P. Michler

Cavity-quantum electrodynamics using a single InAs quantum dot in a microdisk structure

We investigate cavity-quantum electrodynamics (QED) effects in an all-semiconductor nanostructure by tuning a single self-assembled InAs quantum dot (QD) into resonance with a high quality factor microdisk whispering gallery mode (WGM). The stronger temperature dependence of the QD single-exciton (1X) resonance allows us to change the relative energy of the WGM and the 1X transitions by varying the sample temperature. The two coupled resonances exhibit crossing behavior due to the weak coupling cavity-QED regime. We demonstrate exciton lifetime reduction by 6 due to the Purcell effect by tuning the QD into resonance with the WGM. Our experiments also show that single-exciton lifetime is independent of temperature up to 50 K.

Single-photon emission of CdSe quantum dots at temperatures up to 200 K

We report on the generation of triggered single photons obtained from epitaxially grown self-assembled CdSe/Zn(S,Se) quantum dots for temperatures up to 200 K. At low temperatures (T 40 K) an increasing multi-photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring quantum dots is observed. We found that the multi-photon emission probability of a bare quantum dot (background subtracted) is strongly suppressed at 200 K if compared to a Poissonian light source of the same average intensity. Our results demonstrate the large potential of self-assembled CdSe/Zn(S,Se) quantum dots for nonclassical light generation at temperatures up to 200 K.

On-demand generation of indistinguishable polarization-entangled photon pairs

Polarization-entangled photon pairs are generated from an In(Ga)As quantum dot by setting the pump intensity such that the inversion of the quantum dot from the ground to the biexcitonic state is the most probable transition. On-demand generation is demonstrated with an ultrahigh purity, a high entanglement fidelity and high two-photon-interference non-post-selective visibilities.

Laser emission from quantum dots in microdisk structures

We report optically pumped continuous-wave lasing from self-assembled InAs and InGaAs quantum dots (QDs) embedded in high-quality-factor microdisk laser structures. The microdisk emission spectra show lasing on 1–5 well separated modes in the wavelength range between 900 and 990 nm. The estimated threshold pump densities are between 20 and 200 W/cm2. The lasing characteristics are discussed in terms of both inhomogeneously and homogeneously broadened QD transitions.

Triggered polarization-entangled photon pairs from a single quantum dot up to 30 K

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization- entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices.

Post-Selected Indistinguishable Photons from the Resonance Fluorescence of a Single Quantum Dot in a Microcavity

Applying continuous-wave pure resonant s-shell optical excitation of individual quantum dots in a high-quality micropillar cavity, we demonstrate the generation of post-selected indistinguishable photons in resonance fluorescence. Close to ideal visibility contrast of 90% is verified by polarization-dependent Hong-Ou-Mandel two-photon interference measurements. Furthermore, a strictly resonant continuous-wave excitation together with controlling the spontaneous emission lifetime of the single quantum dots via tunable emitter-mode coupling (Purcell) is proven as a versatile scheme to generate close to Fourier transform-limited (T2/(2T1)=0.91) single photons even at 80% of the emission saturation level.

Non-resonant dot|[ndash]|cavity coupling and its potential for resonant single-quantum-dot spectroscopy

Mechanisms of distinct resonance in microcavities driven by strongly detuned single quantum dots are not well understood. Investigation of non-resonant dot–cavity coupling of individual quantum dots in micropillars now suggests a dominant role of phonon-mediated dephasing. This new perspective may have implications for single-photon sources, quantum information applications and spectroscopy.

Dephasing of triplet-sideband optical emission of a resonantly driven InAs/GaAs quantum dot inside a microcavity.

Detailed properties of resonance fluorescence from a single quantum dot in a micropillar cavity are investigated, with particular focus on emission coherence in the dependence on optical driving field power and detuning. A power-dependent series over a wide range reveals characteristic Mollow triplet spectra with large Rabi splittings of |Ω|≤15  GHz. In particular, the effect of dephasing in terms of systematic spectral broadening ∝Ω(2) of the Mollow sidebands is observed as a strong fingerprint of excitation-induced dephasing. Our results are in excellent agreement with predictions of a recently presented model on phonon-dressed quantum dot Mollow triplet emission in the cavity-QED regime.

Triggered polarization-correlated photon pairs from a single CdSe quantum dot

We report on the generation of polarization-correlated photon pairs by the radiative biexciton (XX)-exciton (X) cascade of single CdSe quantum dots (QDs). Under nonresonant optical pulsed excitation (76 MHz) at low temperature (4 K), a high collinear correlation degree of 74.5% was observed from cross-correlation measurements between single XX and X emissions, which reflects an asymmetry-induced exciton fine-structure splitting. In consideration of the excitonic radiative lifetime (250 ps) this effect allows for direct conclusions about the relaxation time (T⩾480 ps) between the corresponding sublevels. Our results also suggest that the biexciton-exciton cascade in a CdSe QD is well suited for triggered single photon and/or photon pair generation rates above 1 GHz.

Interface characteristics of GaInP/GaAs double heterostructures grown by metalorganic vapor phase epitaxy

Abstract The formation of parasitic Ga x In 1− x As y P 1− y intermediate layers when growing GaAs on GaInP or vice versa by MOVPE is investigated by the use of asymmetric GaInP/GaAs/AlGaAs double heterostructures. We found that such intermediate layers are only formed when growing GaAs on GaInP (inverted interface), but not for the reverse case (normal interface), resulting in a strong photoluminescence peak below the GaAs band gap. Time-resolved photoluminescence also indicates a much shorter minority carrier lifetime of the structures containing the inverted GaInP/GaAs interface (∼10 ns) compared to samples containing the normal interface (∼0.5 μs). By investigation of the interface of GaInP/AlGaAs and AlGaInP/GaAs, it is found that In carry-over is mainly responsible for the formation of this layer in our system. By growing thin AlGaAs intermediate layers (1 to 2 nm) at the inverted interface, this low-energy photoluminescence peak could be effectively suppressed. The same could be obtained without any special means in a smaller epitaxial system, probably due to the reduced hot susceptor surface.