Significant progress in quantum light sources for quantum communication applications requires reproducible and symmetric quantum emitters acting as single-photon sources capable of generating entangled photons on demand at specific telecom wavelengths. Here, we propose telecom-emitting epitaxial quantum dots (QDs) fabricated using the local droplet etching (LDE) approach. The resulting well-defined, low-density ($10^9$/cm$^2$) QDs based on In$_{x}$Ga$_{1-x}$As are formed in symmetric LDE nanoholes (in-plane aspect ratio of 1.14) in In$_{0.52}$Al$_{0.48}$As. Detailed transmission electron microscopy provides comprehensive insight into the structural integrity, interface quality, and compositional profiles of the QDs, which underpin their promising optical properties. Photoluminescence spectroscopy reveals narrow emission lines (0.2 meV) and high optical quality, while second-order autocorrelation measurements confirm clear single-photon emission, with $g^{(2)}(0)=0.07\pm0.02$ under above-band continuous-wave excitation and $g^{(2)}(0)=0.16 \pm 0.18$ under pulsed excitation. Precise numerical modeling, combining multiband $\boldsymbol{k} \cdot \boldsymbol{p}$ and configuration-interaction methods, supports the optical characterization and identifies thermal excitation pathways that explain the persistence of emission up to liquid-nitrogen temperatures. These results highlight the versatility of the LDE approach for integrating new material systems and pave the way toward scalable fabrication of quantum light sources with tailored emission properties.

This paperfocuses on the design and optimization of high contrast grating (HCG) used in tunable micro-electromechanical system - vertical-cavity surface emitting laser (MEMS-VCSEL). The optimization consists of mitigating reflectivity dips, that can present when incidence angle effects are not considered, to obtain a broad reflectivity bandwidth. Two new designs are proposed, low-duty-cycle and focusing HCG. These designs are verified using FDTD 3D simulations and then fabricated. From the characterization, the best result was obtained with a low-duty-cycle design, that achieved a maximum tuning range of 67.8 nm and an average tuning range of 55.8 nm surpassing the standard design where we obtained 62.2 nm and 50.2 nm respectively. Furthermore, the new designs displayed better performance in the maximum output power and a better far-field profile compared to the standard design, indicatingimproved light extraction efficiency. These findings demonstrate the promise of optimized HCG, although further fabrication refinement is crucial to consistently achieve and maximize their performance for swept-source optical coherence tomography (SS-OCT) applications.

By harnessing the hyperfine coupling to the hydrogen nuclear spin and a nearby 29Si nuclear spin, a T-centre in silicon forms a combined quantum register and a spin–photon interface that is operational in the telecommunications original band.

Deterministic quantum light sources emitting at telecom wavelengths with vanishing fine-structure splitting (FSS) are essential components for scalable quantum communication. While self-assembled Stranski-Krastanov (SK) quantum dots (QDs) are high-quality emitters, their random positioning and shape-induced anisotropy typically limit their use in entangled-photon applications. In this work, we demonstrate site-controlled SK growth where InAs/InP QDs nucleate at the symmetric apexes of truncated InP nanopyramids. Confining adatom diffusion to a small, symmetric nucleation area suppresses anisotropic growth, promoting the nucleation of highly symmetric QDs with FSS reduced to values below our statistically validated resolution limit of 9.2 μeV. At the same time, lithographically defined nucleation sites enable deterministic control of the QD position, overcoming the limitations of conventional SK growth. The high structural quality of single symmetric QDs is evidenced by the single-photon character of the emission (g^(2)(0)=0.07^+0.27_-0.07) spanning the S, C, and L telecom bands, with no evidence of lithography-induced defects affecting emission dynamics. These results demonstrate that tailoring QD symmetry through nanopyramid growth engineering provides a route toward site-controlled emitters suitable for entangled photon generation and integrated quantum photonics devices.

