Site-controlled Quantum Dots Research Articles

Epitaxial growth and characterization of multi-layer site-controlled InGaAs quantum dots based on the buried stressor method

We report on the epitaxial growth, theoretical modeling, and structural as well as optical investigation of multi-layer, site-controlled quantum dots fabricated using the buried stressor method. This deterministic growth technique utilizes the strain from a partially oxidized AlAs layer to induce site-selective nucleation of InGaAs quantum dots. By implementing strain-induced spectral nano-engineering, we achieve spectral control of emission and a local increase in the emitter density. Furthermore, we achieve a threefold increase in the optical intensity and reduce the inhomogeneous broadening of the ensemble emission by 20% via stacking three layers of site-controlled emitters, which is valuable for using the SCQDs as a gain medium in microlaser applications. Our optimization of site-controlled growth of quantum dots enables the development of high-β microlasers with increased confinement factor.

Single site-controlled inverted pyramidal InGaAs QD–nanocavity operating at the onset of the strong coupling regime

Precise positioning of single site-controlled inverted pyramidal InGaAs quantum dots (QDs) at the antinode of a GaAs photonic crystal cavity with nanometer-scale accuracy holds unique advantages compared to self-assembled QDs and offers great promise for practical on-chip photonic quantum information processing. However, the strong coupling regime in this geometry has not yet been achieved due to the low cavity Q-factor based on the (111)B-oriented membrane structures. Here, we reveal the onset of phonon-mediated coherent exciton–photon interaction on our tailored single site-controlled InGaAs QD–photonic crystal cavity. Our results present the Rabi-like oscillation of luminescence intensity between excitonic and photonic components correlated with their energy splitting pronounced at small detuning. Such Rabi-like oscillation is well reproduced by modeling the coherent exchange of the exciton-photon population. The modeling further reveals an oscillatory two-time covariance at QD-cavity resonance, which indicates that the system operates at the onset of the strong coupling regime. Moreover, by using the cavity mode as a probe of the virtual state of the QD induced by phonon scattering, it reveals an increase in phonon scattering rates near the QD–cavity resonance and asymmetric phonon emission and absorption rate of even around 50 K.

Exciton-polariton dynamics of the single site-controlled quantum dot-nanocavity in the coexisting strong-weak coupling regime

Deterministic positioning single site-controlled high symmetric InGaAs quantum dots (QDs) in (111)B-oriented GaAs photonic crystal cavities with nanometer-scale accuracy provides an idea component for building integrated quantum photonic circuits. However, it has been a long-standing challenge of improving cavity Q-factors in such systems. Here, by optimizing the trade-off between the cavity loss and QD spectral quality, we demonstrate our site-controlled QD-nanocavity system operating in the intermediate coupling regime mediated by phonon scattering, with the dynamic coexistence of strong and weak coupling. The cavity-exciton detuning-dependent micro-photoluminescence spectrum reveals concurrence of a trend of exciton-polariton mode avoided crossing, as a signature of Rabi doublet of the strongly coupled system. Meanwhile, a trend of keeping constant or slight blue shift of coupled exciton–cavity mode(CM) energy across zero-detuning is ascribed to the formation of collective states mediated by phonon-assisted coupling, and their rare partial out-of-synchronization linewidth-narrowing is linked to their coexisting strong-weak coupling regime. We further reveal the pump power-dependent anti-bunching photon statistical dynamics of this coexisting strong-weak coupled system and the optical features of strongly confined exciton-polaritons, and dark-exciton-like states. These observations demonstrate the potential capabilities of site-controlled QD-cavity systems as deterministic quantum nodes for on-chip quantum information processing and provide guidelines for future device optimization for achieving the strong coupling regime.

Mode Interference Effect in Optical Emission of Quantum Dots in Photonic Crystal Cavities

