arXiv subjects

Joonas Hilska

Publications and source records attributed to Joonas Hilska.

13 recordsLinked to original sources

arXiv2026

Temperature Dependence of the Refractive Index for AlAsGaSb

Accurate design and optimization of photonic multilayer structures like distributed Bragg reflectors (DBRs) require precise knowledge of material optical constants, particularly the temperature dependence of the refractive index. While these parameters are well established for widely used semiconductors, for emerging materials such as antimonides they are often limited to room-temperature data, especially for new spectral ranges of interest. Antimonide compounds, in particular GaSb-based alloys, are promising for quantum photonics applications. In this work, we investigated DBRs lattice-matched to GaSb and designed for operation in the third telecommunication window. Reflectivity spectra were measured in the temperature range from 11.5 K to 300 K, and then fitted using the transfer matrix method (TMM), combined with a dedicated recursive numerical fitting algorithm. Initial parameters included layer thicknesses determined by scanning electron microscopy (SEM) and literature values of refractive indices at room temperature. This approach enabled extraction of the temperature-dependent refractive indices of two AlGaAsSb alloys suitable for forming DBR mirrors for 1.5 um wavelengths. The obtained results provide essential input for reliable DBR design, ensuring proper stopband positioning and high reflectivity under cryogenic operating conditions required for efficient quantum emitter performance.

cond-mat.other↗

arXiv2026

Predictive wavelength tailoring of uniform GaSb-based quantum dots for emission at 1.55 um

A detailed study of emission wavelength tailoring of GaSb-based QDs formed by InGaSb-filling of droplet-etched nanoholes in AlGaSb is presented. The study shows that the emission wavelength can be modified from 1.48 um to the center of the telecom C-band at 1.55 mm by independently varying the QD composition and size. More specifically, the optical transition energy shifts linearly as a function of In-content of the QD material at a rate of -4.4 meV/In-percentage, and with the number of monolayers (ML) of material used for filling the nanoholes, at -2.0 meV/ML. These experimentally observed energy shifts are well predicted by simulations yielding rates of -4.3 meV/In-percentage and -2.1 meV/ML, respectively. For the simulation, a uniform In composition, low intermixing, and microscopically measured QD geometry is considered. Additionally, excellent ensemble QD uniformity, with unprecedented inhomogeneous broadening well-below 7 meV across all samples is demonstrated. Finally, photoluminescence of single-QDs reveals narrow excitonic emission lines of 13.8+/-6.7 ueV and low fine-structure splitting values reaching <10 ueV. These results identify GaSb-based LDE QDs as a tunable telecom platform for scaling quantum-photonic applications over long-haul optical fiber networks.

cond-mat.mes-hall↗

arXiv2026

Telecom wavelength single-photon emission from quasi-resonantly excited InGaSb/AlGaSb quantum dots

Deterministic light sources capable of generating quantum states on-demand at wavelengths compatible with fiber optics and atmospheric transmission windows are essential for practical applications in quantum communication, distributed photonic quantum computing, and quantum metrology. Currently, the technology providing semiconductor quantum emitters with the most promising properties is based on filling droplet-etched nanoholes to form quantum dots (QDs). However, the standard GaAs/AlGaAs material system does not offer telecom window emission. Here, we combine this growth method with antimonide-based materials to demonstrate single-photon emission at 1500 nm from a droplet-etched InGaSb QD. Our device with an antimony-based high refractive index contrast back-reflector designed for cryogenic operation and a solid immersion lens improves photon extraction. QD states are protected by a potential barrier limiting the influx of surrounding carriers, which however prevents revealing excitonic fine structure under nonresonant excitation. In this work, we employ a frequency-tunable continuous wave laser to achieve longitudinal optical (LO) phonon-assisted excitation of the QD ground state and resonant excitation of an excited state. These direct approaches for exciting a single InGaSb QD unlock access to its excitonic fine structure. The typical neutral biexciton-exciton cascade exhibits a negative binding energy of 1.4 meV (2.6 nm) and a fine structure splitting of 24.1+/-0.4 ueV. Furthermore, we obtain spectrally isolated emission from a charged exciton with a multi-photon probability of 5 % with LO phonon-assisted two-color excitation. These results represent a major step towards using this novel antimonide-based QD emitters as deterministic quantum light sources in complex quantum secure networks exploiting the wavelength compatibility with standard telecom fibers.

