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A novel class of programmable integrated photonic circuits has emerged over the past years, strongly driven by approaches to tackle unsolved computing problems in the optical domain. Photonic neuromorphic and quantum computing are examples of optical systems implemented in complex photonic circuits, which are reconfigured before and during operation. However, a key building block to enable efficient reconfigurable optical network architectures is still missing: a non-volatile optical phase shifter. Here we demonstrate such an element—compatible with silicon photonics—based on the monolithic integration of BaTiO3 thin films with silicon waveguides. By manipulating ferroelectric domains in BaTiO3 with electrical control signals, we achieve analogue and non-volatile optical phase tuning with no absorption changes. We demonstrate an eight-level long-term-stable photonic device with non-destructive optical readout and switching energy as low as 4.6 pJ. With our results, an analogue non-volatile photonic element is added to the integrated photonics toolbox, enabling a new generation of power-efficient programmable photonic circuits. ISSN:1749-4885 ISSN:1749-4893

A Ferroelectric, two-terminals, analog memristive device is fabricated with a Back-End-Of-Line, CMOS compatible process. A bilayer composed of a ferroelectric material, HfZrO4 (HZO) and a semiconducting oxide, WOx layer is comprised between two TiN electrodes. The devices demonstrate reversible and remanent resistive switching, with a record endurance (>10(10) switching cycles) and ON/OFF ratio up to 10. The analog resistive switching is obtained, with a cycle-to-cycle reproducibility of 90%. The synaptic behavior is explored with pulses of varying sign, length, or amplitude. For the scheme with constant pulse width the linearity coefficients for the potentiation and depression are -1.4 and 4. The dynamics of the resistive switching is shown to be governed by the ferroelectric domains switching. Finally, temperature dependent transport measurements indicate Ohmic conduction at low bias and Modified Schottky Emission at larger bias. They provide insights on the role of defects and oxygen vacancies in the transport and resistive switching mechanisms., IEEE Journal of the Electron Device Society, 9, ISSN:2168-6734

To benefit from the diverse functionalities of perovskite oxides in silicon-based complementary metal oxide semiconductor (CMOS) technology, integrating oxides into a silicon platform has become one of the major tasks for oxide research. Using the deposition of LaMnO3/SrTiO3 (STO) superlattices (SLs) as a case study, we demonstrate that (001) single oriented oxide SLs can be integrated on Si using various template techniques, including a single-layer buffer of STO prepared by molecular beam epitaxy (MBE) and pulsed laser deposition, a multilayer buffer of Y-stabilized zirconia/CeO2/LaNiO3/STO, and STO-coated two-dimensional nanosheets of Ca2Nb3O10 (CNO) and reduced graphene oxide. The textured SL grown on STO-coated CNO nanosheets shows the highest crystallinity, owing to the small lattice mismatch between CNO and STO as well as less clamping from a Si substrate. The epitaxial SL grown on STO buffer prepared by MBE suffers the largest thermal strain, giving rise to a strongly suppressed saturation magnetization but an enhanced coercive field, as compared to the reference SL grown on an STO single crystal. These optional template techniques used for integrating oxides on Si are of significance to fulfill practical applications of oxide films in different fields.

Integrated ferroelectric plasmonic modulators featuring large bandwidths, broad optical operation range, resilience to high temperature and ultracompact footprint are introduced. Measurements show a modulation bandwidth of 70 GHz and a temperature stability up to 250{\deg}C. Mach-Zehnder interferometer modulators with 10-$\mu$m-long phase shifters were operated at 116 Gbit/s PAM-4 and 72 Gbit/s NRZ. Wide and open eye diagrams with extinction ratios beyond 15 dB were found. The fast and robust devices are apt to an employment in industrial environments.

