Journal: ETH Zürich Series in Electromagnetic Fields

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ETH Zurich

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2016 - 201962020 - 20214
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Publications 1 - 10 of 10
  • Reflective Semiconductor Optical Amplifiers (RSOAs) as Colorless Sources in Access Networks
    Item type: Doctoral Thesis
    Gebrewold, Simon A. (2016)
    ETH Zürich Series in Electromagnetic Fields
  • Metamaterial-Based Mid-Infrared Gas Sensing
    Item type: Doctoral Thesis
    Lochbaum, Alexander (2020)
    ETH Zürich Series in Electromagnetic Fields
    In recent years, air pollution finally has been recognized as a global, pressing issue. The resulting increase in environmental awareness leads to a tremendous rise in demand for low-cost CO2 sensors, which are key components of air quality monitoring and "smart home" applications. In order to access these high-volume, price-sensitive markets, technologies that offer low cost (<$10), sensitive (<50 ppm) and compact (mm-sized) CO2 detection are required. Among the large number of different gas sensing schemes, optical absorption sensors are known for their high selectivity, fast response time, and long-term stability. Furthermore, for certain gases such as CO2, optical absorption sensing is the only reliable detection method currently available. Within the field of optical gas sensors, non-dispersive infrared (NDIR) sensing at mid-infrared (mid-IR) wavelengths is a compact and relatively low-cost measurement principle. Owing to its simplicity, it is one of the commercially most relevant optical gas sensing schemes to date. Commercial NDIR CO2 sensors offer sensitivities on the order of 30 ppm in centimeter-scaled systems at prices around $50 - $100. In order to prevail in above-mentioned markets, mid-IR gas sensors must substantially scale down in both size and cost without compromising performance. Unfortunately, the simple scaling of NDIR sensors by sheer size reduction approaches an intrinsic limitation resulting from their use of discrete sensor components and dielectric interference filters. In this thesis, an all-metamaterial optical gas sensing concept has been developed that overcomes the integration limit of conventional NDIR sensors. Key to its design are metamaterial perfect absorbers (MPAs), which serve as optical filter elements that are integrated into the membranes of on-chip thermal emitters and detectors. Combining MPAs on the emitter and the detector side cascades their individual filter functions, yielding a combined narrowband resonance that is matched to the absorption band of the target gas, in this case CO2. The MPAs' angle-independent filter characteristics allow for a non-resonant cavity design that "folds" the required cm-long absorption path into a mm-sized cuboid cavity, thereby reducing the absorption volume by a factor of 30 when compared to conventional cavity designs. The all-metamaterial gas sensor exhibits a decrease in energy consumption by 80% when compared to commercial solutions without compromising performance (CO2 sensitivity 22 ppm, humidity cross sensitivity 1.2 ppm/%rH). The sensor architecture developed in this thesis offers a viable path toward compact and low-cost mid-infrared gas sensors without trade-offs in sensitivity or robustness. This cumulative dissertation is structured as follows: Chapter 1 serves as an introduction to this thesis. After motivating the research, it provides an overview on the state of the art in non-dispersive optical gas sensing. The chapter concludes with the vision of an all-metamaterial optical gas sensor. Chapter 2 summarizes the theoretical backgrounds relevant to the interdisciplinary field of metamaterial optical gas sensing. First, an introduction to absorption spectroscopy and non-dispersive infrared gas sensing is given. The principles of thermal emission and detection are established, followed by an overview on electromagnetics, plasmonics, and the concept of metamaterials. The chapter concludes with an introduction to the theory of integrating spheres. Chapter 3 demonstrates an on-chip thermal light source exhibiting narrowband and efficient mid-infrared emission. The light source's spectral properties are tailored by metamaterial perfect absorbers, which are integrated into the emitter membrane. Employed in a gas sensing setup, the metamaterial light source leads to a 5-fold increase in relative sensitivity when compared to a conventional blackbody emitter. Chapter 4 presents a CMOS-compatible metamaterial thermal detector with a narrowband absorption resonance at 4.29 μm. The high selectivity of the device leads to a 6.5-fold reduction in humidity cross sensitivity when employed in a gas sensing setup. The metamaterial's potential for highest integration densities is showcased by the realization of a dual-band detector on a single thermopile membrane. Chapter 5 demonstrates for the first time an all-metamaterial optical CO2 sensor. By advantageously combining metamaterial thermal emitters and detectors with an efficient non-resonant cavity, the sensor's absorption volume could be decreased by a factor of 30 when compared to conventional non-dispersive infrared gas sensors. The all-metamaterial sensor performs at par with much larger commercial devices, while consuming 80% less energy per measurement. Chapter 6 summarizes the findings of this thesis and gives an outlook on future research.
