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We report on nanoscopic exploration of the luminescence from individual InP quantum dots (QDs) by means of highly spatially resolved cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM). A 7-fold layer stack with high-density InP quantum dots is embedded as an active medium membrane in an external-cavity surface-emitting laser. We characterize the vertical transfer of carriers within the periodic separate confinement heterostructure and determine the capture efficiency of carriers from the cladding layer into the quantum dot layers. Benefiting from the nanoscale resolution of our STEM-CL, we perform single-dot spectroscopy on single isolated QDs in the STEM lamella resolving the details of the excitonic structure of individual quantum dots. Executing highly spatially resolved spectrum line scans within the QD layers, we directly visualize the lateral transport

The GaAs-based material system is well-known for hosting InAs quantum dots (QDs) with outstanding optical properties, typically emitting at a wavelength of around 900 nm. The insertion of a metamorphic buffer (MMB) can shift this emission to the technologically attractive telecom C-band range centered at 1550 nm. However, the thickness of common MMB designs (>1 μm) limits their compatibility with most photonic resonator types. Here, we report on the metal–organic vapor-phase epitaxy (MOVPE) growth of a novel InGaAs MMB with a nonlinear indium content grading profile designed to maximize plastic relaxation within minimal layer thickness. This allows us to achieve the necessary transition of the lattice constant and to provide a smooth surface for QD growth within 180 nm. Single-photon emission at 1550 nm from InAs QDs deposited on top of this thin-film MMB is demonstrated. The strength of the new design is proven by integrating it into a bullseye cavity via nano-structuring techniques. The presented advances in the epitaxial growth of QD/MMB structures form the basis for the fabrication of high-quality telecom nonclassical light sources as a key component of photonic quantum technologies.

Single photon emission from localized excitons in two-dimensional (2D) materials has been extensively investigated because of its relevance for quantum information applications. Prerequisites are the availability of photons with high purity polarization and controllable polarization orientation that can be integrated with optical cavities. Here, deformation strain along edges of prepatterned square-shaped substrate protrusions is exploited to induce quasi-one-dimensional (1D) localized excitons in WSe2 monolayers as an elegant way to get photons that fulfill these requirements. At zero magnetic field, the emission is linearly polarized with 95% purity because exciton states are valley hybridized with equal shares of both valleys and predominant emission from excitons with a dipole moment along the elongated direction. In a strong field, one valley is favored and the linear polarization is converted to high-purity circular polarization. This deterministic control over polarization purity and orientation is a valuable asset in the context of integrated quantum photonics.

Coherent laser arrays compatible with silicon photonics are demonstrated in a waveguide geometry in epitaxially grown semiconductor membrane quantum well lasers transferred on substrates of silicon carbide and oxidised silicon; we record lasing thresholds as low as 60 mW of pump power. We study the emission of single lasers and arrays of lasers in the sub-mm range. We are able to create waveguide laser arrays with modal widths of approximately 5 − 10 µm separated by 10 − 20 µm, using real and reciprocal space imaging we study their emission characteristics and find that they maintain their mutual coherence while operating on either single or multiple longitudinal modes per lasing cavity.

We present a new saturable absorber device principle which has the potential for broad spectral range applications. An active region membrane is separated from the substrate and placed on a dielectric end mirror. By combining the absorbing membrane with the dielectric mirror to one device we get a membrane saturable absorber mirror (MESAM) which is similar to the well-known semiconductor saturable absorber mirror (SESAM) without the restriction of the stop-band reflectivity of the distributed Bragg reflector (DBR). Stable mode-locking with the MESAM was achieved in a red-emitting VECSEL at a pump power of 4.25 W with a pulse duration of 3.06 ps at 812 MHz repetition rate. We compare the performance and pulses of both SESAM and MESAM in a z-shaped VECSEL cavity.

