arXiv subjects

Manfred Bayer

Publications and source records attributed to Manfred Bayer.

101 records · Page 6Linked to original sources

arXiv2014

Giant Rydberg Excitons in Cuprous Oxide

Highly excited atoms with an electron moved into a level with large principal quantum number are fascinating hydrogen-like objects. The giant extension of these Rydberg atoms leads to huge interaction effects. Monitoring these interactions has provided novel insights into molecular and condensed matter physics problems on a single quantum level. Excitons, the fundamental optical excitations in semiconductors consisting of a negatively charged electron and a positively charged hole, are the condensed matter analogues of hydrogen. Highly excited excitons with extensions similar to Rydberg atoms are attractive because they may be placed and moved in a crystal with high precision using microscopic potential landscapes. Their interaction may allow formation of ordered exciton phases or sensing of elementary excitations in the surrounding, also on a quantum level. Here we demonstrate the existence of Rydberg excitons in cuprous oxide, Cu2O, with principal quantum numbers as large as n=25 . These states have giant wave function extensions of more than 2 micrometers, compared to about a nanometer for the ground state. The strong dipole-dipole interaction is evidenced by a blockade effect, where the presence of an exciton prevents excitation of a further exciton in its vicinity.

cond-mat.mes-hall↗

arXiv2014

Polaron-induced lattice distortion of (In,Ga)As quantum dots by optically excited carriers

We report on a high resolution x-ray diffraction study unveiling the effect of carriers optically injected into (In,Ga)As quantum dots on the surrounding GaAs crystal matrix. We find a tetragonal lattice expansion with enhanced elongation along the [001] crystal axis that is superimposed on an isotropic lattice extension. The isotropic contribution arises from excitation induced lattice heating as confirmed by temperature dependent reference studies. The tetragonal expansion on the femtometer scale is attributed to polaron formation by carriers trapped in the quantum dots.

cond-mat.mtrl-sci↗

arXiv2014

Influence of interactions with non-condensed particles on the coherence of a 1D polariton condensate

One-dimensional polariton condensates (PoCos) in a photonic wire are generated through non-resonant laser excitation, by which also a reservoir of background carriers is created. Interaction with this reservoir may affect the coherence of the PoCo, which is studied here by injecting a condensate locally and monitoring the coherence along the wire. While the incoherent reservoir is mostly present within the excitation laser spot, the condensate can propagate ballistically through the wire. Photon correlation measurements show that far from the laser spot the second order correlation function approaches unity value, as expected for the coherent condensed state. When approaching the spot, however, the correlation function increases up to values of 1.2 showing the addition of noise to the emission due to interaction with the reservoir. This finding is substantiated by measuring the first order coherence by a double slit experiment, which shows a reduced visibility of interference at the excitation laser spot.

cond-mat.mes-hall↗

arXiv2013

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 200$-$300 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 $B$, reaching 10 GHz at $B\sim 5$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 $g$-factor $|g_{\rm exc}|=0.2$ and the magneto-spatial constant $V= 5 \times 10^{-12}$ eVcm$\textup{T}^{-1}$.

cond-mat.mtrl-sci↗

arXiv2013

Compressive adaptive computational ghost imaging

Compressive sensing is considered a huge breakthrough in signal acquisition. It allows recording an image consisting of $N^2$ pixels using much fewer than $N^2$ measurements if it can be transformed to a basis where most pixels take on negligibly small values. Standard compressive sensing techniques suffer from the computational overhead needed to reconstruct an image with typical computation times between hours and days and are thus not optimal for applications in physics and spectroscopy. We demonstrate an adaptive compressive sampling technique that performs measurements directly in a sparse basis. It needs much fewer than $N^2$ measurements without any computational overhead, so the result is available instantly.

physics.optics↗

arXiv2012

Correlation and dephasing effects on the non-radiative coherence between bright excitons in an InAs QD ensemble measured with 2D spectroscopy

Exchange-mediated fine-structure splitting of bright excitons in an ensemble of InAs quantum dots is studied using optical two-dimensional Fourier-transform spectroscopy. By monitoring the non-radiative coherence between the bright states, we find that the fine-structure splitting decreases with increasing exciton emission energy at a rate of 0.1 $μ$eV/meV. Dephasing rates are compared to population decay rates to reveal that pure dephasing causes the exciton optical coherences to decay faster than the radiative limit at low temperature, independent of excitation density. Fluctuations of the bright state transition energies are nearly perfectly-correlated, protecting the non-radiative coherence from interband dephasing mechanisms.

