Showing papers by "Sven Höfling published in 2008"
TL;DR: In this paper, the authors reported the first observation of interaction effects on the exciton-polariton condensate and the excitation spectra, which are in quantitative agreement with the Bogoliubov theory.
Abstract: The observation of so-called Bogoliubov excitations provides the first sign of possible superfluid behaviour in an exciton-polariton condensate. Einstein’s 1925 paper predicted the occurrence of Bose–Einstein condensation (BEC) in an ideal gas of non-interacting bosonic particles1. However, particle–particle interaction and peculiar excitation spectra are keys for understanding BEC and superfluidity physics. A quantum field-theoretical formulation for a weakly interacting Bose condensed system was developed by Bogoliubov in 1947, which predicted the phonon-like excitation spectrum2 in the low-momentum regime. The experimental verification of the Bogoliubov theory on the quantitative level was carried out for atomic BEC3 using the two-photon Bragg scattering technique4. Exciton-polaritons in a semiconductor microcavity, which are elementary excitations created by strong coupling between quantum-well excitons and microcavity photons, were proposed as a new BEC candidate in solid-state systems5. Recent experiments with exciton-polaritons have demonstrated several interesting signatures from the viewpoint of polariton condensation, such as quantum degeneracy at non-equilibrium conditions6,7,8, the polariton-bunching effect at the condensation threshold9, long spatial coherence10,11,12 and quantum degeneracy at equilibrium conditions13. The particle–particle interaction and the Bogoliubov excitation spectrum are at the heart of BEC and superfluidity physics, but have only been studied theoretically for exciton-polaritons14,15. In this letter, we report the first observation of interaction effects on the exciton-polariton condensate and the excitation spectra, which are in quantitative agreement with the Bogoliubov theory.
235 citations
TL;DR: In this paper, the authors report on electrically pumped high-Q quantum dot-micropillar cavities with quality factors of up to 16.000 and a special current injection scheme using a ring-shaped upper contact is presented which ensures an efficient light out-coupling through the uncapped upper surface of the micropillars.
Abstract: We report on electrically pumped high-Q quantum dot-micropillar cavities with quality factors of up to 16.000. A special current injection scheme using a ring-shaped upper contact is presented which ensures an efficient light out-coupling through the uncapped upper surface of the micropillar. The devices feature excellent single-quantum dot cavity quantum electrodynamic effects with a Purcell enhancement of about 10 for a micropillar with a diameter of 2.5μm.
151 citations
TL;DR: In this article, the authors studied the response of a photonic crystal cavity to changes of the ambient refractive index and found a linear dependence of the resonance wavelength on the ambient gas.
Abstract: We have studied the response of a photonic crystal cavity to changes of the ambient refractive index. Transmission measurements of the cavity under different gaseous environments and pressures showed a linear dependence of the resonance wavelength on the refractive index of the ambient gas. A change of the refractive index by 10−4 leads to a shift of the resonance by 8pm, which is readily detectable due to the high quality factor of the cavity. The observed wavelength shifts agree well with finite-difference time domain simulations of the cavity.
126 citations
TL;DR: In this paper, a scalable fabrication technology for devices based on single quantum dots (QDs) was proposed, which combines site-controlled growth of QDs with an accurate alignment procedure.
Abstract: We report on a scalable fabrication technology for devices based on single quantum dots (QDs) which combines site-controlled growth of QDs with an accurate alignment procedure. Placement of individual QDs and corresponding device structures with a standard deviation of around 50nm from the target position was achieved. The potential of the technology is demonstrated by fabricating arrays of mesas, each containing one QD at a defined position. The presence of single, optically active QDs in the mesas was probed by scanning microphotoluminescence of the mesa arrays.
106 citations
TL;DR: In this article, low threshold lasing in electrically pumped quantum dot-micropillar cavities with quality factors exceeding 10.000 was reported. But the authors did not consider the effect of the number of quantum dots in the active layer.
