Kartik Gaur

Semiconductor quantum dots (QDs) offer outstanding quantum‐optical performance, making them promising for applications in quantum information technology. However, the simultaneous realization of wide‐range electrical tunability, efficient photon extraction, elevated‐temperature operation, and monolithic integration on silicon in combination with telecom‐wavelength compatibility remains a major unresolved challenge. Here, we demonstrate electrically contacted circular Bragg grating (eCBG) resonators incorporating InGaAs QDs grown on silicon, enabling bright single‐photon emission in the telecom O‐band. Deterministic electron‐beam lithography and a ridge‐based vertical p–i–n diode architecture allow precise device integration and electrical control of individual emitters. The QD‐eCBGs exhibit a quantum‐confined Stark shift of approximately 16 nm (11 meV) at 4 K, which presents a record value for a QD embedded in a nanophotonic structure at telecom wavelengths. This feature is combined with a photon extraction efficiency of (21.7 ± 3.0)%, while preserving excellent single‐photon purity. The measured values are g ⁽²⁾ (0) = 0.0078 ± 0.0012 below saturation and g ⁽²⁾ (0) = 0.0183 ± 0.0021 at saturation under pulsed excitation. Robust antibunching persists up to 77 K, yielding g ⁽²⁾ (0) = 0.0663 ± 0.0056, demonstrating operation within the temperature range accessible to liquid‐nitrogen and user‐friendly Stirling cryocoolers. Moreover, spatially separated QD‐eCBGs can be electrically fine‐tuned into spectral resonance. These results establish a silicon‐compatible telecom O‐band quantum light platform combining spectral tunability, single‐photon purity, and elevated‐temperature operation.

The deterministic realization of quantum light sources operating at telecom wavelengths is essential for long-distance fiber-based quantum communication and distributed quantum computing. In this work, we demonstrate that telecom O-band emission can be achieved from site-controlled InGaAs/GaAs quantum dots (QDs). Our concept utilizes a buried AlAs/AlOx stressor layer with the unique feature of inducing a well-defined and controllable tensile strain field at the growth surface, enabling both site-selective nucleation at the mesa’s centers and a redshift of QD emission to the ∼1.3 μm wavelength range. This concept not only eliminates the need for strain-reducing layers (SRLs), which are known to degrade optical coherence, but also provides spatial control and spectral tunability. The grown telecom QDs show single-photon emission with g(2)(0) = (5.0 ± 1.0) × 10–2 at 4 K and (2.8 ± 0.3) × 10–1 at 77 K, demonstrating the quantum nature and thermal stability of the emitters. The emission characteristics of complex excitonic states are analyzed using 8-band k·p and configuration-interaction modeling, which quantitatively reproduces the experimental observations. Finally, we present a theory-supported strategy to further redshift the emission toward the center of the O-band and beyond by employing a multiburied stressor approach. This combined framework of experiment and theory establishes the buried stressor concept as a scalable route toward highly coherent, position-controlled O-band quantum emitters compatible with scalable photonic integration.

Wafer-scale growth of two-dimensional (2D) semiconductors remains a key challenge for their integration into photonic technologies. While most studies of 2D semiconductors have focused on transition metal dichalcogenides and their scalable fabrication, comparatively little attention has been given to III–VI monochalcogenides. Here, we report large-area growth of gallium selenide (GaSe) by metal-organic chemical vapor deposition (MOCVD) and investigate its structural and optical properties for visible-range classical and quantum light emission. Two samples with thicknesses ranging from a few monolayers to several micrometers, obtained using growth times of 3- and 30 min, were investigated. The 30 min grown sample yields intense, broad emission spanning 1.7–2.0 eV, whereas the thinner 3 min sample exhibits discrete narrow emission lines and single-photon emission. Second-order photon-autocorrelation measurements on multiple localized emission centers yield ( g ( 2 ) ( 0 ) values below 0.5, ranging from ( 0.003 − 0.003 + 0.08 ) to (0.30 ± 0.11), demonstrating reproducible antibunching with emitter to emitter variations. Remarkably, cathodoluminescence mapping reveals pronounced spatial localization of both spectrally narrow and broad emission centers. Together with temperature-dependent power-law analysis and Raman mapping, our results are consistent with defect-induced emission rather than intrinsic excitonic recombination. These results establish large-area MOCVD grown 2D GaSe as a promising platform for integrated classical and quantum light sources compatible with silicon-based architectures.

