Coherently and Incoherently Pumped Telecom C-Band Single-Photon Source with High Brightness and Indistinguishability.

Long-range, terrestrial quantum networks require high-brightness single-photon sources emitting in the telecom C-band for maximum transmission rates. For solid-state quantum emitters, the underlying pumping process, i.e., coherent or incoherent excitation schemes, impacts several photon properties such as photon indistinguishability, single-photon purity, and photon number coherence. These properties play a major role in quantum communication applications, the latter in particular for quantum cryptography. Here, we present a versatile telecom C-band single-photon source that is operated coherently and incoherently using two complementary pumping schemes. The source is based on a quantum dot coupled to a circular Bragg grating cavity, whereas coherent (incoherent) operation is performed via the novel SUPER scheme (phonon-assisted excitation). In this way, high end-to-end-efficiencies (ηend) of 5.36% (6.09%) are achieved simultaneously with a small multiphoton contribution g(2)(0) of 0.076 ± 0.001 [g(2)(0) of 0.069 ± 0.001] for coherent (incoherent) operation.

Highly Indistinguishable Single Photons from Droplet-Etched GaAs Quantum Dots Integrated in Single-Mode Waveguides and Beamsplitters.

Integration of on-demand quantum emitters into photonic integrated circuits (PICs) has drawn much attention in recent years, as it promises a scalable implementation of quantum information schemes. A central property for several applications is the indistinguishability of the emitted photons. In this regard, GaAs quantum dots (QDs) obtained by droplet etching epitaxy show excellent performances, making the realization of these QDs into PICs highly appealing. Here, we show the first implementation in this direction, realizing the key passive elements needed in PICs, i.e., single-mode waveguides (WGs) with integrated GaAs-QDs and beamsplitters. We study the statistical distribution of wavelength, linewidth, and decay time of the excitonic line, as well as the quantum optical properties of individual emitters under resonant excitation. We achieve single-photon purities as high as 1 - g(2)(0) = 0.929 ± 0.009 and two-photon interference visibilities of up to VTPI = 0.953 ± 0.032 for consecutively emitted photons.

Surface relief VCSELs at 670 nm with integrated polymer microlens for highly collimated fundamental-mode emission.

We demonstrate the integration of a wet-chemically etched surface relief on a vertical-cavity surface-emitting laser (VCSEL) emitting in the red spectral range for higher-order mode suppression. With this relief, fundamental-mode emission is achieved over the entire power range from threshold beyond thermal rollover. For collimation of the emitted beam, we implement polymer microlenses fabricated on-chip by a thermal reflow technique. We reduce the angle of divergence for all injected currents to a maximum of 2∘. By measuring high-resolution spectra, we show that Gaussian beam profiles correspond to pure fundamental-mode emission which is preserved after implementation of the polymer microlens onto the etched relief, proving the compatibility of the two processes.

A Unipolar Quantum Dot Diode Structure for Advanced Quantum Light Sources.

Triggered, indistinguishable single photons are crucial in various quantum photonic implementations. Here, we realize a novel n+-i-n++ diode structure embedding semiconductor quantum dots: the gated device enables spectral tuning of the transitions and deterministic control of the charged states. Blinking-free single-photon emission and high two-photon indistinguishability are observed. The line width's temporal evolution is investigated across over 6 orders of magnitude time scales, combining photon-correlation Fourier spectroscopy, high-resolution photoluminescence spectroscopy, and two-photon interference (visibility of VTPI,2ns = (85.8 ± 2.2)% and VTPI,9ns = (78.3 ± 3.0)%). Most of the dots show no spectral broadening beyond ∼9 ns time scales, and the photons' line width ((420 ± 30) MHz) deviates from the Fourier-transform limit by a factor of 1.68. The combined techniques verify that most dephasing mechanisms occur at time scales ≤2 ns, despite their modest impact. The presence of n-doping implies higher carrier mobility, enhancing the device's appeal for high-speed tunable, high-performance quantum light sources.

Membrane saturable absorber mirror (MESAM) in a red-emitting VECSEL for the generation of stable ultrashort pulses.

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.

Coherent waveguide laser arrays in semiconductor quantum well membranes.

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.

InGaAsP VECSEL for watt-level output at a wavelength around 765 nm.

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.

High-power quasi-CW diode-pumped 750-nm AlGaAs VECSEL emitting a peak power of 29.6†W and an average power of 8.5†W.

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.

Bright Purcell Enhanced Single-Photon Source in the Telecom O-Band Based on a Quantum Dot in a Circular Bragg Grating.

