This PDF file contains the front matter associated with SPIE Proceedings Volume 10922, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

INVITED PRESENTATION Distributed feedback (DFB) fiber Bragg grating (FBG) Raman fiber lasers (RFL) were first analysed by Perlin and Winful [1], predicting that meter long uniform FBGs could be used as a Raman oscillator to generate any wavelength with several 10’s watts of pump power. Pi phase shifted distributed feedback grating structures improved the characteristics of the laser significantly [2]. These Raman fiber lasers have improved over the years with a threshold of below 1W. Our own work resulted in several demonstrations of single frequency RFLs and random Raman fiber lasers (rRFL) up to 1 meter long, as well as the lowest reported threshold of 350mW for a RFL. This has been possible only by painstaking analysis and correction of the poor quality of FBGs. The slope efficiency of these lasers, however, is well below that predicted, limited by nonlinear optical effects. This paper will present the state-of-the-art in ultra-long FBGs needed for compact Raman fiber lasers, demonstrate some novel solutions and discuss the limiting issues remaining for improving performance. References: 1. V. E. Perlin and H. G. Winful, "Distributed feedback fiber Raman laser," IEEE Journal of Quantum Electronics 37, 38-47 (2001). 2. Y. Hu and N. G. R. Broderick, "Improved design of a DFB Raman fibre laser," Optics Communications 282, 3356-3359 (2009).

Machine-assisted design of integrated photonic devices (e.g. through optimization and inverse design methods) is opening the possibility of exploring very large design spaces, novel functionalities and non-intuitive geometries. These methods are generally used to optimize performance figures-of-merit. On the other hand, the effect of manufacturing variability remains a fundamental challenge since small fabrication errors can have a significant impact on light propagation, especially in high-index-contrast platforms. Brute-force analysis of these variabilities during the main optimization process can become prohibitive, since a large number of simulations would be required. To this purpose, efficient stochastic techniques integrated in the design cycle allow to quickly assess the performance robustness and the expected fabrication yield of each tentative device generated by the optimization. In this invited talk we present an overview of the recent advances in the implementation of stochastic techniques in photonics, focusing in particular on stochastic spectral methods that have been regarded as a promising alternative to the classical Monte Carlo method. Polynomial chaos expansion techniques generate so called surrogate models by means of an orthogonal set of polynomials to efficiently represent the dependence of a function to statistical variabilities. They achieve a considerable reduction of the simulation time compared to Monte Carlo, at least for mid-scale problems, making feasible the incorporation of tolerance analysis and yield optimization within the photonic design flow.

We report the simulation, design and experimental validation of various PECVD silicon nitride photonic building blocks required for the implementation of a CMOS-compatible photonic integrated circuit technology platform operating in the 850 nm and 600 nm wavelength domain. In particular, we discuss an inverted taper structure for efficient coupling of light to and from the chip, propagation and bend losses as well as broadband power and polarization beam splitters in the 850 nm region. In the 600 nm wavelength region, we demonstrate the realization of an optically pumped integrated dye-doped polymer laser that couples its laser light directly into a silicon nitride waveguide.

Various aspects of grating coupler development are presented to illustrate how the performance and yield of this key component of our silicon photonics platform has improved over recent years. The device design concept, fabrication and testing are discussed in order to give a more general insight into the challenges faced in increasing the maturity level of silicon photonics platforms. In particular, we will highlight some key areas in which tools and solutions developed for the CMOS electronics industry have proven inapplicable in the case of photonics, requiring that established methods be heavily modified or replaced.

Subwavelength gratings (SWG) enable the synthesis of dispersion-engineered photonic metamaterials, leading to unprecedented performance in silicon photonic devices. In this work, we further explore the potential of this technology by presenting an ultra-broadband mode de/multiplexer (DE/MUX) and a polarization beamsplitter (PBS) with a novel approach for anisotropy engineering through tilted SWG structures. The proposed two-mode DE/MUX consists of a SWG-engineered multimode interference coupler (MMI), a 90º phase-shifter and a symmetric Y-junction. SWG structures are also included in fiber-to-chip couplers and in adiabatic transitions between Si-wire interconnect waveguides and the MMI. Simulated insertion losses of the proposed device are less than 0.18 dB in the wavelength range from 1.4 μm to 1.7 μm. These values further decrease down to 0.11 dB for the TE0 mode and 0.07 dB for the TE1 mode in the C-band wavelength range (1.53 – 1.57 μm). Crosstalk of both modes is below -20.6 dB in the wavelength range from 1.4 μm to 1.7 μm and below -36 dB within the C-band. The proposed PBS consists of an MMI incorporating tilted sub-wavelength gratings. This novel anisotropy engineering technique provides independent control on the propagation constant of each polarization, enabling the implementation of shorter devices with improved performance. An extinction ratio over 20 dB and insertion losses below 1.5 dB in a 116-nm-wide bandwidth are demonstrated, for a MMI length under 100 μm.