Radiation properties of a pointlike source of light, such as a molecule or a semiconductor quantum dot, can be tailored by modifying its photonic environment. This phenomenon lies at the core of cavity quantum electrodynamics (CQED). Quantum dots in photonic crystal microcavities have served as a model system for exploring the CQED effects and for the realization of efficient single-photon quantum emitters. Recently, it has been suggested that quantum interference of the exciton recombination paths through the cavity and free-space modes can significantly modify the radiation. In this work, we report an unambiguous experimental observation of this fundamental effect in the emission spectra of site-controlled quantum dots positioned at prescribed locations within a photonic crystal cavity. The observed asymmetry in the polarization-resolved emission spectra strongly depends on the quantum dot position, which is confirmed by both analytical and numerical calculations. We perform quantum interferometry in the near-field zone of the radiation, retrieving the overlap and the position-dependent relative phase between the interfering free-space and cavity-mode-mediated radiative decays. The observed phenomenon is of importance for realization of photonic-crystal light emitters with near unity quantum efficiency. Our results suggest that the full description of light-matter interaction in the framework of CQED requires a modification of the conventional quantum master equation by also considering the radiation mode interference.12 MoreReceived 20 October 2021Revised 12 February 2022Accepted 13 April 2022Corrected 22 June 2022DOI:https://doi.org/10.1103/PhysRevX.12.021042Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCavity quantum electrodynamicsLight-matter interactionNanophotonicsOpen quantum systemsPhotonic crystalsPhotonicsQuantum description of light-matter interactionQuantum interference effectsSingle photon sourcesPhysical SystemsQuantum cavitiesQuantum dotsTechniquesFinite-difference time-domain methodFinite-element methodInterferometryOptical spectroscopyPhotoluminescenceQuantum master equationTime-resolved photoluminescenceCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & OpticalGeneral Physics

Remote excitation between quantum emitters mediated by an optical Fano resonance

Remote coupling between quantum emitters is of great importance for constructing quantum networks. Conventionally, this can be achieved via photon exchange by incorporating the quantum emitters in a waveguide or a large cavity. However, such photonic structures suffer from low quality-factors or large mode volumes, limiting the efficiency of light–matter interaction. Here, we demonstrate remote coupling between two site-controlled semiconductor quantum dot emitters mediated by an optical Fano resonance induced by coupling cavity modes via a continuum waveguide state. Unlike ordinary coupled modes, the Fano mode offers both a spatially extended field and a high local density of optical states at the emitters, enhancing light–matter interaction. This coupling scheme allows the demonstration of mutual excitation between two quantum dots separated in space by > 17 wavelengths. Our approach holds promise for achieving long-distance interaction without compromising interaction efficiency, which is essential for scaling up on-chip integration of quantum networks based on solid-state quantum emitters.

Nanoscale Focus Pinspot for High-Purity Quantum Emitters via Focused-Ion-Beam-Induced Luminescence Quenching.

Epitaxially grown quantum dots (QDs), especially embedded in photonic structures, play an essential role in various quantum photonic systems as on-demand single-photon sources. However, these QDs often suffer from adjacent unwanted emitters, which contribute to the background noise of the QD emission and fundamentally limit the single-photon purity. In this paper, a nanoscale focus pinspot (NFP) technique using focused-ion-beam-induced luminescence quenching enables us to improve single-photon purity from site-controlled QD as a proof-of-concept experiment. The optical quality of the QD emission is not degraded while the signal-to-noise ratio of the QD is improved. Moreover, the QD after the NFP technique reveals the single-photon nature at further elevated temperatures owing to the reduced background noise. As the NFP technique is nondestructive, it retains the apparent physical structures and photonic functions, thereby indicating its promising potential for applying diverse high-purity quantum emitters, particularly integrated in photonic devices and circuits.

Optical properties and symmetry optimization of spectrally (excitonically) uniform site-controlled GaAs pyramidal quantum dots

GaAs quantum dots (QDs) have recently emerged as state-of-the-art semiconductor sources of polarization-entangled photon pairs, however, without site-control capability. In this work, we present a systematic study of epitaxially grown GaAs/AlxGa1-xAs site-controlled pyramidal QDs possessing unrivaled excitonic uniformity in comparison to their InGaAs counterparts or GaAs QDs fabricated by other techniques. We have experimentally and systematically investigated the binding energy of biexcitons, highlighting the importance of the uniformity of all excitonic lines, rather than concentrating solely on the uniformity of the neutral exciton as a typical figure of merit, as it is normally done in the literature. We present optical signatures of GaAs QDs within a range of ∼250 meV with a remarkable uniformity within each individual sample, the ability to excite the biexciton state resonantly, and a systematic study of the fine-structure splitting (FSS) values—features important for polarization entangled photon emission. While, in general, we observe relatively large FSS distribution and associated non-uniformities, we discuss several strategies to suppress the average FSS values to <15 μeV.