Near-zero refractive index (NZI) materials offer a wide range of applications, from classical optics to quantum technologies. The NZI regime can be effectively achieved by engineering the dispersion of photonic crystals to form Dirac-like cones at the Brillouin zone center. This work presents a machine-learning-driven framework for systematically identifying and tuning Dirac-like cones across arbitrary material systems and operating wavelengths. By employing a batched Bayesian optimization strategy, we efficiently map multi-parameter design spaces to locate specific geometric configurations that achieve accidental triple degeneracy. Our findings demonstrate that the dielectric filling factor governs the transition from quadratic to linear Dirac-like dispersion by modulating the inter-modal coupling strength. Expanding to three-dimensional simulations, we establish design rules to tune this degeneracy to specific target wavelengths such as 1550 nm. Ultimately, this versatile approach provides a scalable pathway for developing designs of low-loss, near-zero-index materials for advanced optical applications.

We report lasing from a lithographically defined buried heterostructure with an estimated lateral footprint of (107 nm)2, embedded in an InP photonic-crystal nanobeam cavity. This represents the smallest laterally confined buried heterostructure gain region from which lasing has been observed. Despite etching of the active region during cavity definition and the associated risk of surface-related nonradiative recombination, optically pumped devices exhibit a clear lasing threshold and a narrow linewidth. By systematically varying the buried heterostructure size, we investigate how the lasing threshold depends on the active volume under optical pumping. The estimated intrinsic threshold under ideal carrier injection is 57 nW, comparable to values reported for single quantum-dot nanolasers, highlighting the potential of quantum-dot-scale buried heterostructures as deterministic, scalable gain media for nanophotonic lasers.

The demand for advanced photonics technology is increasing rapidly, fueled by the necessity for high‐performance, cost‐effective optical information processing systems extending into the quantum domain. Silicon, benefiting from its mature fabrication processes, stands as an ideal platform. However, its inherent indirect bandgap leads to inefficient light emission. The integration of III‐V materials is essential to overcome this drawback. These materials are recognized for their efficient light emission and superior bandgap engineering, making them indispensable in photonics and beyond. Here, we present the monolithic integration of small‐volume III‐V nano‐heterostructures with silicon via selective area epitaxy in pyramidal openings etched in (100)‐oriented silicon substrate. Precise positioning of the nano‐heterostructures is achieved using electron beam lithography. Atomic resolution imaging and chemical analysis confirm the epitaxial nature of InP growth, revealing well‐defined heterointerfaces. Each structure incorporates an InAsP quantum dot‐like active medium, and the correlation of the growth parameters with the nanoscale structure is analyzed using advanced electron microscopy. Eight‐band k·p calculations demonstrate energy level quantization in three spatial dimensions. Optical characterization shows that heterostructure emission can be engineered to cover the entire telecom wavelength range. These InAsP/InP nano‐heterostructures can serve as gain medium for silicon‐based hybrid nano‐lasers, nano‐LEDs, and quantum light sources in telecom wavelength range.

The interaction between light and matter can be enhanced by spatially concentrating the light field and extending photon dwell time. Plasmonic structures can provide strong light confinement but suffer from ohmic losses. Recent advances in dielectric nanostructures enable strong light localization without metallic losses. However, previous studies primarily focused on minimizing the optical mode volume without adequately addressing light-matter interactions. Here, we demonstrate a nanolaser that colocalizes photons and excited carriers within a dielectric nanobridge. This extreme dielectric confinement of both light and matter yields a subdiffraction-limited mode volume and a subwavelength carrier volume without lateral quantum confinement. We observe a strong correlation between the mode field and carrier distribution, where enhanced mode localization produces stronger carrier confinement. By suppressing carrier surface recombination, this platform not only enables continuous-wave lasing at room temperature but also achieves a substantially reduced lasing threshold. We quantify the intensified interaction with an interaction volume, generalizing mode volume to a broad class of active media.

Dielectric optical cavities are emerging as viable platforms for efficiently concentrating light within extremely small volumes of sub-wavelength dimensions. This breaks with the notion that only plasmonic nanostructures can achieve this scale of confinement and enables strong light-matter interactions without the losses typically associated with metals. Here, we directly visualize the optical modes of a topology-optimized silicon bowtie nanocavity using multi-orientation electron energy-loss spectroscopy. Tomographic reconstruction of the resulting data sets reveals the three-dimensional profiles of several polarized optical modes in close agreement with simulations. A resonance near the telecom wavelength (∼1550 nm) is shown to be tightly localized at the bowtie bridge, confirming its deep sub-wavelength mode volume. These findings establish electron beam spectroscopy as a powerful tool for mapping three-dimensional field confinement in dielectric photonic cavities with potential applications in future photonic and quantum technologies.

