arXiv subjects

S. Assali

Publications and source records attributed to S. Assali.

6 recordsLinked to original sources

arXiv2026

A Silicon-Compatible Uncooled Compact Broadband Infrared Spectrometer

Infrared spectroscopy is a widely used technique for molecular identification, yet its widespread deployment remains constrained by bulky instrumentation, cryogenic cooling requirements, and limited portability. Here we demonstrate broadband near- to short-wave infrared spectroscopy enabled by a silicon-compatible GeSn photodetector integrated into a compact digital micromirror device-based single-pixel architecture. By combining detector design with targeted optical reconfiguration and wavelength recalibration, we extend the operational range up to 2.4 micrometers while maintaining room-temperature, zero-bias photovoltaic operation. System-level benchmarking quantifies the intrinsic trade-off between spectral extension and detectivity in narrow-bandgap photodetection, yielding a specific detectivity of $1.12 \times 10^{10}$ cm Hz$^{1/2}$ W$^{-1}$ at 1.55 micrometers with an extended wavelength cut-off. We validate the capabilities of the obtained spectrometer through selective identification of representative commodity plastics, demonstrating a lightweight, low-power, and mechanically robust platform for portable and autonomous deployment. Despite the expected increase in generation-limited noise, access to strong polymer combination bands beyond 1.7 micrometers allows enhanced material discrimination. This work establishes a pathway toward scalable distributed molecular sensing and broadens access to infrared spectroscopy for environmental monitoring, industrial process control, and community-level chemical detection.

physics.optics↗

arXiv2026

Extending Silicon Avalanche Photodetection Beyond $2~\mu$m by Direct GeSn Integration

Silicon avalanche photodiodes provide a technologically mature platform for sensitive photodetection, but their spectral response is intrinsically limited by the silicon bandgap. Extending their operation into the infrared requires the integration of narrow-bandgap absorbers while preserving efficient avalanche multiplication and compatibility with silicon processing. Here, we propose and demonstrate a monolithic approach that combines direct, buffer-free growth of GeSn on silicon with a lateral thin-junction separate-absorption-multiplication architecture. The GeSn layer, with a Sn composition reaching 6 at.%, extends optical absorption to a wavelength of $2.6~\mu$m, while avalanche multiplication is spatially confined to an ion-implanted silicon lateral junction. This separation enables independent control of infrared absorption and carrier multiplication without the thick Ge virtual substrates conventionally used for GeSn epitaxy. GeSn-on-Si avalanche photodiodes exhibit low pre-breakdown dark current, stable breakdown at 72 V independent of device diameter, and clear infrared photoresponse extending beyond $2~\mu$m. At 78 K, the devices exhibit external quantum efficiency exceeding 100%, reaching 163% at $1.55~\mu$m, providing direct evidence of avalanche multiplication. These results establish direct integration of narrow-bandgap group-IV absorbers with silicon multiplication regions as a scalable strategy for extending the spectral reach of silicon avalanche photodetection, opening a route toward monolithic infrared detectors for sensing, imaging, communications, LiDAR, and quantum photonics

physics.optics↗

arXiv2023

Nuclear Spin-Depleted, Isotopically Enriched 70Ge/28Si70Ge Quantum Wells

The p-symmetry of the hole wavefunction is associated with a weaker hyperfine interaction as compared to electrons, thus making hole spin qubits attractive candidates to implement long coherence quantum processors. However, recent studies demonstrated that hole qubits in planar germanium (Ge) heterostructures are still very sensitive to nuclear spin bath. These observations highlight the need to develop nuclear spin-free Ge qubits to suppress this decoherence channel and evaluate its impact. With this perspective, this work demonstrates the epitaxial growth of $^\text{73}$Ge-depleted isotopically enriched $^\text{70}$Ge/SiGe quantum wells. The growth was achieved by reduced pressure chemical vapor deposition using isotopically purified monogermane $^\text{70}$GeH$_\text{4}$ and monosilane $^\text{28}$SiH$_\text{4}$ with an isotopic purity higher than 99.9 $\%$ and 99.99 $\%$, respectively. The quantum wells consist of a series of $^\text{70}$Ge/SiGe heterostructures grown on Si wafers using a Ge virtual substrate and a graded SiGe buffer layer. The isotopic purity is investigated using atom probe tomography following an analytical procedure addressing the discrepancies in the isotopic content caused by the overlap of isotope peaks in mass spectra. The nuclear spin background in the quantum wells was found to be sensitive to the growth conditions. The lowest concentration of nuclear spin-full isotopes $^\text{73}$Ge and $^\text{29}$Si in the heterostructure was established at 0.01 $\%$ in the Ge quantum well and SiGe barriers. The measured average distance between nuclear spins reaches 3-4 nm in $^\text{70}$Ge/$^\text{28}$Si$^\text{70}$Ge, which is an order of magnitude larger than in natural Ge/SiGe heterostructures.