Plasmonic-photonic metasurfaces enable large-area, spectrally selective field enhancement for surface-enhanced Raman scattering (SERS). However, identifying low-loss hybrid plasmon-photon modes under confocal, near-normal excitation remains elusive. Angle-resolved SERS provides direct access to their dispersion and degeneracies under stationary, ultrahigh numerical aperture excitation (NA ≈ 1.0 in air) by independently scanning polar and azimuthal incidence angles. SERS additionally demands modes that tolerate defects and periodicity fluctuations. We therefore fabricate colloidal metasurfaces across particle concentrations and lattice periods to quantify robustness. Using template-assisted self-assembly, we reveal a direct correlation between nanoparticle concentration, structural order, and the emergence of hybrid modes. At the optimized periodicity, SERS is maximized (up to 2.32× higher SERS intensity), which we can modulate through resonance detuning. Angle-resolved SERS at 633 and 785 nm under oblique incidence shows maximal enhancement when the hybrid guided-mode resonance is tuned to the laser line, yielding up to 36× higher intensity than off-resonant conditions for the same sample. An analytical model attributes these modes to phase-coupled waveguide coupling and collective plasmonic chain resonances. Thus, we yield practical design rules for robust, tunable colloidal plasmonic-photonic SERS metasurfaces via hybrid-mode dispersion engineering.

The scalable integration of solid-state quantum emitters into photonic nanostructures remains a central challenge for quantum photonic technologies. Here, we demonstrate a robust and streamlined integration strategy that tackles the long-standing issue of deterministic fabrication on randomly positioned self-assembled quantum dots (QDs), leveraging a buried-stressor-based site-controlled InGaAs QD platform. We show that this deterministic growth approach enables precise spatial alignment with circular Bragg grating (CBG) resonators for enhanced emission, eliminating the need for complex and time-consuming deterministic lithography techniques. We fabricated a 6 × 6 SCQD-CBG array with 100% device yield, with 35 devices falling within the radial-offset range where the simulated photon-extraction efficiency (PEE) exceeds 20%, underscoring the spatial precision and scalability of our fabrication concept. A systematically selected subset of five devices with varying radial displacements reveals clear offset-dependent trends in PEE, degree of linear polarization, spectral linewidth, and photon indistinguishability, thereby establishing quantitative bounds on spatial alignment tolerances. In the best-aligned QD-CBG device, we achieve a PEE of (47.1 ± 3.8)% (corresponding to an end-to-end system efficiency of 3.4%), a linewidth of (1.41 ± 0.22) GHz, a radiative decay lifetime of (0.80 ± 0.02) ns, a single-photon purity of (99.58 ± 0.18)%, and a Hong-Ou-Mandel two-photon interference visibility of (81 ± 5)% under quasi-resonant excitation at saturation power. We confirm our conceptual understanding of the effect of emitter-position dependent charge-noise fluctuations in terms of a quantum-optical model for the (quantum-)emission properties. The established nanofabrication platform provides a reproducible, lithography-compatible route to scalable, high-performance single-photon sources (SPS), offering a powerful alternative to conventional lithography-based deterministic integration techniques.

Semiconductor quantum dots (QDs) are key building blocks for photonic quantum technologies, enabling practical sources of non-classical light. A central challenge for scalable integration is the direct epitaxial growth of high-quality emitters on industry-compatible silicon platforms. Furthermore, for long-distance fiber-based quantum communication, emission in the telecom O- or C-band is essential. Here, we demonstrate the direct growth of high-quality InGaAs/GaAs QDs emitting in the telecom O-band using a strain-reducing layer approach on silicon. The deterministic integration of individual QDs into circular Bragg grating resonators is achieved via in situ electron-beam lithography. The resulting devices exhibit strong out-coupling enhancement, with photon extraction efficiencies up to (40 ± 2)%, in excellent agreement with numerical simulations. These results highlight the high material quality of both the epitaxial platform and the photonic nanostructure, as well as the precise lateral positioning of the emitter within 20 nm of the resonator center. At cryogenic temperature (4 K) and low excitation power (0.027 × Psat), the devices show excellent single-photon purity of (99.3 ± 0.1)%. Operation at elevated temperatures of 40 and 77 K, compatible with compact Stirling cryo-coolers and liquid-nitrogen cooling, reveals robust performance, with single-photon purity maintained at (88.4 ± 0.6)% at 77 K. These results demonstrate a practical and scalable route toward silicon-based quantum light sources and provide a promising path for cost-effective fabrication and seamless integration of quantum photonics with classical electronics, representing an important step toward large-scale, chip-based quantum information systems.