Abstract: We report on low threshold lasing in electrically pumped quantum dot-micropillar cavities. Lasing action associated with threshold currents as low as 8μA at 10K is observed for micropillar cavities with quality factors exceeding 10.000. Due to an optimized contact scheme lasing is achieved for pillar structures with diameters as small as 1.5μm, containing on average less than 100 quantum dots in the active layer. The transition from spontaneous to stimulated emission is confirmed by autocorrelation measurements which reveal pronounced photon bunching near threshold.
96 citations
TL;DR: Electro-optical tuning of single quantum dots (QDs) embedded in high-quality (high-Q) micropillar cavities by exploiting the quantum confined Stark effect (QCSE) enables the observation of strong coupling with a vacuum Rabi-Splitting of 63 microeV.
Abstract: 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 In0.3Ga0.7As 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 1.9µm diameter and a quality-factor (Q-factor) of 14,000 enabling the observation of strong coupling with a vacuum Rabi-Splitting of 63µeV.
76 citations
TL;DR: In this paper, a single mode GaInAsSb-GaSb quantum well laser with a 3μm wavelength and a side mode suppression ratio of more than 30dB has been obtained.
Abstract: We report on room temperature continuous wave single mode GaInAsSb-GaSb quantum well lasers emitting beyond 3μm. Quantum well strain and composition were carefully adjusted to enhance the hole confinement without increasing electron confinement in order to avoid inhomogeneous quantum well pumping. In order to realize single mode emission as prerequisite for gas sensing applications, distributed feedback lasers were fabricated. A record single mode emission cw wavelength of 3019nm with a side mode suppression ratio of more than 30dB has been obtained. The room temperature peak power output per facet exceeds 3mW.
62 citations
TL;DR: A scalable method for the fabrication of site-controlled quantum dots (QDs) embedded in optical resonators, demonstrated by the resonant enhancement of the spontaneous emission observed in photoluminescence experiments.
Abstract: We present a scalable method for the fabrication of site-controlled quantum dots (QDs) embedded in optical resonators. The position of the quantum dots is determined by nucleating their growth in an array of nanoholes that is aligned to a set of markers, allowing the precise overlay of the resonator geometry over the QD array. Coupling of the QDs to the resonators is demonstrated by the resonant enhancement of the spontaneous emission observed in photoluminescence experiments.
47 citations
TL;DR: By varying both the Al concentration and the In concentration an independent adjustment of strain and QD band gap is possible and additional degree of freedom can be employed for tailoring AlGaInAs QDs with the desired emission wavelength, surface density and average size.
Abstract: We present a study of the growth, morphology and optical properties of AlxGa1−x−yInyAs quantum dots (QDs) for a wide range of Al and In concentrations (0≤x≤0.34 and 0.43≤y≤0.60). Short emission wavelengths between 660 and 940 nm and QD surface densities up to 1.1 × 1011 cm−2 have been achieved. Our results show that by varying both the Al concentration and the In concentration an independent adjustment of strain and QD band gap is possible. This additional degree of freedom can be employed for tailoring AlGaInAs QDs with the desired emission wavelength, surface density and average size. AlGaInAs QDs thus offer new possibilities for future QD device design.
23 citations
TL;DR: In this paper, the optical properties of columnar quantum dashes are investigated by photoreflectance and photoluminescence and analyzed by comparison with effective mass calculations.
Abstract: Here comes a report on the optical properties of InP based InAs columnar quantum dashes, which are proposed as an alternative for columnar quantum dots in semiconductor optical amplifiers construction since they offer convenient spectral tuning over 1.55μm together with a very broad and high gain. Electronic structure details are investigated by photoreflectance and photoluminescence and analyzed by comparison with effective mass calculations. Columnar quantum dash emission from the cleaved edge is examined by polarization resolved photoluminescence showing a transition of the dominant polarization from transverse electric to transverse magnetic with an increase in the quantum dash vertical dimension.
22 citations
TL;DR: In this paper, a room temperature memory operation of a single InAs quantum dot (QD) layer serving as floating gate is demonstrated, which is attributed to an optimized positioning of the QDs at the site of a local minimum in the AlGaAs conduction band.
