Showing papers by "Sven Höfling published in 2009"
TL;DR: A measurement technique using a streak camera is demonstrated that overcomes this limitation and provides a record of the arrival times of individual photons and identifies regimes of dynamical anti-bunching of photons in agreement with the predictions of a microscopic theory that includes semiconductor-specific effects.
Abstract: Coherent light emission in lasers is reflected in a change of the photon statistics. Here Wiersig et al. demonstrate a streak camera technique with sufficient time resolution to probe the dynamical evolution of correlations between individual photon emission events. This work may lead to novel quantum optical studies addressing the dynamics of correlation functions of light. Lasers are recognized for coherent light emission, the onset of which is reflected in a change in photon statistics; but, until now, attempts to directly measure correlations in the individual photon emission events of semiconductor lasers have been unsuccessful. By using a streak camera technique with sufficient time resolution, the dynamical evolution of correlations between individual photon emission events is now demonstrated. Lasers are recognized for coherent light emission, the onset of which is reflected in a change in the photon statistics1. For many years, attempts have been made to directly measure correlations in the individual photon emission events of semiconductor lasers2,3. 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-up4 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 lasers5 with semiconductor quantum dots2,3,6,7,8 as the active material, operating in the regime of cavity quantum electrodynamics9. 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.
201 citations
TL;DR: In this paper, the authors studied non-resonant dot-cavity coupling of individual quantum dots in micropillars under resonant excitation, revealing a pronounced effect over positive and negative quantum dot mode detunings.
Abstract: Non-resonant emitter–cavity coupling is a fascinating effect recently observed as unexpected pronounced cavity resonance emission even in strongly detuned single quantum dot–microcavity systems1,2,3. This phenomenon indicates strong, complex light–matter interactions in these solid-state systems, and has major implications for single-photon sources4,5,6 and quantum information applications1,2,3,7,9. We study non-resonant dot–cavity coupling of individual quantum dots in micropillars under resonant excitation, revealing a pronounced effect over positive and negative quantum dot mode detunings. Our results suggest a dominant role of phonon-mediated dephasing in dot–cavity coupling, giving a new perspective to the controversial discussions ongoing in the literature. Such enhanced insight is essential for various cavity-based quantum electrodynamic systems using emitters that experience phonon coupling, such as colour centres in diamond10 and colloidal nanocrystals11. Non-resonant coupling is demonstrated to be a versatile ‘monitoring’ tool for observing relevant quantum dot s-shell emission properties and background-free photon statistics. 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.
171 citations
TL;DR: A polarization-independent metamaterial design for the construction of electrically tunable terahertz (THz) devices is presented and the electric resonance and thus the transmission properties of the cross structure can be tuned by an externally applied bias voltage.
Abstract: We present a polarization-independent metamaterial design for the construction of electrically tunable terahertz (THz) devices. The implemented structure consists of an array of gold crosses fabricated on top of an n-doped gallium arsenide (GaAs) layer. Utilizing THz time-domain spectroscopy, we show that the electric resonance and thus the transmission properties of the cross structure can be tuned by an externally applied bias voltage. We further demonstrate the fast amplitude modulation of a propagating THz wave for modulation frequencies up to 100 kHz.
112 citations
TL;DR: In this paper, the authors demonstrate the deterministic integration of single site-controlled quantum dots (SCQDs) into micropillar cavities using cross markers for precise SCQD-cavity alignment.
Abstract: We demonstrate the deterministic integration of single site-controlled quantum dots (SCQDs) into micropillar cavities. Spatial resonance between single positioned QDs and GaAs/AlAs micropillar cavities was achieved using cross markers for precise SCQD-cavity alignment. Cavity effects are clearly reflected in an enhanced photoluminescence intensity when tuning SCQD emission lines through the fundamental cavity resonance. Single photon emission from a spatially and spectrally coupled SCQD-resonator system is confirmed by photon autocorrelation measurements yielding a g(2)(0) value of 0.12.
101 citations
TL;DR: 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.
Abstract: 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 µeV 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 ~110 µeV 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.
97 citations
TL;DR: A transition from strong coupling towards the critical coupling regime is attributed to a reduction of the quantum dot oscillator strength when the magnetic confinement becomes significant with regards to the exciton confinement above 3 T.
Abstract: We have studied a strongly coupled quantum dot-micropillar cavity system subject to an external magnetic field. The large diamagnetic response of elongated In_{0.3}Ga_{0.7}As quantum dots is exploited to demonstrate magneto-optical resonance tuning in the strong coupling regime. Furthermore, the magnetic field provides an additional degree of freedom to in situ manipulate the coupling constant. A transition from strong coupling towards the critical coupling regime is attributed to a reduction of the quantum dot oscillator strength when the magnetic confinement becomes significant with regards to the exciton confinement above 3 T.