This talk begins with a description of how the properties of light become modified for propagation through a material for which the dielectric permittivity epsilon is nearly vanishing. In such a situation, the refractive index also nearly vanishes, and thus both the wavelength of light and the phase velocity of light become nearly infinite. Radiative processes also are strongly modified, with both the Einstein A and B coefficients being dependent on the refractive index of the material. We have recently found that nonlinear optical properties tend to be strongly enhanced in epsilon-near-zero (ENZ) materials [1]. For the case of indium-tin-oxide (ITO), we measured a huge value (10^6 times larger than that of fused silica) of the nonlinear coefficient n_2. In subsequent work, we have fabricated a metasurface consisting of gold nanorods on an ITO substrate, and we have found that the nonlinear coefficient is further enhanced and can be controlled in both magnitude and sign [2]. The talk then turns to a discussion of the implications of the use of ENZ materials as a platform for applications in the field of nanophotonics. 1. Large optical nonlinearity of indium tin oxide in its epsilon-near-zero region, M. Z. Alam, I. D. Leon, and R. W. Boyd, Science 352, 795 (2016).  2. Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material, M. Z. Alam, S. A. Schulz, J. Upham, I. De Leon and R. W. Boyd, Nature Photonics,12 79-83 (2018). 

Semiconductor nanowire lasers have revolutionized the field of photonics offering highly localised coherent light sources at the nanoscale. However, due to the ultra-small dimensions of nanowire lasers, their manipulation and accurate integration at desired locations on targeted surfaces and optoelectronic platforms is a fundamental challenge. This poses critical limitations for the development of complex and tailored nanophotonic circuits using nanowire lasers as building blocks. In this talk, I will present a novel nanoscale transfer printing technique enabling the controllable integration of individually-selected semiconductor nanowires onto multiple receiving substrates (e.g. polymer, silica, metals) and pre-patterned systems. We will show that this technique provides very high positioning accuracy (<1μm) and full control of the orientation angle of the printed nanowires. Hence, this new hybrid nanoscale transfer printing technique offers great potential for the fabrication of bespoke nanophotonic systems with ultra-small nanowire lasers at their core. During the talk we will also present our recent results demonstrating the precise formation of user-defined complex micrometric spatial patterns, such as 1- and 2-Dimensional arrays, using Indium Phosphide (InP) nanowires lasers as building blocks. Furthermore, we will introduce our work on the coupling of InP nanowire lasers onto waveguide systems (built on both planar and mechanically flexible substrates) for on-chip guiding of the nanowire’s emitted light and plasmonic nanoantennas for controlled light directionality. Finally, during the talk we will also review our ongoing activities towards new hybrid nanowire laser systems enabled by our nanoscale transfer printing technique.

Energy efficient 200+ Gbit/s single fiber data transmission systems can be realized by wavelength multiplexing the directly modulated Vertical-Cavity Surface-Emitting Lasers (VCSELs) presented here, emitting at the four wavelengths 850 nm, 880 nm, 910 nm, and 940 nm. Large energy efficiency defined by a heat to data ratio (HBR) of only 240 fJ/bit @ 50 Gb/s is observed. Tuning the cavity photon lifetime is demonstrated to lead to an increase of the data rate in concert with a reduction of the HDR. The large linearity of our L-I-characteristics will allow easily higher order modulation rates. Our results might impact ongoing discussions of new physical layer standards for IEEE 802.3cm and cd coarse wavelength multiplexing standards across OM5 multimode fiber enabling up to 400 Gbit/s error-free transmission.

Hyper scale datacenters drive amazing Ethernet volumes when thousands of servers are connected on one site, which creates high-density interconnect links between servers, switches and routers. Optical technologies are considered as possible solutions to meet the future datacenter requirements in terms of cost, power consumption, port density and performance. In this paper, evolution of optoelectronic integrated circuits technologies for reducing power consumption, improving port density and decreasing the cost of datacenter optical network, such as miniaturization of optoelectronic chips, high density optical coupling, electro-optical co-packaging and integration of nano-scale optical switch, will be outlined, as well as the progresses in research and industrial activities.

Efficient optical frequency mixing typically must accumulate over large interaction lengths because nonlinear responses in natural materials are inherently weak. This limits the efficiency of mixing processes owing to the requirement of phase matching. Here, we report efficient four-wave mixing (FWM) over micrometer-scale interaction lengths at telecommunications wavelengths on silicon. We used an integrated plasmonic gap waveguide that strongly confines light within a nonlinear organic polymer. The gap waveguide intensifies light by nanofocusing it to a mode cross-section of a few tens of nanometers, thus generating a nonlinear response so strong that efficient FWM accumulates over wavelength-scale distances. This technique opens up nonlinear optics to a regime of relaxed phase matching, with the possibility of compact, broadband, and efficient frequency mixing integrated with silicon photonics.

Wavelength-tunable semiconductor lasers have found widespread applications in many fields, such as optical communication systems and high-resolution spectroscopy, due to their wavelength-flexibility and reconfigurability. Wavelength-tunable semiconductor lasers with low phase noise are important in applications where coherent detection scheme is required, so it is therefore imperative to have a good understanding of the phase noise of these devices. This talk will present our investigations on the complicate phase noise characteristics of the wavelength tunable semiconductor lasers, and will specifically demonstrate results with two types of multi-section DBR based laser (the SGDBR and the MGY devices) for their applications in coherent systems.

