Optical pumping of quantum dot micropillar lasers.

Arrays of quantum dot micropillar lasers are an attractive technology platform for various applications in the wider field of nanophotonics. Of particular interest is the potential efficiency enhancement as a consequence of cavity quantum electrodynamics effects, which makes them prime candidates for next generation photonic neurons in neural network hardware. However, particularly for optical pumping, their power-conversion efficiency can be very low. Here we perform an in-depth experimental analysis of quantum dot microlasers and investigate their input-output relationship over a wide range of optical pumping conditions. We find that the current energy efficiency limitation is caused by disadvantageous optical pumping concepts and by a low exciton conversion efficiency. Our results indicate that for non-resonant pumping into the GaAs matrix (wetting layer), 3.4% (0.6%) of the optical pump is converted into lasing-relevant excitons, and of those only 2% (0.75%) provide gain to the lasing transition. Based on our findings, we propose to improve the pumping efficiency by orders of magnitude by increasing the aluminium content of the AlGaAs/GaAs mirror pairs in the upper Bragg reflector.

Design optimization for bright electrically-driven quantum dot single-photon sources emitting in telecom O-band.

A combination of advanced light engineering concepts enables a substantial improvement in photon extraction efficiency of micro-cavity-based single-photon sources in the telecom O-band at ∼1.3 µm. We employ a broadband bottom distributed Bragg reflector (DBR) and a top DBR formed in a dielectric micropillar with an additional circular Bragg grating in the lateral plane. This device design includes a doped layer in pin-configuration to allow for electric carrier injection. It provides broadband (∼8-10 nm) emission enhancement with an overall photon-extraction efficiency of ∼83% into the upper hemisphere and photon-extraction efficiency of ∼79% within numerical aperture NA=0.7. The efficiency of photon coupling to a single-mode fiber reaches 11% for SMF28 fiber (with NA=0.12), exceeds 22% for 980HP fiber (with NA=0.2) and reaches ∼40% for HNA fiber (with NA=0.42) as demonstrated by 3D finite-difference time-domain modeling.

Photon-Number-Resolved Measurement of an Exciton-Polariton Condensate.

We measure the full photon-number distribution emitted from a Bose condensate of microcavity exciton polaritons confined in a micropillar cavity. The statistics are acquired by means of a photon-number-resolving transition edge sensor. We directly observe that the photon-number distribution evolves with the nonresonant optical excitation power from geometric to quasi-Poissonian statistics, which is canonical for a transition from a thermal to a coherent state. Moreover, the photon-number distribution allows one to evaluate the higher-order photon correlations, shedding further light on the coherence formation and phase transition of the polariton condensate. The experimental data are analyzed in terms of thermal-coherent states, which gives direct access to the thermal and coherent fraction from the measured distributions. These results pave the way for a full understanding of the contribution of interactions in light-matter condensates in the coherence buildup at threshold.

Triggered high-purity telecom-wavelength single-photon generation from p-shell-driven InGaAs/GaAs quantum dot.

We report on the experimental demonstration of triggered single-photon emission at the telecom O-band from In(Ga)As/GaAs quantum dots (QDs) grown by metal-organic vapor-phase epitaxy. Micro-photoluminescence excitation experiments allowed us to identify the p-shell excitonic states in agreement with high excitation photoluminescence on the ensemble of QDs. Hereby we drive an O-band-emitting GaAs-based QD into the p-shell states to get a triggered single photon source of high purity. Applying pulsed p-shell resonant excitation results in strong suppression of multiphoton events evidenced by the as measured value of the second-order correlation function at zero delay of 0.03 (and ~0.005 after background correction).

Exploring Dephasing of a Solid-State Quantum Emitter via Time- and Temperature-Dependent Hong-Ou-Mandel Experiments.

We probe the indistinguishability of photons emitted by a semiconductor quantum dot (QD) via time- and temperature-dependent two-photon interference (TPI) experiments. An increase in temporal separation between consecutive photon emission events reveals a decrease in TPI visibility on a nanosecond time scale, theoretically described by a non-Markovian noise process in agreement with fluctuating charge traps in the QD's vicinity. Phonon-induced pure dephasing results in a decrease in TPI visibility from (96±4)% at 10 K to a vanishing visibility at 40 K. In contrast to Michelson-type measurements, our experiments provide direct access to the time-dependent coherence of a quantum emitter on a nanosecond time scale.

Direct observation of correlations between individual photon emission events of a microcavity laser.