cond-mat.mes-hall↗

arXiv2021

High-Bandwidth Extended-SWIR GeSn Photodetectors on Silicon Achieving Ultrafast Broadband Spectroscopic Response

The availability of high-frequency pulsed emitters in the $2-2.5\,\mu$m wavelength range paved the way for a wealth of new applications in ultrafast spectroscopy, free-space and fiber-optical communications, surveillance and recognition, artificial intelligence, and medical imaging. However, developing these emerging technologies and their large-scale use depend on the availability of high-speed, low-noise, and cost-effective photodetectors. With this perspective, here we demonstrate GeSn photodiodes grown on silicon wafers featuring a high broadband operation covering the extended-SWIR range with a peak responsivity of 0.3 A/W at room temperature. These GeSn devices exhibit a high bandwidth reaching 7.5 GHz at 5 V bias with a 2.6 $\mu$m cutoff wavelength, and their integration in ultrafast time-resolved spectroscopy applications is demonstrated. In addition to enabling time-resolved electro-luminescence at 2.3 $\mu$m, the high-speed operation of GeSn detectors was also exploited in the diagnostics of ultra-short pulses of a supercontinuum laser with a temporal resolution in the picosecond range at 2.5 $\mu$m. Establishing these capabilities highlights the potential of manufacturable GeSn photodiodes for silicon-integrated high-speed extended-SWIR applications.

physics.app-ph↗

arXiv2020

Extended Short-Wave Infrared Absorption in Group IV Nanowire Arrays

Engineering light absorption in the extended short-wave infrared (e-SWIR) range using scalable materials is a long-sought-after capability that is crucial to implement cost-effective and high-performance sensing and imaging technologies. Herein, we demonstrate enhanced, tunable e-SWIR absorption using silicon-integrated platforms consisting of ordered arrays of metastable GeSn nanowires with Sn content reaching 9 at.% and variable diameters. Detailed simulations were combined with experimental analyses to systematically investigate light-GeSn nanowire interactions to tailor and optimize the nanowire array geometrical parameters and the corresponding optical response. The diameter-dependent leaky mode resonance peaks are theoretically predicted and experimentally confirmed with a tunable wavelength from 1.5 to 2.2 {\mu}m. A three-fold enhancement in the absorption with respect to GeSn layers at 2.1 {\mu}m was achieved using nanowires with a diameter of 325 nm. Finite difference time domain simulations unraveled the underlying mechanisms of the e-SWIR enhanced absorption. Coupling of the HE11 and HE12 resonant modes to nanowires is observed at diameters above 325 nm, while at smaller diameters and longer wavelengths the HE11 mode is guided into the underlying Ge layer. The presence of tapering in NWs further extends the absorption range while minimizing reflection. This ability to engineer and enhance e-SWIR absorption lays the groundwork to implement novel photonic devices exploiting all-group IV platforms.

physics.app-ph↗

arXiv2013

Harnessing nuclear spin polarization fluctuations in a semiconductor nanowire

Soon after the first measurements of nuclear magnetic resonance (NMR) in a condensed matter system, Bloch predicted the presence of statistical fluctuations proportional to $1/\sqrt{N}$ in the polarization of an ensemble of $N$ spins. First observed by Sleator et al., so-called "spin noise" has recently emerged as a critical ingredient in nanometer-scale magnetic resonance imaging (nanoMRI). This prominence is a direct result of MRI resolution improving to better than 100 nm^3, a size-scale in which statistical spin fluctuations begin to dominate the polarization dynamics. We demonstrate a technique that creates spin order in nanometer-scale ensembles of nuclear spins by harnessing these fluctuations to produce polarizations both larger and narrower than the natural thermal distribution. We focus on ensembles containing ~10^6 phosphorus and hydrogen spins associated with single InP and GaP nanowires (NWs) and their hydrogen-containing adsorbate layers. We monitor, control, and capture fluctuations in the ensemble's spin polarization in real-time and store them for extended periods. This selective capture of large polarization fluctuations may provide a route for enhancing the weak magnetic signals produced by nanometer-scale volumes of nuclear spins. The scheme may also prove useful for initializing the nuclear hyperfine field of electron spin qubits in the solid-state.

cond-mat.mes-hall↗