With the development of micro/nano-scale fabrication technologies, smart active/passive photonic devices have been fabricated by using silicon/polymer materials, which show great potential applications in photonics and optoelectronics. The current fabrication techniques such as electron-beam lithography give a high resolution, but they are expensive and time-consuming. Here, we present some polymer-based photonic devices fabricated by 3D femtosecond laser writing through two-photon polymerization. The resolution can reach up to ~100 nm, which is less than 1/10 wavelength within the C-band. Hence, the fabricated photonic devices can be used for micro lasing and sensing application. In this research, we show the spectral characteristics of several photonic devices such as phase-shifted Bragg grating waveguides. Due to the properties of polymer materials, the devices have a higher sensitivity on acoustic waves that can modify the geometry of the waveguide and thus induce a change in the effective index of the mode, which can be utilized for designing ultrasonic sensors. Although the fabricated quality is lower than that of photonic devices fabricated by the electron-beam lithography, the results show our fabricated devices can be useful for inexpensive sensors for ultrasound detection, demonstrating the usability of the femtosecond laser writing technique for photonic applications.

Detection of terahertz (THz) radiation with high sensitivity and efficiency remains a challenging problem for THz technology development. A photoconductive THz detector based on the Auston switch concept is among the most sensitive and widely-used room-temperature coherent THz detectors. Plasmonic nano-antennas and gratings, and ultrathin optical cavities recently were introduced in the structure of THz optoelectronic devices to improve their efficiencies. It allowed obtaining higher photocurrents and smaller capacitance, which lead to improvements in the response speed and efficiency of THz detectors. Plasmonic nanostructures however introduce Ohmic losses, which can substantially reduce the responsivity of THz detectors. Alternatively, efficiency of photoconductive THz detectors can be enhanced by using all-dielectric metasurfaces. A metasurface containing only dielectric, instead of plasmonic nanostructures, can be designed to trap the incident light in a selected spectral range and thus enhance optical absorption. We designed an optically-thin photoconductive channel as an all-dielectric metasurface, which exhibited enhanced optical absorption at laser excitation wavelength. The metasurface comprised an array of low-temperature grown GaAs nanobeams and a sub-surface distributed Bragg reflector. Integrated into a photoconductive (Auston) switch, the metasurface improved the efficiency and sensitivity of the THz detector. Specifically, the detector produced photocurrents over one order of magnitude higher compared to a similar detector with unstructured surface with only 0.5 mW of optical excitation, while exhibiting high dark resistance required for low-noise detection in THz time-domain spectroscopy and imaging. At that level of optical excitation the metasurface detector showed a high signal to noise ratio of 10^6. We will discuss mechanisms responsible for the efficiency improvement, as well as the application of THz detectors with all-dielectric metasurfaces for enabling more sensitive integrated THz near-field probes.

Polaritons offer us the potential for strong light-matter interactions and nanoscale manipulation and confinement of light, by coupling light with oscillating free-charge carriers. A wide variety of optical phenomena result from such polaritonic media and these serve as a basic building block for most approaches towards infrared nanophotonic and metamaterial approaches. Recently, the strong interactions between oscillators of various types with polaritonic resonances and/or propagating modes have been reported in the context of enhanced molecular sensing and tailored absorber designs. Here I will discuss polaritonic strong coupling utilizing epsilon-near-zero and polaritonic thin films, demonstrating the potential to achieve ultrastrong coupling between these modes. These results will be discussed in the context of both surface plasmon and surface phonon polaritons, and the implications of the coupling effects and means to control the strength of the coupling also reported. Additionally, the such strong coupling between molecular absorbers and surface phonon polariton systems will be discussed, with discussions of how the strength of the vibrational-polariton coupling influences enhanced spectroscopy provided.

Wireless optical communication is a viable alternative to conventional RF technology. Our novel design combines optical communication and energy harvesting in one device with a size of 30 x 10 x 5 mm using the latest innovations in lowpower electronics and solar cell technology. In our study, we implement visible light communication between a sensor module and a smartphone. The proposed system design and a communication protocol are specifically developed for environments with illumination levels of 100 – 500 lux, like industrial halls. The sensor integrated into the module can vary according to application requirements. As an example, in our work, we use a temperature and pressure sensor and an accelerometer. A bright flash from a smartphones build-in LED activates the module. The module takes measurements and sends the result in form of an optical data signal, which is then received by the smartphone camera. This technique is able to provide reliable communication despite low-power restrictions of energy harvesting. By using a smartphone this approach offers more convenience to a user and enables flexible deployment of the modules in industrial machinery.

Silicon photonics is considered to be the leading platform to achieve faster data transfer speeds on-chip. However, the weak electro-optic coefficient of silicon limits the maximum achievable single channel data rates. A hybrid solution consisting of a silicon photonic backbone and an incorporated optical phase change material that provides improved optical functionality may provide the solution for realizing broadband, low power, small footprint on-chip photonic devices capable of achieving record modulation speed. In this presentation, we discuss theoretical and experimental work integrating vanadium dioxide and GST in thermo-optic, electro-optic, and all-optical silicon photonic devices. Future directions will also be discussed.

