We experimentally demonstrate an autonomous, fully tunable and scalable optical neural network of 400+ parallel nodes based on a large area, multimode semiconductor laser. We implement hardware compatible, online learning strategies based on reinforcement learning and evolutionary strategies and evaluate them in terms of performance and energy cost. Our system achieves high performance and a high classification bandwidth of 15KHz for the MNIST dataset. Our approach is highly scalable both in terms of classification bandwidth and neural network size due to our device's short response time (nanosecond).
We design, fabricate, characterize, and compare 980 nm vertical cavity surface emitting lasers (VCSELs) with monolithic high contrast gratings (MHCGs) as top coupling mirrors. The MHCG is a series of parallel, rectangular stripes etched into a uniform GaAs epitaxial surface layer via electron-beam lithography and inductively coupled reactive ion etching, with specific grating period, height, and fill factor (defined as the grating bar width divided by the grating period). To boost the MHCG’s optical power reflectance at 980 nm and the width of the optical stopband we add a 5.5-period p-doped distributed Bragg reflector (DBR) beneath the MHCG grating, thus forming a composite DBR plus MHCG top coupling mirror. The bottom n-doped DBR is a conventional all-semiconductor AlGaAs/GaAs DBR with 37-periods on a GaAs substrate. We fabricate single 980 nm DBR MHCG VCSELs with two oxide aperture diameters on quarter wafer pieces from starting 3- inch diameter VCSEL epitaxial wafers. Each quarter wafer contains six complete unit cells, and each unit cell is a twodimensional array of single VCSELs in 16 rows and 15 columns. We for example set a constant but different grating period in five of the unit cells and vary the grating fill factors from column to column and we vary the oxide aperture diameters from 1 to 9 Pm in the rows, thus yielding a large variety of VCSEL diodes with differing MHCG parameters for us to compare. We perform room temperature on-wafer probe testing of the static optical output power-current-voltage (LIV) characteristics and emission spectra and compare the impact of the grating designs on these test results. We report record static LIV performance for our DBR MHCG VCSELs with threshold current below 1 mA and optical output power exceeding 1.3 mW. We observe room temperature bias current dependent mode emission for example single mode wavelength tuning ranges up to 12 nm.
We investigate the calculation of resonance modes of a VCSEL with a Riesz projection eigenvalue solver. The eigenvalue solver is based on the principle of contour integration where for the solution of scattering problems physical right sides are used. Here, it is investigated how numerical parameters impact the performance of the method, where we focus on the computation of the fundamental VCSEL mode.
Photonic neural networks are a highly promising computational system for AI-inspired future information processing. We have recently demonstrated the first fully implemented, photonic neural network realized in multimode semicondcutor lasers. The numerous laser modes acts as the systems neurons, which carrier diffusion and intra-cavity diffraction creating recurrent connections. I will discuss our recent result, where we push the realtime data-rate of the neural network towards GHz levels and use such systems to address highly relevant photonic-technology applications.
We demonstrated a large-scale space-time-multiplexed homodyne optical neural network (ONN) using arrays of high-speed (GHz) vertical-cavity surface-emitting lasers (VCSELs). Injection locking enables precise phase control over tens of VCSEL devices simultaneously, facilitating photoelectric-multiplication-based matrix operations and all-optical nonlinearity, operating at the quantum-noise limit. Our VCSEL transmitters exhibit ultra-high electro-optic conversion efficiency (Vπ=4 mV), allowing neural encoding at 5 attojoule/symbol. Three-dimensional neural connectivity allows parallel computing. The full-system energy efficiency reaches 7 fJ/operation, which is >100-fold better than the state-of-the-art digital microprocessors and other ONN demonstrations. Digit classification is achieved with an accuracy of 98% of the group truth.
Spin-controlled lasers are highly interesting photonic devices and have been shown to provide ultra-fast polarization dynamics in excess of 200 GHz. Another class of modern semiconductor lasers are high-beta emitters which benefit from enhanced light-matter interaction due to strong mode confinement in low-mode-volume microcavities. We combine the advantages of both laser types to demonstrate spin-lasing in high-beta microlasers for the first time. For this purpose, we realize bimodal high-beta quantum dot micropillar lasers for which the mode splitting and the polarization-oscillation frequency can be engineered via the pillar cross-section. The microlasers show very pronounced spin-lasing effects with polarization oscillation frequencies up to 15 GHz.
