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The recent progress on (Si)GeSn optoelectronic devices holds a great promising for photonic integration on the Si substrate. In parallel to the development of bulk devices, the (Si)GeSn based quantum wells (QWs) have been investigated aiming to improve the device performance. While the multiple QW structure is preferred for the device application, the single quantum well (SQW) is more suitable for optical property study. In this work, a comprehensive study of a SiGeSn/GeSn SQW was conducted. The calculated band diagram provided the band alignment and energies of possible transitions. The SQW features the direct bandgap well with L-{\Gamma} valley energy separation of 50 meV, and the barrier heights for both electron and hole are greater than 80 meV. Using two continuous-wave and two pulsed pumping lasers, the analysis of PL spectra allows for identifying different transitions and for better understanding the SQW optical properties. The study could provide the guidance for advancing the future QW design towards device applications., Comment: 26 pages, 8 figures, and 3 tables

GeSn lasers enable monolithic integration of lasers on the Si platform using all-group-IV direct-bandgap materials. Although optically pumped GeSn lasers have made significant progress, the study of the electrically injected lasers has just begun only recently. In this work, we present explorative investigations of electrically injected GeSn heterostructure lasers with various layer thicknesses and material compositions. The cap layer total thickness was varied between 240 and 100 nm. At 10 K, a 240-nm-SiGeSn capped device had a threshold current density Jth = 0.6 kA/cm2 compared to Jth = 1.4 kA/cm2 of a device with 100-nm-SiGeSn cap due to an improved modal overlap with the GeSn gain region. Both devices had a maximum operating temperature Tmax = 100 K. Device with cap layers of Si0.03Ge0.89Sn0.08 and Ge0.95Sn0.05, respectively, were also compared. Due to less effective carrier (electron) confinement, the device with a 240-nm-GeSn cap had a higher threshold Jth = 2.4 kA/cm2 and lower maximum operating temperature Tmax = 90 K, compared to those of the 240-nm-SiGeSn capped device with Jth = 0.6 kA/cm2 and Tmax = 100 K. In the study of the active region material, the device with Ge0.85Sn0.15 active region had a 2.3 times higher Jth and 10 K lower Tmax, compared to the device with Ge0.89Sn0.11 in its active region. This is likely due to higher defect density in Ge0.85Sn0.15 rather than an intrinsic issue. The longest lasing wavelength was measured as 2682 nm at 90 K. The investigations provide guidance to the future structure design of GeSn laser diodes to further improve the performance., Comment: 21 pages, 5 figures, and 2 tables

The significant progress of GeSn material development has enabled a feasible solution to the long-desired monolithically integrated lasers on the Si platform. While there are many reports focused on optically pumped lasers, GeSn lasers through electrical injection have not been experimentally achieved yet. In this work, we report the first demonstration of electrically injected GeSn lasers on Si. A GeSn/SiGeSn heterostructure diode grown on a Si substrate was fabricated into ridge waveguide laser devices and tested under pulsed conditions. Special considerations were given for the structure design to ensure effective carrier confinement and optical confinement that lead to lasing. Lasing was observed at temperatures from 10 to 100 K with emission peaks at around 2300 nm. The minimum threshold of 598 A/cm2 was recorded at 10 K and the threshold increased to 842 A/cm2 at 77 K. The spectral linewidth of a single peak was measured as small as 0.13 nm (0.06 meV). The maximum characteristic temperature was extracted as 99 K over the temperature range of 10-77 K.

The fruit fly's natural visual environment is often characterized by light intensities ranging across several orders of magnitude and by rapidly varying contrast across space and time. Fruit fly photoreceptors robustly transduce and, in conjunction with amacrine cells, process visual scenes and provide the resulting signal to downstream targets. Here we model the first step of visual processing in the photoreceptor-amacrine cell layer. We propose a novel divisive normalization processor (DNP) for modeling the computation taking place in the photoreceptor-amacrine cell layer. The DNP explicitly models the photoreceptor feedforward and temporal feedback processing paths and the spatio-temporal feedback path of the amacrine cells. We then formally characterize the contrast gain control of the DNP and provide sparse identification algorithms that can efficiently identify each the feedforward and feedback DNP components. The algorithms presented here are the first demonstration of tractable and robust identification of the components of a divisive normalization processor. The sparse identification algorithms can be readily employed in experimental settings, and their effectiveness is demonstrated with several examples.

This paper reports a comprehensive study of Si-based GeSn mid-infrared photodetectors, which includes: 1) the demonstration of a set of photoconductors with Sn compositions ranging from 10.5% to 22.3%, showing the cut-off wavelength has been extended to 3.65 um. The measured maximum D* of 1.1x10^10 cmHz^(1/2)W(-1) is comparable to that of commercial extended-InGaAs detectors; 2) the development of surface passivation technique on photodiode based on in-depth analysis of dark current mechanism, effectively reducing the dark current. Moreover, mid-infrared images were obtained using GeSn photodetectors, showing the comparable image quality with that acquired by using commercial PbSe detectors., Comment: 25 pages, 8 figures

A Si-based monolithic laser is highly desirable for full integration of Si-photonics. Lasing from direct bandgap group-IV GeSn alloy has opened a completely new venue from the traditional III-V integration approach. We demonstrated optically pumped GeSn lasers on Si with broad wavelength coverage from 2 to 3 {\mu}m. The GeSn alloys were grown using newly developed approaches with an industry standard chemical vapor deposition reactor and low-cost commercially available precursors. The achieved maximum Sn composition of 17.5% exceeded the generally acknowledged Sn incorporation limits for using similar deposition chemistries. The highest lasing temperature was measured as 180 K with the active layer thickness as thin as 260 nm. The unprecedented lasing performance is mainly due to the unique growth approaches, which offer high-quality epitaxial materials. The results reported in this work show a major advance towards Si-based mid-infrared laser sources for integrated photonics., Comment: 34 pages, 12 figures

We investigate the sparse functional identification of complex cells and the decoding of visual stimuli encoded by an ensemble of complex cells. The reconstruction algorithm of both temporal and spatio-temporal stimuli is formulated as a rank minimization problem that significantly reduces the number of sampling measurements (spikes) required for decoding. We also establish the duality between sparse decoding and functional identification, and provide algorithms for identification of low-rank dendritic stimulus processors. The duality enables us to efficiently evaluate our functional identification algorithms by reconstructing novel stimuli in the input space. Finally, we demonstrate that our identification algorithms substantially outperform the generalized quadratic model, the non-linear input model and the widely used spike-triggered covariance algorithm.