We present recent development on integrated flexible and stretchable photonic devices. Conventional photonic devices are fabricated on rigid semiconductor or dielectric substrates and are therefore inherently incompatible with soft biological tissues. Recently, we have developed a suite of active and passive photonic devices and systems integrated on plastic substrates which can be bent, twisted, and stretched without compromising their optical performance. Key innovations are monolithic multi-material integration and advanced micro-mechanical structures co-designed with photonic devices, which enables devices with extreme mechanical flexibility and excellent optical performance.

Design, fabrication, and measurement aspects of integrated optic devices based on ion-sliced lithium niobate combined with patterned silicon or silicon nitride are presented. Combining sub-micrometer thin films of lithium niobate with patterned thin film materials produces a platform for compact integrated optics with second order susceptibility. Hybrid silicon and lithium niobate waveguides are designed at 1550 nm wavelength with micrometer scale mode field diameter. Bend losses are less than 0.1 dB/cm for radii as small as 10 μm. Two hybrid silicon and lithium niobate electro-optic devices are shown, namely, an RF electric-field sensor with an experimentally demonstrated sensitivity of 4.5 V m^-1 Hz^-1/2 and an electro-optic ring modulator with experimentally demonstrated digital modulation of 4.5 Gb/s at 4.5 dB extinction ratio. A hybrid silicon nitride and lithium niobate device is also presented for quasi-phase matched second harmonic generation. Periodic poling of thin films of x-cut magnesium oxide doped lithium niobate has been achieved with a poling period of 7.5 μm. Chip-scale electro-optics and nonlinear optics are envisioned for classical and quantum communications, sensing, and computing applications.

Optical waveguides represent the key element of integrated planar photonic circuitry having revolutionized many fields of photonics ranging from telecommunications, medicine, environmental science and light generation. However, the use of solid cores imposes limitations on applications that demand controlling strong light-matter interaction within low permittivity media such as gases or liquids, which has triggered substantial interest towards the development of hollow core waveguides. Here, we introduce the concept of the on-chip hollow core light cage that consists of free standing arrays of cylindrical dielectric strands surrounding a central hollow core implemented by 3D nanoprinting. The cage operates by the anti-resonant guidance effect and exhibits extraordinary properties such as (1) diffraction-less propagation in “quasi-air” over more than a centimetre distance within the ultraviolet, visible and near-infrared spectral domains, (2) unique side-wise direct access to the hollow core via open spaces between the strands speeding up gas diffusion times by at least a factor of 10.000, and (3) an extraordinary high fraction of modal fields in the hollow section (> 99.9%). With these properties, the light cage can overcome the limitations of current planar hollow core waveguide technology, allowing unprecedented future on-chip applications within quantum technology, ultrafast spectroscopy, bioanalytics, acousto-optics, optofluidics and nonlinear optics.

With the exponential growth of bandwidth requirement of data centers and supercomputers, energy already limits our ability to process and communicate information. There is an urgent need to develop low-energy photonic devices and systems that can push the energy efficiency to attojoule/bit level. Traditional silicon photonic modulators rely on the plasma dispersion effect by free-carrier injection or depletion, which occupies a large footprint and consumes relatively high energy for optical interconnects. Here we report an ultra-compact hybrid silicon-conductive oxide electro-optic modulator with total device footprint of 0.6 × 8 μm2. The device was built by integrating voltage-switched transparent conductive oxide with one-dimensional silicon photonic crystal nanocavity. The active modulation volume is only 0.06 µm3, which is less than 2% of the lambda-cubic volume. The device operates in the dual mode of cavity resonance and optical absorption by exploiting the refractive index modulation from both the conductive oxide and the silicon waveguide induced by the applied gate voltage. Such a metal-free, hybrid silicon-conductive oxide nanocavity modulator also demonstrates only 0.5 dB extra optical loss, high E-O efficiency of 250pm/V, and low energy consumption of 3fJ/bit. In addition, we will discuss strategy to further improve the energy efficiency below 1fJ/bit and to achieve high-speed modulation above 10Gbps. The combined results achieved through the holistic design opened a new route for the development of next generation electro-optic modulators that can be used for future on-chip optical interconnects.

Integrated quantum photonic circuits are promising for an on-chip realization of a quantum advantage. It is desirable to develop a platform which allows dense integration of functionalities, which includes sources, photon processing units and detectors on the single photon level. Among the different material platforms currently being investigated, direct-bandgap semiconductors and particularly gallium arsenide offer the widest range of functionalities, including single and entangled-photon generation by radiative recombination, low-loss routing, electro-optic modulation, and single-photon detection. We summarize the potential and current status in the field of quantum integrated photonic components and circuits based on the GaAs technology platform.

