arXiv subjects

Grzegorz Sęk

Publications and source records attributed to Grzegorz Sęk.

At least 19 recordsLinked to original sources

arXiv2026

Optimization of InAs/InAlGaAs/InP quantum dots for O-band laser applications

The O-band telecommunication spectral range combines minimal chromatic dispersion with low transmission loss, making it highly suitable for optical communications. Semiconductor telecom lasers utilizing low-dimensional active regions, such as quantum dots, offer low threshold power and high thermal stability. However, long-wavelength quantum dot emitters often suffer from thermal carrier escape through excited states when the energy separation between the ground and excited states is insufficient. Here we demonstrate the optimization of InAs quantum dot emitters grown on InP by molecular-beam epitaxy for O-band laser active regions. By optimizing the design for better carrier capture efficiency and tuning the emission wavelength to the O-band, we show ways of improving the carrier confinement. Absorption spectroscopy on the finalized structure confirms substantial energy separation between the quantum dot ground and excited states, effectively suppressing thermal carrier escape even at elevated temperatures. These results highlight critical design for improving carrier capture and retention in long-wavelength quantum dot lasers.

cond-mat.mtrl-sci↗

arXiv2026

Emission dynamics in zincblende InAsxP1-x quantum dots in InP nanowires: influence of quantum dot size, composition and nanowire geometry

Hereby, we present an experimental and theoretical investigation of emission dynamics in zincblende InAsxP1-x quantum dots (QDs) embedded in InP nanowires (NWs) grown via vapour-liquid-solid mechanism by chemical beam epitaxy, using Au nanoparticles as a nucleation catalyst. By measuring time-resolved photoluminescence from an ensemble of QD-NWs it was possible to determine the exciton lifetime dependence on QD composition and height. Changes in the InP shell thickness surrounding the InP NW stem with a QD, brought additional insight into the influence of photonic environment on the carrier dynamics. High-resolution transmission electron microscopy, combined with energy-dispersive X-ray spectroscopy, provided actual structural parameters. The experimentally obtained lifetimes were interpreted in the light of results of 8 band kp calculations combined with configuration-interaction model to take into account the Coulomb interactions and finite-difference time domain photonic simulations to include the effect of optical confinement. The full understanding of the experimental results required considering both, the changes in the QD potential and the Purcell effect, the latter leading to spontaneous emission inhibition in the case of NWs with thin InP shell.

cond-mat.mes-hall↗

arXiv2026

A Fiber-pigtailed Quantum Dot Device Generating Indistinguishable Photons at GHz Clock-rates

Solid-state quantum light sources based on semiconductor quantum dots (QDs) are increasingly employed in photonic quantum information applications. Especially when moving towards real-world scenarios outside shielded lab environments, the efficient and robust coupling of nanophotonic devices to single-mode optical fibers offers substantial advantage by enabling "plug-and-play" operation. In this work we present a fiber-pigtailed cavity-enhanced source of flying qubits emitting single indistinguishable photons at clock-rates exceeding $1\,$GHz. This is achieved by employing a fully deterministic technique for fiber-pigtailing optimized QD-devices based on hybrid circular Bragg grating (hCBG) micro-cavities. The fabricated fiber-pigtailed hCBGs feature radiative emission lifetimes of $<80\,$ps, corresponding to a Purcell factor of $\sim$9, a suppression of multi-photon emission events with $g^{(2)}(0)<1\%$, a photon-indistinguishability $>80\%$ and a measured single-photon coupling efficiency of 53$\%$ in a high numerical aperture single-mode fiber, corresponding to 1.2 Megaclicks per second at the single-photon detectors under $80\,$MHz excitation clock-rates. Furthermore, we show that high multi-photon suppression and indistinguishability prevail for excitation clock-rates exceeding $1\,$GHz. Our results show that Purcell-enhanced fiber-pigtailed quantum light sources based on hCBG cavities are a prime candidate for applications of quantum information science.

quant-ph↗

arXiv2025

Generation of C-band entangled photon pairs by biexciton-exciton cascade from symmetric InAs/InP quantum dots