Abstract: Room temperature (RT) memory operation of a single InAs quantum dot (QD) layer serving as floating gate is demonstrated. In an in-plane gated quantum-wire transistor, the charge state of the self-assembled InAs QDs is controlled by the applied gate voltage. Due to the floating-gate function of the QDs on a nearby transport channel, threshold hysteresis exceeding 200mV and storage times of several minutes are observed. The RT operation is attributed to an optimized positioning of the QDs at the site of a local minimum in the AlGaAs conduction band.
TL;DR: Using a multi section laser in coupled cavity injection grating design based on 1.3 microm InGaAs/GaAs quantum dot (QD) active region the authors were able to enhance the 3 dB modulation bandwidth well beyond the inherent material modulation bandwidth.
Abstract: Using a multi section laser in coupled cavity injection grating design based on 1.3 µm InGaAs/GaAs quantum dot (QD) active region we were able to enhance the 3 dB modulation bandwidth well beyond the inherent material modulation bandwidth. The material bandwidth was determined by measurements on distributed feedback (DFB) devices to approximately 8 GHz. The special multisectional design allows interaction between the lasing mode and a second mode used as catalyst and enables a high resonance frequency of the device. Based on active QD material this approach allowed us to reach a cut off frequency of 20 GHz in the small signal response of the device.
TL;DR: In this article, the authors investigated the impact of p-type modulation doping on QDash lasers on (100) InP and found that the differential gain was increased more than 50% for doping concentrations of 50 acceptors per QDash for constant cavity length lasers.
Abstract: p-type modulation doping in the range of 0–100 acceptors per quantum dash (QDash) has been carried out to investigate the impact on QDash lasers on (100) InP. The differential gain was found to increase more than 50% for doping concentrations of 50 acceptors per QDash for constant cavity length lasers. However, this benefit is overcompensated by enhanced gain compression and enlarged thermal heating due to high internal losses in highly p-doped devices. The maximum modulation bandwidth of 8GHz in continuous wave operation at room temperature is, therefore, obtained for a moderate p-doping level of 10 holes per QDash.
TL;DR: In this article, the linewidth enhancement factor (LEF) of a quantum-dot distributed-feedback laser emitting around 1.3 mum above and below threshold was measured.
Abstract: We present detailed measurements of the linewidth enhancement factor (LEF) of a quantum-dot distributed-feedback laser emitting around 1.3 mum above and below threshold. The above threshold method is based on the measurement of the frequency modulation and amplitude modulation response of the laser and shows significant dependence on modulation frequency. Drive current dependent measurements with both methods yield LEF values between 1 well below and 3 well above threshold, converging around 2 close to threshold.
TL;DR: In this article, a comparative study of the gain spectra of quantum-well (QW) and quantum-dot (QD) lasers is presented, where the authors point out the differences between the gain function of a QD laser and a QW laser and give a qualitative description of the effect that leads to the high wavelength stability of QD lasers.
Abstract: In this paper, a comparative study of the gain spectra of quantum-well (QW) and quantum-dot (QD) lasers is presented. We point out the differences between the gain function of a QD laser and a QW laser and give a qualitative description of the effect that leads to the high wavelength stability of QD lasers. Furthermore, we demonstrate, by means of the gain spectra of an InGaAs and AlInGaAs QD laser, that devices with a high wavelength stability can be manufactured over a wide range of emission energies. The experimentally obtained data are fitted with a theoretical model that describes the gain of a QD ensemble. The characteristic features resulting in the high wavelength stability of QD lasers of 0.072 nm/K are analyzed and discussed.
TL;DR: In this article, the room temperature continuous-wave operation of distributed feedback GalnNAs quantum well laser diodes on GaAs in the 1.5 mum wavelength range is demonstrated.
Abstract: Room-temperature continuous-wave operation of distributed feedback GalnNAs quantum well laser diodes on GaAs in the 1.5 mum wavelength range is demonstrated for the first time. Singlemode emission with a sidemode suppression ratio of more than 45 dB is obtained at 1486 nm with a threshold current of 44 mA and an external efficiency of 0.06 W/A.