Research in nanophotonic materials and design is yielding advances that are opening conceptually new paths to address “grand challenges” that have not previously been achievable. One of these is the realization of comprehensively tunable nanoantenna arrays, which can enable dynamic, active control of all of the key constitutive properties of light – amplitude, phase, wavevector and polarization – opening new applications such as phased-array optical beam steering, visible light modulation for communication and thermal radiation management. Electronic tuning of the complex dielectric function of nanostructured materials is enabling i) scientific exploration of new two-dimensional and layered materials such as graphene and black phosphorus and transition metal dichalcogenide and also ii) plasmonic and nanophotonic device applications for dynamic wavefront control, including electronic phase and amplitude modulators for the near infrared (conducting oxides) and mid infrared (graphene). I will discuss advances in the realization of dynamically tunable metasurfaces in the near-infrared and mid-infrared with tunable spontaneous emission rate, tunable emissivity, > π phase modulation, tunable control of polarization, and ‘perfect’ absorption approaching 100% in submonolayer thickness structures.

Photonic neural networks (PNN) are a promising alternative to electronic GPUs to perform machine-learning tasks. The PNNs value proposition originates from i) near-zero energy consumption for vector matrix multiplication once trained, ii) 10-100 ps short interconnect delays, iii) weak required optical nonlinearity to be provided via fJ/bit efficient emerging electrooptic devices. Furthermore, photonic integrated circuits (PIC) offer high data bandwidth at low latency, with competitive footprints and synergies to microelectronics architectures such as foundry access. This talk discusses recent advances in photonic neuromorphic networks and provides a vision for photonic information processors. Details include, 1) a comparison of compute performance technologies with respect to compute efficiency (i.e. MAC/J) and compute speed (i.e. MAC/s), 2) a discussion of photonic neurons, i.e. perceptrons, 3) architectural network implementations, 4) a broadcast-and-weight protocol, 5) nonlinear activation functions provided via electro-optic modulation, and 6) experimental demonstrations of early-stage prototypes. The talk will open up answering why neural networks are of interest, and concludes with an application regime of PNN processors which reside in deep-learning, nonlinear optimization, and real-time processing.

As a result of recent developments in nanofabrication techniques, the dimensions of metallic building blocks of plasmonic devices continue to shrink down to nanometer range thicknesses. The optical and electronic properties of ultra-thin plasmonic films are expected to have a strong dependence on the film thickness, composition, and strain, as well as an increased sensitivity to external optical and electrical perturbation. This unique tailorability establishes ultra-thin plasmonic films as an attractive material for the design of tailorable and dynamically switchable metasurfaces. Due to their epitaxial growth on lattice matched substrates, TiN is an ideal material to investigate the tailorable properties of plasmonic films with thicknesses of just a few monolayers. MXenes, a class of two-dimensional (2D) nanomaterials formed of transition metal carbides and carbon nitrides, are yet another promising material platform for tailorable plasmonic metamaterials. MXenes have been widely explored in a variety of applications, such as electromagnetic shielding and SERS. However, investigations of MXenes in the context of nanophotonics and plasmonics have been limited leading to this current exploration of MXenes as building blocks for plasmonic and metamaterial devices. In this study, we investigate these two emerging classes of materials, MXenes and ultra-thin transition metal nitrides, as potential material platforms for tailorable plasmonic metamaterials. We report on the strain and oxidation dependent optical properties of ultrathin TiN. Applications of MXenes as a broadband plasmonic metamaterial absorber and a random laser device are also discussed.

Programmable multifunctional integrated nanophotonics (PMIN) is a new paradigm that aims at designing common integrated optical hardware configurations, which by suitable programming can implement a variety of functionalities that can be elaborated for basic or more complex operations in many application fields. The strength of PMIN relies on the suitable interconnection of field-programmable waveguide arrays. Here, we review the recent advances reported in the field of PMIN, paying special attention to outlining the design principles, material platforms, synthesis algorithms and practical constraints of these structures. Finally, we discuss their applicability to different fields.

Silicon photonics attracted much attention in past decades, but it is challenging for silicon laser source because of its indirect bandgap. The long-wavelength InAs/GaAs quantum dot (QD) laser monolithically grown on Ge substrate has been reported. Promising performance was reported by solid source molecular beam epitaxy (MBE). In this paper, gas source MBE was tried for the growth of InAs QD lasers on Ge. InAs QD laser is demonstrated in continuous wave mode at room temperature, with wavelengths covering 1.0-1.3 microns. The lowest threshold current density was obtained as 48 A/cm2 with an output power of several tens of mW.

We have successfully fabricated electronic and photonic components on the same substrate using a standard process offered by public silicon photonics foundry service. As we adopted the standard process parameters offered by A*STAR’s Institute of Microelectronics (IME), the design and fabrication of the n- and p-regions of the electronic devices are limited to the specific parameters optimized for creating optical waveguide modulators and photodetectors. In this work, we chose to fabricate and integrate Lateral Bipolar Transistors (LBJTs) and Junction Field Effect Transistors (JFETs) transimpedance amplifiers with Ge waveguide photodetectors, as well as on-chip photonic components such as grating couplers, submicron strip waveguides and PN depletion modulators. Our experimental results shows rectifying Ge photodiode characteristics and demonstrates full integration for sensing applications.