cond-mat.mes-hall↗

arXiv2026

Time-domain optical coherence tomography at 2 $μ\mathrm{m}$ using GaSb-based broadband superluminescent diode

We report a time-domain optical coherence tomography (TD-OCT) system operating in the 2 $μ\mathrm{m}$ spectral region, enabled by a GaSb-based superluminescent diode (SLD). The spectrum emitted by the SLD exhibits a full-width half-maximum (FWHM) of $\sim$80 nm centred near 2.1 $μ\mathrm{m}$. For OCT operation, stable amplified spontaneous emission with low spectral ripple ($<20\%$) is maintained at drive currents below 150 mA. The SLD is fiber coupled and integrated into a fiber-based Michelson interferometer. In the OCT system, the measured coherence envelope yields an axial resolution of approximately 300 $μ$m in air and enables depth-resolved imaging of scattering paint-based coating samples. In contrast to OCT implementations at 2 $μ\mathrm{m}$ wavelength region that commonly rely on supercontinuum sources, the use of GaSb-based SLDs offers a compact practical alternative, leveraging the maturity and scalability of electrically driven semiconductor light sources packaged in a standard "butterfly" module. This report represents the first demonstration of TD-OCT imaging at 2 $μ\mathrm{m}$ using a GaSb-based SLD source and establishes its suitability for compact and scalable mid-IR OCT instrumentation targeting non-biological, low-water-content materials.

physics.optics↗

arXiv2025

Design of broadband optical gain in GaSb-based waveguide amplifiers with asymmetric quantum wells

A design strategy for achieving broadband optical gain in GaSb-based semiconductor amplifiers operating beyond 2 μm is presented. By employing asymmetric GaInSb/AlGaAsSb quantum wells (QWs) of varying thicknesses, a flat and wide gain spectrum is demonstrated. The approach leverages carrier density and transition energy tuning across QWs to access various energy levels at specific current densities. Simulations using "Harold" self-consistent environment predict a full-width at half-maximum (FWHM) gain bandwidth exceeding 340 nm for a structure comprising one 7 nm and three 13 nm-thick QWs. The modelling parameters were validated against experimental data, ensuring a robust framework for designing broadband amplifiers and superluminescent diodes for mid-infrared applications.

physics.optics↗

arXiv2025

Fast Recovery Dynamics of GaSbBi-based SESAMs for high-fluence operation

Modelocked lasers operating at 2-3 um wavelength region are interesting for various spectroscopic applications. To this end, GaSb-based semiconductor saturable absorber mirrors (SESAMs) are developing fast as a practical technology for passive modelocking. Yet, such SESAMs suffer from either too high two-photon absorption or slow absorption recovery dynamics. This study introduces GaSbBi quantum wells (QWs) as a novel platform to ensure a larger material selection for engineering GaSb-based SESAMs with decreased two-photon absorption and ultrafast absorption recovery time. Three GaSbBi QW SESAM designs were fabricated to compare their performance against conventional GaInSb QW SESAMs. The first structure makes use of typical GaSb barriers and exhibits comparable characteristics to the conventional design, including a saturation fluence of 1.09 uJ/cm^2, modulation depth of 1.41%, and a fast interband recovery time of 6.03 ps. The second design incorporated AlAs0.08Sb0.92 barriers, achieving reduced two-photon absorption, though at the cost of higher non-saturable losses due to unintended Bi droplet formation during growth of the AlAs0.08Sb0.92/GaSbBi QW heterostructure. Importantly, it maintained a fast interband recovery time (30 ps), overcoming the slow recovery dynamics exhibited by standard GaInSb QW SESAMs with AlAs0.08Sb0.92 barriers. The third design explored GaSbBi QWs with higher Bi content targeted for longer wavelength operation at 2.3 um, which exhibited fast recovery times and good nonlinear reflectivity characteristics. However, the higher Bi content resulted in elevated non-saturable losses. These results highlight the potential of GaSbBi QWs for SWIR SESAMs, opening the path for further epitaxial optimization to enhance their performance.