TL;DR: In this article, a quantum cascade ridge waveguide laser with coupled ring resonators has been fabricated, achieving side mode suppression ratios of up to 26 dB over a temperature range of 140 K. The tuning rate of the microring resonator laser is 0.40 nm/K.
Abstract: Quantum cascade ridge waveguide lasers with coupled ring resonators have been fabricated. Coupling of an actively pumped microring resonator to a ridge waveguide device allows for filtering the numerous Fabry–Perot modes emerging in the ridge waveguide. Due to the large free spectral range of the ring resonators mode selection is accomplished, resulting in stable single mode emission for an optimized design. Thus, side mode suppression ratios of up to 26 dB over a temperature range of 140 K are attained by on-chip coupling of a ridge waveguide device with a microsquare ring resonator. The tuning rate of the microring resonator laser is 0.40 nm/K. Output powers of several milliwatts are obtained.
TL;DR: Laterally coupled, complex distributed feedback lasers based on InAs/ InGaAs/InAlGaAs+InP quantum dash-in-a-well layers emitting in the 19 mum wavelength range were fabricated in this article.
Abstract: Laterally coupled, complex distributed feedback lasers based on InAs/ InGaAs/InAlGaAs/InP quantum dash-in-a-well layers emitting in the 19 mum wavelength range were fabricated Total CW powers above 25 mW at room temperature and sidemode suppression ratios of more than 35 dB are demonstrated, which makes the devices promising for gas sensing applications
04 May 2008
TL;DR: In this paper, the authors report on high quality electrically driven quantum dot micropillar cavities with Q-factors up to 16.000, which allow the observation of pronounced single dot resonance effects with a Purcell enhancement of about 10.
Abstract: We report on high quality electrically driven quantum dot micropillar cavities with Q-factors up to 16.000. The high Q-factors allow the observation of pronounced single dot resonance effects with a Purcell enhancement of about 10.
TL;DR: In this paper, a tuning of the emission wavelength in GaAs∕AlxGa1−xAs bound-to-continuum quantum cascade lasers with different Al Mole fractions (x) is reported.
Abstract: Tuning of the emission wavelength in GaAs∕AlxGa1−xAs bound-to-continuum quantum cascade lasers with different Al mole fractions (x) is reported. By varying x in the range of 0.37–0.52, a shift of the emission wavelengths of over 4μm has been observed. Using this method, laser action in the range of 11.2–15.3μm at temperatures T⩾260K has been demonstrated with a record value of ∼340K for GaAs based QCLs operating at 13.5μm.
TL;DR: Under variation of driving current the lasers exhibit both discrete and continuous tuning behavior over a wide current range very well explicable by simulation of the sub-threshold spectra, rendering the devices especially interesting for multi-gas sensing by absorption spectroscopy.
Abstract: We report on single-mode emitting coupled cavity ridge waveguide lasers on the GaSb material system in the 2??m spectral range using two-dimensional (2D) photonic crystals (PhCs). Eight rows of 2D PhCs lateral to the ridge waveguides act as intermediate mirrors and are used to create two coupled cavities. This leads to preferential emission at one single longitudinal mode in the emission spectrum with side mode suppression ratios of 30?35?dB. Monolithic integration of high reflectivity 2D PhC back mirrors allows the realization of cavity lengths as short as 300??m with threshold currents as low as 18.5?mA while reaching output powers well above 18?mW. Under variation of driving current the lasers exhibit both discrete and continuous tuning behavior over a wide current range very well explicable by simulation of the sub-threshold spectra, rendering the devices especially interesting for multi-gas sensing by absorption spectroscopy.
TL;DR: In this paper, the authors report on the high-temperature performance of high-power GalnNAs broad area laser diodes with different waveguide designs emitting in the 1220-1240-nm wavelength range.
Abstract: We report on the high-temperature performance of high-power GalnNAs broad area laser diodes with different waveguide designs emitting in the 1220-1240-nm wavelength range. Large optical cavity laser structures enable a maximum continuous-wave output power of >8.9 W at T = 20degC with emission at 1220 nm and are characterized by low internal losses of 0.5 cm-1 compared to 2.9 cm-1 for the conventional waveguide structures. High-power operation up to temperatures of 120deg C is observed with output powers of >4 W at T = 90degC. This laser diode showed characteristic temperatures of To = 112 K and T1 = 378 K.