64 citations
TL;DR: In this paper, Fourier-transformed photoreflectance and photoluminescence have been used to study the optical transitions in type II quantum wells (QWs) ranging up to almost 5 µm.
Abstract: Fourier-transformed photoreflectance and photoluminescence have been used to study the optical transitions in type II quantum wells (QWs) ranging up to almost 5 µm. High signal-to-noise ratio spectral features resulting from fundamental and excited state transitions have been detected for molecular beam epitaxially grown GaSb/AlSb/InAs/InGaSb/InAs/AlSb/GaSb "W"-shaped QW structures designed for laser-based gas sensing applications in the mid-infrared. The spectral features' dependence on arsenic pressure during growth process and on InAs confining-layer thickness could be followed unambiguously at room temperature.
44 citations
TL;DR: In this article, a GaSb-based type-II quantum well (QW) structures and interband cascade lasers (ICLs) were investigated with regards to the dependence of emission wavelength on active QW thicknesses.
Abstract: GaSb-based type-II quantum well (QW) structures and interband cascade lasers (ICLs) are investigated with regards to the dependence of emission wavelength on active QW thicknesses. Experimentally derived photoluminescence data and electrically driven ICL device data accompanied by theoretical calculations yield an average tuning rate of 0.55 μm per monolayer InAs in the range between 2.97 and 4.16 μm. Together with a temperature dependent ICL tuning behavior of 1.88 nm/K, the presented results provide the means for reliable and accurate emission wavelength control of ICLs in the 3–4 μm wavelength span which is of major importance for gas sensing applications.
37 citations
TL;DR: In this paper, the optical properties of GaSb/AlSb and InAs/InAs/Sb type II quantum wells (QWs) have been investigated and the broad emission wavelength tunability in the mid-infrared range has been demonstrated.
Abstract: Photoreflectance and photoluminescence, supported by the energy level calculations in the eight-band k⋅p model including strain, have been used to study the optical properties of GaSb/AlSb/InAs/InGaSb/AlSb/GaSb type II quantum wells (QWs). The broad emission wavelength tunability in the midinfrared range has been demonstrated by the control of InAs layer thickness. The temperature dependent measurements have shown that the emission can still be efficient at room temperature in such structures, and that the temperature shift of the fundamental type II optical transition between 10 and 300 K can be significantly smaller than for type I QW systems.
29 citations
TL;DR: In this paper, the authors report on the observation of oscillatory variations in the quality (Q) factor of quantum dot-micropillar cavities based on planar Bragg reflectors.
Abstract: We report on the observation of oscillatory variations in the quality (Q) factor of quantum dot-micropillar cavities based on planar Bragg reflectors. The oscillatory behavior in the Q versus diameter dependence appears in the diameter range between 1.0 and 4.0 μm, has a characteristic period of a few hundred nanometers and increases in amplitude with increasing reflectivity of the planar microcavity structures. The experimental results are well reproduced by numerical calculations which support the interpretation that the Q oscillations are caused by coupling of propagating Bloch modes of different orders at the mirror interfaces.
29 citations
TL;DR: A strongly coupled quantum dot-micropillar cavity system is studied under variation of the excitation power, which yields a transition from strong coupling towards weak coupling which is mainly attributed to an exciton-photon coupling constant decrease.
Abstract: 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 µeV 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.
TL;DR: In this paper, a single quantum dash was detected revealing a biexciton binding energy of about 0.4 meV, and the dependence of the photoluminescence intensity versus the excitation power density was determined and analyzed using the three level rate equation model.
Abstract: Molecular beam epitaxy grown InAs/InGaAlAs/InP quantum dashes designed for the 1.5 μm range were investigated by microphotoluminescence spectroscopy. The exciton and biexciton emission from a single quantum dash was detected revealing a biexciton binding energy of about 0.4 meV. The dependence of the photoluminescence intensity versus the excitation power density was determined and analyzed using the three level rate equation model, which allowed to confirm that the observed lines originate from the same single quantum dash.
TL;DR: In this article, the state of the art in the experimental implementations of the main elements of semiconductor spin qubits: qubit initialization, single-qubit gates, two-qubits gates, entanglement distribution, projective measurement, quantum memory and indistinguishable single-photon generation.