The combination of semiconductor quantum dots (QDs) with photonic cavities is a promising way to realize non-classical light sources with state-of-the-art performances in terms of brightness, indistinguishability and repetition rate. In the present work we demonstrate the coupling of an InGaAs/GaAs QDs emitting in the telecom O-band to a circular Bragg grating cavity. We demonstrate a broadband geometric extraction efficiency enhancement by investigating two emission lines under above-band excitation, inside and detuned from the cavity mode, respectively. In the first case, a Purcell enhancement of 4 is attained. For the latter case, an end-to-end brightness of 1.4% with a brightness at the first lens of 23% is achieved. Using p-shell pumping, a combination of high count rate with pure single-photon emission (g(2)(0) = 0.01 in saturation) is achieved. Finally a good single-photon purity (g(2)(0) = 0.13) together with a high detector count rate of 191kcps is demonstrated for a temperature of up to 77K.

Photon-number resolving (PNR) single-photon detectors are of interest for a wide range of applications in the emerging field of photon based quantum technologies. Especially photonic integrated circuits will pave the way for a high complexity and ease of use of quantum photonics. Superconducting nanowire single-photon detectors (SNSPDs) are of special interest since they combine a high detection efficiency and a high timing accuracy with a high count rate and they can be configured as PNR-SNSPDs. Here, we present a PNR-SNSPD with a four photon resolution suitable for waveguide integration operating at a temperature of 4 K. A high statistical accuracy for the photon number is achieved for a Poissonian light source at a photon flux below 5 photons/pulse with a detection efficiency of 22.7 +- 3.0% at 900 nm and a pulse rate frequency of 76 MHz. We demonstrate the ability of such a detector to discriminate a sub-Poissonian from a Poissonian light source.

We investigated the optical gain properties and lasing characteristics of a pulsed electrically pumped laser structure, which consists of a single layer of self-assembled InP quantum dots (QDs) assembled in (Al 0.10 Ga 0.90 ) 0.52 In 0.48 P barriers. The optical gain and absorption spectra were obtained by analyzing the amplified spontaneous emission as a function of the excitation length. At a current density of 1.2 kA/cm 2 , an internal optical loss value of 5 ± 2 cm -1 and a peak modal gain of 39.3 cm -1 corresponding to a material gain of approximately 2675 cm -1 per QD layer were determined at room temperature. For a 2.24-mm-long laser with uncoated facets emitting at 660 nm, a low threshold current density of 281 A/cm 2 and an external differential quantum efficiency of 34.2% were determined. The internal quantum efficiency of 66% and the transparent current density of 65.8 A/cm 2 for a single layer of QDs were also demonstrated.

Solid-state quantum emitters with manipulable spin-qubits are promising platforms for quantum communication applications. Although such light-matter interfaces could be realized in many systems only a few allow for light emission in the telecom bands necessary for long-distance quantum networks. Here, we propose and implement an optically active solid-state spin-qubit based on a hole confined in a single InAs/GaAs quantum dot grown on an InGaAs metamorphic buffer layer emitting photons in the C-band. We lift the hole spin-degeneracy using an external magnetic field and demonstrate hole injection, initialization, read-out and complete coherent control using picosecond optical pulses. These results showcase a solid-state spin-qubit platform compatible with preexisting optical fiber networks.

The spectral range between 700 nm and 800 nm has lately been of increasing interest for several applications e.g. in medicine. State-of-the-art vertical external-cavity surface-emitting lasers (VECSELs) with direct emission in that range are usually pumped using solid-state lasers at 532 nm. They are still limited with respect to the output power [1] , [2] , owing mostly to the large temperature dependence of the electronic and optical material properties and a limited durability. Reducing the quantum defect by in-well pumping proved to strongly diminish thermal effects [3] and using diode lasers in a multi-pass pumping scheme opened the way for further power scaling [4] .

Many quantum communication schemes require sources of single, indistinguishable and entangled photon pairs with high brightness. Semiconductor quantum dots (QDs) hold great promise to meet these requirements offering the additional key advantage of the possibility of shifting the emission wavelength to the telecom C-band (centered around 1550 nm). This wavelength range is especially sought-after for fiber-based applications due to the global absorption minimum in standard glass fibers which maximizes the attainable communication distance.

