We report an investigation on the photo-response from a GeSn-based photodetector using a tunable laser with a range of incident light power. An exponential increase in photocurrent and an exponential decay of responsivity with increase in incident optical power intensity were observed at higher optical power range. Time-resolved measurement provided evidence that indicated monomolecular and bimolecular recombination mechanisms for the photo-generated carriers for different incident optical power intensities. This investigation establishes the appropriate range of optical power intensity for GeSn-based photodetector operation.
Waveguides were fabricated from n-type doped GeSn layers with Sn content in the range 4.4-7.0 % Sn grown on Ge-buffered Si substrates. The waveguides were optically pumped a 976nm continuous wave laser, and their power output was measured as a function of pump power. The output power dependence indicates light amplification through stimulated emission, and that GeSn acts as a gain medium. Under the experimental conditions, the cavity gain did not exceed the lasing threshold, but amplified spontaneous emission was still observed. This demonstration of optical gain at room temperature is an important first step to achieving room temperature lasing in GeSn.
Object detection is one of the fundamental tasks in computer vision. Although excellent progress has been made, there still exist challenges for objects with dense distribution, fuzzy feature, and small size. This paper presents a detector for object detection of small size in High-Resolution remote sensing images, namely SF-YOLOV4. The proposed SF-YOLOV4 exploits information for shallow layers and contextual information along with spatial attention to address the above challenges. Specifically, a shallow semantic information extraction network(SFN) is designed which introduces low-level semantic information into the backbone, to alleviate the loss of the small object features. Meanwhile, we replace the original Nick with multi-scale context feature pyramid (MSC-FPN) to improve the utilization of lower layers information and integrate the context information, we also add spatial attention module to find attention region at different scales. Experiments on two remote sensings public datasets DIOR and RSOD show the good detection performance of SF-YOLOV4.
The development of Si-based lasers has been a goal of the photonics industry for many years. A number of methods have been explored for achieving lasers on Si platforms, and a few attempts, such as the Si Raman laser, have achieved marginal success, but typically suffer from low efficiency. The silicon photonics problem requires the realization of high-efficiency lasers on CMOS-compatible platforms. One possible solution to this problem is to employ the use of direct band gap materials that can be grown on Si. Of course, the epitaxial growth of such materials has been extremely difficult, and the efforts have mostly concentrated on the use of III-V materials like InGaAs. These materials typically suffer from high defect densities when grown directly on silicon, making them unsuitable for use in lasers. Additionally, the introduction of Group III and Group V materials into a CMOS fabrication facility can cause severe problems in device production due the electronic properties of these atoms, which are typically used as dopants in Si. GeSn alloys present a possible path forward for the realization of Si-based lasers, as these materials are grown directly on Si or Ge-buffered Si and have a band structure that is more suited for efficient optical emission. A laser based on tensile strained Ge grown directly on Si has been demonstrated at room temperature, but the efficiency is limited by the indirect nature of the Ge band gap. More recently, demonstration of lasing at low temperature in GeSn (12.6% Sn) was achieved using optical pumping of a GeSn waveguide. While this represents a first step to the actualization of GeSn lasers, room temperature operation is a necessary condition for widespread adoption and incorporation into everyday devices. The approach for demonstration of lasing in GeSn and Ge-on-Si has typically relied on measuring the optical emission spectrum at very high resolution to see the narrowing of the emission linewidth and the presence of cavity modes while optically-pumping a waveguide with a pulsed laser via topside illumination. We have designed a new system for optically pumping waveguides by coupling into the waveguide edge, which increases the absorption length by orders of magnitude, thereby increasing optical emission intensity and relaxing the constraints under which lasing is achieved. We employ a CW laser and erbium-doped fiber amplifier at 1550 nm as the pump source, which serves to decrease the cost of the experiment and increase the pump power. The edge emission from the waveguide is sent into a Fabry-Perot cavity with an InGaAs detector, and the optical pump power is increased until coherence is achieved, which results in the observance of modes in the Fabry-Perot cavity. This measurement can be performed with much lower pump power than a measurement using a high-resolution spectrometer. In this work, we demonstrate the use of this method for measuring the optical emission from GeSn waveguides with Sn concentrations of 4-7%.
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