Tailoring the optical properties of dilute nitride semiconductors at the nanometer scale

We report on the innovative approaches we developed for the fabrication of site-controlled semiconductor nanostructures [e.g. quantum dots (QDs), nanowires], based on the spatially selective incorporation and/or removal of hydrogen in dilute nitride semiconductor alloys [e.g. Ga(AsN) and (InGa)(AsN)]. In such systems, the formation of stable nitrogen-hydrogen complexes removes the effects nitrogen has on the alloy properties, which in turn paves the way to the direct engineering of the material’s electronic—and, thus, optical—properties: not only the bandgap energy, but also the refractive index and the polarization properties of the system can indeed be tailored with high precision and in a reversible manner. Here, lithographic approaches and/or plasmon-assisted optical irradiation—coupled to the ultra-sharp diffusion profile of hydrogen in dilute nitrides—are employed to control the hydrogen implantation and/or removal process at a nanometer scale. This results in a highly deterministic control of the spatial and spectral properties of the fabricated nanostructures, eventually obtaining semiconductor nanowires with controlled polarization properties, as well as site-controlled QDs with an extremely high control on their spatial and spectral properties. The nanostructures fabricated with these techniques, whose optical properties have also been simulated by finite-element-method calculations, are naturally suited for a deterministic coupling in optical nanocavities (i.e. photonic crystal cavities and circular Bragg resonators) and are therefore of potential interest for emerging quantum technologies.

Biexciton initialization by two-photon excitation in site-controlled quantum dots: The complexity of the antibinding state case

In this work, we present a biexciton state population in (111)B oriented site-controlled InGaAs quantum dots (QDs) by resonant two photon excitation. We show that the excited state recombines emitting highly pure single photon pairs entangled in polarization. The discussed cases herein are compelling due to the specific energetic structure of pyramidal InGaAs QDs—an antibinding biexciton—a state with a positive binding energy. We demonstrate that resonant two-photon excitation of QDs with antibinding biexcitons can lead to a complex excitation-recombination scenario. We systematically observed that the resonant biexciton state population is competing with an acoustic-phonon assisted population of an exciton state. These findings show that under typical two-photon resonant excitation conditions, deterministic biexciton state initialization can be compromised. This complication should be taken into account by the community members aiming to utilize similar epitaxial QDs with an antibinding biexciton.

N−nH complexes in GaAs studied at the atomic scale by cross-sectional scanning tunneling microscopy

Hydrogenation of nitrogen (N) doped GaAs allows for reversible tuning of the bandgap and the creation of site controlled quantum dots through the manipulation of N-nH complexes, N-nH complexes, wherein a nitrogen atom is surrounded by n hydrogen (H) atoms. Here we employ cross-sectional scanning tunneling microscopy (X-STM) to study these complexes in the GaAs (110) surface at the atomic scale. In addition to that we performed density functional theory (DFT) calculations to determine the atomic properties of the N-nH complexes. We argue that at or near the (110) GaAs surface two H atoms from N-nH complexes dissociate as an H$_2$ molecule. We observe multiple features related to the hydrogenation process, of which a subset is classified as N-1H complexes. These N-1H related features show an apparent reduction of the local density of states (LDOS), characteristic to N atoms in the GaAs (110) surface with an additional apparent localized enhancement of the LDOS located in one of three crystal directions. N-nH features can be manipulated with the STM tip. Showing in one case a switching behavior between two mirror-symmetric states and in another case a removal of the localized enhancement of the LDOS. The disappearance of the bright contrast is most likely a signature of the removal of an H atom from the N-nH complex.

Development of site-controlled quantum dot arrays acting as scalable sources of indistinguishable photons

We report on the realization of an array of 28 × 28 mesas with site-controlled InGaAs quantum dots acting as single-photon sources for potential applications in photonic quantum technology. The site-selective growth of quantum dots is achieved by using the buried stressor approach where an oxide aperture serves as the nucleation site in the center of each mesa. Spectroscopic maps demonstrate the positioning of quantum dots with an inhomogeneous broadening of the ensemble emission of only 15.8 meV. Individual quantum dots are characterized by clean single-quantum-dot spectra with narrow exciton, biexciton, and trion lines, with a best value of 27 μeV and an ensemble average of 120 μeV. Beyond that, Hanbury Brown and Twiss and Hong-Ou-Mandel measurements validate the quantum nature of emission in terms of high single-photon purity and photon indistinguishability with a g(2)(0) value of (0.026 ± 0.026) and a post-selected two-photon interference visibility V = (87.1 ± 9.7)% with an associated coherence time of τc = (194 ± 7) ps.