Artificial neural networks (ANNs) have become a staple computing technique in many fields. Yet, they differ from classical computing hardware by taking a connectionist and parallel approach to computing and information processing. Here, we present a high performance, scalable, fully parallel, and autonomous PNN based on a large area vertical-cavity surface-emitting laser (LA-VCSEL). We implement 300+ hardware nodes and train the network to perform up to 6-bit header recognition, XOR classification and digital to analog conversion. Moreover, we investigate the impact of different physical parameters, namely, injection wavelength, injection power, and bias current on performance, and link these parameters to the general computational measures of consistency and dimensionality.
Highly efficient electrically–driven single photon sources (SPSs) with a narrow far–field emission pattern suitable for coupling to a single mode fiber are critical components for applications in quantum communication. We address modern concepts of the design of SPSs suitable for such applications: (i) Quantum dot (QD) in a micropillar, where a proper reflectivity balance of top and bottom distributed Bragg reflectors (DBRs) and cavity design allows non–resonant highly directional light source. (ii) Dielectric multilayer structure acting as photon extracting microcavity including a passive cavity design and a deeply etched circular Bragg grating enabling a high efficiency of the light extraction and a narrow far field pattern. The light source is not resonant in wavelength and allows narrow far field distribution at a low series resistance. (iii) Resonant light sources based on broadband high–contrast dielectric DBRs optimized for O–band 1300 nm operation with photon–extraction efficiency above 90% and the coupling efficiency to a single mode fiber of 76%. Resonant tuning of the cavity and QD emission allows Purcell effect–enhancement of the QD photon emission rate.
Spin-controlled lasers are highly interesting photonic devices and have been shown to provide ultra-fast polarization dynamics in excess of 200 GHz. Another class of modern semiconductor lasers are high-beta emitters which benefit from enhanced light-matter interaction due to strong mode confinement in low-mode-volume microcavities. We combine the advantages of both laser types to demonstrate spin-lasing in high-beta microlasers for the first time. For this purpose, we realize bimodal high-beta quantum dot micropillar lasers for which the mode splitting and the polarization-oszillation frequency can be engineered via the pillar cross-section. The microlasers show very pronounced spin-lasing effects with polarization oscillation frequencies up to 16 GHz.
Artificial Neural Networks (ANNs) have become a staple computing technique. Their flexibility allows them to excel in a wide range of tasks and they benefit from highly parallelized architecture by design. We experimentally demonstrate a fully parallel photonic neural network using spatially distributed modes of a large-area vertical-cavity surface-emitting laser (LA-VCSEL). All components of the ANN are fully realized in parallel hardware. We train the readout weights to perform 2 and 3-bit header recognition, XOR classification, and digital to analog conversion, and obtain low error rates for all tasks. Our system uses readily available components, is scalable to much larger sizes and to bandwidths in excess of 20 GHz.
The deterministic integration of quantum emitters into on-chip photonic elements is crucial for the implementation of scalable on-chip quantum circuits. Here, we report on the deterministic integration of single quantum dots (QD) into tapered multimode interference beam splitters using in-situ electron beam lithography (EBL). We demonstrate the functionality of the deterministic QD-waveguide structures by µPL spectroscopy and by studying the photon cross-correlation between the two MMI output ports. The latter confirms single-photon emission and on-chip splitting associated with g(2)(0) << 0.5. Moreover, the deterministic integration of QDs enables the demonstration and controlled study of chiral light-matter effects and directional emission in QD-WGs, and the realization of low-loss heterogenous QD-WG systems with excellent quantum optical properties.
Quantum optical information systems offer the potential for secure communication and fast quantum computation. To fully characterise a quantum optical system one has to use quantum tomography.1 The integration of quantum optics onto photonic chips provides advantages such as miniaturisation and stability, significantly improving quantum tomography using both re-configurable, and more recently, simpler static designs. These on-chip designs have, so far, only used probabilistic single photon sources. Here we are working towards quantum tomography using a true deterministic source - an InGaAs quantum dot.