Plasmonics has emerged as a promising technological solution for realizing high-performance nanoscale communication photonic devices. This paper reports our recent advances on high-performance plasmonic modulators and photodetectors. ISSN:0277-786X

A Ferroelectric Analog Non-Volatile Memory based on a WOx electrode and ferroelectric HfZrO 4 layer is fabricated at a low thermal budget (∼375°C), enabling BEOL processes and CMOS integration. The devices show suitable properties for integration in crossbar arrays and neuralnetwotk inference: analog potentiation/depression with constant field or constant pulse width schemes, cycle to cycle and device to device variation

We demonstrate a non-volatile optical memory element integrated in silicon photonics for low-power reconfigurable photonic circuits and neural networks. Stable transmission states are set by manipulating ferroelectric domains in BaTiO3 films embedded in photonic waveguides.

A Ferroelectric Analog Non-Volatile Memory based on a WO x electrode and ferroelectric HfZrO 4 layer is fabricated at a low thermal budget (~375°C), enabling BEOL processes and CMOS integration. The devices show suitable properties for integration in crossbar arrays and neural network inference: analog potentiation/depression with constant field or constant pulse width schemes, cycle to cycle and device to device variation

We enable a new class of electro-optic components based on a strong Pockels effect (>900 pm/V) in photonic integrated circuits. Integration concepts and examples of high-speed electro-optic modulators and ultra-low power tuning elements are discussed.

Physical Review Materials, 4 (3), ISSN:2475-9953

The integration of oxides on Si remains challenging, which largely hampers the practical applications of oxide-based electronic devices with superior performance. Recently, $\mathrm{LaMn}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (LMO/STO) heterostructures have gained renewed interest for the debating origin of the ferromagnetic-insulating ground state as well as for their spin-filter applications. Here we report on the structural and magnetic properties of high-quality LMO/STO heterostructures grown on silicon. The chemical abruptness across the interface was investigated by atomic-resolution scanning transmission electron microscopy. The difference in the thermal expansion coefficients between LMO and Si imposed a large biaxial tensile strain to the LMO film, resulting in a tetragonal structure with $c/a\ensuremath{\sim}0.983$. Consequently, we observed a significantly suppressed ferromagnetism along with an enhanced coercive field, as compared to the less distorted LMO film ($c/a\ensuremath{\sim}1.004$) grown on STO single crystal. The results are discussed in terms of tensile-strain enhanced antiferromagnetic instabilities. Moreover, the ferromagnetism of LMO on Si sharply disappeared below a thickness of 5 unit cells, in agreement with the LMO/STO case, pointing to a robust critical behavior irrespective of the strain state. Our results demonstrate that the growth of oxide films on Si can be a promising way to study the tensile-strain effects in correlated oxides, and also pave the way towards the integration of multifunctional oxides on Si with atomic-layer control.

Neuromorphic computing architectures enable the dense co-location of memory and processing elements within a single circuit. This co-location removes the communication bottleneck of transferring data between separate memory and computing units as in standard von Neuman architectures for data-critical applications including machine learning. The essential building blocks of neuromorphic systems are non-volatile synaptic elements such as memristors. Key memristor properties include a suitable non-volatile resistance range, continuous linear resistance modulation and symmetric switching. In this work, we demonstrate voltage-controlled, symmetric and analog potentiation and depression of a ferroelectric Hf$_{57}$Zr$_{43}$O$_{2}$ (HZO) field effect transistor (FeFET) with good linearity. Our FeFET operates with a low writing energy (fJ) and fast programming time (40 ns). Retention measurements have been done over 4-bits depth with low noise (1%) in the tungsten oxide (WO$_{x}$) read out channel. By adjusting the channel thickness from 15nm to 8nm, the on/off ratio of the FeFET can be engineered from 1% to 200% with an on-resistance ideally >100 kOhm, depending on the channel geometry. The device concept is using earth-abundant materials, and is compatible with a back end of line (BEOL) integration into complementary metal-oxidesemiconductor (CMOS) processes. It has therefore a great potential for the fabrication of high density, large-scale integrated arrays of artificial analog synapses., 14 pages, 5 figures, supplementary information available, submitted to ACS Applied Materials & Interfaces