Hereby, we study the generation of pairs of polarization-entangled photons at telecom C-band by biexciton-exciton cascade from non-resonantly excited epitaxial InAs/InP quantum dots (QDs). It is realized without external tuning of the fine structure splitting (FSS), which does not exceed 10 $μ$eV in as-grown nanostructures, due to their high in-plane symmetry. Excitonic complexes are identified by means of excitation power-dependent and polarization-resolved magneto)microphotoluminescence. Their origin from different carrier configurations confined in the same QD is confirmed by time correlations of emitted photons. Experimental results are supported by 8-band kp calculations, followed by the configuration interaction method to include excitonic effects. This comparison reveals the structure of higher energy states, allowing for the reconstruction of the QD structural parameters. To verify and quantify the entanglement, we perform quantum state tomography and reconstruct the two-photon density matrix. Its diagonalization allows a detailed analysis of the entangled state and reveals an unfavourable interplay between phase accumulation and decoherence, pointing to a clear route for boosting entanglement by using a XX resonant, pulsed excitation scheme and shortening the radiative lifetimes.

quant-ph↗

arXiv2025

High-reflectivity homoepitaxial distributed Bragg reflectors for photonic applications

Distributed Bragg reflectors (DBRs) are one of the basic photonic structures used to define microcavities for fundamental light-matter coupling studies, as well as to optimize performance of optoelectronic and photonic devices, e.g., lasers or non-classical light sources. The reflectivity of these structures depends critically on the refractive index contrast between the two quarter-wavelength thick layers constituting the DBR. At the same time, epitaxial fabrication process limits the choice of materials to those with the same, or very similar lattice constant to avoid strain accumulation in the relatively thick multilayer structure. This becomes very often a bottleneck for the DBR designs at certain wavelengths or for some of the material systems. Therefore, we explore theoretically DBR designs employing the reflective index contrast between undoped and doped layers of the same material, making the entire growth process homoepitaxial. The refractive index for a doped layer is calculated taking into account the free carrier absorption, carrier-carrier interaction, the Burnstein-Moss and plasma effects. The reflectivity spectrum of a DBR is further calculated using transfer matrix method. Exemplary results for three application relevant materials - hBN, InP and Si suitable for different spectral ranges, i.e. ultraviolet, telecommunication and mid-infrared, respectively, are presented. We report reflectivities on the level of 90\% for technologically achievable doping concentrations and moderate number of layer pairs.

cond-mat.other↗

arXiv2025

Monolithic Integration of Sub-50 nm III-V Nano-Heterostructures on Si (001) for Telecom Photonics

The demand for advanced photonics technology is increasing rapidly, fueled by the necessity for high-performance and 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 has been proven essential to overcome this drawback. These materials are recognized for their efficient light emission and superior bandgap engineering capabilities, making them indispensable in photonics and beyond. Here, we present the monolithic integration of small-volume III-V nanoheterostructures with silicon via selective area epitaxy in the pyramidal openings etched in (100)-oriented silicon substrate. The precise positioning of the nano-heterostructures is achieved using electron beam lithography. Our atomic resolution imaging and chemical analysis confirms 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 was analyzed using advanced electron microscopy. The 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. Consequently, these InAsP/InP nano-heterostructures could serve as a gain medium for silicon-based hybrid nano-lasers and nano-LEDs and quantum light sources in the telecom wavelength range.

physics.optics↗

arXiv2024

Xenon plasma-focused ion beam milling as a method to deterministically fabricate bright and high-purity single-photon sources operating at C-band

Electron beam lithography is a standard method for fabricating photonic nanostructures around semiconductor quantum dots (QDs), which are crucial for efficient single and indistinguishable photon sources in quantum information processing. However, this technique lacks direct 3D control over the nanostructure shape, complicating the design and enlarging the 2D footprint to suppress in-plane photon leakage while directing photons into the collecting lens aperture. Here, we present an alternative approach to employ Xenon plasma-focused ion beam (Xe-PFIB) technology as a reliable method for the 3D shaping of photonic nanostructures containing low-density self-assembled InAs/InP quantum dots emitting in the C-band range of the 3rd telecommunication window. We explore both deterministic and non-deterministic fabrication approaches, resulting in mesas naturally shaped as truncated cones. As a demonstration, we fabricate mesas using a heterogeneously integrated structure with a QDs membrane atop an aluminum mirror and silicon substrate. Finite-difference time-domain (FDTD) simulations show that the angled sidewalls significantly increase photon extraction efficiency, achieving η = 0.89 for NA = 0.65. We demonstrate experimentally a high purity of pulsed single-photon emission (~99%) and a high extraction efficiency of η = 0.24, with the latter surpassing the highest reported values obtained using electron beam lithography in the C-band.