The capability of realizing quantum photonics implementations on-chip will have a strong impact on several aspects of quantum computation and information processing. The implementation of large scale integrated quantum photonic technologies requires on-chip reconfigurable waveguide circuits as well as single-photon sources and detectors. Many successes have been made in recent years to demonstrate complex quantum circuits in many material platforms. Among all these platforms, the Silicon-on-Insulator (SOI) platform is outstanding as it is well developed for classical applications and therefore deploys a high component density, mature fabrication techniques and compatibility with the telecom regime and the complementary metal-oxide-semiconductor (CMOS) technology. However, the lack of on-demand non-classical light sources, directly incorporated in silicon chips, forces the use of nonlinear processes to generate non-classical light states: the probabilistic nature of the emission process sets a limit in the maximally achievable experimental complexity. The use of on-demand sources will have a strong impact on the device’s scalability.

Single photons from solid-state quantum emitters are playing a crucial role in the development of photonic quantum technologies and, by extension, higher order states, such as N-photon Fock states, allow for applications, e.g., in quantum-enhanced sensing. To verify the applicability of these states in future quantum technological implementations involving photon-atom interactions (i.e., storage of a quantum state in alkali vapor and photon delay) we utilize in the present study the dispersion of a hot cesium vapor at the ${D}_{1}$ line to realize a temporal delay for two-photon Fock states as a result of the slow-light effect. Single photons are generated on demand from an InGaAs quantum dot, while their quantum interference at a beam splitter is used to generate a two-photon Fock state. We verify the successful propagation and the preservation of the two-photon Fock states after the interaction with the slow-light medium, while a significant temporal delay (five times the initial photon length) is achieved with a high vapor transmission of $90%$.

Tunable cavities have proven to be highly attractive systems in cavity quantum electrodynamics thanks to their performance and flexibility. The possibility to form a cavity around any emitter while simultaneously spectrally matching the chosen transitions makes these cavities an important tool in photonic quantum technology. In this paper, we report on the experimental realization and theoretical description of a fiber-based resonator with two spatially and spectrally distinct cavity modes. The careful design of the structures was performed via finite-element simulations. Thanks to the intrinsic tunability of the system, one mode can be brought in resonance with the second one, forming a supermode resulting in a hybridized two-mode pattern in the emission spectrum. For its realization, we combine a monolithic bottom cavity, formed by two distributed Bragg reflectors, with a top movable fiber mirror forming an externally tunable cavity mode. When tuning the top cavity in resonance with the monolithic bottom one, the cavity modes exhibit a pronounced anticrossing behavior typical for mode hybridization. Differently from a standard open cavity, when embedding single emitters---in this case, InGaAs quantum dots---in the structure, the Purcell factor is not uniform for each wavelength. Furthermore, we find a strong influence of the simulated Purcell factor, as well as of the measured light extraction, depending on the placement of the emitter in the top or in the bottom cavity. The discussed two-mode cavity could be employed for simultaneously Purcell-enhancing multiple transitions of an emitter while preserving the single-photon nature of the emitters inside the device.

We present an optically addressed non-pixelated spatial light modulator. The system is based on reversible photoalignment of a LC cell using a red light sensitive novel azobenzene photoalignment layer. It is an electrode-free device that manipulates the liquid crystal orientation and consequently the polarization via light without artifacts caused by electrodes. The capability to miniaturize the spatial light modulator allows the integration into a microscope objective. This includes a miniaturized 200 channel optical addressing system based on a VCSEL array and hybrid refractive-diffractive beam shapers. As an application example, the utilization as a microscope objective integrated analog phase contrast modulator is shown. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement, Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 1001

Long-distance fiber-based quantum communication relies on efficient non-classical light sources operating at telecommunication wavelengths. Semiconductor quantum dots are promising candidates for on-demand generation of single photons and entangled photon pairs for such applications. However, their brightness is strongly limited due to total internal reflection at the semiconductor/vacuum interface. Here we overcome this limitation using a dielectric antenna structure. The non-classical light source consists of a gallium phosphide solid immersion lens in combination with a quantum dot nanomembrane emitting single photons in the telecom O-band. With this device, the photon extraction is strongly increased in a broad spectral range. A brightness of 17 % (numerical aperture of 0.6) is obtained experimentally, with a single photon purity of g ( 2 ) ( 0 ) = 0.049 ± 0.02 at saturation power. This brings the practical implementation of quantum communication networks one step closer.

