Chapter 3 - Spatial Heterodyne Fourier-Transform Waveguide Spectrometers

Silicon photonics is emerging as leading technology for development of photonic integrated circuits (PIC) which finds number of applications in various fields. Since CMOS fabrication process can be used to manufacture photonic integrated circuits, large scale integration and high volume production of PIC are possible at low cost. Due to various optical and material characteristics of crystalline silicon, silicon-on-insulator (SOI) is now widely used platform for silicon photonics. In this paper, we review the silicon micro-ring resonators with it's promising application in field of medical, communication and quantum electronics. We begin with the theory of silicon micro-ring resonators which is required to understand the basic principle of ring-resonators. Further, micro-ring resonators based applications such as label-free biosensors, modulators, filters, switches and application in quantum photonics is reviewed.

Silicon photonics research can be dated back to the 1980s. However, the previous decade has witnessed an explosive growth in the field. Silicon photonics is a disruptive technology that is poised to revolutionize a number of application areas, for example, data centers, high-performance computing and sensing. The key driving force behind silicon photonics is the ability to use CMOS-like fabrication resulting in high-volume production at low cost. This is a key enabling factor for bringing photonics to a range of technology areas where the costs of implementation using traditional photonic elements such as those used for the telecommunications industry would be prohibitive. Silicon does however have a number of shortcomings as a photonic material. In its basic form it is not an ideal material in which to produce light sources, optical modulators or photodetectors for example. A wealth of research effort from both academia and industry in recent years has fueled the demonstration of multiple solutions to these and other problems, and as time progresses new approaches are increasingly being conceived. It is clear that silicon photonics has a bright future. However, with a growing number of approaches available, what will the silicon photonic integrated circuit of the future look like? This roadmap on silicon photonics delves into the different technology and application areas of the field giving an insight into the state-of-the-art as well as current and future challenges faced by researchers worldwide. Contributions authored by experts from both industry and academia provide an overview and outlook for the silicon waveguide platform, optical sources, optical modulators, photodetectors, integration approaches, packaging, applications of silicon photonics and approaches required to satisfy applications at mid-infrared wavelengths. Advances in science and technology required to meet challenges faced by the field in each of these areas are also addressed together with predictions of where the field is destined to reach.

Special issue “optical communications, sensing, and laser applications”

Silicon photonic transceivers in the field of optical communication

The high index contrast silicon-on-insulator platform is the dominant CMOS compatible platform for photonic integration. The successful use of silicon photonic chips in optical communication applications has now paved the way for new areas where photonic chips can be applied. It is already emerging as a competing technology for sensing and spectroscopic applications. This increasing range of applications for silicon photonics instigates an interest in exploring new materials, as silicon-on-insulator has some drawbacks for these emerging applications, e.g., silicon is not transparent in the visible wavelength range. Silicon nitride is an alternate material platform. It has moderately high index contrast, and like silicon-on-insulator, it uses CMOS processes to manufacture photonic integrated circuits. In this paper, the advantages and challenges associated with these two material platforms are discussed. The case of dispersive spectrometers, which are widely used in various silicon photonic applications, is presented for these two material platforms.