physics.optics↗

arXiv2024

Electronic properties of metamorphic GaSbBi films on GaAs

We report on the electronic, structural, and optical properties of epitaxial GaSbBi films with varying Bi-concentration (up to 7%Bi) grown on semi-insulating GaAs(100) substrates. The 1 $μ$m thick GaSbBi epilayers exhibit fully relaxed narrow X-ray diffraction peaks and smooth surface morphology comparable to that of high-quality GaSb epilayers on GaAs. Low temperature photoluminescence spectra exhibit band gap shrinkage consistent with Bi alloying. Electrical Hall measurements indicate reduction of hole concentration and no change in the hole mobilities with increasing Bi content for the nominally undoped GaSbBi alloy. The residual hole concentration reduces from $10^{18} cm^{-3}$ level for a reference GaSb sample, to low $10^{17} cm^{-3}$ level with increasing Bi content. Hole mobility values of around $300 cm^{2}/Vs$ are observed independent of the Bi content. These dependencies are attributed to the Bi surfactant effect and Bi-induced defect formation.

cond-mat.mtrl-sci↗

arXiv2023

Electronic structure of GaSb/AlGaSb quantum dots formed by filling droplet-etched nanoholes

Epitaxially-grown semiconductor quantum dots (QDs) provide an attractive platform for the development of deterministic sources of high-quality quantum states of light. Such non-classical light sources are essential for quantum information processing and quantum communication. QDs emitting in the telecom wavelengths are especially important for ensuring compatibility with optical fiber systems required to implement quantum communication networks. To this end, GaSb QDs fabricated by filling local-droplet etched nanoholes are emerging as a viable approach, yet the electronic properties of such nanostructures have not been studied in detail. In this article, an insight into the electronic structure and carrier dynamics in GaSb/AlGaSb QDs is provided through a systematic experimental analysis of their temperature-dependent photoluminescence behavior. A steady-state rate equation model is used to reveal the relevant energy barriers for thermally activated carrier capture and escape processes. Furthermore, results of detailed theoretical simulations of quantum-confined energy states using the multi-band k.p model and the effective mass method are presented. The purpose of the simulations is to reveal the direct and indirect energy states, carrier wavefunctions, and allowed optical transitions for GaSb QDs with different physical dimensions.

cond-mat.mes-hall↗

arXiv2023

Strain-free GaSb quantum dots as single-photon sources in the telecom S-band

Creating single photons in the telecommunication wavelength range from semiconductor quantum dots (QDs) and interfacing them with spins of electrons or holes has been of high interest in recent years, with research mainly focusing on indium based QDs. However, there is not much data on the optical and spin properties of galliumantimonide (GaSb) QDs, despite it being a physically rich system with an indirect to direct bandgap crossover in the telecom wavelength range. Here, we investigate the (quantum-) optical properties of GaSb quantum dots, which are fabricated by filling droplet-etched nanoholes in an aluminum-galliumantimonide (AlGaSb) matrix. We observe photoluminescence (PL) features from isolated and highly symmetric QDs that exhibit narrow linewidth in the telecom S-band and show an excitonic fine structure splitting of $ΔE=(12.0\pm0.5)μeV$. Moreover, we perform time-resolved measurements of the decay characteristics of an exciton and measure the second-order photon autocorrelation function of the charge complex to $g^{(2)}(0)=0.16\pm0.02$, revealing clear antibunching and thus proving the capability of this material platform to generate non-classical light.

cond-mat.mes-hall↗

arXiv2022

Widely tunable 2 $μ$m hybrid laser using GaSb semiconductor optical amplifiers and Si3N4 photonics integrated reflector