physics.optics↗

arXiv2024

Effects of dislocation filtering layers on optical properties of third telecom window emitting InAs/InGaAlAs quantum dots grown on silicon substrates

Integrating light emitters based on III-V materials with silicon-based electronics is crucial for further increase in data transfer rates in communication systems since the indirect bandgap of silicon prevents its direct use as a light source. We investigate here InAs/InGaAlAs quantum dot (QD) structures grown directly on 5° off-cut Si substrate and emitting light at 1.5 micrometers, compatible with established telecom platform. Using different dislocation defects filtering layers, exploiting strained superlattices and supplementary QD layers, we mitigate the effects of lattice constant and thermal expansion mismatches between III-V materials and Si during growth. Complementary optical spectroscopy techniques, i.e. photoreflectance and temperature-, time- and polarization- resolved photoluminescence, allow us to determine the optical quality and application potential of the obtained structures by comparing them to a reference sample -- state-of-the-art QDs grown on InP. Experimental findings are supported by calculations of excitonic states and optical transitions by combining multiband k.p and configuration-interaction methods. We show that our design of structures prevents the generation of a considerable density of defects, as intended. The emission of Si-based structures appears to be much broader than for the reference dots, due to the creation of different QD populations which might be a disadvantage in particular laser applications, however, could be favourable for others, e.g. in broadly tunable devices, sensors, or optical amplifiers. Eventually, we identify the overall most promising combination of defect filtering layers, discuss its advantages and limitations, and prospects for further improvements.

cond-mat.mes-hall↗

arXiv2023

Impact of MBE-grown (In,Ga)As/GaAs metamorphic buffers on excitonic and optical properties of single quantum dots with single-photon emission tuned to the telecom range

Tuning GaAs-based quantum emitters to telecom wavelengths makes it possible to use the existing mature technology for applications in, e.g., long-haul ultra-secure communication in the fiber networks. A promising method re-developed recently is to use a metamorphic InGaAs buffer that redshifts the emission by reducing strain. However, the impact of such a buffer causes also a simultaneous modification of other quantum dot properties. Knowledge of these effects is crucial for actual implementations of QD-based non-classical light sources for quantum communication schemes. Here, we thoroughly study single GaAs-based quantum dots grown by molecular-beam epitaxy on specially designed, digital-alloy InGaAs metamorphic buffers. With a set of structures varying in the buffer indium content and providing quantum dot emission through the telecom spectral range up to 1.6 $μ$m, we analyze the impact of the buffer and its composition on QD structural and optical properties. We identify the mechanisms of quantum dot emission shift with varying buffer composition. We also look into the charge trapping processes and compare excitonic properties for different growth conditions with single-dot emission successfully shifted to both, the second and the third telecom windows.

cond-mat.mes-hall↗

arXiv2023

Heterogeneous integration of single InAs/InP quantum dots with the SOI chip using direct bonding

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 $μ$m-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%.

physics.optics↗

arXiv2023

Crossover from exciton-polariton condensation to photon lasing in an optical trap