Broadband enhancement of light-matter interaction in photonic crystal cavities integrating site-controlled quantum dots

The fabrication of integrated quantum dot (QD)-optical microcavity systems is a requisite step for the realization of a wide range of nanophotonic experiments (and applications) that exploit the ability of QDs to emit nonclassical light, e.g., single photons. Thanks to their \ensuremath{\sim}20-nm positioning accuracy and to their proven potential for single-photon operation, the QDs obtained by spatially selective hydrogen irradiation of dilute-nitride semiconductors ---such as Ga(AsN) and Ga(PN)---are uniquely suited for integration with photonic nanodevices. In the present work, we demonstrate the ability to deterministically integrate single, site-controlled Ga(AsN)/Ga(AsN):H QDs within a photonic crystal (PhC) cavity. The properties of the fabricated QD-PhC cavity systems are then probed by photon correlation---providing clear evidence of single-photon emission---and time-resolved microphotoluminescence spectroscopy. Detailed information on the dynamics of our integrated nanodevices can be inferred by comparing these experiments to the solutions of a rate-equations system, developed by taking into account all the main processes leading to the capture, relaxation, and recombination of carriers in and out of the QD. This allows us to follow the evolution of the relevant recombination rates in our system for varying energy detuning, \ensuremath{\Delta}$E$, between the QD and the PhC cavity. When the QD exciton transition is nearly resonant with the cavity mode, a large (gtenfold) enhancement of the spontaneous emission rate is observed, in substantial agreement with Jaynes-Cummings (JC) theory. For intermediate detunings (\ensuremath{\Delta}$E$ \ensuremath{\sim} 1.5--3.5 meV), on the other hand, the observed enhancement is significantly larger than that predicted by JC theory, due to the important role played by acoustic phonons in mediating the QD-PhC cavity coupling in a solid-state environment. Apart from its fundamental interest, the observation of such phonon-mediated, broadband enhancement of light-matter interaction significantly relaxes the requirements for the realization of a large variety of cavity QED-based experiments and applications. These include many photonic devices for which the use of site-controlled Ga(AsN)/Ga(AsN):H QDs would be inherently advantageous, such as those based on the coupling between more than one QD and a single cavity mode (e.g., few-QD nanolasers and QD solids).

Advanced technologies for quantum photonic devices based on epitaxial quantum dots.

Quantum photonic devices are candidates for realizing practical quantum computers and networks. The development of integrated quantum photonic devices can greatly benefit from the ability to incorporate different types of materials with complementary, superior optical or electrical properties on a single chip. Semiconductor quantum dots (QDs) serve as a core element in the emerging modern photonic quantum technologies by allowing on-demand generation of single-photons and entangled photon pairs. During each excitation cycle, there is one and only one emitted photon or photon pair. QD photonic devices are on the verge of unfolding for advanced quantum technology applications. In this review, we focus on the latest significant progress of QD photonic devices. We first discuss advanced technologies in QD growth, with special attention to droplet epitaxy and site-controlled QDs. Then we overview the wavelength engineering of QDs via strain tuning and quantum frequency conversion techniques. We extend our discussion to advanced optical excitation techniques recently developed for achieving the desired emission properties of QDs. Finally, the advances in heterogeneous integration of active quantum light-emitting devices and passive integrated photonic circuits are reviewed, in the context of realizing scalable quantum information processing chips.

Tilted-potential photonic crystal cavities for integrated quantum photonics.

We propose and investigate a new type of photonic crystal (PhC) cavity for integrated quantum photonics, which provides tailored optical modes with both confined and extended spatial components. The structures consist of elongated PhC cavities in which the effective index of refraction is varied quasi-linearly along their axis, implemented by systematic lateral shifts of the PhC holes. The confined modes have approximately Airy-function envelopes, exhibiting single peaks and extended tails, which is useful for optimizing single photon extraction and transmission in integrated quantum photonic devices. The measured spectrally resolved near-field patterns of such devices show the expected spatial and resonance wavelength behavior, in agreement with numerical simulations of the Airy-Bloch modes. The effects of fabrication-induced disorder on the mode features are also analyzed and discussed. Selective excitation of specific Airy-Bloch modes using integrated, site-controlled quantum dots as localized light sources is demonstrated. Based on the tilted-potential cavity, multiple-QD single photon emitters exploiting wavelength division multiplexing are proposed.