The combination of semiconductor quantum dots with photonic cavities is a promising way to realize nonclassical light sources with state-of-the-art performances regarding brightness, indistinguishability, and repetition rate. Here we demonstrate the coupling of 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 191 kcps is demonstrated for a temperature of up to 77 K.

Microcavity-enhanced Kerr nonlinearity in a vertical-external-cavity surface-emitting laser: erratum.

We correct a mistake in [Opt. Express27, 11914 (2019)10.1364/OE.27.011914] when calculating the focal length of the Kerr lens with the measured values of the nonlinear refractive index n2 and parameters of a prototypical self-mode-locking VECSEL cavity. We therefore update Fig. 1 of the original publication. The new calculation yields a significantly larger value of the Kerr lens focal length leading to a smaller perturbation of the cavity beam profile.

Quantum dot-based broadband optical antenna for efficient extraction of single photons in the telecom O-band.

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.

Gaussian-like transverse-mode profile characteristics of high-power large-area red-emitting VCSELs.

We demonstrate a large-area red-emitting vertical-cavity surface-emitting laser (VCSEL) structure with significant improvement in the uniformity of charge carrier distribution by adopting a Si-doped $ {{\rm Al}_{0.20}}{\rm GaInP} $Al0.20GaInP current spreading layer and a bottom disk contact. The new structure emitting at 670 nm with a bottom disk contact diameter of 20 µm was compared with the conventional oxide-confined top-emitting structure with a similar aperture size. The maximum output peak power increased from 8.8 mW to 22.5 mW under pulsed-mode operation at room temperature. The far field improved from a strong multiple-mode pattern to a Gaussian-like profile. The corresponding divergence angle of the far-field pattern at $ 2{\rm {I}}_{\rm{th}} $2Ith injection current reduced from 16.2° to 10.9°.

Microcavity-enhanced Kerr nonlinearity in a vertical-external-cavity surface-emitting laser.

Self-mode-locking has become an emerging path to the generation of ultrashort pulses with vertical-external-cavity surface-emitting lasers. In our work, a strong Kerr nonlinearity that is so far assumed to give rise to mode-locked operation is evidenced and a strong nonlinearity enhancement by the microcavity is revealed. We present wavelength-dependent measurements of the nonlinear absorption and nonlinear refractive index change in a gain chip using the Z-scan technique. We report negative nonlinear refraction up to 5x10-12 cm2/W in magnitude in the (InGa)As/Ga(AsP) material system close to the laser design wavelength, which can lead to Kerr lensing. We show that by changing the angle of incidence of the probe beam with respect to the gain chip, the Kerr nonlinearity can be wavelength-tuned, shifting with the microcavity resonance. Such findings may ultimately lead to novel concepts with regard to tailored self-mode-locking behavior achievable by peculiar Kerr-lens chip designs for cost-effective, robust and compact fs-pulsed semiconductor lasers.

Fully On-Chip Single-Photon Hanbury-Brown and Twiss Experiment on a Monolithic Semiconductor-Superconductor Platform.

Fully integrated quantum photonic circuits show a clear advantage in terms of stability and scalability compared to tabletop implementations. They will constitute a fundamental breakthrough in integrated quantum technologies, as a matter of example, in quantum simulation and quantum computation. Despite the fact that only a few building blocks are strictly necessary, their simultaneous realization is highly challenging. This is especially true for the simultaneous implementation of all three key components on the same chip: single-photon sources, photonic logic, and single-photon detectors. Here, we present a fully integrated Hanbury-Brown and Twiss setup on a micrometer-sized footprint consisting of a GaAs waveguide embedding quantum dots as single-photon sources, a waveguide beamsplitter, and two superconducting nanowire single-photon detectors. This enables a second-order correlation measurement on the single-photon level under both continuous-wave and pulsed resonant excitation. The presented proof-of-principle experiment proves the simultaneous realization and operation of all three key building blocks and therefore a major step towards fully integrated quantum optical chips.

Bragg grating cavities embedded into nano-photonic waveguides for Purcell enhanced quantum dot emission.

In the present work, we demonstrate the fabrication and optical properties of Bragg grating cavities that are directly integrated into ridge waveguides along with the Purcell enhanced emission from integrated quantum dots. Measured Q-factors up to 4600 are observed in combination with resonances of the fundamental mode within a ± 0.11 nm range along the full fabricated chip. The measured Purcell enhancement up to a factor of 3.5 ± 0.5 shows the potential utility for state-of-the-art on-chip quantum optical experiments as realized in off-chip implementations. Our measurements are fully supported via FDTD simulations giving a theoretical Purcell enhancement up to a factor of 20 with a highly directional ßdir-factor of 70 %. The straightforward upscaling and robust design of the investigated Bragg grating cavity in combination with a substantial Purcell enhancement represents a major step towards large scale on-chip quantum photonic circuits.