Optical trapping has been proven to be an effective method of separating exciton-polariton condensates from the incoherent high-energy excitonic reservoir located at the pumping laser position. This technique has significantly improved the coherent properties of exciton-polariton condensates, when compared to a quasi-homogeneous spot excitation scheme. Here, we compare two experimental methods on a sample, where a single spot excitation experiment allowed only to observe photonic lasing in the weak coupling regime. In contrast, the ring-shaped excitation resulted in the two-threshold behavior, where an exciton-polariton condensate manifests itself at the first and photon lasing at the second threshold. Both lasing regimes are trapped in an optical potential created by the pump. We interpret the origin of this confining potential in terms of repulsive interactions of polaritons with the reservoir at the first threshold and as a result of the excessive free-carrier induced refractive index change of the microcavity at the second threshold. This observation offers a way to achieve multiple phases of photonic condensates in samples, e.g., containing novel materials as an active layer, where two-threshold behavior is impossible to achieve with a single excitation spot.

physics.optics↗

arXiv2023

Optical characteristics of cavity structures with Al0.2Ga0.8As/Al0.9Ga0.1As distributed Bragg reflectors and In0.37Ga0.63As quantum dots as the active region

We characterized optically In0.37Ga0.63As/GaAs quantum dots and Al0.2Ga0.8As/Al0.9Ga0.1As distributed Bragg reflector based cavities. The main mechanisms of quantum dots photoluminescence quenching were identified. We measured reflectivity spectra of the cavity structure which allowed us to verify the proposed layer design and its fabrication by comparing them with the results of simulations within the transfer matrix method.

physics.app-ph↗

arXiv2022

A Quantum Key Distribution Testbed using a Plug&Play Telecom-wavelength Single-Photon Source

Deterministic solid-state quantum light sources are considered key building blocks for future communication networks. While several proof-of-principle experiments of quantum communication using such sources have been realized, most of them required large setups often involving liquid helium infrastructure or bulky closed-cycle cryotechnology. In this work, we report on the first quantum key distribution (QKD) testbed using a compact benchtop quantum dot single-photon source operating at telecom wavelengths. The plug\&play device emits single-photon pulses at O-band wavelengths ($1321\,$nm) and is based on a directly fiber-pigtailed deterministically-fabricated quantum dot device integrated into a compact Stirling cryocooler. The Stirling is housed in a 19-inch rack module including all accessories required for stand-alone operation. Implemented in a simple QKD testbed emulating the BB84 protocol with polarization coding, we achieve an antibunching of $g^{(2)}(0) = 0.10\pm0.01$ and a raw key rate of up to $(4.72\pm0.13)\,$kHz using an external pump laser. In this setting, we further evaluate the performance of our source in terms of the quantum bit error ratios, secure key rates, and tolerable losses expected in full implementations of QKD also accounting for finite key size effects. Furthermore, we investigate optimal settings for a two-dimensional temporal acceptance window applied on receiver side, resulting in predicted tolerable losses up to $23.19\,$dB. Not least, we compare our results with previous proof-of-concept QKD experiments using quantum dot single-photon sources. Our study represents an important step forward in the development of fiber-based quantum-secured communication networks exploiting sub-Poissonian quantum light sources.

quant-ph↗

arXiv2020

Thermal stability of emission from single InGaAs/GaAs quantum dots at the telecom O-band

Single-photon sources are key building blocks in most of the emerging secure telecommunication and quantum information processing schemes. Semiconductor quantum dots (QD) have been proven to be the most prospective candidates. However, their practical use in fiber-based quantum communication depends heavily on the possibility of operation in the telecom bands and at temperatures not requiring extensive cryogenic systems. In this paper we present a temperature-dependent study on single QD emission and single-photon emission from metalorganic vapour-phase epitaxy-grown InGaAs/GaAs QDs emitting in the telecom O-band. Micro-photoluminescence studies reveal that trapped holes in the vicinity of a QD act as reservoir of carriers that can be exploited to enhance photoluminescence from trion states observed at elevated temperatures up to at least 80 K. The luminescence quenching is mainly related to the promotion of holes to higher states in the valence band and this aspect must be primarily addressed in order to further increase the thermal stability of emission. Photon autocorrelation measurements yield single photon emission with a purity of $g_{50\mathrm{K}}^{(2)}\left(0\right)=0.13$ up to 50 K. Our results imply that these nanostructures are very promising candidates for single-photon sources at elevated temperatures in the telecom O-band and highlight means for improvements in their performance.