Light sources for applications in quantum information, quantum-enhanced sensing and quantum metrology are attracting increasing scientific interest. To gain inside into the underlying physical processes of quantum light generation, efficient photon detectors and experimental techniques are required to access the photon statistics. In this work, we employ photon-number-resolving (PNR) detectors based on superconducting transition-edge sensors (TESs) for the metrology of photonic microstructures with semiconductor quantum dots (QDs) as emitters. For the PNR analysis, we developed a state of the art PNR detection system based on fiber-coupled superconducting TESs. Our stand-alone system comprises six tungsten TESs, read out by six 2-stage-SQUID current sensors, and operated in a compact detector unit integrated into an adiabatic demagnetization refrigerator. This PNR detection system enables us to directly access the photon statistics of the light field emitted by our photonic microstructures. In this contribution, we focus on the PNR study of deterministically fabricated quantum light sources emitting single indistinguishable photons as well as twin-photon states. Additionally, we present a PNR-analysis of electrically pumped QD micropillar lasers exhibiting a peculiar bimodal behavior. Employing TESs our work provides direct insight into the complex emission characteristics of QD- based light sources. We anticipate, that TES-based PNR detectors, will be a viable tool for implementations of photonic quantum information processing relying on multi-photon states.
We present an alignment procedure which allows for precise gluing of a structure with an optically pumped quantum emitter to the end face of zirconia ferrule with a specially fabricated high numerical aperture single-mode fiber. The proposed method is an important step towards building a single-photon source based on an InGaAs quantum dot emitting in 1.3 μm range and located deterministically in a microlens fabricated by in-situ electron beam lithography and plasma etching to improve the photon extraction efficiency. Since single QDs are very dim at room temperature which hinders QD-fiber adjustment by maximizing the collected photoluminescence signal, the developed method uses light back-reflected from the top surface of the sample with microlens as a feedback signal. Using this approach, we were able to position the high-NA fiber over the center of the microlens with an accuracy of about 150 nm in a lateral direction and 50 nm in a vertical direction. The alignment accuracy was confirmed by following the room temperature emission from quantum wells embedded in a reference microlens. We also present initial low temperature tests of the coupling system mounted in a compact and portable Stirling cryocooler.
The development and physical understanding of high-beta nanolasers operating in regime of cavity-quantum-electrodynamics (cQED) is a highly interdisciplinary field of research, involving important aspects of nanotechnology, quantum optics, and semiconductor physics. Of particular interest is the quantum limit of operation, in which a few or even a single emitter act as gain material.
The regime of strong light-matter coupling is typically associated with weak excitation. With current realizations of cQED systems, strong coupling may persevere even at elevated excitation levels sufficient to cross the threshold to lasing. In the presence of stimulated emission, the vacuum-Rabi doublet in the emission spectrum is modified and the established criterion for strong coupling no longer applies.
Based on an analytic approach, we provide a generalized criterion for strong coupling and the corresponding emission spectrum that includes the influence of higher Jaynes-Cummings states. The applicability is demonstrated in a theory-experiment comparison of a state-of-the-art few-emitter quantum-dot (QD)–micropillar laser as a particular realization of the driven dissipative Jaynes-Cummings model [1]. Furthermore, we address the question if and for which parameters true single-emitter lasing can be achieved. By using a master-equation approach for up to 8 QDs coupled to the mode, we provide evidence for the coexistence of strong coupling and lasing in our system in the presence of background emitter contributions by identifying signatures in the mean-photon number, the photon-autocorrelation function, and the emission linewidth.
[1] C. Gies et al., accepted for publication in PRA, arxiv:1606.05591
Despite their name polariton lasers do not rely on stimulated emission of cavity photons. The less stringent threshold conditions are the cause that bosonic polariton lasers can outperform standard lasers in terms of their threshold currents. The part-light and part-matter quasiparticles called polaritons, can undergo a condensation process into a common energy state. The radiated light from such a system shares many similarities with the light emitted from a conventional photon laser, even though the decay of the polaritons out of the finite lifetime cavity is a spontaneous process. We discuss properties of polariton condensates in GaAs based microcavities. The system’s response to an external magnetic field is used as a reliable tool to distinguish between polariton laser and conventional photon laser. In particular, we will discuss the realization of an electrically pumped polariton laser, which manifests a major step towards the exploitation of polaritonic devices in the real world.
