The advancements in integrated thin-film lithium niobate on insulator (LNOI) platform have significantly enhanced the performance of various integrated electro-optic devices, including modulators, external cavity diode lasers, and optical frequency comb generators. Additionally, the development of LNOI has facilitated applications at shorter wavelengths, due to its wide transparency window. The coupling efficiency between laser diodes (LDs) and LN chips is commonly enhanced by designing spot size converters (SSCs). However, achieving high-efficiency SSCs is more challenging at shorter wavelength due to the smaller mode area and the increased confinement of the optical field in LN waveguides. In this study, we present a spot size converter based on hybrid SiN-LN structure, for low-loss light coupling between a III– V gain chip and a LNOI waveguide at 780nm. The parameters of SiN waveguide, LN taper and SiO2 spacing layer has been optimized in order to enhance the matching of effective refractive indices. The entire SSC structure can be fabricated with two steps of photolithography and etching, demonstrating high fabrication tolerance. Simulations indicate that the coupling losses between the output mode of the LD and the fundamental mode of the LN waveguide are 0.41dB/facet for TE mode and 0.55dB/facet for TM mode at a wavelength of 780nm. Our design is intended to offer efficient light coupling from LD to LNOI chips at short wavelength range, characterized by its simple process and high fabrication tolerance.
KEYWORDS: Quantum chromodynamics, Transition metals, Quantum efficiency, Absorption, Sensors, Electric field sensors, Doping, Modulation, Quantum wells, Electron transport
This work proposes a novel way to regulate the electron quantum states of quantum cascade detectors (QCDs) by utilizing localized built-in electric field introduced by modulation doping. The mechanism that how the localized built-in electric field influences extraction efficiency is studied by analyzing the quantum transitions in a simplified three-quantum-well model. The calculation results show that, by introducing the localized built-in electric field, a transition energy close to the LO phonon energy can be more easily realized with almost unchanged transition matrix element. The transition matrix element can be enlarged by the localized built-in electric field with almost unchanged transition energy. The calculated extraction efficiency is below 65% for the standard QCD structures without localized built-in electric field, whereas for the structures with localized built-in electric field, the extraction efficiency can reach above 80%. From experimental results, a higher extraction efficiency of photo-generated electrons of 89% is obtained for the proposed QCD structure, comparing with 63% for the standard QCD structure. The peak response wavelengths of two structures are both around 4.5 μm. At temperatures ranging from 40K to 210K, the photocurrents of the structure with localized built-in electric field are over 55% larger than those of the standard structure. To sum up, the localized built-in electric field can be utilized to regulate the electron states besides the layer thickness and material composition of QCDs.
In this paper, we investigate the fabrication of high aspect ratio photonic crystal air holes in AlGaAs materials using general inductively coupled plasma (ICP) dry etching system. We propose dividing the long etching process into multiple short-time etching segments during the ICP etching process, so that there is enough time to exhaust the etch products out from the bottom of the holes before the next etching segment, which is beneficial for deep air hole etching. Simultaneously, a novel method to suppress lateral penetration of holes by in-situ sidewall passivation is proposed, which can be realized by inserting one oxygen plasma treatment between two etching segments. This method also allows the optimization of etch rate to be achieved independent of sidewall passivation. Our experiment results show that the sidewall passivation has a crucial influence on the etched morphology of air holes. Without sidewall passivation, the air holes are lateral penetrated in the middle. While with appropriate oxidation for sidewall passivation, deep air holes with high verticality are obtained. Finally, high-aspect-ratio air holes with a diameter of 130 nm and a depth of over 1.5 μm are successfully manufactured.
A hybrid integration method of back-illuminated modified uni-traveling carrier photodiode (MUTC-PD) on silicon-oninsulator (SOI) is demonstrated. Compared with the die-to-die bonding of unprocessed III-V die, this hybrid bonding method, implemented by a flip-chip bonding machine, is more convenient and flexible, thus providing a more direct path to utilizing high-speed PDs in integrated microwave photonics on SOI. As a result, the integrated photodetector exhibits a 3-dB bandwidth of 30 GHz, showing no degradation compared with the bandwidth before bonding.
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