Silicon Photonic Components Research Articles

(Invited) Defect Engineering in Ge QDs for Light Emission in Si Photonics

The search for a practical, low-cost and monolithic light source that allows for straightforward implementation on CMOS-based microprocessors and that can be easily coupled to existing active and passive Silicon photonics components is ongoing. In this presentation, we will demonstrate an alternative approach to hybrid III-V on Si and strained bulk group-IV (Ge, GeSn) light emitters. This concept makes use of epitaxially grown Ge quantum dots (QDs) in a defect-free crystalline Si matrix, as they are known since the early nineties [1-3]. However, we will show that the light emission properties of such Ge/Si QDs can be boosted by intentional, low-dose and low-energy implantation of heavy ions (such as Ge and Si) into the QDs. We demonstrate CMOS-compatible lasing from these novel QDs in which defined point-defects states spatially confine electrons after their tunnelling into the nanostructures [1-4]. This leads to optically direct transitions and, hence, efficient light emission. In contrast to conventional epitaxial Ge/Si QDs - these defect-enhanced QDs (DEQDs) exhibit dramatically shortened carrier lifetimes and negligible thermal quenching of the photoluminescence above room-temperature (RT) [1]. Embedding the DEQDs into microdisk resonators leads to lasing characteristics up to RT upon optical excitation [3]. Contact doping and hence fabrication of electrically driven devices is straightforward in this nanosystem since DEQDs are embedded into a defect-free Si matrix. LEDs containing DEQDs show bright emission up to 100°C, the temperature limit of the measurement setup [5]. [1] Grydlik, M.; Hackl, F.; Groiss, H.; Glaser, M.; Halilovic, A.; Fromherz, T.; Jantsch, W.; Schäffler, F.; Brehm, M. ACS Photonics 2016, 3 (2), 298–303 [2] Grydlik, M.; Lusk, M. T.; Hackl, F.; Polimeni, A.; Fromherz, T.; Jantsch, W.; Schäffler, F.; Brehm, M. Nano Lett. 2016, 16 (11), 6802–6807 [3] Brehm M. and Grydlik, M. Nanotechnology 2017, 28, 392001 [4] Groiss H., Spindlberger L., Oberhumer P., Schäffler F., Fromherz T., Grydlik M. and Brehm M, Semicond. Sci. Technol. 2017 32 (2), 02LT01 [5] Rauter P., Spindlberger L., Schäffler F., Fromherz T., Freund J., and Brehm M., ACS Photonics 2018, 5 (2), pp 431–438

Silicon chip-scale space-division multiplexing: from devices to system

Space-division multiplexing (SDM) technique has attracted increasing attentions recently, because it provides an effective way to increase transmission capacity. With the continuous and exponential increase in data demands, high-density integration of silicon photonic components is of significant interest in terms of link price, performance and power consumption. The multimode/mutlicore devices applied to achieve diverse functionalities are key building blocks to construct a chip-scale SDM system based on a silicon on insulator (SOI) platform. This study reviews the recent progress of multimode/multicore devices, which enable coupling, multiplexing/demultiplexing, transmitting switching, as well as modulation and detection. Based on these devices, a complete on-chip SDM system is constructed and discussed.

Engineering of Germanium Tunnel Junctions for Optical Applications

Data transfer across millimeter-scale electrical wires is limited by both data rates and power density, which creates a performance bottleneck for semiconductor microchips in modern computer systems. Hence, silicon based platforms for optical communication are diligently explored for an on-chip optical data transfer. Semiconductor optical amplifiers provide signal recovery and loss compensation in advanced photonic circuits and are, thus, indispensable components for such platforms. However, silicon photonic components have to operate at much lower voltages and energy-per-bit metrics before it is worth integrating them on-chip with a CPU. The usage of tunnel junctions to control carrier injection can provide a fast and energy-saving alternative. Here, we present experimental results on direct conduction band carrier modulation in the indirect semiconductor Ge by Zener tunnel injection. Electrons are injected by a reverse-biased p-n Zener tunnel diode and recombine radiatively with holes injected by a forward biased p-i-n diode. This injection mechanism favors tunneling of electrons into the direct conduction band valley with concomitant improvements of optoelectronic properties. Benchmarking the performance, 2.42 dB transmission change at 0.9 V bias (1660 nm) at 300 K confirm the working principle. Our device can serve as a starting point to investigate the benefits of tunnel injection for silicon photonic devices.