  • Non-Resonant and Resonant Surface Plasmon Polariton Modulators for Optical Communications
    Item type: Doctoral Thesis
    Haffner, Christian (2018)
    ETH Zürich Series in Electromagnetic Fields
    The ongoing technical revolution demands for a continuous growth of computational power. This demand is addressed by reducing the size of electronic circuits in order to increase their computational power, as predicted by Moore’s law. However, the communication speed of electronic circuits is limited to data rates of several Gbit/s and cannot be increased due to energy dissipation of electrons. This bottleneck is the major cause for slowing down the growth of computational power. Photonic communication is envisioned to overcome this limitation. Photons can transmit data at rates of Tbit/s with negligible energy dissipation, which is not possible for electrons. Thus, integrating photonics within electronic circuits on a single platform is the scope of current research. Such a platform uses photonics for communication and electronics for local computation. A fundamental component of this platform is the electro-optic (EO) modulator, which encodes the computed electrical signal onto an optical carrier. Ideally, EO-modulators should encode data at highest speed, with lowest energy consumption and on the most compact footprint. These characteristics are improved when photons and electronic signals are confined to smallest area. Plasmonics achieve the tightest confinement among all photonic technologies and promises high-speed and energy efficient EO-modulators. The tight confinement is enabled by coupling photons to the electron gas of a metal to form surface-plasmon polartion (SPP). This coupling forces electrons to oscillate with the frequency of light. However, moving electrons experience Ohmic losses due to scattering processes. This attenuates the optical carrier and diminishes the performance of the optical link. For instance, current state-of-the-art high-speed plasmonic modulators feature insertion loss (IL) in excess of 10 dB. This hampers practical implementation of plasmonics, despite the promises of high-speed and compactness. In the course of this thesis low-loss plasmonic EO-modulators were developed. These devices feature low IL (~2.5 dB), a high-speed modulation capability (>>100GHz), a compact footprint (several square microns) and a low electrical energy consumption (~10fJ/bit). The following outstanding results have been achieved. First, the modulation efficiency scales more strongly with the confinement of SPP than the Ohmic losses increase. This allows one to reduce the device length, and thus, IL as they scale with length. This is contrary to other approaches which try to minimize device losses by reducing the confinement. Based on this approach, the first plasmonic high-speed Mach-Zehnder modulator (MZM) was realized with a record voltage-length product (modulation efficiency) of 40 Vmicrons, as well as reduced IL of 8 dB. Second, the modulation efficiency is increased two fold when utilizing the EO-material’s resonances. These are harnessed by operating the modulator in close proximity to the absorption peak of the EO-material. Normally, this is avoided for photonic approaches as losses increase too fast when approaching the material resonances, however, these losses are negligible in comparison to Ohmic losses in plasmonic devices. Third, employing a hybrid resonant modulator schema comprising of a plasmonic cavity and a photonic bus waveguide. This allows one to bypass light with a photonic waveguide in the on-state (max. transmission desired), while in the off-state (min. transmission desired) light is converted to SPPs confined to a closed loop plasmonic ring. All the other demonstrated high-speed plasmonic EO-modulator utilize non-resonant schemes. While exploring this route, three first-of-their-kind high-speed modulator types have been realized, namely a plasmonic MZM, a plasmonic IQ-modulator and a plasmonic ring-modulator. These realizations demonstrate the unique integration density enabled by plasmonics. For instance, the footprint of the MZM and IQ-modulator presented here is two and three orders of magnitude smaller, respectively, compared to their photonic counterparts. All devices are capable of generating high-speed data streams of 72 Gbit/s (MZM, ring modulator) to 144 Gbit/s (IQ-modulator). The experimental results prove that losses of high-speed devices can be reduced from 10 dB down to 2.5 dB by utilizing stronger plasmonic confinement and most importantly bypassing Ohmic losses with the help of a resonant approach. Furthermore, the experiments show that material resonances can be harnessed to reduce device losses, and thus, ultimately achieving loss of approximately 1dB. The results of this thesis provide an outlook on the potential of plasmonics to generate unique data-rates on smallest footprint.
