Showing papers by "Johann Peter Reithmaier published in 2020"

High‐Purity Triggered Single‐Photon Emission from Symmetric Single InAs/InP Quantum Dots around the Telecom C‐Band Window

Abstract: The authors demonstrate pure triggered single-photon emission from quantum dots (QDs) around the telecommunication C-band window, with characteristics preserved under non-resonant excitation at saturation, that is, the highest possible, lifetime-limited emission rates. The direct measurement of emission dynamics reveals photoluminescence decay times in the range of (1.7–1.8) ns corresponding to maximal photon generation rates exceeding 0.5 GHz. The measurements of the second-order correlation function exhibit, for the best case, a lack of coincidences at zero time delay—no multiple photon events are registered within the experimental accuracy. This is achieved by exploiting a new class of low-density and in-plane symmetric InAs/InP QDs grown by molecular beam epitaxy on a distributed Bragg reflector, perfectly suitable for non-classical light generation for quantum optics experiments and quantum-secured fiber-based optical communication schemes.

Novel Ultra Localized and Dense Nitrogen Delta-Doping in Diamond for Advanced Quantum Sensing.

Abstract: We introduce and demonstrate a new approach for nitrogen-vacancy (NV) patterning in diamond, achieving a deterministic, nanometer-thin, and dense delta-doped layer of negatively charged NV centers in diamond. We employed a pure nitridation stage using microwave plasma and a subsequent in situ diamond overgrowth. We present the highest reported nitrogen concentration in a delta-doped layer (1.8 × 1020 cm-3) while maintaining the pristine diamond crystal quality. This result combined with the large optically detected magnetic resonance contrast can pave the way toward highly sensitive NV-based magnetometers. We further employed this delta-doping technique on high-quality fabricated diamond nanostructures for realizing a topographic NV patterning in order to enhance the sensing and hyperpolarization capabilities of NV-based devices.

Optical and Electronic Properties of Symmetric In As / ( In , Al , Ga ) As / In P Quantum Dots Formed by Ripening in Molecular Beam Epitaxy: A Potential System for Broad-Range Single-Photon Telecom Emitters

Abstract: We present a detailed experimental optical study supported by theoretical modeling of $\mathrm{In}\mathrm{As}$ quantum dots (QDs) embedded in an $(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Al},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga})\mathrm{As}$ barrier lattice matched to $\mathrm{In}\mathrm{P}$(001) grown with the use of a ripening step in molecular beam epitaxy. The method leads to the growth of in-plane symmetric QDs of low surface density, characterized by a multimodal size distribution resulting in a spectrally broad emission in the range of $1.4$--$2.0\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$, essential for many near-infrared photonic applications. We find that, in contrast to the $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ system, the multimodal distribution results here from a two-monolayer difference in QD height between consecutive families of dots. This may stem from the long-range ordering in the quaternary barrier alloy that stabilizes QD nucleation. Measuring the photoluminescence (PL) lifetime of the spectrally broad emission, we find a nearly dispersionless value of $1.3\ifmmode\pm\else\textpm\fi{}0.3$ ns. Finally, we examine the temperature dependence of emission characteristics. We underline the impact of localized states in the wetting layer playing the role of carrier reservoir during thermal carrier redistribution. We determine the hole escape to the $(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Al},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga})\mathrm{As}$ barrier to be a primary PL quenching mechanism in these QDs.

Functionalised phosphonate ester supported lanthanide (Ln = La, Nd, Dy, Er) complexes.

Abstract: A series of phosphonate ester supported lanthanide complexes bearing functionalities for subsequent immobilisation on semiconductor surfaces are prepared. Six phosphonate ester ligands (L1–L6) with varying aromatic residues are synthesised. Subsequent complexation with lanthanide chloride or -nitrate precursors (Ln = La, Nd, Dy, Er) affords the corresponding mono- or dimeric lanthanide model complexes [LnX3(L1–L3 or L5–L6)3]n (X = NO3, Cl; n = 1 (Nd, Dy, Er), 2 (La, Nd)) or [LnCl2Br(L4-Br)2(L4-Cl)]n (n = 1 (Nd, Dy, Er), 2 (La, Nd)) (1–32). All compounds are thoroughly characterised, and their luminescence properties are investigated in the visible and NIR spectral regions, where applicable.

Mode properties of telecom wavelength InP-based high-(Q/V) L4/3 photonic crystal cavities.

Abstract: We present finite-difference time domain simulations and optical characterizations via micro-photoluminescence measurements of InP-based L4/3 photonic crystal cavities with embedded quantum dots (QDs) and designed for the M1 ground mode to be emitting at telecom C-band wavelengths. The simulated M1 Q-factor values exceed 106, while the M1 mode volume is found to be 0.33 × (λ/n)3, which is less than half the value of the M1 mode volume of a comparable L3 cavity. Low-temperature micro-photoluminescence measurements revealed experimental M1 Q-factor values on the order of 104 with emission wavelengths around 1.55 μm. Weak coupling behavior of the QD exciton line and the M1 ground mode was achieved via temperature-tuning experiments.

