High-yield, wafer-scale fabrication of ultralow-loss, dispersion-engineered silicon nitride photonic circuits

Low-loss photonic integrated circuits and microresonators have enabled a wide range of applications, such as narrow-linewidth lasers and chip-scale frequency combs. To translate these into a widespread technology, attaining ultralow optical losses with established foundry manufacturing is critical. Recent advances in integrated Si3N4 photonics have shown that ultralow-loss, dispersion-engineered microresonators with quality factors Q>10x10(6) can be attained at die-level throughput. Yet, current fabrication techniques do not have sufficiently high yield and performance for existing and emerging applications, such as integrated travelling-wave parametric amplifiers that require meter-long photonic circuits. Here we demonstrate a fabrication technology that meets all requirements on wafer-level yield, performance and length scale. Photonic microresonators with a mean Q factor exceeding 30x10(6), corresponding to 1.0 dBm(-1) optical loss, are obtained over full 4-inch wafers, as determined from a statistical analysis of tens of thousands of optical resonances, and confirmed via cavity ringdown with 19 ns photon storage time. The process operates over large areas with high yield, enabling 1-meter-long spiral waveguides with 2.4 dBm(-1) loss in dies of only 5x5 mm(2) size. Using a response measurement self-calibrated via the Kerr nonlinearity, we reveal that the intrinsic absorption-limited Q factor of our Si3N4 microresonators can exceed 2x10(8). This absorption loss is sufficiently low such that the Kerr nonlinearity dominates the microresonator's response even in the audio frequency band. Transferring this Si3N4 technology to commercial foundries can significantly improve the performance and capabilities of integrated photonics. For widespread technological application of nonlinear photonic integrated circuits, ultralow optical losses and high fabrication throughput are required. Here, the authors present a CMOS fabrication technique that realizes integrate photonic microresonators on waver-level with mean quality factors exceeding 30 million and 1 dB/m optical losses.

Fabrication and Characterization of Freestanding Single Crystal Diamond and Silicon Microresonators

Teodoro Graziosi

Microresonators are fundamental building blocks of any photonic integrated circuit, as they can be used as filters, modulators, sensors, and to enhance light emission. If these resonators are suspended and are free to oscillate, we can exploit the interaction of the optical and mechanical resonance. Demonstration of the fundamental interactions between the optical and mechanical fields, as well as applications based on this physics, were already presented for other material systems such as silicon, silicon oxide, or silicon nitride. Single crystal diamond is a strong candidate to realise high quality resonators due to the excellent material properties. The mechanical properties, such as stiffness, low intrinsic damping, and high thermal conductivity, are critical for mechanical oscillators with high frequency and high quality factor, which is strongly correlated to the noise of the oscillator. The wide transparency range and the low absorption enable operation in a large wavelength range spanning from near ultraviolet to far infrared. Diamond can host very bright emitters, based on defects of the diamond lattice, which can be employed as single photon sources, quantum memories, or very sensitive magnetic sensors. Chemical resistance to most acids or bases permit operations in aggressive environments. For these reasons, single crystal diamond is an attractive platform for integrated photonics and micro- or nanomechanics, and it should be possible to realize low noise optomechanical resonators. However, compared to more established material systems, microfabrication of single crystal diamond is not easy. High quality single crystal diamond substrates are available, to date, only as bulk plates, therefore it is important to develop fabrication strategies that isolate optically and mechanically a thin diamond layer from the rest of the diamond substrate. Over the last years, several approaches have been proposed and in this work two methods will be employed: 3D milling using a focused ion beam to create micrometer sized disks supported by a narrow pillar; and etching of the bulk diamond using a plasma which is selective to particular crystal planes of the single crystal diamond. Using these processes, single crystal diamond microresonators were realized, measuring optical quality factors of 5700 and 42000 at telecom wavelengths, respectively, and of 3100 and 10500 at visible wavelengths, respectively. Single crystal silicon is similar in many aspects to single crystal diamond, with the material properties being slightly worse. However, silicon benefits from decades of knowledge developed for integrated electronic circuits and integrated silicon photonics circuits. Silicon photonics is now extensively used in telecommunications to interface the optical links to electronics. Commercial foundries started to offer silicon photonics services, either within the CMOS fabrication or with dedicated processes. Micro Electro Mechanical Systems represent a way to expand on the capability of photonic integrated circuits by providing low power and/or reconfigurable components. In this context, optomechanical oscillators were realized by postprocessing an IMEC iSiPP50G silicon photonics chip. Optical quality factors of 300000 were measured at telecom wavelengths. Mechanical oscillations are supported at 250 MHz, with quality factors of 1800 measured at ambient conditions.