Lasers are recognized for coherent light emission, the onset of which is reflected in a change in the photon statistics. For many years, attempts have been made to directly measure correlations in the individual photon emission events of semiconductor lasers. Previously, the temporal decay of these correlations below or at the lasing threshold was considerably faster than could be measured with the time resolution provided by the Hanbury Brown/Twiss measurement set-up used. Here we demonstrate a measurement technique using a streak camera that overcomes this limitation and provides a record of the arrival times of individual photons. This allows us to investigate the dynamical evolution of correlations between the individual photon emission events. We apply our studies to micropillar lasers with semiconductor quantum dots as the active material, operating in the regime of cavity quantum electrodynamics. For laser resonators with a low cavity quality factor, Q, a smooth transition from photon bunching to uncorrelated emission with increasing pumping is observed; for high-Q resonators, we see a non-monotonic dependence around the threshold where quantum light emission can occur. We identify regimes of dynamical anti-bunching of photons in agreement with the predictions of a microscopic theory that includes semiconductor-specific effects.

Room temperature polariton light emitting diode with integrated tunnel junction.

We present a diode incorporating a large number (12) of GaAs quantum wells that emits light from exciton-polariton states at room temperature. A reversely biased tunnel junction is placed in the cavity region to improve current injection into the device. Electroluminescence studies reveal two polariton branches which are spectrally separated by a Rabi splitting of 6.5 meV. We observe an anticrossing of the two branches when the temperature is lowered below room temperature as well as a Stark shift of both branches in a bias dependent photoluminescence measurement.

Bloch-wave engineering of quantum dot micropillars for cavity quantum electrodynamics experiments.

We have employed Bloch-wave engineering to realize submicron diameter high quality factor GaAs/AlAs micropillars (MPs). The design features a tapered cavity in which the fundamental Bloch mode is subject to an adiabatic transition to match the Bragg mirror Bloch mode. The resulting reduced scattering loss leads to record-high vacuum Rabi splitting of the strong coupling in MPs with modest oscillator strength quantum dots. A quality factor of 13, 600 and a splitting of 85 µeV with an estimated visibility v of 0.41 are observed for a small mode volume MP with a diameter d{c} of 850 nm.

Density and size control of InP/GaInP quantum dots on GaAs substrate grown by gas source molecular beam epitaxy.

We demonstrate a method to controllably reduce the density of self-assembled InP quantum dots (QDs) by cyclic deposition with growth interruptions. Varying the number of cycles enabled a reduction of the QD density from 7.4 × 10(10) cm(-2) to 1.8 × 10(9) cm(-2) for the same total amount of deposited InP. Simultaneously, a systematic increase of the QD size could be observed. Emission characteristics of different-sized InP QDs were analyzed. Excitation power dependent and time-resolved measurements confirm a transition from type I to type II band alignment for large InP quantum dots. Photon autocorrelation measurements of type I QDs performed under pulsed excitation reveal pronounced antibunching (g((2))(τ = 0) = 0.06 ± 0.03) as expected for a single-photon emitter. The described growth routine has great promise for the exploitation of InP QDs as quantum emitters.

Up on the Jaynes-Cummings ladder of a quantum-dot/microcavity system.

In spite of their different natures, light and matter can be unified under the strong-coupling regime, yielding superpositions of the two, referred to as dressed states or polaritons. After initially being demonstrated in bulk semiconductors and atomic systems, strong-coupling phenomena have been recently realized in solid-state optical microcavities. Strong coupling is an essential ingredient in the physics spanning from many-body quantum coherence phenomena, such as Bose-Einstein condensation and superfluidity, to cavity quantum electrodynamics. Within cavity quantum electrodynamics, the Jaynes-Cummings model describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling, a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently reported. Here we use advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton-cavity system. We measure and simulate its four-wave mixing response, granting direct access to the coherent dynamics of the first and second rungs of the Jaynes-Cummings ladder. The agreement of the rich experimental evidence with the predictions of the Jaynes-Cummings model is proof of the quantum strong-coupling regime in the investigated solid-state system.

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.

Observation of non-Markovian dynamics of a single quantum dot in a micropillar cavity.

We measure the detuning-dependent dynamics of a quasiresonantly excited single quantum dot coupled to a micropillar cavity. The system is modeled with the dissipative Jaynes-Cummings model where all experimental parameters are determined by explicit measurements. We observe non-Markovian dynamics when the quantum dot is tuned into resonance with the cavity leading to a nonexponential decay in time. Excellent agreement between experiment and theory is observed with no free parameters providing the first quantitative description of an all-solid-state cavity QED system based on quantum dot emitters.

Time-resolved photoluminescence investigations on HfO2-capped InP nanowires.