Epitaxial growth of SrTiO3 (STO) on silicon greatly accelerates the monolithic integration of multifunctional oxides into the mainstream semiconductor electronics. However, oxide superlattices (SLs), the birthplace of many exciting discoveries, remain largely unexplored on silicon. In this work, LaNiO3/LaFeO3 SLs are synthesized on STO-buffered silicon (Si/STO) and STO single-crystal substrates, and their electronic properties are compared using dc transport and X-ray absorption spectroscopy. Both sets of SLs show a similar thickness-driven metal-to-insulator transition, albeit with resistivity and transition temperature modified by the different amounts of strain. In particular, the large tensile strain promotes a pronounced Ni (Formula presented.) orbital polarization for the SL grown on Si/STO, comparable to that reported for LaNiO3 SL epitaxially strained to DyScO3 substrate. Those results illustrate the ability to integrate oxide SLs on silicon with structure and property approaching their counterparts grown on STO single crystal, and also open up new prospects of strain engineering in functional oxides based on the Si platform. © 2020 The Authors. Advanced Materials published by Wiley-VCH GmbH

We demonstrate PIC-based non-volatile optical synaptic elements, an essential building block in large non-von Neumann circuits realized in integrated photonics. The impact of non-idealities on the performance of a photonic recurrent neural networks is evaluated.

We demonstrate for the first time the heterogeneous co-integration of Si photonics, BTO/Si for high-speed modulation and III-V materials for photodetection and emission. We show light coupling with losses

Microelectronics is as pervasive as it is invisible. It is however impacting every moment and aspect of our daily lives and has radically transformed all industries. Yet, this adventure began only about 60 years ago, with the first integrated circuits by J. Kilby and R. Noyce.

To develop a new generation of high-speed photonic modulators on silicon-technology-based photonics, new materials with large Pockels coefficients have been transferred to silicon substrates. Previous approaches focus on realizing stand-alone devices on dedicated silicon substrates, incompatible with the fabrication process in silicon foundries. In this work, we demonstrate monolithic integration of electro-optic modulators based on the Pockels effect in barium titanate (BTO) thin films into the back-end-of-line of a photonic integrated circuit (PIC) platform. Molecular wafer bonding allows fully PIC-compatible integration of BTO-based devices and is, as shown, scalable to 200 mm wafers. The PIC-integrated BTO Mach-Zehnder modulators outperform conventional Si photonic modulators in modulation efficiency, losses, and static tuning power. The devices show excellent V{\pi}L (0.2 Vcm) and V{\pi}L{\alpha} (1.3 VdB), work at high speed (25 Gbps), and can be tuned at low static power consumption (100 nW). Our concept demonstrates the possibility of monolithic integration of Pockels-based electro-optic modulators in advanced silicon photonic platforms. {\c} 2019 Optical Society of America. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved. https://www.osapublishing.org/jlt/abstract.cfm?URI=jlt-37-5-1456 Publication date: March 1, 2019 This work was supported in part by the European Union (EU) under Horizon 2020 grant agreements no. H2020-ICT-2015-25-688579 (PHRESCO) and H2020-ICT-2017-1-780997 (plaCMOS).

Integrated photonics is a key technology platform for optical communication, sensing, and data processing. Driven by the success of the CMOS industry and the resulting maturity of silicon-based fabrication methods, silicon photonics has evolved as an important candidate to realize integrated photonic circuits (PICs) in a cost-efficient and scalable way. Electrical control of the optical properties is critical in PICs: Fast electro-optical modulators are essential to reach high data rates, low-loss switches are needed to dynamically reconfigure networks, and low-power tuning elements are important to compensate temperature fluctuations. Traditionally, such components are implemented in silicon photonics by exploiting the plasma-dispersion effect [1] or Joule heating [2], which are, however, intrinsically linked with optical absorption or high power consumption. These challenges could be solved by using the Pockels effect as an electro-optic switching mechanism. However, because silicon lacks a Pockels effect, other materials with a non-vanishing Pockels effect need to be integrated on the technology platform.