Heterogeneous Silicon/III–V Semiconductor Optical Amplifiers

We report high output power and high-gain semiconductor optical amplifiers integrated on a heterogeneous silicon/III–V photonics platform. The devices produce 25 dB of unsaturated gain for the highest gain design, and 14 dBm of saturated output power for the highest output power design. The amplifier structure is also suitable for lasers, and can be readily integrated with a multitude of silicon photonic circuit components. These devices are useful for a wide range of photonic integrated circuits. We show a design method for optimizing the amplifier for the desired characteristics. The amplifier incorporates a low loss and low reflection transition between the heterogeneous active region and a silicon waveguide, and we report transition loss below 1 dB across the entire measurement range and parasitic reflection coefficient from the transition below $1\cdot 10^{ - 3}$ .

Ultra‐broadband nanophotonic beamsplitter using an anisotropic sub‐wavelength metamaterial

AbstractNanophotonic beamsplitters are fundamental building blocks in integrated optics, with applications ranging from high speed telecom receivers to biological sensors and quantum splitters. While high‐performance multiport beamsplitters have been demonstrated in several material platforms using multimode interference couplers, their operation bandwidth remains fundamentally limited. Here, we leverage the inherent anisotropy and dispersion of a sub‐wavelength structured photonic metamaterial to demonstrate ultra‐broadband integrated beamsplitting. Our device, which is three times more compact than its conventional counterpart, can achieve high‐performance operation over an unprecedented 500 nm design bandwidth exceeding all optical communication bands combined, and making it one of the most broadband silicon photonics components reported to date. Our demonstration paves the way toward nanophotonic waveguide components with ultra‐broadband operation for next generation integrated photonic systems. image

All‐optical wavelet‐based orthogonal frequency division multiplexing system based on silicon photonic integrated components

In this study, the authors discuss in detail the implementation of all-optical orthogonal frequency-division multiplexing system based on discrete wavelet packet transform (DWPT) by using silicon-on-insulator (SOI) cascaded Mach–Zehnder interferometers (MZIs). DWPT can be used in the multiplexing and de-multiplexing of data channels in coherent optical communication in order to improve spectral efficiency excluding a cyclic prefix. DWPT and inverse-DWPT may be realised in an integrated-photonic MZI platform. They demonstrated experimentally the post-fabrication tuning of 1 × 4 cascaded MZI de-multiplexing filters. Tuning procedures effectively overcome fabrication tolerances and meet the target transfer function of the devices. The fabrication platform represents an essential stepping-stone toward the all-optical realisation of DWPT-based multiplexing.

Review—Multifunctional Technology with Monolithic Integrated THz-, Photonic- and μ-Fluidic Modules

The purpose of this work is to review requirements and challenges for the monolithic integration of different devices and modules in a baseline CMOS-process with the intention to create a multifunctional technology platform. We consider integrated modules like THz devices, i.e. complementary SiGe heterojunction bipolar transistors, silicon photonics components in particular Ge-PIN photo detectors and the wafer-level integration of micro-fluidic channels for sensor applications. However, the monolithic integration of devices and modules for novel functionalities is still a trade-off. On the one side there are requirements of the integration concept and therefore to the process steps of the dedicated module to achieve a maximum device performance. On the other side there are influences of the process steps for the module integration to the performance of the devices from the underlying baseline technology like bipolar- and CMOS-transistors or passive components. Both aspects have to be considered to enable the co-integration of new modules in order to get a technology platform for high performances and novel functionalities. We review process adaptations of the baseline technologies to enable an efficient co-integration of different modules. Moreover we discuss critical process steps with a certain thermal budget and their influence to device performances. Finally we give recommendations for process sequences to enable a wafer level co-integration of microfluidic channels into a SiGe-BiCMOS technology platform.