Abstract: Implementations of quantum information processing systems based on optically controlled electron spins in semiconductor quantum dots are particulary appealing due to several features. These features include inherent ultrafast gate operation times, reasonably long decoherence times, small optical control power and a natural ability to link to optical fiber communication networks. We will discuss the current state of the art in the experimental implementations of the main elements of semiconductor spin qubits: qubit initialization, single-qubit gates, two-qubit gates, entanglement distribution, projective measurement, quantum memory and indistinguishable single-photon generation.
TL;DR: In this paper, columnar quantum dash (CQDash) structures on (100) InP have been realized by gas source molecular beam epitaxy for stacking numbers of up to 24.
Abstract: InAs columnar quantum dash (CQDash) structures on (100) InP have been realized by gas source molecular beam epitaxy for stacking numbers of up to 24. Laser devices show low threshold current densities between 0.73 and 3.5 kA/cm2, dependent on the CQDash orientation within the cavity. Photoluminescence and electroluminescence measurements confirm a strong relationship between the polarization degree of the emission and the orientation of the CQDashes. Eventually, the polarization of the CQDash emission could be changed from predominantly transverse electric to transverse magnetic by simply altering the dash alignment relative to the light propagation axis.
TL;DR: InAs/GaAs(N) QDs can be considered as to be a promising system for use as single photon sources emitting in the 1.3 microm telecommunication band, with prospects for an extension to even longer wavelengths.
Abstract: We demonstrate an optimized molecular beam epitaxial growth procedure of InAs quantum dots (QDs) capped by a low nitrogen content GaInAs(N) quantum well to obtain single QD emission at telecommunication wavelengths. Technical separation of the nitrogen radio frequency plasma source to a second chamber does allow formation of InAs QDs without nitrogen incorporation. Thereby, optical quality degradation is avoided and by additional careful separation of the GaInAsN cap from the InAs QD layer with a partial GaInAs cap of nominal 4 nm thickness we achieve comparatively bright single dot emission above 1300 nm at 8 K. Micro-photoluminescence spectroscopy on single QDs reveal excitonic and biexcitonic emission at 939.8 meV (~1.319 µm) and 934.6 meV (~1.327 µm), respectively. Hence, InAs/GaAs(N) QDs can be considered as to be a promising system for use as single photon sources emitting in the 1.3 µm telecommunication band, with prospects for an extension to even longer wavelengths.
TL;DR: In this paper, a GaInNAs-based high-power laser diodes with emission in the 1220-1240 nm wavelength range are presented, allowing high continuous-wave room-temperature output powers of almost 9 W and emission at a wavelength of 1220 nm.
Abstract: GaInNAs-based high-power laser diodes with emission in the 1220-1240 nm wavelength range are presented. Broad-area (BA) devices show very low internal losses of only 0.5 cm-1, allowing high continuous-wave room-temperature output powers of almost 9 W and emission at a wavelength of 1220 nm. Based on the needs for applications like, e.g., pumping of Raman amplifiers, wavelength-stabilized tapered laser diodes are shown with maximum output powers of 1 W together with a high beam quality. Beam propagation factors M 2 down to 1.4, high brightness of up to 23 MW/cm2middotsr, and nearly spatial single-mode emission have been obtained.
TL;DR: In this paper, the linewidth enhancement factor above threshold is evaluated by means of the frequency-modulation/amplitude modulation method in dependence on modulation frequency and drive current, exhibiting a value of 2.5 slightly above threshold.
Abstract: InAs-InGaAlAs-InP quantum-dash lasers have been fabricated showing continuous-wave operation up to 100degC with a characteristic temperature of 88 K between 25degC and 85degC and output powers above 27 mW at room temperature (RT). The small-signal modulation bandwidth of 10 GHz at RT amounts still to 4 GHz at 85degC. The linewidth enhancement factor above threshold is evaluated by means of the frequency-modulation/amplitude-modulation method in dependence on modulation frequency and drive current, exhibiting a value of 2.5 slightly above threshold.
TL;DR: Both structural and optical properties of Ga(x)In(1-x)P quantum dots grown by molecular beam epitaxy (MBE) for various Ga concentrations are promising for future applications of the herein reported QDs in visible wavelength optoelectronic devices.
Abstract: We present a study of the growth, morphology and optical properties of Ga(x)In(1-x)P quantum dots (QDs) grown by molecular beam epitaxy (MBE) for various Ga concentrations x. QD areal densities up to 10(11) cm(-2) have been achieved showing strong dependence on the amount of gallium supplied. Structural properties are evaluated using scanning electron microscopy (SEM) and atomic force microscopy (AFM) and are related to photoluminescence properties of the QDs. Both structural and optical properties are promising for future applications of the herein reported QDs in visible wavelength optoelectronic devices.