Light emission from Si and Ge nanostructures (NSs) has been of great interest in recent years (1) owing to the need for silicon-based light sources for applications in silicon optoelectronics and photonics (2). Optical interconnects are required these days for on-chip technology as an alternative to metal wires, because of data transmission bottlenecks introduced by their unavoidable delay times, significant signal degradation, problems with power dissipation, and electromagnetic interference. Two major avenues toward optical interconnects on a chip include a hybrid approach with III-V densely packaged optoelectronic components and the all-group-IV approach (mainly Si, Ge and SiGe), where the all major components, e.g., light emitters, modulators, waveguides and photodetectors, are monolithically integrated into the CMOS environment. Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters (1). Band gap engineering employing quantum wells, quantum wires or quantum dots has been proposed as one way to overcome this limitation and Si/Ge or Si/SiGe-alloy thin multilayer quantum well structures grown on Si have been produced on this principle, and although light emission with greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation (1–3). Recently, through employing novel band-gap engineering strategies, we have prepared several different entirely new bright light-emitting Si/Ge NSs including one possessing a direct gap. The latter structure is based on constructing a new super unit cell comprised of multiple planar epitaxial layers of Si and Ge grown on (001) Si0.4Ge0.6 (4). Others are based on silicon-germanium layers grown epitaxially on silicon in such a way as to form multiple layer three-dimensional NSs (quantum dots) (5, 6). We have also developed a simple and efficient electrochemical process that combines galvanic reaction and focused-ion-beam lithography to selectively synthesize gold nanoparticles that are consequently used for the growth of ordered SiGe nanowire arrays with predefined diameter (200 nm) and position (7, 8). Here, the light emitting properties of these and other recent similar Si/Ge NSs that have been found to luminesce efficiently at wavelengths in the important spectral range of 1.1–1.6 μm are compared for possible applications in optoelectronics and photonics. D. J. Lockwood, Light Emission in Silicon, Academic Press, San Diego (1998).L. Pavesi and D. J. Lockwood, Silicon Photonics, Springer, Berlin (2004).D. J. Lockwood and L. Tsybeskov, IEEE J. Sel. Top. Quantum Electronics 20(4), 8200807 (2014).D. J. Lockwood, N. L. Rowell, A. Gouyé, L. Favre, A. Ronda and I. Berbezier, ECS Trans. 61(5), 31 (2014).S. A. Mala, L. Tsybeskov D. J. Lockwood, X. Wu and J.-M. Baribeau, Appl. Phys. Lett. 103, 033103 (2013).S. A. Mala, L. Tsybeskov, D. J. Lockwood, X. Wu and J.-M. Baribeau, Physica B 453, 29 (2014).A. Benkouider, A. Ronda, A. Gouye, C. Herrier, L. Favre, D. J. Lockwood, N. L. Rowell, A. Delobbe, P. Sudraud, and I. Berbezier, Nanotechnology 25(33), 335303 (2014).D. J. Lockwood, N. L. Rowell, A. Benkouider, A. Ronda, L. Favre and I. Berbezier, Beilstein J. Nanotechnol. 2014(5), 2498 (2014).

The primary goal of this article is to provide an overview of silicon photonics technology and its applications in the design and improvement of current and future computing systems. We start by reviewing silicon photonics technology and introducing some of its benefits and challenges as well as providing some background on it. Next, we introduce some fundamental silicon photonic components in the design of silicon photonic integrated circuits (PICs) and optical interconnect for computing systems as well as their operating principles and applications. These components can be active, such as photodetectors and optical modulators, or passive, such as silicon‐on‐insulator (SOI) waveguides. Subsequently, we discuss the application of silicon photonics to improve the communication and computation infrastructure in future computing systems, while reviewing the state‐of‐the‐art and some design and implementation challenges. Finally, we discuss several research opportunities to push forward the application of silicon photonics in the design of future computing systems.