EPFL2019

Microstructure and ferroelectricity of BaTiO3 thin films on Si for integrated photonics

Patrik Hoffmann, Chiara Marchiori, Michael Oliver Reinke

Significant progress has been made in integrating novel materials into silicon photonic structures in order to extend the functionality of photonic circuits. One of these promising optical materials is BaTiO3 or barium titanate (BTO) that exhibits a very large Pockels coefficient as required for high-speed light modulators. However, all previous demonstrations show a noticable reduction of the Pockels effect in BTO thin films deposited on silicon substrates compared to BTO bulk crystals. Here, we report on the strong dependence of the Pockels effect in BTO thin films on their microstructure, and provide guidelines on how to engineer thin films with strong electro-optic response. We employ several deposition methods such as molecular beam epitaxy and chemical vapor deposition to realize BTO thin films with different morphology and crystalline structure. While a linear electro-optic response is present even in porous, polycrystalline BTO thin films with an effective Pockels coefficient r(eff). = 6 pmV(-1), it is maximized for dense, tetragonal, epitaxial BTO films (r(eff) = 140 pm V-1). By identifying the key structural predictors of electro-optic response in BTO/Si, we provide a roadmap to fully exploit the linear electro-optic effect in novel hybrid oxide/semiconductor nanophotonic devices.

Institute of Physics2017

Low-noise frequency-agile photonic integrated lasers for coherent ranging

Jijun He, Tobias Kippenberg, Junqiu Liu, Vladimir Shadymov, Anat Siddharth, Viacheslav Snigirev, Andrey Voloshin, Rui Ning Wang, Wenle Weng

Stable and tunable integrated lasers are fundamental building blocks for applications from spectroscopy to imaging and communication. Here the authors present a narrow linewidth hybrid photonic integrated laser with low frequency noise and fast linear wavelength tuning. They then provide an efficient FMCW LIDAR demonstration. Frequency modulated continuous wave laser ranging (FMCW LiDAR) enables distance mapping with simultaneous position and velocity information, is immune to stray light, can achieve long range, operate in the eye-safe region of 1550 nm and achieve high sensitivity. Despite its advantages, it is compounded by the simultaneous requirement of both narrow linewidth low noise lasers that can be precisely chirped. While integrated silicon-based lasers, compatible with wafer scale manufacturing in large volumes at low cost, have experienced major advances and are now employed on a commercial scale in data centers, and impressive progress has led to integrated lasers with (ultra) narrow sub-100 Hz-level intrinsic linewidth based on optical feedback from photonic circuits, these lasers presently lack fast nonthermal tuning, i.e. frequency agility as required for coherent ranging. Here, we demonstrate a hybrid photonic integrated laser that exhibits very narrow intrinsic linewidth of 25 Hz while offering linear, hysteresis-free, and mode-hop-free-tuning beyond 1 GHz with up to megahertz actuation bandwidth constituting 1.6 x 10(15) Hz/s tuning speed. Our approach uses foundry-based technologies - ultralow-loss (1 dB/m) Si3N4 photonic microresonators, combined with aluminium nitride (AlN) or lead zirconium titanate (PZT) microelectromechanical systems (MEMS) based stress-optic actuation. Electrically driven low-phase-noise lasing is attained by self-injection locking of an Indium Phosphide (InP) laser chip and only limited by fundamental thermo-refractive noise at mid-range offsets. By utilizing difference-drive and apodization of the photonic chip to suppress mechanical vibrations of the chip, a flat actuation response up to 10 MHz is achieved. We leverage this capability to demonstrate a compact coherent LiDAR engine that can generate up to 800 kHz FMCW triangular optical chirp signals, requiring neither any active linearization nor predistortion compensation, and perform a 10 m optical ranging experiment, with a resolution of 12.5 cm. Our results constitute a photonic integrated laser system for scenarios where high compactness, fast frequency actuation, and high spectral purity are required.