Fabrication and Characterization of Single-Crystal Diamond Membranes for Quantum Photonics with Tunable Microcavities

Abstract: The development of quantum technologies is one of the big challenges in modern research. A crucial component for many applications is an efficient, coherent spin–photon interface, and coupling single-color centers in thin diamond membranes to a microcavity is a promising approach. To structure such micrometer thin single-crystal diamond (SCD) membranes with a good quality, it is important to minimize defects originating from polishing or etching procedures. Here, we report on the fabrication of SCD membranes, with various diameters, exhibiting a low surface roughness down to 0.4 nm on a small area scale, by etching through a diamond bulk mask with angled holes. A significant reduction in pits induced by micromasking and polishing damages was accomplished by the application of alternating Ar/Cl2 + O2 dry etching steps. By a variation of etching parameters regarding the Ar/Cl2 step, an enhanced planarization of the surface was obtained, in particular, for surfaces with a higher initial surface roughness of several nanometers. Furthermore, we present the successful bonding of an SCD membrane via van der Waals forces on a cavity mirror and perform finesse measurements which yielded values between 500 and 5000, depending on the position and hence on the membrane thickness. Our results are promising for, e.g., an efficient spin–photon interface.

Temperature resistant fast InxGa1-xAs / GaAs quantum dot saturable absorber for the epitaxial integration into semiconductor surface emitting lasers.

Abstract: Quantum-dot-based semiconductor saturable absorber mirrors (SESAMs) with fast response times were developed by molecular beam epitaxy (MBE). Using quantum dots (QDs) in the absorber region of the SESAMs instead of quantum wells, enables additional degrees of freedom in the design, the control of saturation parameters and the recovery dynamics. However, if one wants to integrate such a SESAM element into semiconductor surface emitting lasers such as a mode-locked integrated external-cavity surface-emitting laser (MIXSEL), the saturable absorber layers have to withstand a longer high-temperature growth procedure for the epitaxial formation of distributed Bragg reflectors (DBR). Typically defect related SESAMs will be annealed at those growth temperatures and lose their high-speed performance. Here we present a systematic study on the growth parameters and post-growth annealing of SESAMs based on high-quality InxGa1-xAs/GaAs quantum dots (QDs) grown by MBE at growth temperatures of 450 °C or higher. The good quality enables the QDs to survive the long DBR overgrowth at 600 °C with only minimal shifts in the designed operation wavelength of 1030 nm required for growth of MIXSEL devices. The introduction of recombination centers with p-type modulation doping and additional post-growth annealing improves the absorption of the high-quality QDs. Hence, low saturation fluences < 10 µJ/cm2 and a reduction of the τ1/e recovery time to values < 2 ps can be achieved.

Room Temperature Quantum Coherent Revival in an Ensemble of Artificial Atoms

Abstract: We report a demonstration of the hallmark concept of quantum optics: periodic collapse and revival of quantum coherence (QCR) in a room temperature ensemble of quantum dots (QD). Control over quantum states, inherent to QCR, together with the dynamical QD properties present an opportunity for practical room temperature building blocks of quantum information processing. The amplitude decay of QCR is dictated by the QD homogeneous linewidth, thus, enabling its extraction in a double-pulse Ramsey-type experiment. The more common photon echo technique was also invoked and yielded the same linewidth. Measured electrical bias and temperature dependencies of the transverse relaxation times enable to determine the two main decoherence mechanisms: carrier-carrier and carrier-phonon scatterings.

X-ray mapping in a scanning transmission electron microscope of InGaAs quantum dots with embedded fractional monolayers of aluminium

Abstract: We investigate AlGaAs/GaAs superlattices as well as InGaAs/GaAs quantum wells (QWs) and epitaxial quantum dots (QDs) where during the molecular beam epitaxy of InGaAs QDs the aluminium flux cell was opened briefly to incorporate fractional monolayers of Al into the InGaAs. We show that X-ray mapping with a large collection angle is capable to detect 0.3-0.4 fractional Al monolayers with a resolution of just under 1nm.

Resonant and Nonresonant Tunneling Injection Processes in Quantum Dot Optical Gain Media

Abstract: Resonant and non-resonant tunneling injection of charge carriers in a quantum dot optical gain medium in the form of an optical amplifier operating in the telecom wavelength range were identified using multi wavelength pump probe measurements. Since the inhomogeneously broadened ensemble of quantum dots exhibits a very wide spectrum of transition energies, both tunneling injection processes take place simultaneously at different spectral locations and thus can be identified. These findings highlight the fact that at least one of the tunneling processes must occur at a wavelength at or near the gain peak, where laser oscillations take place. An energetic misalignment leads to reduced laser performance since in that case, the tunneling process becomes a carrier loss mechanism.

