We report on the theoretical investigation of using an amorphous Ge0.83Si0.17 lateral taper to enable a low-loss small-footprint optical coupling between a Si3N4 waveguide and a low-voltage Ge-based Franz–Keldysh optical modulator on a bulk Si substrate using 3D Finite-Difference Time-Domain (3D-FDTD) simulation at the optical wavelength of 1550 nm. Despite a large refractive index and optical mode size mismatch between Si3N4 and the Ge-based modulator, the coupling structure rendered a good coupling performance within fabrication tolerance of advanced complementary metal-oxide semiconductor (CMOS) processes. For integrated optical modulator performance, the Si3N4-waveguide-integrated Ge-based on Si optical modulators could simultaneously provide workable values of extinction ratio (ER) and insertion loss (IL) for optical interconnect applications with a compact footprint.

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The behavior of holes in the valence band of BaTiO 3 is investigated using hybrid densityfunctional calculations. We find that holes tend to self-trap, localizing on individual O atoms and causing local lattice distortions, forming small hole-polarons. This takes place even in the absence of intrinsic defects or impurities. The self-trapped hole (STH) is more energetically favorable than the delocalized hole in the valence band. The calculated emission peak energy corresponding to the recombination of a conduction band electron with a STH can explain the observed photoluminescence at low temperatures. The stability of the STH, its migration barrier, and the related emission peak are then compared to those of SrTiO 3 .

We report on the design and simulation of a waveguide-integrated Ge/SiGe quantum-confined Stark effect (QCSE) optical modulator based on the use of a Ge-rich SiGe relaxed buffer on a graded buffer as an optical waveguide. Despite the promising potential of this waveguide platform, efficient and wideband optical integration with a Ge-based active device has not been properly addressed so far. In this paper, via 3D finite-difference time domain simulation, we demonstrate that a simple 2D taper is sufficient to enable adiabatic optical coupling from the fundamental mode of the input SiGe waveguide to the fundamental mode of the Ge/SiGe multiple quantum well (MQW) modulator without the excitation of higher-order modes in Ge/SiGe MQWs. The 2D taper shows good fabrication tolerance considering critical variations in its dimensions. Significantly, wideband optical modulation performance in terms of extinction ratio and insertion loss is presented over the whole low-loss spectral range of the Ge/SiGe MQWs at different electrical bias values, device lengths, and numbers of quantum wells in order to comprehensively report its potential for Si-based optical modulators.

Si3N4 photonic integrated circuits have gain significant and rapid interest in different photonic applications thanks to its superior passive performance. Nevertheless, optical integration between Si3N4 and Ge-based optical components remains critically challenging especially for optical modulation. In this paper, via 3D-FDTD calculations we investigate the optical integration between Si3N4 and Ge-based waveguides using vertical coupling configuration employing amorphous Si (α-Si) as an optical bridge showing efficient and robust coupling efficiency, which can be maintained according to the tolerant analysis with respect to the variations in optical wavelengths and critical parameters of the coupling structure. In addition, with respect to the recent theoretically-optimized SOI waveguide-integrated Ge-based optical modulators, we found that the studied coupling structure could be employed to enable a low-voltage Si3N4 waveguide-integrated Gebased optical modulator with a competitive extinction ratio/insertion loss performance, increasing the prospect of Si3N4-based photonic integrated circuits for low-energy optical interconnects.