TL;DR: In this article, double GaAs quantum wells (QWs) embedded between modulation-doped AlGaAs barriers with different aluminium (Al) contents were grown by molecular beam epitaxy.
Abstract: Double GaAs quantum wells (QWs) embedded between modulation-doped AlGaAs barriers with different aluminium (Al) contents were grown by molecular beam epitaxy. Independent electric contacts to each well were realized by applying different etching techniques without substrate removal. In particular, the lower quantum well was electrically pinched off by a local undercut of the lower AlGaAs barrier exploiting an Al selective etching process. In contrast, the upper quantum well was locally depleted by top etched trenches. Transistor operation of quantum wires defined in such bilayers is demonstrated at room temperature with one GaAs layer used as conducting channel controlled by the other nearby GaAs layer as efficient quantum gate.
26 Sep 2008
TL;DR: In this paper, the authors realized high-quality electrically pumped mircopillar cavities featuring pronounced cQED effects, such as weak coupling and high-beta lasing.
Abstract: We realized high-quality electrically pumped mircopillar cavities featuring pronounced cQED effects, such as weak coupling and high-beta lasing. The high quality of the electrically contacted micropillars allowed the observation of strong coupling via electro-optical resonance-tuning.
TL;DR: In this article, all optical spatially resolved spin diffusion experiments in an unstrained, unbiased n-GaAs layer were conducted to study the impact of optical disturbance on spin transport.
Abstract: We report on all optical spatially resolved spin diffusion experiments in an unstrained, unbiased n-GaAs layer. Optical pump and probe intensities are varied over a wide range to study the impact of optical disturbance on spin transport. Both quantities have a considerable influence on the measured spin diffusion length and spin lifetime. Furthermore, an effective spin diffusion coefficient was obtained as a function of temperature.
Proceedings Article•
04 May 2008TL;DR: In this paper, the authors measured time-resolved photoluminescence from large quantum dots near a semiconductor-air interface and found that the results were consistent with dipole theory but near the interface they question the validity of the dipole approximation.
Abstract: We measured time-resolved photoluminescence from large quantum dots near a semiconductor-air interface. Far from the interface our data are consistent with dipole theory, but near the interface they question the validity of the dipole approximation.
30 Sep 2008
TL;DR: In this article, a single mode emission of tapered lasers was achieved by an on-chip distributed Bragg reflector, and the quantum dot-based devices showed stable single-mode emission around 920 nm, good beam quality and output powers up to 2 W.
Abstract: Single mode emission of tapered lasers was achieved by an on-chip distributed Bragg reflector. The quantum dot based devices show stable single mode emission around 920 nm, good beam quality and output powers up to 2 W.
26 Sep 2008
TL;DR: In this paper, the authors present detailed measurements of the Henry factor (linewidth enhancement factor) above and below threshold of a quantum dot laser and the current dependence of the above-threshold method based on modulation experiments.
Abstract: We present detailed measurements of the Henry factor (linewidth enhancement factor) above and below threshold of a quantum dot laser. Modulation frequency and current dependence of the above-threshold method based on modulation experiments is shown.
26 Sep 2008
TL;DR: In this paper, a scalable process to incorporate InAs quantum dots in spatially resonant devices is described, which combines site controlled quantum dot growth with an accurate alignment of the device to the single QDs.
Abstract: We report on a scalable process to incorporate InAs quantum dots in spatially resonant devices. This process combines site controlled quantum dot growth with an accurate alignment of the device to the single QDs.
26 Sep 2008
TL;DR: In this article, the authors discuss quantum dot micropillar cavities with electrical injection and quality factors in excess of 10.000. Weak and strong coupling effects and lasing are investigated.
Abstract: The talk discusses quantum dot micropillar cavities with electrical injection and quality factors in excess of 10.000. Weak and strong coupling effects and lasing are investigated. Challenges and the potential for device applications are discussed.