Solid-state quantum emitters are playing a crucial role in the development of photonic quantum technologies. However, several decoherence mechanisms are known to impact the device performances and scalability. In various quantum emitter materials the fluctuation of the two-level system has been investigated by different methods to capture the evolution of coherence reduction. However, resolving the dynamics of spectral diffusion in the fundamental framework of on-demand single-photon generation remained unsolved. Hereby, the challenge is to observe with high time and energy resolution the impact of fluctuations directly in the properties of the emitted photons. In this Rapid Communication, we demonstrate the use of dispersion in a slow-light medium to map the photons frequency domain into time domain, observed as frequency-dependent time-of-flight. This allows for the measurement of the emission spectrum and the quantification of the spectral diffusion dynamics in one intensity correlation measurement. On exemplary semiconductor quantum dots, the impact of charge and spin noise on the spectral diffusion are revealed to follow an Ornstein-Uhlenbeck process. By a single measurement, broadening from the excitation repetition up to the stationary limit is resolved. This enables one to extract time-dependent two-photon interference visibilities for various timescales, which is a key performance measure for quantum emitters.

We demonstrate a deep-red-emitting vertical external-cavity surface-emitting laser (VECSEL) with an emission wavelength around λ = 765 nm based on InGaAsP/GaInP quantum wells. The quaternary material system was characterized with x-ray diffraction of thin films as the basis for InGaAsP quantum wells, which are incorporated into an 11 × 1 quantum well active region. The surface morphology of the fabricated VECSEL structure is analyzed with atomic force microscopy and the laser is evaluated in a linear cavity for various heatsink temperatures resulting in a watt-level output power of Pmax,−15°C = 1.71 W in a fundamental transverse mode.

A peak output power of 29.6 W and an average output power of 8.5 W at a wavelength of 750 nm were demonstrated in quasi-CW multi-mode operation using an AlGaAs-based vertical external-cavity surface-emitting laser (VECSEL) diode-pumped at a wavelength of 675 nm. The comparatively low bandgap of the barrier material that was tuned to the pump-photon energy allowed a good compromise between low heat generation due to the quantum defect and strong absorptance of the pump radiation. The limitations for the average output power came mainly from insufficient heat flow from the intra-cavity heat spreader to the heat sink. These results show the potential for power scaling of diode-pumped VECSELs and the importance of effective heat removal.

Narrow-band filtering of light is widely used in optical spectroscopy. Atomic filters, which rely on the Faraday effect, allow for GHz-wide transmission spectra, which are intrinsically matched to an atomic transition. We present an experimental realization and a theoretical study of a Faraday filter based on cesium and its D1-line-transition ( $6^{2}S_{1/2}\rightarrow 6^{2}P_{1/2}$ ) around 894 nm. We also present the prospects and visions for combining this filter with the single photon emission of a single quantum dot, which matches with the atomic transition. The option to lock the spectral position of a quantum dot is discussed at the end of this letter.

The well-established silicon-on-insulator platform is very promising for large-scale integrated photonic and quantum photonic technologies due to the mature manufacturing technology and integration density. Here, we present an efficient and stable fiber-to-chip coupling, which enables the injection of single photons from telecom quantum dots into a silicon-on-insulator photonic chip. Two additional fibers further couple the chip to single-photon detectors. The approach chosen to achieve steady fiber-chip coupling is based on the use of grating couplers steadily packaged with angled single-mode fibers. Using this technique, coupling efficiencies between the fiber and the SOI chip as high as 69.1% per grating coupler (including the taper losses) are reached. The effective interface between the quantum light generated by quantum dots and the silicon components is verified via the measurement of the second-order correlation function using a Hanbury–Brown and Twiss setup. With g(2)(0)=0.051±0.001, it clearly proves the single-photon nature of the injected QD photons. This demonstrates the reliability of the interfacing method and opens the route to employ telecom quantum dots as non-classical light sources with high complexity silicon photonic functionalities.