Developments in semiconductor nanotechnology have allowed the experimental realization of a new generation of semiconductor lasers with cavity sizes on the scale of the optical wavelength or smaller. Such semiconductor nanolasers present new opportunities in information technology with extremely low energy consumption, e.g. for on-chip optical communications. As the characteristic dimensions of the laser shrink to the nanoscale, assumptions that hold well for macroscopic semiconductor lasers must be revisited. The paper presents recent progress on semiconductor nanolasers, specifically emphasizing three topics: photonic crystal nanolasers with ultra-low threshold, semiconductor lasers with deep subwavelength light confinement, and semiconductor Fano lasers.

Fiber-based long-haul quantum communication would greatly benefit from a robust and deterministically integrated source of quantum states. Here, we report the design, fabrication, and optical characterization of InAs/InP quantum dots in the InP H1 point-defect 2D photonic crystal cavity, integrated with a standard single-mode fiber employing microtransfer printing. The device is placed in a compact cryocooler maintaining a cryogenic temperature of 15 K, and it exhibits a low multiphoton contribution to emission, with g(2)(0) = 0.14(14) in the all-fiber-based optical link between two laboratory nodes. In this way, we demonstrate a plug-and-play all-fiber single-photon source operating in the third telecom window, where standard telecommunication fiber networks can be used as a low-loss medium.

Deterministic nanoscale emitters are much needed for scalable quantum technology and integrated photonics. Here, we experimentally investigate ultra-small buried heterostructures (BHs) with in-plane sizes down to 20x20 nm. The BH platform has proven its value in laser applications [1]. Recently, a 400 by 440 nm BH enabled a record-low threshold in a nanolaser [2].

We demonstrate the first bidirectional 1310 nm MEMS VCSELs with MHz-range sweep rates, fabricated on a silicon platform by wafer-bonding InP epitaxy onto a processed and polished SOI wafer. Two device configurations are analyzed, projecting a tuning range exceeding 100 nm, making them highly suitable for optical coherence tomography applications. A semiconductor cavity dominant MEMS VCSEL, fabricated via vacuum bonding, achieved a continuous tuning range of 58 nm at a resonant frequency of 2.4 MHz. In comparison, a device incorporating an anti-reflection coating at the bonding interface achieved a continuous tuning range of 30.5 nm at a resonant frequency of 2.48 MHz.

Solid-state quantum emitters operating in the telecom wavelength range are pivotal for the development of scalable quantum information processing technologies. In this review, we provide a comprehensive overview of the state-of-the-art solid-state emitters of single photons targeting quantum information processing in the discrete-variable regime and telecom wavelength range. We focus on quantum dots, color centers, and erbium ion dopants, detailing their synthesis methods and their applications. The review addresses the strategies for the integration of these quantum emitters into photonic devices alongside the associated challenges. We also discuss their applications in quantum technologies, examining current limitations, including performance constraints, decoherence, and scalability. Finally, we propose future directions for advancing photonic-based quantum technologies.

Surface effects can significantly impact the performance of nanophotonic and quantum photonic devices, especially as the device dimensions are reduced. In this work, we propose and investigate a novel approach to surface passivation to mitigate these challenges in photonic nanostructures with III-As(P) quantum wells defined by a dry etching process. The nanostructures are annealed under the phosphine (PH_3) ambient inside a metal-organic vapor phase epitaxy chamber to eliminate surface and subsurface defects induced during the dry etching and subsequent oxidation of the etched sidewalls. Moreover, encapsulation of the active material with a wider bandgap material allows for maintaining the band structure of the device, mitigating band bending effects. Our findings reveal an almost order of magnitude reduction in the surface recombination velocity from 2 × 10^3 cm/s for the PH_3 annealing compared to 1.5 × 10^4 cm/s for the non-passivated structures and 5 × 10^3 cm/s for the standard method based on (NH_4)_2S wet treatment followed by Al_2O_3 encapsulation. A further reduction to 5 × 10^2 cm/s is achieved for the InP-regrown samples. Additionally, we develop a model accounting for the impact of surface charges in the analysis of time-resolved photoluminescence curves and demonstrate that the proposed passivation method effectively reduces the surface charge density on the sidewalls of the studied quantum well-based photonic nanostructures.