Polariton Lasers do not rely on stimulated emission of photons, a criterion that sets stringent conditions on the threshold current in a conventional laser. Therefore, they have the capability to outperform photon lasers in the weak coupling regime in terms of the threshold power consumption. We present the first successful realization of an electrically pumped polariton laser based on a GaAs/AlAs distributed Bragg reflector cavity. We have furthermore identified the system’s response to an applied magnetic field as a sensitive tool to distinguish a polariton laser from a standard VCSEL device in the weak light-matter coupling regime
The semiconductor micropillar is attractive for cavity QED experiments. For strong coupling, the figure of merit is proportional to Q/√V, and a design combining a high Q and a low mode volume V is thus desired. However, for the standard submicron diameter design, poor mode matching between the cavity and the DBR Bloch mode limits the Q. We present a novel adiabatic design where Bloch-wave engineering is employed to improve the mode matching, allowing the demonstration of a record-high vacuum Rabi splitting of 85 μeV and a Q of 13600 for a 850 nm diameter micropillar.
Studying cavity quantum electrodynamical effects is an emerging and important field of research for the understanding
of the many body quantum theory as well as for the generation of a new type of efficient lasers. Here we report a
dramatic change in the photon statistics of quantum dot based micropillar lasers where a finite fraction of the emission is
reflected back into the microcavity after a roundtrip time τ in an external cavity, where τ greatly exceeds the coherence
time. Photon bunching was observed above the threshold current where the second order autocorrelation function g(2)(τ)
at zero-lag can reach values up to 3.51±0.06. The change in the photon statistics of the two non-degenerated fundamental
modes were found to be correlated, indicating non-trivial interactions between both cavity modes. Furthermore the
optical feedback led to revivals of the bunching signal in integer multiples of the round trip time of the external cavity
and to a decrease in the coherence time of the laser. These phenomena compare well with milliwatt chaotic lasers
induced by an external feedback, indicating that chaos might occur in the nanowatt lasing regime where fluctuations in
the photon statistics are in the leading order.
Efficient sources of indistinguishable single photons are a key resource for various applications in fields like quantum
sensing, quantum metrology and quantum information processing. In this contribution we report on single photon
generation based on III-V semiconductor quantum dots (QDs). To increase the emission efficiency of single photons, it
is essential to tailor the radiative properties of the quantum dot emitters by engineering their photonic environment. We
present optimized single photon emitters being based on both micropillar and photonics crystal cavities, for applications
in a vertical platform and on-chip in-plane platform, respectively.
Electrically driven single photon sources with self assembled semiconductor QDs embedded into GaAs/AlAs
micropillar cavities emit on demand net rates of ~35 MHz single photons, thus being well exploitable in quantum key
distribution systems. In order to establish also a spatially deterministic fabrication platform, position controlled quantum
dots are integrated into p-i-n micropillar cavities and single photon emission of a coupled QD-micropillar diode system
is observed. Efficient broadband coupling of single photons into photonic crystal waveguides provides the basis for all
on-chip quantum information processing, and an according approach is reported.
Semiconductor quantum dots (QDs) are fascinating nanoscopic structures for photonics and future quantum information
technology. However, the random position of self-organized QDs inhibits a deterministic coupling in devices relying on
cavity quantum electrodynamics (cQED) effects which complicates, e.g., the large scale fabrication of quantum light
sources. As a result, large efforts focus on the growth and the device integration of site-controlled QDs. We present the
growth of low density arrays of site-controlled In(Ga)As QDs where shallow etched nanoholes act as nucleation sites.
The nanoholes are located relative to cross markers which allows for a precise spatial alignment of the site-controlled
QDs (SCQDs) and the photonic modes of high quality microcavites with an accuracy better than 50 nm. We also
address the optical quality of the SCQDs in terms of the single SCQD emission mode linewidth, the oscillator strength
and the quantum efficiency. A stacked growth of strain coupled SCQDs forming on wet chemically etched nanoholes
provide the smallest linewidth with an average value of 210 μeV. Using time resolved photoluminescence studies on
samples with a varying thickness of the capping layer we determine a quantum efficiency of the SCQD close to 50 % and an oscillator strength of about 10. Finally, single photon emission with associated with g(2)(0) = 0.12 of a weakly
coupled SCQD - micropillar system will be presented.
