain.
The polarization effects including linear and circular birefringence are characterized and explained by a detailed model.
Exciton polaron formation is studied in a semimagnetic CdMnSe quantum well surround by CdMgSe barriers.
Here, unusual slow exciton magnetic polaron formation is observed and successfully attributed to autolocalization, a positive feedback mechanism between polaron formatation and the confinement of the hole of the exciton.
This is enabled by the presence of Mn inside the quantum well and its absence in the barrier.
This leads to a stronger confinement of the hole to the quantumn well if an external magnetic field is applied or an exciton magnetic polaron is formed.
This additional confinement is confirmed using time resolved magneto spectrosopy by a reduction of exciton lifetime with the application of a Faraday magnetic field and a magnetic field induced anisotropy of the exciton g-factor
Magnetic field induced nutation of the exciton-polariton polarization in (Cd,Zn)Te crystals
We study the polarization dynamics of exciton-polaritons propagating in
sub-mm thick (Cd,Zn)Te bulk crystals using polarimetric time-of-flight
techniques. The application of a magnetic field in Faraday geometry leads to
synchronous temporal oscillations of all Stokes parameters of an initially
linearly or circularly polarized, spectrally broad optical pulse of 150 fs
duration propagating through the crystal. Strong dispersion for photon energies
close to the exciton resonance leads to stretching of the optical pulse to a
duration of 200300 ps and enhancement of magneto-optical effects such as the
Faraday rotation and the non-reciprocal birefringence. The oscillation
frequency of the exciton-polariton polarization increases with magnetic field
, reaching 10 GHz at T. Surprisingly, the relative contributions of
Faraday rotation and non-reciprocal birefringence undergo strong changes with
photon energy, which is attributed to a non-trivial spectral dependence of
Faraday rotation in the vicinity of the exciton resonance. This leads to
polarization nutation of the transmitted optical pulse in the time domain. The
results are well explained by a model that accounts for Faraday rotation and
magneto-spatial dispersion in zinc-blende crystals. We evaluate the exciton
-factor and the magneto-spatial constant eVcm.Comment: 11 pages, 6 figure
Light-emitting GaAs nanowires on a flexible substrate
Semiconductor nanowire-based devices are among the most promising structures used to meet the current challenges of electronics, optics and photonics. Due to their high surface-to-volume ratio and excellent optical and electrical properties, devices with low power, high efficiency and high density can be created. This is of major importance for environmental issues and economic impact. Semiconductor nanowires have been used to fabricate high performance devices, including detectors, solar cells and transistors. Here, we demonstrate a technique for transferring large-area nanowire arrays to flexible substrates while retaining their excellent quantum efficiency in emission. Starting with a defect-free self-catalyzed molecular beam epitaxy (MBE) sample grown on a Si substrate, GaAs core–shell nanowires are embedded in a dielectric, removed by reactive ion etching and transferred to a plastic substrate. The original structural and optical properties, including the vertical orientation, of the nanowires are retained in the final plastic substrate structure. Nanowire emission is observed for all stages of the fabrication process, with a higher emission intensity observed for the final transferred structure, consistent with a reduction in nonradiative recombination via the modification of surface states. This transfer process could form the first critical step in the development of flexible nanowire-based light-emitting devices
Light-Emitting GaAs Nanowires on a Flexible Substrate
Semiconductor nanowire-based
devices are among the most promising
structures used to meet the current challenges of electronics, optics
and photonics. Due to their high surface-to-volume ratio and excellent
optical and electrical properties, devices with low power, high efficiency
and high density can be created. This is of major importance for environmental
issues and economic impact. Semiconductor nanowires have been used
to fabricate high performance devices, including detectors, solar
cells and transistors. Here, we demonstrate a technique for transferring
large-area nanowire arrays to flexible substrates while retaining
their excellent quantum efficiency in emission. Starting with a defect-free
self-catalyzed molecular beam epitaxy (MBE) sample grown on a Si substrate,
GaAs core–shell nanowires are embedded in a dielectric, removed
by reactive ion etching and transferred to a plastic substrate. The
original structural and optical properties, including the vertical
orientation, of the nanowires are retained in the final plastic substrate
structure. Nanowire emission is observed for all stages of the fabrication
process, with a higher emission intensity observed for the final transferred
structure, consistent with a reduction in nonradiative recombination
via the modification of surface states. This transfer process could
form the first critical step in the development of flexible nanowire-based
light-emitting devices
Reduced Charge Transfer Exciton Recombination in Organic Semiconductor Heterojunctions by Molecular Doping
We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer–fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by ∼70%. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states
Reduced Charge Transfer Exciton Recombination in Organic Semiconductor Heterojunctions by Molecular Doping
Emergence of Highly Linearly Polarized Interlayer Exciton Emission in MoSe<sub>2</sub>/WSe<sub>2</sub> Heterobilayers with Transfer-Induced Layer Corrugation
The availability of accessible fabrication methods based on deterministic
transfer of atomically thin crystals has been essential for the rapid expansion
of research into van der Waals heterostructures. An inherent issue of these
techniques is the deformation of the polymer carrier film during the transfer,
which can lead to highly non-uniform strain induced in the transferred
two-dimensional material. Here, using a combination of optical spectroscopy,
atomic force and Kelvin probe force microscopy, we show that the presence of
nanometer scale wrinkles formed due to transfer-induced stress relaxation can
lead to strong changes in the optical properties of MoSe/WSe
heterostructures and the emergence of the linearly polarized interlayer exciton
photoluminescence. We attribute these changes to the local breaking of crystal
symmetry in the nanowrinkles, which act as efficient accumulation centers for
the interlayer excitons due to the strain-induced interlayer band gap
reduction. The surface potential images of the rippled heterobilayer samples
acquired using Kelvin probe force microscopy reveal the variation of the local
work function consistent with the strain-induced band gap modulation, while the
potential offset observed at the ridges of the wrinkles shows a clear
correlation with the value of the tensile strain estimated from the wrinkle
geometry. Our findings highlight the important role of the residual strain in
defining optical properties of van der Waals heterostructures and suggest novel
approaches for interlayer exciton manipulation by local strain engineering