Integrate LEDs and CMOS circuits on large Si wafers can enable numerous new applications and add new functions to Si integrated circuits. In the past efforts on the integration of AlGaInP LEDs and CMOS circuits on 200 mm Si wafers, we have solved fundamental problems such as III-V semiconductor heteroepitaxy on Si substrates, wafer bow control, and bonding of LED wafers with CMOS wafers. Our latest achievement in this work is the demonstration of working devices processed on 200 mm LED wafers. We will present our efforts on the development of CMOS-compatible Ohmic contacts, 200 mm wafer-scale processing, and characteristics of the devices. We have evaluated different metals as CMOS-compatible low-resistance Ohmic contacts to the AlGaInP LEDs. We will compare the performance of the LEDs using the different metal contacts. We will present our progress on the process of CMOS-bonded LED wafers. Different from the LED-only wafers, the process of CMOS-bonded LED wafers can only be done in opened trenches, which adds extra difficulties. In addition, we will show the method we have developed for the re-entry of the CMOS-LED integrated wafers to the CMOS foundries for the end-of-line metal interconnections. Finally, potential applications using the CMOS-integrated LEDs will be discussed.

Prof. Josè Azaña and I are pleased to accept the invitation of Prof. Jianji Dong to speak at the special session, focused on Heterogeneous Integration and Microwave photonics, at the Smart Photonic and Optoelectronic Integrated Circuits XXI of the Photonics West 2019. We look forward to the conference.

Various future system applications that involve photonic technology rely on our ability to integrate it on a chip to augment and/or interact with other signals (e.g., electrical, chemical, biomedical, etc.). For example, future computing and communication systems will need integration of photonic circuits with electronics and thus require miniaturization of photonic materials, devices and subsystems. Another example, involves integration of microfluidics with nanophotonics, where former is used for particle manipulation, preparation and delivery, and the latter in a large size array form parallel detection of numerous biomedical reactions useful for healthcare applications. To advance the nanophotonics technology we established design, fabrication and testing tools at UCSD. The design tools need to incorporate not only the electromagnetic equations, but also the material, quantum physics, thermal, etc. equations to include near field interactions. These designs are integrated with device fabrication and characterization to validate the device concepts and optimize their performance. Our research work emphasizes the construction of passive (e.g., engineered composite metamaterials, filters, etc.) and active (e.g., nanolasers) components on-chip, with the same lithographic tools as electronics. In this talk, we discuss progress in passive and active integrated photonic devices, circuits and systems that recently have been demonstrated in our labs.

Kerr frequency combs based on integrated microresonators are very attractive to microwave photonics for their compact size, broad spectrum, and high repetition rate. Promising applications include low-phase-noise microwave generation, radio over fiber transmission, signal processing, channelized receiver, etc. For microwave photonic applications, the power conversion efficiency, which means the ratio of the pump power converted to the comb lines, is a very important metric, because it is closely related to the optical-microwave conversion loss and noise figure of the microwave photonic link. In this talk, we discuss the mechanisms that limit the conversion efficiency and the configurations for highefficiency Kerr comb generation. An example of Kerr comb based microwave true-time-delay beamforming network is also introduced.

Emerging RF systems utilize multiple frequency bands to facilitate multi-function operations and to adapt to dynamic transmission conditions, making multiband RF systems an essential infrastructure for applications in the commercial, defense, and civilian federal marketplace. While multiband RF systems are the backbone for intelligence, surveillance, and reconnaissance, as well as for supporting data-intensive physical weaponry in the battlefield; Civilians also rely on multiband RF systems for all types of day-to-day applications including smart home system control, entertainment, virtual reality and augmented reality learning. With the recent development of 5G networks, the spectrum of multiband networks could spend from hundreds of MHz to tens of GHz range, which could support new applications and improve the quality of services. The benefits associated with using multiband and wideband RF technologies can only be realized if it is possible to dynamically manipulate the ultra-wide multiband spectrum to ensure high-quality transmission performance. This is challenging, however, as the bandwidth of multiband RF signal could be as wide as several GHz with a center frequency from hundreds of MHz to tens of GHz range, and neither RF electronics nor digital signal processing are capable of dynamically manipulating spectrum of GHz wide. In this paper, we will present our recent advancement on novel photonic systems for dynamically manipulating the wide RF spectrum for multiband and wideband emerging RF systems.

First, we demonstrate, for the first time, an on-chip multipurpose microwave frequency identification system (MFIS). We believe that we have made a breakthrough in this area in the following aspects: 1) Our work can identify complex microwave signals, including multiple-frequency, chirped and frequency-hopping microwave signals, even their combinations. 2) Our work is implemented using an on-chip high-Q-factor (600,000) silicon microring resonator to implement frequency-to-time mapping. Therefore, the frequency measurement range is ultra large, from 1 to 30 GHz, with a high resolution of 375 MHz and a low measurement error of 237.3 MHz. Second, a new scheme adopting an ultra-compact reflector for doubling group delay is proposed and verified, achieving a large group delay of 400 ps and a large dispersion value up to 5.5 million ps/nm/km within bandwidth of 12 nm.

High-frequency microwave signal generation and processing are highly needed in future multi-functional radar and nextgeneration wireless communication systems. It is extremely difficult or impossible for electronics to fulfil the tasks. As an enabling technology, photonics has been considered a promising solution for the generation and processing of highfrequency microwave signals. In this paper, photonic integrated solutions for microwave signal generation and processing will be discussed. Specifically, a monolithically integrated silicon-photonic frequency-tunable microwave bandpass filter and a frequency-tunable low phase-noise optoelectronic oscillator are discussed. This successful demonstration of the two integrated microwave photonic systems marks a significant step forward for large-scale implementation of integrated microwave photonic systems for future radar and wireless communication applications.

