Quantifying the impact of upscaled parameters on radionuclide transport in three-dimensional fracture-matrix systems.

Assessing the long-term safety of geological repositories for high-level radioactive waste is critically dependent on understanding radionuclide transport in multi-scale fractured rocks. This study explores the influence of upscaled parameters on radionuclide movement within a three-dimensional fracture-matrix system using a discrete fracture-matrix (DFM) model. The developed numerical simulation workflow includes creating a random discrete fracture network, meshing of the fractures and matrix, assigning upscaled parameters, and conducting finite element simulations. We simulated the spatiotemporal evolution of radionuclide concentrations in the fractures and matrix over a century, revealing significant spatial heterogeneity driven by a heterogeneous seepage field. Employing geostatistics-based upscaling methods, we predicted the effective ranges of crucial solute transport parameters at the field scale. The matrix diffusion coefficient, matrix distribution coefficient, and longitudinal dispersivity were upscaled by factors of 2.0-3.0, 2.5-4.0, and 10-104, respectively, based on laboratory-scale measurements. Incorporating these upscaled parameters into the DFM model, we analyzed their impact on radionuclide transport. Our findings demonstrate that an upscaled matrix diffusion coefficient and matrix distribution coefficient result in a delayed transport of radionuclides in fractures by enhancing mass transfer between the fractures and rock matrix, while an upscaled longitudinal dispersivity accelerates transport by advancing the positions of concentration peaks. Sensitivity analysis revealed that the matrix distribution coefficient is the most impactful, followed by dispersivity and matrix diffusion coefficient. These insights are important for minimizing parameter uncertainties and enhancing the accuracy of predictions concerning radionuclide transport in multi-scale fractured rocks.

Quantitative characterization of solute transport in fractures with different surface roughness based on ten Barton profiles.

In order to understand the transport mechanism of solute through naturally fractured rocks, it is important to quantitatively characterize the influence of varying surface roughness on fluid flow and solute transport. Rough-walled fracture geometry models with different joint roughness coefficients (JRC) were generated based on MATLAB pixel analyses of ten standard Barton profiles. Fluid flow and solute transport in the 2D rough-walled fractures were simulated by solving the Navier-Stokes-equation (NSE) and the transport equation for different pressure drops (i.e., 5, 10, and 20 Pa). The simulation results show that the evolution of the solute-concentration field within rough-walled fractures is significantly dependent on surface roughness. Analysis of the breakthrough curves (BTCs) and residence time distributions (RTDs) indicated that rough fracture surfaces with large JRCs played a significant role in weakening the non-Fickian transport characteristics (i.e., early arrival and long tail) under the same pressure drop. It was found that the solute-concentration-distribution index (CDI), i.e., a metric for quantifying the longevity of the tail, increased with the JRCs and decreased with an increase in pressure drops. This result demonstrates that decreasing the surface roughness increases the Péclet number (Pe) and enhances advection process in solute transport, resulting in an increase in the non-uniform concentration distribution and shortened the long tail. Inverse modeling of the BTCs shows that rough fracture surfaces with large JRCs decrease the effective dispersion coefficient and Pe, suggesting that rough fracture surfaces decrease the advection and dispersion processes and delay the early arrival. These results provide more comprehensive understanding of the role of surface roughness in solute transport through fractures. Based on the relationships between JRC value and effective dispersion coefficient, a prediction method was established to predict the non-Fickian transport and the JRC value, and the practical cases further proved the feasibility of the prediction method.

III/V-on-Si MQW lasers by using a novel photonic integration method of regrowth on a bonding template.

Silicon photonics is becoming a mainstream data-transmission solution for next-generation data centers, high-performance computers, and many emerging applications. The inefficiency of light emission in silicon still requires the integration of a III/V laser chip or optical gain materials onto a silicon substrate. A number of integration approaches, including flip-chip bonding, molecule or polymer wafer bonding, and monolithic III/V epitaxy, have been extensively explored in the past decade. Here, we demonstrate a novel photonic integration method of epitaxial regrowth of III/V on a III/V-on-SOI bonding template to realize heterogeneous lasers on silicon. This method decouples the correlated root causes, i.e., lattice, thermal, and domain mismatches, which are all responsible for a large number of detrimental dislocations in the heteroepitaxy process. The grown multi-quantum well vertical p-i-n diode laser structure shows a significantly low dislocation density of 9.5 × 104 cm-2, two orders of magnitude lower than the state-of-the-art conventional monolithic growth on Si. This low dislocation density would eliminate defect-induced laser lifetime concerns for practical applications. The fabricated lasers show room-temperature pulsed and continuous-wave lasing at 1.31 µm, with a minimal threshold current density of 813 A/cm2. This generic concept can be applied to other material systems to provide higher integration density, more functionalities and lower total cost for photonics as well as microelectronics, MEMS, and many other applications.

5 x 20 Gb/s heterogeneously integrated III-V on silicon electro-absorption modulator array with arrayed waveguide grating multiplexer.

We present a five-channel wavelength division multiplexed modulator module that heterogeneously integrates a 200 GHz channel-spacing silicon arrayed-waveguide grating multiplexer and a 20 Gbps electro-absorption modulator array, showing the potential for 100 Gbps transmission capacity on a 1.5x0.5 mm² footprint.

High-speed silicon modulator based on cascaded microring resonators.

A high-speed silicon modulator based on cascaded double microring resonators is demonstrated in this paper. The proposed modulator experimentally achieved 40 Gbit/s modulation with an extinction ratio of 3.9 dB. Enhancement of the modulator achieves with an ultra-high optical bandwidth of 0.41 nm, corresponding to 51 GHz, was accomplished by using cascaded double ring structure. The described modulator can provides an ultra-high-speed optical modulation with a further improvement in electrical bandwidth of the device.

High speed silicon Mach-Zehnder modulator based on interleaved PN junctions.

A high speed silicon Mach-Zehnder modulator is proposed based on interleaved PN junctions. This doping profile enabled both high modulation efficiency of V(π)L(π) = 1.5~2.0 V·cm and low doping-induced loss of ~10 dB/cm by applying a relatively low doping concentration of 2 × 10(17) cm(-3). High speed operation up to 40 Gbit/s with 7.01 dB extinction ratio was experimentally demonstrated with a short phase shifter of only 750 µm.

25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions.

A high-speed depletion-mode silicon-based microring modulator with interleaved PN junctions optimized for high modulation efficiency and large alignment tolerance is demonstrated. It is fabricated using standard 0.18 µm complementary metal-oxide-semiconductor processes and provides low V(π)L(π)s of 0.68 V·cm to 1.64 V·cm with a moderate doping concentration of 2 × 10(17) cm(-3). The measured modulation efficiency decreases by only 12.4% under ± 150 nm alignment errors. 25 Gbit/s non-return-zero modulation with a 4.5 dB extinction ratio is experimentally realized at a peak-to-peak driving voltage of 2 V, demonstrating the excellent performance of the novel doping profile.