TL;DR: In this article, the authors demonstrate electrical readout of high quality quantum dot micropillars by means of photocurrent (PC) spectroscopy under resonant excitation and apply this technique to enable a high spectral resolution mapping of the optical mode spectrum of the microplar revealing quality factors of up to 11'000 for a 3'μm diameter device.
Abstract: We demonstrate electrical readout of high quality quantum dot micropillars by means of photocurrent (PC) spectroscopy under resonant excitation. Applying this technique enables a high spectral resolution mapping of the optical mode spectrum of the micropillar revealing quality factors of up to 11 000 for a 3 μm diameter device. PC spectroscopy also shows that the contacted micropillars can act as light sensors with highly wavelength selective and photon sensitive detection capabilities down to 20 nW incident power. Moreover, bias voltage dependent PC studies provide an effective tool to study the competition between carrier tunneling out of the quantum dots and the radiative recombination.
Abstract: Direct current and alternating current characteristics of three-terminal nanojunctions (TTJs) are studied at room temperature. The TTJs are based on a modulation-doped GaAs∕AlGaAs heterostructure and were structured by applying mask techniques and wet chemical etching. Devices with lateral dimensions of a few tens of nanometers and with narrow gold contacts were fabricated and transistor characteristics with maximum transconductance values exceeding 100μA∕V are demonstrated. By analyzing the scattering parameters of the TTJs, power gain up to 1.5GHz is observed. This gigahertz amplification is related to the implemented narrow gold contacts which control the quantum capacitance of the electron reservoirs.
TL;DR: In this article, the influence of the arsenic flux during molecular beam epitaxy growth of 1.5-1.6-μm-emitting GaInNAs quantum wells on the annealing properties is studied.
TL;DR: In this article, a quasi-two-dimensional surrounding of columnar quantum dashes formed during the close stacking self assembled growth is presented, which induces a significant heavy hole-light hole valence states mixing leading to equal transverse electric and transverse magnetic components of the optical transition intensity.
Abstract: We report on polarization independent 1.55 µm room temperature edge emission from an InGaAs immersion layer, being a quasi-two-dimensional surrounding of columnar quantum dashes formed during the close stacking self assembled growth. Calculations performed in an 8 band kp model revealed that the in-plane strain distribution across the immersion layer induces a significant heavy hole–light hole valence states mixing leading to equal transverse electric and transverse magnetic components of the optical transition intensity.
TL;DR: In this paper, the field effect transistor operation of monolithic three-terminal junctions (TTJ) is demonstrated at room temperature for channel width smaller than 200 nm, drain currents of up to 9.3 muA and a maximum transconductance exceeding 15.7 muA/V are observed.
Abstract: The field-effect transistor operation of monolithic three-terminal junctions (TTJs) is demonstrated at room temperature. The TTJs are based on a modulation-doped GaAs/AlGaAs heterostructure with a 2-D electron gas situated 33 nm below the surface. By applying mask technology and wet chemical etching, several TTJs were fabricated, and the interplay between the TTJ geometry and the transistor characteristics is analyzed. For channel width smaller than 200 nm, drain currents of up to 9.3 muA and a maximum transconductance exceeding 15.7 muA/V are observed in the transfer characteristics. It is also demonstrated that engineering the channel and gate branch allows one to control the device currents and the threshold voltages over a wide range.
TL;DR: In this article, an optimized current injection scheme is employed, which enables simultaneously efficient electrical pumping and efficient out-coupling of light: a ring-shaped electrical contact leaving the upper facet of the pillars free from any absorbing material is used for efficient current injection and additionally serves as an aperture.
Abstract: We present cavity quantum electrodynamics (cQED) effects in electrically driven high quality (high-Q) quantum dot-micropillar cavities. An optimized current injection scheme is employed, which enables simultaneously efficient electrical pumping and efficient out-coupling of light: A ring-shaped electrical contact leaving the upper facet of the pillars free from any absorbing material is used for efficient current injection and additionally serves as an aperture. This approach helps to suppress the detection of spontaneous emission from QDs through the sidewalls of the micropillars and leads to very clean electroluminescence spectra. Temperature tuning of single quantum dots (QDs) excitons allows us to observe Purcell-enhancement of the spontaneous emission for QDs interaction with the fundamental mode as well as higher order modes. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
20 Nov 2009
TL;DR: In this paper, a low threshold laser emission from a low number of InGaAs quantum dots embedded in optically and electrically pumped high-Q micropillar laser structures was demonstrated.