  • Tip-enhanced atomic memristor – energy-efficient circuits and photon emission
    Item type: Doctoral Thesis
    Cheng, Bojun (2021)
    ETH Zürich Series in Electromagnetic Fields
  • Integrated Plasmonic Detectors and Mixers for Microwave and Terahertz Applications
    Item type: Doctoral Thesis
    Salamin, Yannick (2019)
    ETH Zürich Series in Electromagnetic Fields
    In this dissertation, new integrated plasmonic electro-optic devices on a silicon photonics platform were developed for optical interconnects, microwave photonics and terahertz applications. Photodetectors compatible with the CMOS technology have shown great potential in implementing active silicon photonics circuits at infrared wavelengths. However, current technologies are facing fundamental bandwidth limitations. Here, we propose and experimentally demonstrate two plasmonic photodetectors operating at highest speed. First, a germanium-plasmonic waveguide photodetector simultaneously achieving beyond 100 GHz bandwidth, an internal quantum efficiency of 36% and low footprint. High-speed data reception at 72 Gbit/s is demonstrated. Such superior performance is attributed to the sub-wavelength confinement of the optical energy in a photoconductive based plasmonic-germanium waveguide detector enabling shortest drift paths for photo-generated carriers and a very small resistance-capacitance product. We show that combining plasmonic waveguides with an absorbing semiconductor enables efficient photodetection at highest operation speeds. Along the same line, graphene holds great promises for high-speed photodetection. Yet, the responsivity of graphene-based photodetectors is commonly limited by the weak absorption of the atomically thin structure. In the following, we propose and experimentally demonstrate a plasmonically enhanced waveguide-integrated graphene photodetector. The device, which combines a 6 um long monolayer of graphene with field-enhancing plasmonic structures, features at the same time a high external responsivity of 0.55 A/W and a fast photoresponse going beyond 110 GHz. The high efficiency and fast response of the device enables 100 Gbit/s PAM 2 and 100 Gbit/s PAM 4 data reception in an optical link experiment. Furthermore, microwave photonics and terahertz technologies are attracting a great interest due to a high demand for increased wireless capacity. To cope with the high bandwidth requirements, wireless carrier frequencies are shifting towards the millimeter-wave and terahertz bands. Nevertheless, optical fibers are carrying the global data traffic around the world. Ideally, future communication networks would offer full transparency and flexibility to switch between the optical and wireless domains. To this end, efficient, low-cost fiber-wireless transmitters and receivers are of crucial importance. In this work, we demonstrate for the first time a passive, all-optical, wireless-to-optical receiver in a transparent fiber-wireless-fiber link. We successfully transmit 20 Gbit/s over a wireless distance of 1 m and 10 Gbit/s over a 5 m distance at a carrier frequency of 60 GHz. This breakthrough has become possible by directly mapping the wireless information onto plasmonic signals by means of an antenna-coupled plasmonic modulator. By the direct wireless-to-optical mixing we can overcome any potential speed limitations associated with the electronics. Furthermore, the plasmonic scheme with its subwavelength feature and pronounced field confinement not only provides a built-in field enhancement of up to 90’000 over the incident field but also an ultra-compact design in a CMOS compatible structure. Finally, in the last decades terahertz waves that typically extend from the 100 GHz to the 10 THz frequency range enabled a large variety of new applications from astronomy to biology and medical sciences as well as information and communications technologies, among others. Still, most terahertz systems rely on bulky free-space optics. Their limited capabilities, high complexity and high cost strongly hinder the development of practical systems for a broader range of applications. Most prominently, chip-size high-performance terahertz sources and detectors would offer significant advantages in a multitude of areas. