Showing papers by "Stephan Reitzenstein published in 2016"

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

Abstract: 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.

An electrically driven cavity-enhanced source of indistinguishable photons with 61% overall efficiency

Abstract: We report on an electrically driven efficient source of indistinguishable photons operated at pulse-repetition rates f up to 1.2 GHz. The quantum light source is based on a p-i-n-doped micropillar cavity with integrated self-organized quantum dots, which exploits cavity quantum electrodynamics effects in the weak coupling regime to enhance the emission of a single quantum emitter coupled to the cavity mode. We achieve an overall single-photon extraction efficiency of (61 ± 11) % for a device triggered electrically at f = 625 MHz. Analyzing the suppression of multi-photon emission events as a function of excitation repetition rate, we observe single-photon emission associated with g(2)HBT(0) values between 0.076 and 0.227 for f ranging from 373 MHz to 1.2 GHz. Hong-Ou-Mandel-type two-photon interference experiments under pulsed current injection at 487 MHz reveal a photon-indistinguishability of (41.1 ± 9.5) % at a single-photon emission rate of (92 ± 23) MHz.

Mode-Switching induced super-thermal bunching in quantum-dot microlasers

Abstract: The super-thermal photon bunching in quantum-dot (QD) micropillar lasers is investigated both experimentally and theoretically via simulations driven by dynamic considerations. Using stochastic multi-mode rate equations we obtain very good agreement between experiment and theory in terms of intensity profiles and intensity-correlation properties of the examined QD micro-laser's emission. Further investigations of the time-dependent emission show that super-thermal photon bunching occurs due to irregular mode-switching events in the bimodal lasers. Our bifurcation analysis reveals that these switchings find their origin in an underlying bistability, such that spontaneous emission noise is able to effectively perturb the two competing modes in a small parameter region. We thus ascribe the observed high photon correlation to dynamical multistabilities rather than quantum mechanical correlations.

Emission from quantum-dot high- microcavities: transition from spontaneous emission to lasing and the effects of superradiant emitter coupling

Abstract: Measured and calculated results are presented on the emission properties of a new class of emitters operating in the cavity quantum electrodynamics regime. The structures are based on high-finesse GaAs/AlAs micropillar cavities, each with an active medium consisting of a layer of InGaAs quantum dots and distinguishing feature of having substantial fraction of spontaneous emission channeled into one cavity mode (high-beta factor). This paper shows that the usual criterion for lasing with a conventional (low-beta factor) cavity, a sharp nonlinearity in an input-output curve accompanied by noticeable linewidth narrowing, has to be reinforced by the equal-time second-order photon autocorrelation function for confirming lasing. It will also show that the equal-time second-order photon autocorrelation function is useful for recognizing superradiance, a manifestation of the correlations possible in high- microcavities operating with quantum dots. In terms of consolidating the collected data and identifying the physics underlying laser action, both theory and experiment suggest a sole dependence on intracavity photon number. Evidence for this comes from all our measured and calculated data on emission coherence and fluctuation, for devices ranging from LEDs and cavity-enhanced LEDs to lasers, lying on the same two curves: one for linewidth narrowing versus intracavity photon number and the other for g(2)(0) versus intracavity photon number.

Quantum dot micropillar cavities with quality factors exceeding 250,000

Abstract: We report on the spectroscopic investigation of quantum dot micropillar cavities with unprecedented quality factors. We observe a pronounced dependency of the quality factor on the measurement scheme and find that significantly larger quality factors can be extracted in photoreflectance compared to photoluminescence measurements. While the photoluminescence spectra of the microcavity resonances feature a Lorentzian lineshape and Q-factors up to 184,000 (±10,000), the reflectance spectra have a Fano-shaped asymmetry and feature significantly higher Q-factors in excess of 250,000 resulting from a full saturation of the embedded emitters. The very high quality factors in our cavities promote strong light-matter coupling with visibilities exceeding 0.5 for a single QD coupled to the cavity mode.

Impact of Phonons on Dephasing of Individual Excitons in Deterministic Quantum Dot Microlenses.