This work presents the study and the design of optical switch based on a hybrid plasmonic-vanadium dioxide based waveguide. The power-attenuating mechanism takes the advantage of the phase change properties of vanadium dioxide that exhibits a change in the real and complex refractive indices upon switching from the dielectric phase to the metallic phase. The proposed switch designed to operate under the telecommunication wavelength. The switch was analyzed by 3D full electro-magnetic simulations. An ER per unit length of 4.32 dB/μm and IL per unit length of 0.88 dB/μm are realized for the proposed electro-optical switch. The proposed electro-optical switch has the advantages of small device foot-print, compatible with the existing VLSI-CMOS technology and broadband operation.

Semiconductor laser frequency comb sources such as type-I quantum well cascade diode lasers, interband cascade lasers and quantum cascade lasers have recently shown tremendous potential as spectroscopic sources for chemical sensing. By simultaneously providing broadband coverage and high-spectral resolution in the mid-IR spectral region, they enable detection of large molecules with broadband absorption spectra and small molecules with well-resolved spectral lines. There is a strong interest in technologies that can provide sensitive spectroscopic detection of chemicals using a compact integrated photonics systems, and semiconductor sources offer unique opportunity for system integration. In this paper I will demonstrate results from a dual comb spectroscopy (DCS) systems based on semiconductor sources that effectively down-convert mid-IR spectra to the RF domain where one can perform spectroscopic signal detection followed by chemical concentration retrieval algorithms. We utilize phase and timing correction algorithms to allow for coherent averaging of data generated by free-running lasers over extended time-scales. Examples of high-resolution spectroscopic detection of small and large molecules in a gas phase will be presented. Current limitations and future directions towards fully integrated photonics DCS systems will be discussed.

(Invited) Subwavelength grating metamaterial waveguide devices are becoming established as key building blocks for silicon photonic integrated circuits. The novel optical properties found in these structures and ability to control their optical responses with unprecedented accuracy are opening new prospects for controlling and manipulating light in planar waveguides. Subwavelength grating metamaterial waveguides have attracted a strong research interest in academia and industry and many advanced devices with unprecedented performance have been demonstrated. The subwavelength engineered silicon waveguides have been adopted by industry for fiber-chip coupling and subwavelength engineered structures in general are likely to become key building blocks for the next generation of integrated photonic circuits. In this invited talk we will present an overview of our recent advances in this exciting field, including silicon-based subwavelength structures for highly efficient fiber-chip couplers, ultra-broadband waveguide devices, Bragg filters with high spectral sensitivity and nanophotonic waveguides with engineered anisotropy.

Semiconducting single walled carbon nanotubes (s-SWNT) have an immense potential for the development of active optoelectronic functionalities in ultra-compact hybrid photonic circuits. Specifically, s-SWNT have been identified as a very promising solution to implement light sources and detectors in the silicon photonics platform. Here we present our most recent results on the hybrid integration of s-SWNT with silicon resonators, including micro-rings, micro-disks and photonic crystal cavities. We show chirality-wise resonant enhancement of s-SWNT emission, allowing on-chip selection of (8,6) or (8,7) SWNT chiralities present in a high-purity polymer-sorted solution. We also demonstrate that, opposite to the common knowledge, the longitudinal component in transverse-magnetic optical modes can efficiently interact with drop-casted s-SWNTs arranged along the chip surface. The proposed hybrid integration approach unlocks new tools to optimize light-SWNT interaction in silicon photonic circuits and open a new route towards single-chirality selection in hybrid Si-SWNT devices, even if the SWNT solution contains various chiralities. Hence, these results stand as an important step towards the implementation of s-SWNT-based devices for the silicon photonics platform.

Hybrid III-V/Si laser integration on silicon photonic platform has been demonstrated several time using III-V direct bonding on top of patterned silicon [1-3]. Most of these former works have been achieved using small wafer diameter III-V fabrication line for post bonding process steps. The expected low-cost added value of silicon photonics cannot be sustained with such integration scheme. More recently, we present III-V laser integration with a CMOS compatible process using wafer to wafer bonding and 1 level of contact [4]. In this paper, we present the technological progresses on a 200mm fully CMOS compatible hybrid III-V/Si laser technology. We introduced an improved backend of line for hybrid lasers with 2 interconnection levels, W-plugs and fully planarized process offering a state of the art access resistance and a homogeneous current density distribution over the gain material. Second, in order to optimize the use of the costly III-V material and enable the laser large scale integration on silicon we present fabrication process with die to wafer molecular bonding with high bonding yield at wafer scale. These process features will be detailed and the impact of laser performances will be presented. Finally, the scalability towards 300mm for the overall platform will be discussed.