Abstract: We report on laser emission from high quality quantum dot micropillar cavities. In these structures cavity quantum electrodynamics (cQED) effects are exploited to realize high efficient, low threshold lasing. We demonstrate that cQED effects allow for the observation of low threshold laser emission from a low number of InGaAs quantum dots embedded in optically and electrically pumped high-Q micropillar laser structures. For instance, lasing with threshold currents as low as 4 µA (160 A/cm2) is observed for electrically pumped microlasers at cryogenic temperatures. Moreover, single quantum dot controlled lasing effects are achieved in optically pumped micropillar lasers with particular high quality factors.
TL;DR: In this paper, a distributed Bragg reflector was used as a wavelength-selective element for tapered quantum dot (QD) laser devices with 1.4mm-long tapers.
Abstract: Wavelength control of tapered quantum dot (QD) lasers was achieved by adding a distributed Bragg reflector as wavelength-selective element to the devices. The Bragg wavelength of around 920 nm is matched to the gain of the single layer of InGaAs QDs that is used as active material. Devices with 1.4-mm-long tapers have reached output powers of over 1 W, and over 2 W were obtained from lasers with 3-mm-long tapers. The emission of the devices is restricted to a very narrow spectral range, with side-mode suppression ratios of over 40 dB.
02 Jun 2009
TL;DR: In this paper, a polarization-insensitive, electrically tunable metamaterial operating at terahertz (THz) frequencies and demonstrating the fast modulation of a propagating THz wave was presented.
Abstract: We present a polarization-insensitive, electrically tunable metamaterial operating at terahertz (THz) frequencies and demonstrate the fast modulation of a propagating THz wave. The structure is composed of gold crosses on n-doped gallium arsenide (GaAs).
14 Jun 2009
TL;DR: In this paper, the authors investigated weak and strong coupling in quantum dot-micropillar systems in electrically contacted devices in order to investigate their properties under strict resonant excitation where the elimination of stray light from the excitation laser is a critical issue in resonance fluorescence studies.
Abstract: The investigation of quantum electrodynamics effects such as weak and strong coupling in quantum dot-micropillar systems in electrically contacted devices has recently become possible due to advances in nano-processing [1-2]. This is of high interest for practical applications, e.g. the realization of compact single photon sources. Moreover, compared to simple optically excited structures electrically contacted high quality (Q) quantum dot-micropillar cavities provide an additional degree of freedom to either control the emission properties of the system via the quantum confined Stark effect [2] or to read out its optical properties by means of photocurrent (PC) spectroscopy. This has particular implications when probing the system's properties under strict resonant excitation where the elimination of stray light from the excitation laser is a critical issue in resonance fluorescence studies [3].
TL;DR: In this paper, the authors presented a detailed overview of semiconductor cavity quantum electrodynamics with micropillar cavities, including experiments in high-quality factor micro-cavities containing semiconductor quantum dots.
Abstract: Semiconductor cavity quantum electrodynamics (cQED) with single quantum dots as artificial atoms and high quality factor cavies is a rapidly developing research field Solid state platforms are a prerequisite for more wide spread exploitation of cQED effects, which hold promise for the realization of several key devices for quantum information processing and nano-optoelectronics, like single photon sources or ultra-low threshold lasers Semiconductor systems are particularly attractive because they offer routes for electrical injection or electro-optical tuning This talk will mainly focus on the presentation of cQED experiments in high-quality factor micropillar cavities containing semiconductor quantum dots (see Fig 1) After a general introduction into semiconductor cQED with micropillar cavities, the recent progress made in our group on electrically driven micropillar cavities will be reported [1, 2] By developing a process for planarizing and contacting quantum dot micropillars we fabricated devices with quality factors up to 16000 for pillar diameters of 4 μm Such devices feature pronounced cQED effects, like enhanced spontaneous emission, photon antibunching, high-β lasing and electro-optical tuning in both the weak or strong (see Fig 2) coupling regime Spatial determinism in cQED is of major importance for the exploitation of related effects on larger scale or for the realization of more complex device functionalities, eg the deterministic photonic coupling of remote quantum dots via spatially separated cavities and waveguides Another main part of the talk will discuss our recent progress made in the field of device integration of site-controlled quantum dots into photonic crystal as well as micropillar cavities (see Fig 3) [3, 4]
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TL;DR: In this paper, a related and re-worked paper will be submitted at a later date, which will clarify several aspects of the paper and will be published in a later publication.
Abstract: This work has been withdrawn as we wish to rework and clarify several aspects of the paper. A related and reworked paper will be submitted at a later date.