We have employed time-resolved photoluminescence (PL) spectroscopy to study the impact of HfO(2) surface capping by atomic layer deposition (ALD) on the optical properties of InP nanowires (NWs). The deposition of high-kappa dielectrics acting as a gate oxide is of particular interest in view of possible applications of semiconductor NWs in future wrap-gated field effect transistors (FETs). A high number of charged states at the NW-dielectrics interface can strongly degrade the performance of the FET which explains the strong interest in high quality deposition of high-kappa dielectrics. In the present work we show that time-resolved spectroscopy is a valuable and direct tool to monitor the surface quality of HfO(2)-capped InP NWs. In particular, we have studied the impact of ALD process parameters as well as surface treatment prior to the oxide capping on the NW-dielectrics interface quality. The best results in terms of the surface recombination velocity (S(0) = 9.5 x 10(3) cm s(-1)) were obtained for InP/GaP core/shell NWs in combination with a low temperature (100 degrees C) ALD process. While the present report focuses on the InP material system, our method of addressing the surface treatment for semiconductors with high-kappa dielectrics will also be applicable to nanoelectronic devices based on other III/V material systems such as InAs.

Strong coupling in a single quantum dot-semiconductor microcavity system.

Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140 microeV.

The role of optical excitation power on the emission spectra of a strongly coupled quantum dot-micropillar system.

A strongly coupled quantum dot-micropillar cavity system is studied under variation of the excitation power. The characteristic double peak spectral shape of the emission with a vacuum Rabi splitting of 85 microeV at low excitation transforms gradually into a single broad emission peak when the excitation power is increased. Modelling the experimental data by a recently published formalism [Laussy et al., Phys. Rev. Lett. 101, 083601 (2008)] yields a transition from strong coupling towards weak coupling which is mainly attributed to an excitation power driven decrease of the exciton-photon coupling constant.

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.

Single site-controlled In(Ga)As/GaAs quantum dots: growth, properties and device integration.

Results obtained by an advanced growth of site-controlled quantum dots (SCQDs) on pre-patterned nanoholes and their integration into both photonic resonators and nanoelectronic memories are summarized. A specific technique has been pursued to improve the optical quality of single SCQDs. Quantum dot (QD) layers have been vertically stacked but spectrally detuned for single SCQD studies. Thereby, the average emission linewidth of single QDs could be reduced from 2.3 meV for SCQDs in a first QD layer close to the etched nanoholes down to 600 microeV in the third InAs QD layer. Accurate SCQD nucleation on large QD distances is maintained by vertical strain induced QD coupling throughout the QD stacks. Record narrow linewidths of individual SCQDs down to approximately 110 microeV have been obtained. Experiments performed on coupled photonic SCQD-resonator devices show an enhancement of spontaneous emission. SCQDs have also been integrated deterministically in high electron mobility heterostructures and flash memory operation at room temperature has been observed.

Demonstration of strong coupling via electro-optical tuning in high-quality QD-micropillar systems.

We demonstrate electro-optical tuning of single quantum dots (QDs) embedded in high-quality (high-Q) micropillar cavities by exploiting the quantum confined Stark effect (QCSE). Combining electrically contacted high-Q micropillars and large In(0.3) Ga(0.7)As QDs with high oscillator strength facilitates the realization of strong coupling. In our experiments a single QD exciton was electrically tuned on resonance with a cavity mode of a micropillar with 1.9 microm diameter and a quality-factor (Q-factor) of 14,000 enabling the observation of strong coupling with a vacuum Rabi-Splitting of 63 microeV.

Single quantum dot controlled lasing effects in high-Q micropillar cavities.

Lasing effects based on individual quantum dots have been investigated in optically pumped high-Q micropillar cavities. We demonstrate a lowering of the threshold pump power from a off-resonance value of 37 microW to 18 microW when an individual quantum dot exciton is on-resonance with the cavity mode. Photon correlation studies below and above the laser threshold confirm the single dot influence. At resonance we observe antibunching with g((2))(0) = 0.36 at low excitation, which increases to 1 at about 1.5 times the threshold. In the off-resonant case, g((2))(0) is about 1 below and above threshold.

High Q whispering gallery modes in GaAs/AlAs pillar microcavities.

We report the observation of whispering gallery modes (WGM) in high quality GaAs/AlAs pillar microcavities defined by electron-beam lithography and electron cyclotron resonance reactive ion etching. Photoluminescence experiments, conducted using InAs quantum dots as an internal probe, reveal a remarkably simple WGM spectrum, consisting of a single series of TE modes. For diameters ranging from 3 to 4 mum, Q-factors in excess of 15 000 were measured, allowing for WGM lasing. Noticeably, sub-micron diameter micropillars also display high Qs (~ 1000), close to the limit set by intrinsic radiative losses. These results open the way to the development of original microlasers and improved quantum-dot single photon sources.