We experimentally demonstrate continuous-wave lasing at room temperature in a topology-optimized InP nanolaser with extreme dielectric confinement.

Fiber-based long-haul quantum communication would greatly benefit from a robust and deterministically integrated source of quantum state. Here, we report the design, fabrication, and optical characterization of InAs/InP quantum dots in the InP H1 point defect 2D photonic crystal cavity, integrated with the standard single-mode fiber using a micro-transfer printing technique. The device was placed in a compact cryocooler maintaining a cryogenic temperature of 15 K with single-photon emission characterized by g^(2)(0)=0.14(14) and reliable and stable emission (intensity fluctuations given by a standard deviation σ = 0.13), so that an all-fiber based connection between two laboratory nodes through an open area was established and utilized for testing the quantum channel. In this way, we demonstrate a plug-and-play all-fiber single-photon source operating in the third telecom window, where standard telecommunication fiber networks can be used as a low-loss medium.

In our paper, the design of a high contrast grating is optimized to obtain a wider reflectivity bandwidth. HCGs present an incidence angle dependence that can affect the reflectivity bandwidth. This dependence was mitigated by reducing the duty cycle of the HCG. A further optimized model was done by matching the reflectivity phase with the phase of the incident Gaussian wavefront. The broader bandwidth of the new designs was calculated using simulations. Subsequently the MEMS VCSELs with the new designs of HCG were fabricated. The highest tuning range obtained for the focusing design was 45nm and for the low duty cycle design 38nm. These ranges are lower than the standard design due to differences from the design. Further improvements in the fabrication process are required to demonstrate the new designs proposed.

Quantum information processing with photons in small-footprint and highly integrated silicon-based photonic chips requires incorporating non-classical light sources. In this respect, self-assembled III-V semiconductor quantum dots (QDs) are an attractive solution, however, they must be combined with the silicon platform. Here, by utilizing the large-area direct bonding technique, we demonstrate the hybridization of InP and SOI chips, which allows for coupling single photons to the SOI chip interior, offering cost-effective scalability in setting up a multi-source environment for quantum photonic chips. We fabricate devices consisting of self-assembled InAs QDs embedded in the tapered InP waveguide (WG) positioned over the SOI-defined Si WG. Focusing on devices generating light in the telecom C-band compatible with the low-loss optical fiber networks, we demonstrate the light coupling between InP and SOI platforms by observing photons outcoupled at the InP-made circular Bragg grating outcoupler fabricated at the end of an 80 µm-long Si WG, and at the cleaved edge of the Si WG. Finally, for a device with suppressed multi-photon generation events exhibiting 80% single photon generation purity, we measure the photon number outcoupled at the cleaved facet of the Si WG. We estimate the directional on-chip photon coupling between the source and the Si WG to 5.1%.

We experimentally demonstrate an InP nanocavity with a mode volume of 0.26 (λ/2n)3. This is an order of magnitude smaller than the mode volumes previously demonstrated in photonic crystal point-defect cavities realized in III-V materials and four times smaller than what is often referred to as the diffraction-limited volume, Vλ= (λ/2n)3. The nanocavity is designed using topology optimization, taking into account fabrication limitations, which are pushed compared to the state-of-the-art. This work thus introduces a new class of cavities featuring extreme dielectric confinement (EDC) into the realm of III-V semiconductors, offering order-of-magnitude Purcell-enhancement of the radiative rate. EDC nanocavities may thus be employed to significantly improve the properties of nanolasers, nanoLEDs and single-photon sources, among other applications.