Micropillar lasers with site-controlled quantum dots as active medium

The development of ultimate microcavity lasers requires precise engineering of the gain medium. Of particular interest are microlasers based on discrete gain centers, which are aligned to the field maximum of the cavity mode to maximize the modal gain. Here, we report on micropillar lasers with a gain medium composed of site-controlled quantum dots (SCQDs). Adjusting the size of a buried stressor, we define the number of high-quality SCQDs located at the antinode of the fundamental cavity mode. Our deterministic nanoprocessing platform allows us to tightly control the emission properties of high-β microlasers operating in the few-QD regime.

On-demand single-photons from electrically-injected site-controlled pyramidal quantum dots

We report on the performance of electrically-injected pyramidal quantum dots (PQDs) in terms of single-photon emission. We previously presented the generation of entangled photon pairs from similarly structured devices. Here we show that it is also possible to obtain single-photons upon continuous wave excitation as well as pulsed excitation, obtaining a low g2(0) of 0.088 ± 0.059, by discarding re-excitation events within a single excitation pulse by applying time-gating techniques.

Laser induced nano-patterning with atomic-scale thickness on an InAs/GaAs surface

Nano-patterned substrates fabricated by different techniques have been widely applied in high performance semiconductor devices. For the influence of surface morphology on semiconductor electronic properties and epitaxy growth, there is an increasing interest to realize atomic-scale surface modification by a simple and effective method. In this study, laser induced nano-patterning with atomic-scale thickness on an InAs/GaAs surface was studied in both atmosphere and vacuum environments. Periodic nano-grooves with an average depth of 1 to 2 atomic layers were fabricated by focused laser spot scanning in the air. The depth and width of nano-grooves increase with the laser energy intensity. The result of nano-patterning is attributed to atom desorption stimulated by photon-induced electronic excitation. To explore the situation in the vacuum, in situ two-beam laser interference was applied during InAs/GaAs sample MBE growth. Periodic InGaAs nano-islands were fabricated, which show significant influence on subsequent crystal growth. The formation of ordered InAs quantum dots agreeing with the atomic layer nano-pattern demonstrates the potential application of the technique in site-controlled quantum dot growth.

Vanishing biexciton binding energy from stacked, MOVPE grown, site-controlled pyramidal quantum dots for twin photon generation

We characterized stacked double-pyramidal quantum dots which showed biexciton binding energies close to zero by means of photoluminescence and cross-correlation measurements. It was possible to obtain a sequence of two photons with (nearly) the same energy from the biexciton-exciton-ground state cascade, as corroborated by a basic rate-equation model. This type of two-photon emission is both of relevance for fundamental quantum information theory studies as well as for more exotic applicative fields such as quantum biology.

Site-Controlled Quantum Emitters in Dilute Nitrides and their Integration in Photonic Crystal Cavities

We review an innovative approach for the fabrication of site-controlled quantum emitters (i.e., single-photon emitting quantum dots) based on the spatially selective incorporation and/or removal of hydrogen in dilute nitride semiconductors (e.g., GaAsN). In such systems, the formation of stable N-H complexes removes the effects that nitrogen has on the alloy properties, thus enabling the in-plane engineering of the band bap energy of the system. Both a lithographic approach and/or a near-field optical illumination—coupled to the ultra-sharp diffusion profile of H in dilute nitrides—allow us to control the hydrogen implantation and/or removal on a nanometer scale. This, eventually, makes it possible to fabricate site-controlled quantum dots that are able to emit single photons on demand. The strategy for a deterministic spatial and spectral coupling of such quantum emitters with photonic crystal cavities is also presented.

Site-Controlled Single-Photon Emitters Fabricated by Near-Field Illumination.

Many of the most advanced applications of semiconductor quantum dots (QDs) in quantum information technology require a fine control of the QDs' position and confinement potential, which cannot be achieved with conventional growth techniques. Here, a novel and versatile approach for the fabrication of site-controlled QDs is presented. Hydrogen incorporation in GaAsN results in the formation of N-2H and N-2H-H complexes, which neutralize all the effects of N on GaAs, including the N-induced large reduction of the bandgap energy. Starting from a fully hydrogenated GaAs/GaAsN:H/GaAs quantum well, the NH bonds located within the light spot generated by a scanning near-field optical microscope tip are broken, thus obtaining site-controlled GaAsN QDs surrounded by a barrier of GaAsN:H (laterally) and GaAs (above and below). By adjusting the laser power density and exposure time, the optical properties of the QDs can be finely controlled and optimized, tuning the quantum confinement energy over more than 100 meV and resulting in the observation of single-photon emission from both the exciton and biexciton recombinations. This novel fabrication technique reaches a position accuracy <100 nm and it can easily be applied to the realization of more complex nanostructures.