cond-mat.mes-hall↗

arXiv2020

Optical and electronic properties of symmetric InAs/InGaAlAs/InP quantum dots formed by a ripening process in molecular beam epitaxy: a promising system for broad-range single-photon telecom emitters

We present a detailed experimental optical study supported by theoretical modeling of InAs quantum dots (QDs) embedded in an InAlGaAs barrier lattice-matched to InP(001) grown with the use of a ripening step in molecular beam epitaxy. The method leads to the growth of in-plane symmetric QDs of low surface density, characterized by a multimodal size distribution resulting in a spectrally broad emission in the range of $1.4-2.0$ $μ$m, essential for many near-infrared photonic applications. We find that, in contrast to the InAs/InP system, the multimodal distribution results here from a two-monolayer difference in QD height between consecutive families of dots. This may stem from the long-range ordering in the quaternary barrier alloy that stabilizes QD nucleation. Measuring the photoluminescence (PL) lifetime of the spectrally broad emission, we find a nearly dispersionless value of $1.3\pm0.3$ ns. Finally, we examine the temperature dependence of emission characteristics. We underline the impact of localized states in the wetting layer playing the role of carrier reservoir during thermal carrier redistribution. We determine the hole escape to the InAlGaAs barrier to be a primary PL quenching mechanism in these QDs.

cond-mat.mes-hall↗

arXiv2020

Observation of gain-pinned dissipative solitons in a microcavity laser

We demonstrate an experimental approach to create dissipative solitons in a microcavity laser. In particular, we shape the spatial gain profile of a quasi-one-dimensional microcavity laser with a nonresonant, pulsed optical pump to create spatially localised structures, called gain-pinned dissipative solitons that exist due to the balance of gain and nonlinear losses and are confined to a diffraction-limited volume. The ultrafast formation dynamics and decay of the gain-pinned solitons are probed directly, showing that they are created on a picosecond timescale, orders of magnitude faster than laser cavity solitons. All of the experimentally observed features and dynamics are reconstructed by using a standard complex Ginzburg-Landau model.

physics.optics↗

arXiv2020

Interplay between emission wavelength and s-p splitting in MOCVD-grown InGaAs/GaAs quantum dots emitting above 1.3 μm

The electronic structure of strain-engineered single InGaAs/GaAs quantum dots emitting in the telecommunication O band is probed experimentally by photoluminescence excitation spectroscopy. Observed resonances can be attributed to p-shell states of individual quantum dots. The determined energy difference between s-shell and p-shell shows an inverse dependence on the emission energy. The experimental data are compared with the results of confined states calculations, where the impact of the size and composition in the investigated structures is simulated within the 8-band $\mathbf{k}\cdot\mathbf{p}$ model. On this basis, the experimental observation is attributed mainly to changes in indium content within individual quantum dots, indicating a way of engineering and selecting a desired quantum dot, whose electronic structure is the most suitable for a given nanophotonic application.

cond-mat.mes-hall↗

arXiv2019

Excitonic complexes in MOCVD-grown InGaAs/GaAs quantum dots emitting at telecom wavelengths

Hereby, we present a comprehensive experimental and theoretical study of the electronic structure and optical properties of excitonic complexes in strain-engineered InGaAs/GaAs quantum dots (QDs) grown by metal-organic chemical vapour deposition and emitting at the 1300 nm telecommunication window. Single QD properties have been determined experimentally for a number of nanostructures by means of excitation-power-dependent and polarization-resolved microphotoluminescence and further compared with the results of confined states calculations employing the 8-band kp theory combined with the configuration interaction method. The origin of excitonic complexes has been exemplarily confirmed based on magnetooptical and correlation spectroscopy study. Understanding the influence of structural parameters and compositions (of QDs themselves as well as in the neighbouring strain reducing layer) allows to distinguish which of them are crucial to control the emission wavelength to achieve the telecommunication spectral range or to affect binding energies of the fundamental excitonic complexes. The obtained results provide deeper knowledge on control and on limitations of the investigated structures in terms of good spectral isolation of individual optical transitions and the spatial confinement that are crucial in view of QD applications in single-photon sources of high purity at telecom wavelengths.

cond-mat.mes-hall↗