Abstract: Optimized light–matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques Using in situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs), achieving their efficient coupling to the external light field This enables performing four-wave mixing microspectroscopy of single QD excitons, revealing the exciton population and coherence dynamics We infer the temperature dependence of the dephasing in order to address the impact of phonons on the decoherence of confined excitons The loss of the coherence over the first picoseconds is associated with the emission of a phonon wave packet, also governing the phonon background in photoluminescence (PL) spectra Using theory based on the independent boson model, we consistently explain the initial coherence decay, the zero-phonon line fraction, and the line

Generating single photons at gigahertz modulation-speed using electrically controlled quantum dot microlenses

Abstract: We report on the generation of single-photon pulse trains at a repetition rate of up to 1 GHz. We achieve this speed by modulating the external voltage applied on an electrically contacted quantum dot microlens, which is optically excited by a continuous-wave laser. By modulating the photoluminescence of the quantum dot microlens using a square-wave voltage, single-photon emission is triggered with a response time as short as (281 ± 19) ps, being 6 times faster than the radiative lifetime of (1.75 ± 0.02) ns. This large reduction in the characteristic emission time is enabled by a rapid capacitive gating of emission from the quantum dot, which is placed in the intrinsic region of a p-i-n-junction biased below the onset of electroluminescence. Here, since our circuit acts as a rectifying differentiator, the rising edge of the applied voltage pulses triggers the emission of single photons from the optically excited quantum dot. The non-classical nature of the photon pulse train generated at GHz-speed is proven by intensity autocorrelation measurements with g(2)(0) = 0.3 ± 0.1. Our results combine optical excitation with fast electrical gating and thus show promise for the generation of indistinguishable single photons at rates exceeding the limitations set by the intrinsic radiative lifetime.

On thresholdless lasing features in high-$\beta$ nitride nanobeam cavities: a quantum optical study

Abstract: Exploring the limits of spontaneous emission coupling is not only one of the central goals in the development of nanolasers, it is also highly relevant regarding future large-scale photonic integration requiring energy-efficient coherent light sources with a small footprint. These studies are accompanied by a vivid debate on how to prove and interpret lasing in the high-$\beta$ regime. We investigate close-to-ideal spontaneous emission coupling in GaN nanobeam lasers grown on silicon. Due to their high optical quality, such nanobeam cavities allow for efficient funneling of spontaneous emission from the quantum well gain material into the laser mode. By performing a comprehensive optical and quantum-optical characterization, supported by microscopic modeling of the nanolasers, we identify high-$\beta$ lasing at room temperature and show a lasing transition in the absence of a threshold nonlinearity at 156 K. This peculiar characteristic is explained in terms of a temperature and excitation power dependent interplay between 0D and 2D gain contributions.

Cavity assisted emission of single, paired and heralded photons from a single quantum dot device

Abstract: The photon emission into different spatial directions of a quantum dot in a micropillar cavity is theoretically analyzed. We propose two types of photon emission statistics from a single quantum light device: (i) single photon emission into the axial, strong coupling direction and a two-photon emission into the lateral, weak coupling direction, as well as (ii) the simultaneous use of both emission directions for the temporally ordered generation of two photons within a defined time-bin constituting a heralded single photon source. Our results open up exciting perspectives for solid state based quantum light sources, which can be generalized to any quantum emitter-microcavity system featuring spatially distinct emission channels between the resonator and unconfined modes.

CSAR 62 as negative-tone resist for high-contrast e-beam lithography at temperatures between 4 K and room temperature

Abstract: The temperature dependence of the electron-beam sensitive resist CSAR 62 is investigated in its negative-tone regime. The writing temperatures span a wide range from 4 K to room temperature with the focus on the liquid helium temperature regime. The importance of low temperature studies is motivated by the application of CSAR 62 for deterministic nanophotonic device processing by means of in situ electron-beam lithography. At low temperature, CSAR 62 exhibits a high contrast of 10.5 and a resolution of 49 nm. The etch stability is almost temperature independent and it is found that CSAR 62 does not suffer from peeling which limits the low temperature application of the standard electron-beam resist polymethyl methacrylate. As such, CSAR 62 is a very promising negative-tone resist for in situ electron-beam lithography of high quality nanostructures at low temperature.