Silicon (Si) photonics is a highly promising platform for integrated optoelectronics. The ability to leverage existing CMOS infrastructure and the high index contrast between silicon and its native oxide have enabled compact, low-loss, and relatively low-cost photonic integrated circuits. Recently, there has been a drive towards integrating other CMOS-compatible materials in the silicon photonics platform, with silicon nitride (SiN) on the forefront. SiN offers a wider transparency range, lower temperature sensitivity and lower linear and nonlinear losses than Si. Many potential applications would benefit from having efficient, low-loss and fast phase modulators in these platforms. However, current solutions still lack in performance, plasma dispersion-based modulators in silicon are intrinsically lossy and suffer from spurious amplitude modulation. On SiN, simply no viable solutions with bandwidths beyond the MHz range exist. We tackle these issues by co-integration of thin films of lead zirconate titanate (PZT) onto integrated photonic structures. PZT is known to exhibit a strong Pockels effect, as well as related second order optical nonlinearities. This enables us to harness these effects for electro-optic modulation and other applications on existing Si and SiN platforms. In this work, we summarize our recent results. Including the demonstration of phase modulation using PZT-on-SiN waveguides in both the C- and the O-band, with bias-free operation, data rates of at least 40 Gbps and bandwidths beyond 33 GHz. We moreover demonstrate efficient phase modulation with a half-wave voltage length product of 3.2 Vcm and low propagation losses. Simulations indicate that further improvements are possible.

Silicon photonics is being considered as the future photonic platform for low power consumption optical communications. However, Silicon is a centrosymmetric semiconductor, which cannot exhibit any second order optical nonlinearities, like second harmonic generation nor the linear electro-optic effect (i.e. Pockels effect). Nonetheless, by means of strain gradients, generated by depositing a stressed layer (typically SiN) on silicon waveguides, this restriction vanishe. Hence, for years, many attempts on characterizing the second order nonlinear susceptibility tensor through Pockels effect have been performed. However, due to the semiconductor nature of silicon, its analysis has been wrongly carried out. Indeed, carriers in Si, at the Si/SiN interface and in SiN have a screening effect when performing electro-optic modulation, which have led to overestimations of the second order nonlinear susceptibility and eventually rose a controversy on the real existence of Pockels effect in strained silicon waveguides. Here, we report on unambiguous experimental characterization of Pockels effect in the microwave domain, by taking advantage of the inherent limitation of carrier effect in high frequency range. Recent results on high-speed measurements will be presented and discussed. Both charge effects and Pockels effect induced under an electric field will be also analysed.

The coupling between a silicon nanowire (NW) and a single mode fiber (SMF) is challenging. Design and optimization of compact spot-size converters (SSCs) for silicon photonics devices are presented by using numerically efficient and rigorous full-vectorial finite-element based approaches. The multi-Poly-Silicon layers based SSCs are proposed and optimized for the quasi-TE and quasi-TM polarizations sequentially. The coupling losses can be reduced to 2.72 and 2.45 dB for the quasi-TE and quasi-TM polarizations, respectively by using an eleven Poly-Si layers based SSC. A polarizationindependent SSC is also proposed based on the phase-matched multi-Poly-Silicon-layer and lower taper waveguide for both the quasi-TE and quasi-TM polarizations. Coupling to a lensed fiber with the radius of 2 μm, the optimized polarization-independent SSC is with the coupling losses of 0.34 and 0.25 dB for the quasi-TE and quasi-TM polarizations, respectively. The on-chip integrated SSC opens up the feasibility of a low cost passive aligned fiber-pigtailed electronicphotonics integrated circuits platform.

We are reporting on a Multi-Color Laser Engine (MLE) multiplexing four wavelengths (405 nm, 488 nm, 561 nm, 640 nm) by means of a Photonic Integrated Circuit (PIC) with Silicon Nitride (SiN) waveguides. Multiple building blocks are tested that allow manipulating the light in the waveguides to achieve fiber switching and variable optical attenuation. To slow down facet degradation and extend chip lifetime at near Ultra-Violet (UV) wavelengths (405 nm), a lateral endcap is implemented on chip and tested for reliability. Reasonable coupling and on-chip losses have been achieved in view of a practical use of the technology.

The optical beam steering device is essential for LiDARs and non-mechanical ones have been developed extensively. We have studied the one based on a Si photonic crystal waveguide (PCW) that guides slow light. In LiDARs, the beam hits a distant object. Then, reflected light is scattered hemispherically and a part of it is returned and received by the PCW. In this process, a long PCW aperture is expected to increase the reception intensity. However, since the PCW has a propagation loss of the order of 10 dB/cm, the reception intensity is not increased by simply lengthening the PCW. In this study, in order to suppress the total loss of the PCW, we proposed and fabricated a serial array of PCWs, in which light is received by multiple and short PCWs and then summed by using Si wire waveguide and coupler. We first estimate the transmission and reception characteristics of the PCW array. The effective aperture radiating light is lengthened by dividing the PCW, so the beam divergence becomes small and the reception intensity is improved. Also, we measured the transmission characteristics of the PCW array. We obtained a 0.046° beam divergence by controlling the phase between the PCWs. In the beam steering by the wavelength scanning or heating, we confirmed that the phase matching angular step appears stepwise. If we use the angular step as a resolution point, we can obtain the beam steering without the phase control.