The generation of indistinguishable single photons is a fundamental requirement for photonic quantum technologies. However, spectral fluctuations, often induced by charge noise in epitaxial quantum dots (QDs), lead to exciton dephasing, thereby limiting their practical usage in quantum applications. We present a straightforward approach to mitigate charge noise-induced decoherence in droplet-etched GaAs QDs embedded in an n − i − p diode structure and integrated deterministically into an electrically contacted circular Bragg grating resonator for emission enhancement. The quantum device allows for the stabilization of the charge environment by applying an external electrical field while producing a photon extraction efficiency of ( 37 ± 2 ) % . Hong-Ou-Mandel two-photon interference measurements reveal a strong dependence of the exciton dephasing time and interference visibility on the applied bias, in excellent agreement with our theoretical predictions. Notably, the reduction in visibility from a maximum, charge stabilized corrected value of 97% at the optimum bias point follows an inverse square dependence ( ∝ 1 / I 2 ) with increasing diode current ( I ) in the forward direction. Under a quasi-resonant excitation scheme, we achieve a maximum exciton dephasing time ( T 2 * ) of approximately ( 6.8 ± 0.5 ) ns, reaching nearly the Fourier limit ( T 2 = 2 T 1 ) without the need for complex echo schemes like Ramsey or Carr-Purcell-Meiboom-Gill sequences. These findings are consistent with theoretical predictions from rate equation modeling and quantum optical analysis, as well as voltage-dependent linewidth measurements, demonstrating optimized electrical control of exciton dephasing.

The controlled integration of quantum dots (QDs) as single‐photon emitters into quantum light sources is essential for the implementation of large‐scale quantum networks. In this study, we employ the deterministic in situ electron‐beam lithography (iEBL) nanotechnology platform to integrate individual QDs with high accuracy and process yield into a circular Bragg grating (CBG) resonators. Notably, CBG devices comprising just 3 to 4 rings exhibit photon extraction efficiencies comparable to those of structures with more rings. This facilitates faster fabrication, reduces the device footprint, and enables compatibility with electrical contacting. To demonstrate the scalability of this process, we present results of 95 optically active QD‐CBG devices fabricated across two lithography sessions. These devices exhibit bright, narrow‐linewidth single‐photon emission with excellent optical quality. To evaluate QD placement accuracy, we apply a powerful characterization technique that combines cathodoluminescence (CL) mapping and scanning electron microscopy. Statistical analysis of these devices reveals that our iEBL approach enables high alignment accuracy and a process yield of over across various CBG geometries. Our findings highlight a reliable route toward the scalable fabrication of high‐performance QD‐based single‐photon sources for use in photonic quantum technology applications.

Interfacing light from solid-state single-photon sources with scalable and robust room-temperature quantum memories has been a long-standing challenge in photonic quantum information technologies due to inherent noise processes and time-scale mismatches between the operating conditions of solid-state and atomic systems. Here, we demonstrate storage of single photons from a semiconductor quantum dot (QD) device in a room-temperature atomic vapor memory and their on-demand retrieval. A deterministically fabricated InGaAs QD light source emits single photons at the wavelength of the cesium D1 line at 895 nm which exhibit an inhomogeneously broadened linewidth of 5.1(7) GHz and are subsequently stored in a low-noise ladder-type cesium vapor memory. We show control over the interaction between the single photons and the atomic vapor, allowing for variable retrieval times of up to 19.8(3) ns. A maximum internal efficiency of η int = 0.6 ( 1 ) % is achieved. Our results expand the application space of both room-temperature vapor memories and semiconductor QDs in future quantum network architectures.

In laser-based optical gas sensing, to exploit the multi-line response of analyzed gases, one needs to employ light sources with wide spectral tunability. This is a complex requirement, since it requires a gain medium with bright and spectrally broad emission. A matrix of tunable vertical-cavity surface-emitting lasers (VCSELs), with an active region made of epitaxially-grown quantum dots (QDs), might satisfy the abovementioned requirements better than conventional quantum wells with a narrowband gain spectrum. Here, we report on growth development, as well as structural and optical characterization, of atypical self-assembled, low-strain InGaAs/GaAs QDs, obtained by metal-organic chemical vapor deposition and optimized to achieve emission in the range of 935–955 nm, where strong and distinct absorption lines of water are found. As the InGaAs/GaAs QDs do not naturally emit at these wavelengths at room temperature, a special structure design and growth procedure, plus a post-processing step involving rapid thermal annealing, are required. Thanks to these steps, we achieved an emission level below 980 nm, which is a milestone on the way to manufacturing a low-cost VCSEL-based spectroscopic water vapor sensing system crucial for many industrial and environmental applications.