Tunable lasers emitting at a 2-3 $μ$m wavelength range and compatible with photonic integration platforms are of great interest for sensing applications. To this end, combining GaSb-based semiconductor gain chips with Si$_3$N$_4$ photonic integrated circuits offers an attractive platform. Herein, we exploit the low-loss features of Si$_3$N$_4$ waveguides and demonstrate a hybrid laser comprising a GaSb gain chip with an integrated tunable Si$_3$N$_4$ Vernier mirror. At room temperature, the laser exhibited a maximum output power of 15 mW and a tuning range of 80 nm (1937-2017 nm). The low-loss performance of several fundamental Si$_3$N$_4$ building blocks for photonic integrated circuits is also validated. More specifically, the single-mode waveguide exhibit transmission loss as low as 0.15 dB/cm, the 90$^\circ$ bend has 0.008 dB loss, and the 50/50 Y-branch has an insertion loss of 0.075 dB.

physics.optics↗

arXiv2021

Highly uniform GaSb quantum dots with indirect-direct bandgap crossover at telecom range

We demonstrate a new quantum-confined semiconductor material based on GaSb quantum dots (QDs) embedded in single-crystalline AlGaSb matrix by filling droplet-etched nanoholes. The droplet-mediated growth mechanism allows formation of low QD densities required for non-classical single-QD light sources. The photoluminescence (PL) experiments reveal that the GaSb QDs have an indirect-direct bandgap crossover at telecom wavelengths. This is due to the alignment of the Γ and L valleys in the conduction band as a result of quantum confinement controlled by dimensions of the nanostructure. We show that in the direct bandgap regime close to 1.5 um wavelength, the GaSb QDs have a type I band alignment and exhibit excitonic emission with narrow spectral lines and very low inhomogeneous broadening of PL emission owing to the high material quality and dimensional uniformity. These properties are extremely promising in terms of applications in infrared quantum optics and quantum photonic integration.

cond-mat.mes-hall↗

arXiv2021

Nanohole etching in AlGaSb with Ga droplets

We demonstrate nanohole formation in AlGaSb by Ga droplet etching within a temperature range from 270°C to 500°C, allowing a wide range of tunability of the nanohole density. By leveraging the low vapor pressure of Sb, we can obtain high degree of control over droplet formation and nanohole etching steps and reveal the physics of adatom diffusion in these processes. Furthermore, by combining the experimental results and a geometric diffusion-based model, we can determine the temperature and Sb-flux-dependencies of the critical monolayer coverage of Sb atoms required for driving the droplet etching process to completion. These findings provide new insight into the droplet formation and etching process present in the droplet-mediated synthesis of semiconductor nanostructures and represent a significant step towards development of telecom-emitting quantum dots in the GaSb system.

physics.app-ph↗

arXiv2019

Epitaxial phases of high Bi content GaSbBi alloys

GaSbBi alloys have recently emerged as attractive materials for mid-infrared optoelectronics owing to strong band gap reduction enabled by Bi incorporation into the GaSb matrix. The fundamental understanding of the epitaxial process required to demonstrate high quality crystals is in an early-developmental phase. From this perspective, we report on the key role played by the Sb/Ga flux ratio in controlling the structural quality and incorporation of high Bi content (up to 14.5 %-Bi), revealing three distinct epitaxial phases. The first phase (below stoichiometric Sb/Ga) exhibits Ga-Bi compound droplets, low crystal quality, and reduced Bi content. At the second phase (above stoichiometric Sb/Ga), the crystal exhibits smooth surfaces and excellent crystallinity with efficient Bi incorporation. The last phase corresponds to exceeding a Sb/Ga threshold that leads to reduced Bi incorporation, Bi droplets and degraded crystallinity. This threshold value that defines the optimal growth window is controlled by the temperature as well as the Bi/Ga ratio. Increasing temperature increases the threshold, albeit simultaneously reducing Bi incorporation. Conversely, increasing the Bi/Ga flux ratio increases Bi incorporation, while narrowing down and ultimately closing the window. This study provides a general framework enabling development of high quality GaSbBi heterostructures for emerging mid-infrared optoelectronics.

physics.app-ph↗