Injection locking of quantum dot microlasers operating in the few photon regime

Abstract: We experimentally and theoretically investigate injection locking of quantum dot (QD) microlasers in the regime of cavity quantum electrodynamics (cQED). We observe frequency locking and phase-locking where cavity enhanced spontaneous emission enables simultaneous stable oscillation at the master frequency and at the solitary frequency of the slave microlaser. Measurements of the second-order autocorrelation function prove this simultaneous presence of both master and slave-like emission, where the former has coherent character with $g^{(2)}(0)=1$ while the latter one has thermal character with $g^{(2)}(0)=2$. Semi-classical rate-equations explain this peculiar behavior by cavity enhanced spontaneous emission and a low number of photons in the laser mode.

Using low-contrast negative-tone PMMA at cryogenic temperatures for 3D electron beam lithography

Abstract: We report on a 3D electron beam lithography (EBL) technique using polymethyl methacrylate (PMMA) in the negative-tone regime as a resist. First, we briefly demonstrate 3D EBL at room temperature. Then we concentrate on cryogenic temperatures where PMMA exhibits a low contrast, which allows for straightforward patterning of 3D nano- and microstructures. However, conventional EBL patterning at cryogenic temperatures is found to cause severe damage to the microstructures. Through an extensive study of lithography parameters, exposure techniques, and processing steps we deduce a hypothesis for the cryogenic PMMA's structural evolution under electron beam irradiation that explains the damage. In accordance with this hypothesis, a two step lithography technique involving a wide-area pre-exposure dose slightly smaller than the onset dose is applied. It enables us to demonstrate a >95% process yield for the low-temperature fabrication of 3D microstructures.

Injection Locking of Quantum-Dot Microlasers Operating in the Few-Photon Regime

Abstract: Injection locking is a common technique to control the frequency of a conventional, macroscopic oscillator, but how does a tiny device operating in the quantum regime behave? To answer, the authors study external control of microscopic lasers governed by cavity quantum electrodynamics. Surprisingly, in such devices $b\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}h$ stationary oscillations synchronized to the external signal $a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d$ oscillations at the solitary frequency occur $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}y$. This ``partial injection'' phenomenon is unique to nonlinear oscillators excited with a few tens of quanta, and is relevant to applications in optomechanics, spintronics, and integrated photonics.

Time reordering of paired photons in a dressed three-level cascade

Abstract: The two-photon dressing of a "three-level ladder" system, here the ground state, the exciton and the biexciton of a semiconductor quantum dot, leads to new eigenstates and allows one to manipulate the time ordering of the paired photons without unitary post processing We show that, after spectral post-selection of the single dressed states, the time ordering of the cascaded photons can be removed or conserved Our joint experimental and theoretical study demonstrates the high potential of a "ladder" system to be a versatile source of orthogonally polarized, bunched or antibunched pairs of photons

Resonance fluorescence of a site-controlled quantum dot realized by the buried-stressor growth technique

Abstract: Site-controlled growth of semiconductor quantum dots (QDs) represents a major advancement to achieve scalable quantum technology platforms. One immediate benefit is the deterministic integration of quantum emitters into optical microcavities. However, site-controlled growth of QDs is usually achieved at the cost of reduced optical quality. Here, we show that the buried-stressor growth technique enables the realization of high-quality site-controlled QDs with attractive optical and quantum optical properties. This is evidenced by performing excitation power dependent resonance fluorescence experiments at cryogenic temperatures showing QD emission linewidths down to 10 $\mu$eV. Resonant excitation leads to the observation of the Mollow triplet under CW excitation and enables coherent state preparation under pulsed excitation. Under resonant $\pi$-pulse excitation we observe clean single photon emission associated with $g^{(2)}(0)=0.12$ limited by non-ideal laser suppression.