(Invited) Multifunctional Technology with Monolithic Integrated THz-, Photonic- and µ-Fluidic Modules

In this paper we present a multifunctional technology platform with different monolithic integrated modules like THz devices, i.e. complementary SiGe heterojunction bipolar transistors, silicon photonics components in particular Ge-PIN photo detectors and finally the wafer-level integration of µ-fluidic channels for sensor applications. Challenges during the monolithic integration of these different modules were reviewed and necessary process adaptations discussed in order to enable a co-integration.

A tunable photonic temporal integrator with ultra-long integration time windows based on Raman-gain assisted phase-shifted silicon Bragg gratings

A tunable photonic temporal integrator based on Raman-gain assisted phase-shifted silicon Bragg gratings is proposed and theoretically demonstrated. The proposed temporal photonic integrator is constructed using a silicon-on-insulator (SOI) π-phase-shifted Bragg grating, with a 25V reverse bias applied to the p–i–n rib waveguide. Key feature of our design is that the length of integration time window could be widely tuned by simply changing the optical power of the pump light and could be extended to very long when the pump power is approaching lasing threshold. In addition, this scheme also has the potential for on-chip integration with other silicon photonics components.

Heterogeneous Silicon Photonic Integrated Circuits

We review recent breakthroughs in the silicon photonic technology and components, and describe progress in silicon photonic integrated circuits. Heterogeneous silicon photonics has recently demonstrated performance that significantly outperforms native III/V components. The impact active silicon photonic integrated circuits could have on interconnects, telecommunications, sensors, and silicon electronics is reviewed.

(Invited) Multifunctional Technology with Monolithic Integrated THz-, Photonic- and µ-Fluidic Modules

Beside the use of the continuously scaled silicon-based integrated-circuit technologies there is an increased demand of “More-than-Moore” (MtM) components, modules and technologies to serve the requirements of new applications e.g. in the communication market or in big data processing centers. The MtM approach describes the diversification of functionality on a reasonable scaling level and is according to the ITRS one of the key drivers for future communication products. The strategies to increase the functionality of an integrated circuit or system by the integration of MTM components can be distinguished to monolithic and heterogeneous integration concepts, respectively. The latter one enables in general a high flexibility for the combination of different functionalities as CMOS, MEMS (micro-electro-mechanical systems) or III-V devices. However, a reliable packaging and therefore a sufficient yield can be a major concern for these systems. The monolithic integration of modules and devices with advanced functionality in a baseline CMOS process can overcome this concern. However these integration concepts have to consider issues during the integration of different modules like the interaction of process steps to the performance of the devices. In this paper we present a multifunctional technology platform with different monolithic integrated modules like THz devices, i.e. complementary SiGe heterojunction bipolar transistors, silicon photonics components as waveguides, photo detectors and modulators, and finally the wafer-level integration of µ-fluidic channels. The continuous improvement of SiGe-BiCMOS technologies have proven their demand for instance in the automotive radar market. Mid- and long-range radar systems at 77 GHz can be realized by using HBTs with maximum oscillation frequencies (fmax) of 300 GHz. The power consumption of these systems can be significantly reduced by the use of available SiGe-BiCMOS technologies with fmax of 500 GHz. Addditional new applications as THz circuits for gas spectroscopy and chipsets for sensing and short-range wireless communication have been demonstrated with these technologies. In future novel systems as short-range-radar (SRR) or autonomous cruise control (ACC) at frequencies of 120 and 150GHz can be targeted by the use of next generation SiGe-BiCMOS technologies. According to the ITRS one main challenge of group-IV bipolar technologies is the determination of a reasonable “plateau technology” i.e. the best ratio of fT/fmax of the fast npn-HBTs as well as the realization of complementary devices. In this work we review the continuous progress of high-speed npn-HBTs and the realization of the fastest pnp-HBT with maximum oscillation frequencies of 265GHz in an IHP-BiCMOS process. The demand for opto-electronic technologies in the market of big data processing centers increase continuously because of the necessity to handle the steadily growing data volumes. Recently we introduced the monolithic integration of photonic components as silicon-waveguides, high speed germanium-photo detectors and silicon based mach-zehnder-modulators (MZM) with high speed SiGe-BiCMOS devices in a joint technology. This new electro-photonic-integrated-circuit (EPIC) technology enables new applications as fully integrated linear 40Gbps receiver or SP-QPSK receiver for 28Gbaud operation. These circuits are among best in class and outperform monolithically Si integrated receivers fabricated elsewhere, e.g. with a Photonics-CMOS platform. However, there are several challenges during the monolithic integration of these photonic components in a SiGe-BiCMOS environment as the realization of a mixed substrate (SOI and bulk) or process optimizations to enable high-performance SiGe-HBTs and high speed Ge-photo detectors on the same wafer. The last part of the paper describes the wafer level integration of micro-fluidic channels in a SiGe-BiCMOS technology. The silicon channels were realized by a deep silicon substrate etching from the wafer backside after the SiGe-BiCMOS process on the front side of the wafer is almost finished. The local backside etching (LBE) process requires specific conditions on the front side to enable a save and reliable process on the backside (chemical-mechanical-polishing (CMP), photo-lithography, reactive-ion etching (RIE)). The in- and outlet of the fluidic channels are realized on a separate silicon wafer by different deep RIE processes. Finally both wafers were bonded permanently. With this approach high frequency circuits which were realized in the SiGe-BiCMOS front- and back end can be used for sensor applications to investigate small volumes of fluids in the µ-channels for instance in bio-medical applications. The paper reviews the monolithic integration of different modules and components to realize a multifunctional technology. We show challenges and benefits of this monolithic integration approach for the realization of a complex “More-than-Moore” technology.