Light emission from Si nanostructures has been of great interest for some time now owing to the need for silicon-based light sources for applications in silicon photonics. Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters. Band gap engineering has been proposed as one way to overcome this limitation and although light emission with greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation. Recently, through employing novel band gap engineering computations, entirely new Si/Ge [M. d’Avezac, J.-W. Luo, T. Chanier, and A. Zunger, Phys. Rev. Lett. 108, 027401 (2012)] and Si [H. J. Xiang, B. Huang, E. Kan, S.-H. Wei, and X. G. Gong, Phys. Rev. Lett. 110, 118702 (2013)] supercell structures possessing direct gaps have been proposed. According to d’Avezac et al., a SiGe2Si2Ge2SiGen superstructure should have a direct and dipole-allowed gap of 0.863 eV, which is ideally suited for optical fiber data transmission applications. Here we report on the growth of such a structure and its optical properties. Two similar samples were prepared by different growth methods: molecular beam epitaxy (MBE) and solid phase epitaxy (SPE). In both samples the superstructure (terminated with n = 12 monolayers of Ge) was grown on a 30-nm thick buffer layer of Si0.4Ge0.6 on a 5 µm thick relaxed buffer layer of Si0.4Ge0.6on a 750 µm thick (001) Si substrate. The photoluminescence (PL) spectra were measured using a Bomem DA3 FTIR spectrometer, with the samples excited at low temperatures with either 70 mW of 405 nm or 35 mW of 458 nm laser light.Photoluminescence (PL) spectra obtained at 6 K with excitation at 405 nm are shown in Fig. 1. Similar spectra were obtained with 458 nm excitation. No PL was detected from the two samples in the energy range 1000-1850 meV or at room temperature. The sharp drop at low energy near 700 meV is due to the cut-off in the instrumental response. A strong low-energy PL doublet is seen, with peaks near 780 and 820 meV, together with a much weaker peak at 872 meV. The ratio of intensities of the strong and weak peaks is the same in both samples. The intensities of all three PL peaks decrease with increasing temperature up to 25 K, but the weak peak decreases in intensity faster than that of the strong peaks. The weak peak at 872 meV is most likely the dipole-allowed direct-gap transition expected at 0.863 eV in the superstructure. The small difference in energy between theory and experiment could be the result of a difference in strain within the layer in the sample compared with the ideal (perfect) modeled structure or from assumptions in parameter values in the model. The strong peaks at 820 and 780 meV are assigned to the no-phonon and transverse-optic-phonon emission lines, respectively, of the Si0.4Ge0.6 buffer layer. The ~40 meV separation between the two strong peaks is characteristic of the phonon energies in SiGe alloys. The energies of the peaks, however, are much lower than that expected for a bulk Si0.4Ge0.6 alloy (~0.97 eV). The energies and general appearance of these peaks is reminiscent of what has been obtained from PL studies of SiGe nanostructures imbedded in Si. It is therefore likely that this PL arises predominately at the Si0.4Ge0.6/superstructure interface where there is type-II band alignment. In conclusion, we have obtained experimental evidence of the predicted direct-gap optically-allowed transition in a special supercell comprised of a number of ultrathin layers of Si and Ge.

This paper presents the design, fabrication, and experimental demonstration of a silicon photonic phase tuner that leverages slow-light effects in a suspended photonic crystal (PC) slot waveguide. The phase is tuned by electrostatic comb actuation which modulates the slot width within the PC defect thus achieving a high phase sensitivity enhancement relative to a conventional slot waveguide. The device is fabricated using a silicon photonic foundry process followed by post-processing steps to remove the oxide cladding and suspend the PC slot waveguide and electrostatic actuation combs. The design exhibits up to a 16-fold improvement in phase sensitivity, enabling sub-volt V π with a nominal slot width of 200 nm. A small actuation length of just 33 μm results in a V π L of 20.13 Vμm. The results demonstrate phase tuning with complementary metal-oxide-semiconductor (CMOS) compatible core voltages for high density, high speed applications in silicon photonics, such as optical phased arrays and photonic accelerators.

In recent years silicon photonics has become a mature technology enabling the integration of a variety of optical and optoelectronic functions by means of advanced CMOS technology. While most efforts in this field have gone to telecom and datacom/interconnect applications, there is a rapidly growing interest in using the same technology for sensing applications, ranging from refractive index sensing to spectroscopic sensing. In this paper the prospect of silicon photonics for absorption, fluorescence and Raman spectroscopy on-a-chip will be discussed. To allow spectroscopy in the visible and near infrared the silicon photonics platform is extended with silicon nitride waveguides.