Photon-statistics excitation spectroscopy of a single two-level system

Abstract: We investigate the influence of the photon statistics on the excitation dynamics of a single two-level system. A single semiconductor quantum dot represents the two-level system and is resonantly excited either with coherent laser light, or excited with chaotic light, with photon statistics corresponding to that of thermal radiation. Experimentally, we observe a reduced absorption cross section under chaotic excitation in the steady state. In the transient regime, the Rabi oscillations observable under coherent excitation disappear under chaotic excitation. Likewise, in the emission spectrum, the well-known Mollow triplet, which we observe under coherent drive, disappears under chaotic excitation. Our observations are fully consistent with theoretical predictions based on the semiclassical Bloch equation approach.

Pump-power-driven mode switching in a microcavity device and its relation to Bose-Einstein condensation

Abstract: We investigate the switching of the coherent emission mode of a bimodal microcavity device, occurring when the pump power is varied. We compare experimental data to theoretical results and identify the underlying mechanism to be based on the competition between the effective gain on the one hand and the intermode kinetics on the other. When the pumping is ramped up, above a threshold the mode with the largest effective gain starts to emit coherent light, corresponding to lasing. In contrast, in the limit of strong pumping it is the intermode kinetics that determines which mode acquires a large occupation and shows coherent emission. We point out that this latter mechanism is akin to the equilibrium Bose-Einstein condensation of massive bosons. Thus, the mode switching in our microcavity device can be viewed as a minimal instance of Bose-Einstein condensation of photons. We, moreover, show that the switching from one cavity mode to the other occurs always via an intermediate phase where both modes are emitting coherent light and that it is associated with both superthermal intensity fluctuations and strong anticorrelations between both modes.

On-chip light detection using monolithically integrated quantum dot micropillars

Abstract: We demonstrate the on-chip detection of light using photosensitive detectors based on quantum dot micropillar cavities. These microscale detectors are applied exemplarily to probe the emission of a monolithically integrated, electrically pumped whispering gallery mode microlaser. Light is detected via the photocurrent induced in the electrically contacted micropillar detectors under reverse-bias. In order to demonstrate the high potential and applicability of the microdetector presented, we determine the threshold current of an integrated microlaser to be (54 ± 4) μA, in very good agreement with the value of (53 ± 4) μA inferred from the optical data. Within this work, we realize the monolithic integration of a laser and a detector in a single device operating in the regime of cavity-quantum electrodynamics. Our results thus advance the research on microscale sensor technology towards the few-photon quantum limit and pave the way for on-chip opto-electronic feedback experiments.

All-optical neuromorphic computing in optical networks of semiconductor lasers

Abstract: Networks of interconnected nodes are at the heart of every neural network concept. While neural networks have been implemented in various hardware systems, the efficient realization of such networks still represents a major challenge. We demonstrate the implementation of an all-optical network scheme based on holographic coupling and induce complex spatio-temporal transients with Gigahertz bandwidth. Our scheme illustrates the potential of such all-optical systems for future neural network implementations.

Polariton condensate coherence in planar microcavities in a magnetic field

Abstract: In planar GaAs microcavities in a magnetic field up to 5 T perpendicular to the structure growth plane, under conditions of resonant pulsed pumping to a point close to the inflection point of the lower dispersion curve, Zeeman splitting of the spin sublevels of the polariton condensate is observed. This is accompanied by a significant change in the degree of circular polarization and the second-order correlator g2(0). It is found that the correlator is different for the spin sublevels of the polariton condensate, split in a magnetic field. In particular, correlator measurements indicate different condensation thresholds for the spin sublevels. The correlator values initially differing in terms of the absence of a field increase, reach a maximum, and then decrease and become equal for different polarizations in a field of 5 T.