cond-mat.mes-hall↗

arXiv2012

Optical measurements of spin noise as a high resolution spectroscopic tool

The intrinsic fluctuations of electron spins in semiconductors and atomic vapors generate a small, randomly-varying "spin noise" that can be detected by sensitive optical methods such as Faraday rotation. Recent studies have demonstrated that the frequency, linewidth, and lineshape of this spin noise directly reveals dynamical spin properties such as dephasing times, relaxation mechanisms and g-factors without perturbing the spins away from equilibrium. Here we demonstrate that spin noise measurements using wavelength-tunable probe light forms the basis of a powerful and novel spectroscopic tool to provide unique information that is fundamentally inaccessible via conventional linear optics. In particular, the wavelength dependence of the detected spin noise power can reveal homogeneous linewidths buried within inhomogeneously-broadened optical spectra, and can resolve overlapping optical transitions belonging to different spin systems. These new possibilities are explored both theoretically and via experiments on spin systems in opposite limits of inhomogeneous broadening (alkali atom vapors and semiconductor quantum dots).

cond-mat.mes-hall↗

arXiv2012

Modulation of a surface plasmon-polariton resonance by sub-terahertz diffracted coherent phonons

Coherent sub-THz phonons incident on a gold grating that is deposited on a dielectric substrate undergo diffraction and thereby induce an alteration of the surface plasmon-polariton resonance. This results in efficient high-frequency modulation (up to 110 GHz) of the structure's reflectivity for visible light in the vicinity of the plasmon-polariton resonance. High modulation efficiency is achieved by designing a periodic nanostructure which provides both plasmon-polariton and phonon resonances. Our theoretical analysis shows that the dynamical alteration of the plasmon-polariton resonance is governed by modulation of the slit widths within the grating at the frequencies of higher-order phonon resonances.

cond-mat.mtrl-sci↗

arXiv2011

Distinguishing photon and polariton lasing from GaAs microcavities by spectral and temporal analysis of the two-threshold behavior

We compare polariton lasing with photon lasing of a planar GaAs/GaAlAs microcavity with zero detuning between the bare cavity mode and the bare exciton mode. For the emission from the lower energy-momentum dispersion branch we find a two-threshold behavior of the ground state in the input-output curve where each transition is accompanied by characteristic changes of the in-plane mode dispersion. In particular, we show that the thresholds are unambiguously evidenced in the photon statistics of the emission based on the second-order correlation function. Moreover, the distinct two-threshold behavior is confirmed in the evolution of the emission pulse duration.

cond-mat.quant-gas↗

arXiv2010

First- and second-order coherence properties of a quantum-dot micropillar laser

We present investigations on the coherence of the emission from the polarization split fundamental mode of an AlGaInAs/GaAs quantum-dot microcavity laser. Using polarization insensitive measurements of the first-order field-correlation function $g^{(1)}(τ)$ we determine the power-dependent degree of polarization. Whereas the orthogonally-polarized components of the fundamental mode exhibit comparable strength below the lasing threshold, a degree of linear polarization of 0.99 is observed in the coherent regime. This is also observed for increasing temperatures up to 77K. Furthermore, by measuring $g^{(1)}(τ)$ for both modes separately the stronger mode is found to reveal a record coherence time of 20ns. Finally, based on a theoretical model it is possible to fully characterize the cavity emission in terms of first- and second-order coherence using auxiliary data from $g^{(2)}(τ)$ measurements performed with a recently developed streak camera technique.

cond-mat.mes-hall↗

arXiv2008

Manifestations of the hyperfine interaction between electron and nuclear spins in singly-charged (In,Ga)As/GaAs quantum dots

The nuclear spin fluctuations (NSF) as well as the dynamic nuclear polarization (DNP) and their effects on the electron spins in negatively charged (In,Ga)As/GaAs quantum dots have been studied by polarized pump-probe and photoluminescence spectroscopy techniques. The effective magnetic field of the NSF is about 30 mT at low excitation power. The NSF distribution becomes highly anisotropic at strong optical excitation by circularly polarized light with periodically alternating helicity. This phenomenon is attributed to a decrease of the nuclear spin entropy due to the hyperfine interaction with polarized electron spins. The DNP is limited to small values for intense, but short photoexcitation.

cond-mat.mes-hall↗