Reservoir computing (RC) is a promising implementation of a non-von Neumann computing architecture. It is well-suited to solve time-dependent and dynamic problems like speech recognition or bitwise operations [1]. In principal integrated photonic RC offers energy-efficient, high-speed signal processing capabilities [1,2]. However, current photonic RC systems perform the linear combination of the complex reservoir states in software. Software weights are ideal (high precision, no drift), but limit the processing speed and energy efficiency of the RC system [2]. Here, we discuss a hardware implementation of photonic weights compatible with silicon photonic RC architectures. First, we demonstrate non-volatile synaptic weights based on ferroelectric barium titanate (BTO) thin films. Second, we explore how imperfections in these hardware weights impact the reservoir performance.

We demonstrate the first electro-optic switch operating at cryogenic temperatures of 4 K with a high electrooptic bandwidth of > 18 GHz. Our novel technology exploits the Pockels effect in barium titanate thin films co-integrated with silicon photonics and offers low losses, pure phase modulation, and sub-pW electro-optic tuning.

The electro-optical Pockels effect is an essential nonlinear effect used in many applications. The ultrafast modulation of the refractive index is, for example, crucial to optical modulators in photonic circuits. Silicon has emerged as a platform for integrating such compact circuits, but a strong Pockels effect is not available on silicon platforms. Here, we demonstrate a large electro-optical response in silicon photonic devices using barium titanate. We verify the Pockels effect to be the physical origin of the response, with r42 = 923 pm V−1, by confirming key signatures of the Pockels effect in ferroelectrics: the electro-optic response exhibits a crystalline anisotropy, remains strong at high frequencies, and shows hysteresis on changing the electric field. We prove that the Pockels effect remains strong even in nanoscale devices, and show as a practical example data modulation up to 50 Gbit s−1. We foresee that our work will enable novel device concepts with an application area largely extending beyond communication technologies. Electro-optic modulators based on epitaxial barium titanate (BTO) integrated on silicon exhibit speeds up to 50 Gbit s–1 while the Pockels coefficient of the BTO film is found to be approaching the bulk value.

Integrated electrical and photonic circuits (PIC) operating at cryogenic temperatures are fundamental building blocks required to achieve scalable quantum computing, and cryogenic computing technologies. Optical interconnects offer better performance and thermal insulation than electrical wires and are imperative for true quantum communication. Silicon PICs have matured for room temperature applications but their cryogenic performance is limited by the absence of efficient low temperature electro-optic (EO) modulation. While detectors and lasers perform better at low temperature, cryogenic optical switching remains an unsolved challenge. Here we demonstrate EO switching and modulation from room temperature down to 4 K by using the Pockels effect in integrated barium titanate (BaTiO3)-based devices. We report the nonlinear optical (NLO) properties of BaTiO3 in a temperature range which has previously not been explored, showing an effective Pockels coefficient of 200 pm/V at 4 K. We demonstrate the largest EO bandwidth (30 GHz) of any cryogenic switch to date, ultra-low-power tuning which is 10^9 times more efficient than thermal tuning, and high-speed data modulation at 20 Gbps. Our results demonstrate a missing component for cryogenic PICs. It removes major roadblocks for the realisation of novel cryogenic-compatible systems in the field of quantum computing and supercomputing, and for interfacing those systems with the real world at room-temperature.

Silicon based photonic integrated circuits (Si-PIC) are becoming a key contender for data center related transceivers. Available Si-PIC platforms use fast and well-integrated modulators based on the plasma-dispersion effect. However, they intrinsically suffer from high-loss and coupled amplitude and phase modulation. Using the Pockels effect enables highly efficient, low-loss phase shifters without residual amplitude modulation. Hybrid barium titanite (BaTiO3) Siphotonics technology with a strong Pockels effect on large-scale silicon substrates has emerged as a platform to implement such devices. The platform enables scalable, integrated Si-BaTiO3 devices with large Pockels coefficients for electro-optic modulation. Based on this platform, various electro-optic devices such as low-power tuning elements, monolithically integrated Si-BaTiO3 modulators, and ultra-compact plasmonic BaTiO3 modulators have been demonstrated.