Fabrication of Diamond AFM Tips for Quantum Sensing

Abstract: Diamond attracts an ever-increasing scientific interest not only due to its outstanding properties, but also as host material for the so-called color centers. In particular, the nitrogen-vacancy (NV) center is a promising candidate for applications in quantum sensing on a nanoscale. Incorporating such centers in sharp diamond tips, allows the fabrication of a controllable sensor for magnetic and electric fields. In this regard, we present two different ways to fabricate diamond atomic force microscope (AFM) probes. One approach is based on a bottom-up method, first structuring a silicon substrate by photolithography and anisotropic wet etching and subsequently depositing a nanocrystalline diamond (NCD) film onto the pre-patterned silicon substrate. The second approach is based on electron beam lithography (EBL) and reactive ion etching (RIE), which is also applicable to monocrystalline diamond (MCD). To this end we show our first results in fabricating NV-containing MCD tips by He+ ion implantation and annealing. We demonstrate the fabrication of bottom-up NCD probes with tip radii in the range of ca. 25 nm, and top-down fabricated NCD AFM probes with tip radii even below 10 nm.

Telecom wavelength InP-based L3 photonic crystal cavities: Properties of the cavity ground mode

Abstract: We report on FDTD simulation, fabrication and optical characterization of InP-based L3 photonic crystal (PhC) cavities embedded with InAs/InP quantum dots (QDs) emitting at telecom wavelengths. Temperature tuning experiments show evidence of a weak coupling of an exciton line and M1 ground mode with Q-factor value exceeding 8000. The effect of the cavity designs and extractor structures on M1 Q-factor and mode intensity emitting at 1.55 µm are investigated. The used extractor structures result in a slight decrease in M1 Q-factors and an increased M1 mode intensity by a factor of 20, which is a promising step for Purcell-enhanced single-photon sources at the telecom wavelengths.

On the differences in dynamical properties of quantum-dot lasers with and without p-doping in the active region and tunneling injection quantum wells

Abstract: The small and large signal responses of InP-based 1.55 μm high-speed quantum dot (QD) lasers with and without tunnel-injection (TI) quantum well (QW) and/or p-type doping in the active region (incorporating nominally identical QDs) were designed, manufactured and compared. The structures were grown by a molecular beam epitaxy system equipped with group-V valved cracker cells. In all cases, the active region consisted of six QD or TI-QD structures, which were embedded in InAlGaAs barriers lattice matched to InP. The InGaAs TI-QWs were separated by a thin InAlGaAs tunnel barrier from the InAs QDs. The laser structures were processed into ridge waveguide lasers and analyzed. The results show, that the bandwidth and maximum data rates were reduced by incorporation of TI-QWs. P-doping resulted in slightly worse performance of the simple QD laser, but in an improvement of the TI QD laser. Furthermore, the large signal response of the tunneling injection QD laser is one of the first reports of digital modulation of such a laser. An optimization of the doping profile is promising to further improve the laser performance over the undoped counterparts.

Optical and electronic properties of symmetric InAs/InGaAlAs/InP quantum dots formed by a ripening process in molecular beam epitaxy: a promising system for broad-range single-photon telecom emitters

Abstract: We present a detailed experimental optical study supported by theoretical modeling of InAs quantum dots (QDs) embedded in an InAlGaAs barrier lattice-matched to InP(001) grown with the use of a ripening step in molecular beam epitaxy. The method leads to the growth of in-plane symmetric QDs of low surface density, characterized by a multimodal size distribution resulting in a spectrally broad emission in the range of $1.4-2.0$ $\mu$m, essential for many near-infrared photonic applications. We find that, in contrast to the InAs/InP system, the multimodal distribution results here from a two-monolayer difference in QD height between consecutive families of dots. This may stem from the long-range ordering in the quaternary barrier alloy that stabilizes QD nucleation. Measuring the photoluminescence (PL) lifetime of the spectrally broad emission, we find a nearly dispersionless value of $1.3\pm0.3$ ns. Finally, we examine the temperature dependence of emission characteristics. We underline the impact of localized states in the wetting layer playing the role of carrier reservoir during thermal carrier redistribution. We determine the hole escape to the InAlGaAs barrier to be a primary PL quenching mechanism in these QDs.

Co-Occurrence of Resonance and Non-Resonance Tunneling Injection Processes in Quantum Dot Gain Media

Abstract: Resonance and non-resonance tunneling processes in tunneling injection quantum dot gain media were demonstrated, to occur simultaneously. Our finding sheds light on the basic tunneling injection schemes and their proper utilization in quantum dot lasers.

Coherence Time Prolongation in a Room Temperature Quantum Dot Ensemble

Abstract: Significant prolongation of the coherence time by means of photon echo has been observed in a room temperature quantum dot amplifier operating at telecom wavelength. A record-long room temperature pure dephasing time of 4.7 ps was demonstrated using a detection technique suitable for practical applications.