An ultra-compact broadband silicon polarizing beam splitter is proposed based on a tilted nano-grating structure. A light cross coupling can be realized for transverse-magnetic mode, while the transverse-electric light can almost completely output from the through port. The length of the coupling region is only 6.8 μm, while an extinction ratio of 23.76 dB can be realized at a wavelength of 1550 nm. As a proof of concept, the device was fabricated by a commercial silicon photonic foundry. It can realize a 19.84 dB extinction ratio and an 80 nm working bandwidth with an extinction ratio of larger than 10 dB. The presented device also shows a good fabrication tolerance to the structure deviations, which is favorable for its practical applications in silicon photonics.

Silicon photonics is playing a key role in areas as diverse as high-speed optical communications, neural networks, supercomputing, quantum photonics, and sensing, which demand the development of highly efficient and compact light-processing devices. The lithographic segmentation of silicon waveguides at the subwavelength scale enables the synthesis of artificial materials that significantly expand the design space in silicon photonics. The optical properties of these metamaterials can be controlled by a judicious design of the subwavelength grating geometry, enhancing the performance of nanostructured devices without jeopardizing ease of fabrication and dense integration. Recently, the anisotropic nature of subwavelength gratings has begun to be exploited, yielding unprecedented capabilities and performance such as ultrabroadband behavior, engineered modal confinement, and sophisticated polarization management. Here we provide a comprehensive review of the field of subwavelength metamaterials and their applications in silicon photonics. We first provide an in-depth analysis of how the subwavelength geometry synthesizes the metamaterial and give insight into how properties like refractive index or anisotropy can be tailored. The latest applications are then reviewed in detail, with a clear focus on how subwavelength structures improve device performance. Finally, we illustrate the design of two ground-breaking devices in more detail and discuss the prospects of subwavelength gratings as a tool for the advancement of silicon photonics.

Self-collimation in photonic crystals has received a lot of attention in the literature, partly due to recent interest in silicon photonics, yet no performance metrics have been proposed. This paper proposes a figure of merit (FOM) for self-collimation and outlines a methodical approach for calculating it. Performance metrics include bandwidth, angular acceptance, strength, and an overall FOM. Two key contributions of this work include the performance metrics and identifying that the optimum frequency for self-collimation is not at the inflection point. The FOM is used to optimize a planar photonic crystal composed of a square array of cylinders. Conclusions are drawn about how the refractive indices and fill fraction of the lattice impact each of the performance metrics. The optimization is demonstrated by simulating two spatially variant self-collimating photonic crystals, where one has a high FOM and the other has a low FOM. This work gives optical designers tremendous insight into how to design and optimize robust self-collimating photonic crystals, which promises many applications in silicon photonics and integrated optics.

In this presentation I will give an overview of Rockley Photonics’ real-time, non-invasive biomarker sensing-on-the-wrist integrated photonics technology, and the application of silicon photonics plus AI for smart consumer wearable health sensing applications. I will describe the silicon photonics building blocks needed to make the necessary integrated photonics platform as well as application of AI to provide multi-application, health-related sensing and monitoring applications. I will describe how this platform can provide personalized monitoring of multiple biophysical and biochemical biomarkers, by combining laser-based spectrophotometer capabilities with on-device analytics and a cloud platform which augments stand-alone capabilities with predictive analytics based on biomarker trends.

Integrated Mach-Zehnder (MZ) modulators are important components in silicon photonic devices that rely on a reverse-biased p-n junction to modulate the optical signal via a change of the waveguide refractive index [1]. Reducing their energy consumption is a crucial step towards the application of silicon photonics, especially in connection with the growing traffic volumes in data centers. In this work, we combine band-edge slow light structures consisting of silicon grating waveguides [2] with periodic (interlaced) p-n junctions [3,4] to maximize spatial matching between the optical field mode and the depletion region of the p-n junction. The two effects together will result in an improved modulation efficiency, leading to a strongly reduced energy dissipation per bit.