User-friendly single-photon sources with high photon-extraction efficiency are crucial building blocks for photonic quantum applications. For many of these applications, such as long-distance quantum key distribution, the use of single-mode optical fibers is mandatory, which leads to stringent requirements regarding the device design and fabrication. We report on the on-chip integration of a quantum dot (QD) microlens with a 3D-printed micro-objective in combination with a single-mode on-chip fiber coupler. The practical quantum device is realized by the deterministic fabrication of the QD-microlens via in situ electron-beam lithography and the 3D two-photon laser writing of the on-chip micro-objective and fiber chuck. A QD with a microlens is an efficient single-photon source, whose emission is collimated by the on-chip micro-objective. A second polymer microlens is located at the end facet of the single-mode fiber and ensures that the collimated light is efficiently coupled into the fiber core. For this purpose, the fiber is placed in an on-chip fiber chuck, which is precisely aligned to the QD-microlens thanks to the sub-micrometer processing accuracy of high-resolution two-photon direct laser writing. The resulting quantum device has a broadband photon extraction efficiency, a single-mode fiber-coupling efficiency of 22%, a measured single-photon flux of 42 kHz (8.9 kHz) under cw (pulsed) optical excitation, which corresponds to 1.5 MHz (0.3 MHz) at the single-mode fiber output, and a multi-photon probability in terms of g(2)(0) = 0.00±0.04/0.00 (0.13 ± 0.05) under cw (pulsed) optical excitation. The stable design of the developed fiber-coupled quantum device makes it highly attractive for integration into user-friendly plug-and-play quantum applications.

Mode-locked vertical-external-cavity surface-emitting lasers (VECSELs) are an emerging class of ultrafast lasers capable of emitting pulses in the fs-time scale and kW peak output powers [1]. Based on well-established III-V semiconductor technology, they constitute a promising driving source for frequency combs in the GHz range and multi-photon imaging.

Quantum dot (QD) based semiconductor lasers are the widely used sources in many applications. The unique advantages are their low threshold current density, high characteristic temperature, and high differential material gain, owning to their discrete energy levels. The self-assembled InP QDs in the (AlxGai-x)0.5iIn0.49P (AlxGaInP) barriers are of great interests due to the emitting spectrum from 630 nm to 780 nm [1]. However, limited by the low QD density, the low energy state density, and the small optical confinement factor, a single-sheet of QD only can provide a lower optical modal gain in the laser structure, which limits the laser performances. Meanwhile, the weak charge carrier confinement of the AlGaInP material further makes the devices sensitive to the operation temperature. One of the solutions for above issues is to increase the QD density by vertically stacking the QD layers. In the stacked QDs, the spacer thickness between the QD layers is one of the critical parameters needed to be carefully considered [2].

In this contribution, we investigated the design of an AlGaAs/AlGaInP electrically-pumped VECSEL structure emitting at a wavelength of 665 nm. With the finite element method (FEM) of an electro-thermal numerical model, we analyzed the current density distribution in the active region of different laser structures by changing the structure geometry, doping concentration of current spreading layer, and bottom contact size. A complete flip-chip processing is proposed according to the optimized designed structure. The measured results of the electroluminescence (EL) profile indicate that the diameter of the emission area with quasi-Gaussian distribution can be up to 100 μm, which is in good agreement with the numerical simulation.

Ultra-short pulse generation with saturable-absorber-free vertical-external-cavity surface-emitting-lasers (VECSELs) has raised significant interest in recent years due to the promises it holds for further peak-power scaling and cost-efficiency as well as for the design of more flexible, compact and simpler cavities. Although demonstrated for various devices, the self-mode-locking phenomenon in VECSELs still lacks a consistent explanation. Here, nonlinear lensing in a VECSEL gain chip as a possible mode-locking mechanism, directly measured via Z-scans at laser-relevant wavelengths, and the role of the microcavity resonance on the strength and dispersion of the Kerr nonlinearity are discussed. Furthermore, the impact on self-mode-locking is considered.

In this report, we investigated the optical gain properties and lasing characteristics of a laser structure consisting of one single-layer of self-assembled InP quantum dots in Al0.10GaInP barriers. The optical gain and absorption spectra are obtained by analyzing the amplified spontaneous emission. An internal optical loss value of 5±2 cm−1 , and a maximum peak modal gain of 39.3 cm−1 for a single-sheet of QD were obtained at room temperature. The influence of temperature on the gain properties was studied. A 2.24-mm-long laser with uncoated facets emitting at 660 nm was demonstrated. A low threshold current density of 281 A/cm2 with an external differential quantum efficiency of 34.2% was also achieved.