Silicon Photonics for Advanced Optical Interconnections

After a brief review on silicon photonics research and development in China, the progress of silicon photonic components for coherent transceiver is described in details. The performance of the silicon modulators in high capacity, low-cost advanced intensity modulation and direct detection optical interconnection systems is also reported, and special properties of silicon photonic components, which lead to low power consumption, are discussed. These results show that silicon photonics has great prospect in advanced optical interconnection systems.

Flexible single-crystal silicon nanomembrane photonic crystal cavity.

Flexible inorganic electronic devices promise numerous applications, especially in fields that could not be covered satisfactorily by conventional rigid devices. Benefits on a similar scale are also foreseeable for silicon photonic components. However, the difficulty in transferring intricate silicon photonic devices has deterred widespread development. In this paper, we demonstrate a flexible single-crystal silicon nanomembrane photonic crystal microcavity through a bonding and substrate removal approach. The transferred cavity shows a quality factor of 2.2×10(4) and could be bent to a curvature of 5 mm radius without deteriorating the performance compared to its counterparts on rigid substrates. A thorough characterization of the device reveals that the resonant wavelength is a linear function of the bending-induced strain. The device also shows a curvature-independent sensitivity to the ambient index variation.

Optical Phase Characterization of Photonic Integrated Devices

We propose a relatively simple experimental setup, capable of accurately characterizing the optical phase response of an integrated photonic circuit. The setup is based on a phase-noise reduction scheme using an external heterodyne Mach-Zehnder interferometer. In particular, we characterize the phase response of different silicon photonic components: under- and over-coupled ring resonators, and a slow-light corrugated waveguide.

Low-Loss Silicon Nitride AWG Demultiplexer Heterogeneously Integrated With Hybrid III–V/Silicon Photodetectors

A unique and novel platform that combines III-V and silicon photonic components with ultra-low loss silicon nitride waveguides for monolithic integration of novel photonic circuits is presented. Successful (proof-of-principle) integration of eight hybrid III-V/silicon photodetectors and an arrayed waveguide grating is shown. The InGaAs photodiodes in this platform had average fiber-coupled responsivity of 0.36 A/W at 1550 nm, 30 GHz electrical bandwidth, and operated up to 50 Gb/s. The AWG had an insertion loss of 0.85 dB and adjacent-channel cross-talk less than -38 dB.