Efficient stray-light suppression for resonance fluorescence in quantum dot-micropillars using self-aligned metal apertures

Abstract: Within this work we propose and demonstrate a technological approach to efficiently suppress excitation laser stray-light in resonance fluorescence experiments on quantum dot micropillars. To ensure efficient stray-light suppression, their fabrication process includes a planarization step and subsequent covering with a titanium mask to fabricate self-aligned apertures at the micropillar positions. These apertures aim to limit laser stray-light in the side-excitation vertical-detection configuration, while enabling detection of the optical signal through the top facet of the micropillars. The beneficial effects of these apertures are proven and quantitatively evaluated within a statistical study in which we determine and compare the stray-light suppression of 48 micropillars with and without metal apertures. Actual resonance fluorescence experiments on single quantum dots coupled to the cavity mode prove the relevance of the proposed approach and demonstrate that it will foster further studies on cavity quantum electrodynamics phenomena under coherent optical excitation.

Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses

Abstract: Optimized light-matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques. Using in-situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs) achieving their efficient coupling to the external light field. This enables to perform four-wave mixing micro-spectroscopy of single QD excitons, revealing the exciton population and coherence dynamics. We infer the temperature dependence of the dephasing in order to address the impact of phonons on the decoherence of confined excitons. The loss of the coherence over the first picoseconds is associated with the emission of a phonon wave packet, also governing the phonon background in photoluminescence (PL) spectra. Using theory based on the independent boson model, we consistently explain the initial coherence decay, the zero-phonon line fraction, and the lineshape of the phonon-assisted PL using realistic quantum dot geometries.

Coexistence of lasing and strong coupling in quantum-dot microlasers

Abstract: We demonstrate the coexistence of lasing and strong coupling in a quantum-dot micropillar laser. Comprehensive experimental studies including measurements of the input-output curve, second- order photon-correlation and coherence time are used to identify the transition of a strongly coupled quantum-dot microcavity system to lasing. The experimental results are evaluated on the basis of a microscopic theory that includes contributions from detuned background emitters. Furthermore, we show that both the emission spectrum and the strong coupling condition are strongly modified at the laser threshold due to the higher-order photonic states required to reach lasing. By accounting for these states that become realized under strong pumping, we provide a closed analytic expression that describes the transition from strong to weak coupling across the threshold in agreement with both experiment and a numerical approach.

Injection locking of high-β quantum dot microlasers

Abstract: We experimentally and theoretically investigate injection locking of high-β microlasers and show simultaneous stationary oscillation synchronized to the external signal and at the solitary frequency, where their macroscopic counterparts exhibit perfect synchronization or chaotic dynamics.

Thresholdless Lasing of Nitride Nanobeam Cavities

Abstract: Exploring the limit of thresholdless operation is a central goal in the development of nanolasers. It goes along with a vivid debate about how to prove and interpret thresholdless lasing. Moreover, it is also relevant regarding future applications of nanolasers and large-scale photonic integration requiring energy-efficient coherent light emitters with small footprint. We present close to ideal lasing behavior of GaN nanobeam cavities grown on silicon. Due to their high optical quality, such nanobeam cavities allow for efficient coupling of spontaneous emission from the quantum well gain material into the laser mode. By performing a comprehensive optical and quantum-optical characterization of these nanolasers, we unambiguously identify high-$\beta$ lasing at room temperature and thresholdless lasing at 156 K. The peculiar thresholdless characteristic is explained in terms of a temperature and excitation power dependent interplay between 0D and 2D gain contributions and the impact of non-radiative recombination.

Experimental and theoretical investigations on the nature of spontaneous emission to lasing transition in near-unity spontaneous emission factor emitters

Abstract: We investigated the lasing criterion for high-β emitters, when the customarily-used intensity jump and linewidth narrowing are no longer trustworthy. Spectrally-resolved photoluminscence and photon autocorrelation are measured from AlAs/GaAs micropillars containing InGaAs quantum dots and analyzed using cavity-QED. A physically intuitive lasing criterion applicable to all lasers is proposed.

Thresholdless lasing of nitride nanobeam cavities on silicon

Abstract: We present a temperature dependent optical and quantum-optical characterization of close-to-ideal lasing in GaN-based nanobeam cavities. Measuring the photon statistics of emission allows us to prove high-β lasing at room temperature, and thresholdless lasing at 156K. Thresholdless lasing is explained via temperature dependent carrier redistribution in the 0D/2D gain medium.