The quest for efficient light sources and light detectors is a driving force in the development of semiconductor
quantum dot (QD) devices. Self assembled QDs in bulk material are characterized by high quantum efficiency
and can act as single photon emitters. However, they suffer from a poor light in- and outcoupling efficiency. We
demonstrate highly efficient QD-micropillar based light detectors and single photon emitters exploiting cavity
quantum electrodynamics (cQED) effects. An advanced fabrication technique allows us to realize ultra sensitive
and wavelength selective light detectors as well as triggered, electrically driven single photon sources with photon
outcoupling efficiencies exceeding 60 %.
Light and matter can be unified under the strong coupling regime, creating superpositions of both, called dressed states or polaritons. After initially being demonstrated in bulk semiconductors and atomic systems ,strong coupling phenomena have been realized in solid state optical microcavities. They form an essential ingredient in the exciting physics spanning from many-body quantum coherence phenomena, like Bose-Einstein condensation and superfluidity, to cavity quantum electrodynamics (cQED). A widely used approach within cQED is the
Jaynes-Cummings (JC) model that describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling (QSC), a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently published. In our latest report we applied advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton-cavity
system. Specifically, we measured and simulated its four-wave mixing (FWM) response, granting direct access to the first two rungs of the JC ladder. This paper summarizes the main results of Ref. 15 and adds
FWM experiments obtained on a micropillar cavity in which a doublet of quantum dot (QD) excitons interacts with the cavity mode in the limit of weak to strong coupling.
Quantum light emitters have great application potential in quantum key distribution, precision metrology and
quantum imaging. We present triggered electrically driven single photon sources based on semiconductor quantum dots
in GaAs/AlAs micropillar cavities with on demand single photon rates of 35 MHz while a record outcoupling efficiency
up to 34 % is obtained. Photon autocorrelation measurements reveal g(2)(0) down to 0.13. The high efficiency is
achieved due to an optimized contact scheme which allows for the injection of electrical current into micropillar cavities
which are characterized by low absorption losses and diameters down to 1 μm. By exploiting the established
fabrication procedure, micropillar cavities exhibiting pronounced cavity quantum electrodynamics effects have been
realized. Furthermore, by applying a reverse bias to the micropillar cavities, photocurrent measurements allow for
wavelength selective sensing of light at powers down to 20 nW and further design changes promise photon detection
sensitivities approaching the quantum limit.
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.
We report on experiments where a single quantum dot is strongly coupled to a high-Q mode of a micropillar
cavity. Photon correlation measurements confirm that the observed avoided crossing originates from strong
coupling of a single quantum dot to the cavity mode. Cross-correlations between the cavity mode and the
spectrally detuned quantum dot enabled us to assign the unexpected strong cavity emission to a coupling with
the single quantum dot. The coupled quantum dot-microcavity system displays an Purcell factor of 61 and
represents a single-photon source with an efficiency of 97%.
Properties of atom-like emitters in cavities are successfully described by cavity quantum electrodynamics (cQED). We
report on cavity quantum electrodynamics (cQED) experiments in a single quantum dot semiconductor system. CQED,
which is a very active research field in optics and solid state physics, can be divided into a weak and a strong coupling
regime. In case of weak coupling, the spontaneous emission rate of an atom-like emitter, e.g. a single quantum dot
exciton, can be enhanced or reduced compared to the value in vacuum in an irreversible emission process. In contrast, a
reversible energy exchange between the emitter and the cavity mode takes place when the conditions for strong coupling
are fulfilled. We investigate weak as well as strong coupling in a system based on a low density In0.3Ga 0.7As quantum
dot layer placed as the active layer in a high quality planar AlAs/GaAs distributed Bragg reflector cavity grown by
molecular beam epitaxy. Using electron beam lithography and deep plasma etching, micropillars with high Q-factors
(up to 43.000 for 4 μm diameter) were realized from the planar cavity structure. Due to the high oscillator strength of
the In0.3Ga 0.7As quantum dots together with a small mode volume in high finesse micropillar cavities it is possible to
observe strong coupling characterized by a vacuum Rabi splitting of 140 μeV. The fabrication of high-Q micropillar
cavities as well as conditions necessary to realize strong coupling in the present system are discussed in detail.
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