Efficient WDM Laser Sources Towards Terabyte/s Silicon Photonic Interconnects

Efficient multi-wavelength sources are essential to realize sub-pJ/bit modulated silicon photonic links for multi-TB/s interconnect applications. From a complete link perspective, we discuss the efficiency requirement for silicon WDM on-chip sources. A waveguide-coupled wall-plug efficiency (WPE) better than 10% is important to complement ultra-low power silicon photonic components for practical implementations. We propose a non-invasive hybrid, flip-chip integration approach using surface-normal coupling of an un-cooled III/V gain medium with a tunable wavelength selective silicon reflector. We show that a tunable, external-cavity WDM laser using such a hybrid integration approach can achieve high waveguide-coupled WPE when optimized. A proof-of-concept demonstration achieved a C-band tunable laser with an output power of more than 6 mW in the silicon waveguide, a tuning range of 15 nm, and a WPE of over 4.5%.

Permanent fine tuning of silicon microring devices by femtosecond laser surface amorphization and ablation

We demonstrate the fine tuning capability of femtosecond laser surface modification as a permanent trimming mechanism for silicon photonic components. Silicon microring resonators with a 15 µm radius were irradiated with single 400 nm wavelength laser pulses at varying fluences. Below the laser ablation threshold, surface amorphization of the crystalline silicon waveguides yielded a tuning rate of 20 ± 2 nm/J · cm(-2)with a minimum resonance wavelength shift of 0.10nm. Above that threshold, ablation yielded a minimum resonance shift of -1.7 nm. There was some increase in waveguide loss for both trimming mechanisms. We also demonstrated the application of the method by using it to permanently correct the resonance mismatch of a second-order microring filter.

Colorless Optical Network Unit Based on Silicon Photonic Components for WDM PON

We demonstrate a low-cost colorless optical network unit (ONU) utilizing silicon photonic components for wavelength division multiplexed passive-optical-networks. At the ONU, a waveguide-coupled microring works as a demultiplexer for separating the downstream signal from the centrally distributed continuous-wave (CW) light. The 10-Gb/s downstream signal is received using a waveguide-integrated germanium photodetector while the CW light is further modulated at 5 Gb/s using a silicon microring modulator for upstream signal generation. Error-free transmission over 25-km single mode fiber is achieved with 0.2- and 0.4-dB power penalties for the downstream and upstream signals, respectively. Complementary metal-oxide semiconductor-compatible silicon photonic technology offers the potential for monolithic integration and mass production.

Electron beam lithography writing strategies for low loss, high confinement silicon optical waveguides

The authors present a robust process for fabricating passive silicon photonic components by direct-write electron beam lithography (EBL). Using waveguide transmission loss as a metric, we study the impact of EBL writing parameters on waveguide performance and writing time. As expected, write strategies that reduce sidewall roughness improve waveguide loss and yield. In particular, averaging techniques such as overlap or field shift writing reduce loss, however, the biggest improvement comes from writing using the smaller field-size option of our EBL system. The authors quantify the improvement for each variation and option, along with the tradeoff in writing time.

Progress in Low-Power Switched Optical Interconnects

Optical links have successfully displaced electrical links when their aggregated bandwidth-distance product exceeds ~100 Gb/s-m because their link energy per bit per unit distance is lower. Optical links will continue to be adopted at distances of 1 m and below if link power falls below 1 pJ/bit/m. Providing optical links directly to a switching/routing chip can significantly improve the switched energy/bit. We present an early experimental switched CMOS-vertical-cavity surface-emitting laser (VCSEL) system operating at Gigabit Ethernet line rates that achieves a switched interconnect energy of less than 19 pJ/bit for a fully nonblocking network with 16 ports and an aggregate capacity of 20 Gb/s/port. The CMOS-VCSEL switch achieves an optical bandwidth density of 37 Gb/s/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> even when operating at a modest line rate of 1.25 Gb/s and is capable of scaling to much higher peak bandwidth densities (~350 Gb/s/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) with 5-10 pJ/switched bit. We also review a silicon photonic system design that will lower link energies to 300 fJ/bit, while providing multiterabits per second per square millimeter bandwidth densities. This system will ultimately provide switched optical interconnect at less than a picojoule per switched bit and computer/router system energies of tens of picojoule per bit. We review progress made to date on the silicon photonic components and analyze an energy and bandwidth-density roadmap for future advances toward these goals.