438 research outputs found
Modally Resolved Fabry-Perot Experiment with Semiconductor Waveguides
Based on the interaction between different spatial modes, semiconductor
Bragg-reflection waveguides provide a highly functional platform for non-linear
optics. Therefore, the control and engineering of the properties of each
spatial mode is essential. Despite the multimodeness of our waveguide, the
well-established Fabry-Perot technique for recording fringes in the optical
transmission spectrum can successfully be employed for a detailed linear
optical characterization when combined with Fourier analysis. A prerequisite
for the modal sensitivity is a finely resolved transmission spectrum that is
recorded over a broad frequency band. Our results highlight how the features of
different spatial modes, such as their loss characteristics and dispersion
properties, can be separated from each other allowing their comparison. The
mode-resolved measurements are important for optimizing the performance of such
multimode waveguides by tailoring the properties of their spatial modes.Comment: 8 pages, 7 figure
Multi-dimensional laser spectroscopy of exciton-polaritons with spatial light modulators
We describe an experimental system that allows one to easily access the
dispersion curve of exciton-polaritons in a microcavity. Our approach is based
on two spatial light modulators (SLM), one for changing the excitation angles
(momenta), and the other for tuning the excitation wavelength. We show that
with this setup, an arbitrary number of states can be excited accurately and
that re-configuration of the excitation scheme can be done at high speed.Comment: 4 pages, 5 figure
Verification of band offsets and electron effective masses in GaAsN/GaAs quantum wells : Spectroscopic experiment versus 10-band k.p modeling
Optical transitions in GaAs1-xNx/GaAs quantum wells (QWs) have been probed by two complementary techniques, modulation spectroscopy in a form of photoreflectance and surface photovoltage spectroscopy. Transition energies in QWs of various widths and N contents have been compared with the results of band structure calculations based on the 10-band k.p Hamiltonian. Due to the observation of higher order transitions in the measured spectra, the band gap discontinuities at the GaAsN/GaAs interface and the electron effective masses could be determined, both treated as semi-free parameters to get the best matching between the theoretical and experimental energies. We have obtained the chemical conduction band offset values of 86% for x = 1.2% and 83% for x = 2.2%, respectively. For these determined band offsets, the electron effective masses equal to about 0.09 m(o) in QWs with 1.2% N and 0.15 m(o) for the case of larger N content of 2.2%.Publisher PDFPeer reviewe
Memristive operation mode of a site-controlled quantum dot floating gate transistor
The authors gratefully acknowledge financial support from the European Union (FPVII (2007-2013) under Grant Agreement No. 318287 Landauer) as well as the state of Bavaria.We have realized a floating gate transistor based on a GaAs/AlGaAs heterostructure with site-controlled InAs quantum dots. By short-circuiting the source contact with the lateral gates and performing closed voltage sweep cycles, we observe a memristive operation mode with pinched hysteresis loops and two clearly distinguishable conductive states. The conductance depends on the quantum dot charge which can be altered in a controllable manner by the voltage value and time interval spent in the charging region. The quantum dot memristor has the potential to realize artificial synapses in a state-of-the-art opto-electronic semiconductor platform by charge localization and Coulomb coupling.Publisher PDFPeer reviewe
Exciton lifetime and emission polarization dispersion in strongly in-plane asymmetric nanostructures
We present experimental and theoretical investigation of exciton
recombination dynamics and the related polarization of emission in highly
in-plane asymmetric nanostructures. Considering general asymmetry- and
size-driven effects, we illustrate them with a detailed analysis of
InAs/AlGaInAs/InP elongated quantum dots. These offer a widely varied
confinement characteristics tuned by size and geometry that are tailored during
the growth process, which leads to emission in the application-relevant
spectral range of 1.25-1.65 {\mu}m. By exploring the interplay of the very
shallow hole confining potential and widely varying structural asymmetry, we
show that a transition from the strong through intermediate to even weak
confinement regime is possible in nanostructures of this kind. This has a
significant impact on exciton recombination dynamics and the polarization of
emission, which are shown to depend not only on details of the calculated
excitonic states but also on excitation conditions in the photoluminescence
experiments. We estimate the impact of the latter and propose a way to
determine the intrinsic polarization-dependent exciton light-matter coupling
based on kinetic characteristics.Comment: 11 pages, 8 figure
Rapid onset of molecular friction in liquids bridging between the atomistic and hydrodynamic pictures
Friction in liquids arises from conservative forces between molecules and atoms. Although the hydrodynamics at the nanoscale is subject of intense research and despite the enormous interest in the non-Markovian dynamics of single molecules and solutes, the onset of friction from the atomistic scale so far could not be demonstrated. Here, we fill this gap based on frequency-resolved friction data from high-precision simulations of three prototypical liquids, including water. Combining with theory, we show that friction in liquids emerges abruptly at a characteristic frequency, beyond which viscous liquids appear as non-dissipative, elastic solids. Concomitantly, the molecules experience Brownian forces that display persistent correlations. A critical test of the generalised Stokes–Einstein relation, mapping the friction of single molecules to the visco-elastic response of the macroscopic sample, disproves the relation for Newtonian fluids, but substantiates it exemplarily for water and a moderately supercooled liquid. The employed approach is suitable to yield insights into vitrification mechanisms and the intriguing mechanical properties of soft materials
- …