Mid-Infrared Spectroscopy Platform Based on GaAs/AlGaAs Thin-Film Waveguides and Quantum Cascade Lasers.

The performance and versatility of GaAs/AlGaAs thin-film waveguide technology in combination with quantum cascade lasers for mid-infrared spectroscopy in comparison to conventional FTIR spectroscopy is presented. Infrared radiation is provided by a quantum cascade laser (QCL) spectrometer comprising four tunable QCLs providing a wavelength range of 5-11 µm (1925-885 cm(-1)) within a single collimated beam. Epitaxially grown GaAs slab waveguides serve as optical transducer for tailored evanescent field absorption analysis. A modular waveguide mounting accessory specifically designed for on-chip thin-film GaAs waveguides is presented serving as a flexible analytical platform in lieu of conventional attenuated total reflection (ATR) crystals uniquely facilitating macroscopic handling and alignment of such microscopic waveguide structures in real-world application scenarios.

Generation, guiding and splitting of triggered single photons from a resonantly excited quantum dot in a photonic circuit.

We demonstrate resonance fluorescence from single In-GaAs/GaAs quantum dots embedded in a rib waveguide beamsplitter structure operated under pulsed laser excitation. A systematic study on the excitation laser pulse duration depicts that a sufficiently small laser linewidth enables a substantial improved single-photon-to-laser-background ratio inside a waveguide chip. This manifests in the observation of clear Rabi oscillations over two periods of the quantum dot emission as a function of laser excitation power. A photon cross-correlation measurement between the two output arms of an on-chip beamsplitter results in a g(2)(0)=0.18, demonstrating the generation, guiding and splitting of triggered single photons under resonant excitation in an on-chip device. The present results open new perspectives for the implementation of photonic quantum circuits with integrated quantum dots as resonantly-pumped deterministic single-photon sources.

Low-noise quantum frequency down-conversion of indistinguishable photons.

We present experimental results on quantum frequency down-conversion of indistinguishable single photons emitted by an InAs/GaAs quantum dot at 904 nm to the telecom C-band at 1557 nm. Hong-Ou-Mandel (HOM) interference measurements are shown prior to and after the down-conversion step. We perform Monte-Carlo simulations of the HOM experiments taking into account the time delays of the different interferometers used and the signal-to-background ratio and further estimate the impact of spectral diffusion on the degree of indistinguishability. By that we conclude that the down-conversion step does not introduce any loss of HOM interference visibility. A noise-free conversion-process along with a high conversion-efficiency (> 30 %) emphasize that our scheme is a promising candidate for an efficient source of indistinguishable single photons at telecom wavelengths.

2.5 W continuous wave output at 665 nm from a multipass and quantum-well-pumped AlGaInP vertical-external-cavity surface-emitting laser.

An output power of 2.5 W at a wavelength of 665 nm was obtained from a quantum-well (QW) and multipass-pumped AlGaInP-based vertical-external-cavity surface-emitting laser operated at a heat sink temperature of 10°C. Intracavity frequency doubling resulted in an output power of 820 mW at a wavelength of 333 nm. To the best of our knowledge, these are the highest continuous wave output powers from this type of laser both at the fundamental wavelength and in frequency-doubled operation. In fundamental wavelength operation, further power scaling by increasing the pump-spot size increased the output power to 3.3 W. However, at this power level, the laser was highly unstable. When the laser was operated at 50% pump duty cycle, a reproducible and stable peak output power of 3.6 W was obtained. These results demonstrate the potential of optical QW pumping combined with multipass pumping for the operation of AlGaInP-based semiconductor disk lasers.

Intra-cavity frequency-doubled mode-locked semiconductor disk laser at 325 nm.

We present a passively mode-locked semiconductor disk laser (SDL) emitting at 650nm with intra-cavity second harmonic generation to the ultraviolet (UV) spectral range. Both the gain and the absorber structure contain InP quantum dots (QDs) as active material. In a v-shaped cavity using the semiconductor samples as end mirrors, a beta barium borate (BBO) crystal is placed in front of the semiconductor saturable absorber mirror (SESAM) for pulsed UV laser emission in one of the two outcoupled beams. Autocorrelation (AC) measurements at the fundamental wavelength reveal a FWHM pulse duration of 1.22ps. With a repetition frequency of 836MHz, the average output power is 10mW per beam for the red emission and 0.5mW at 325nm.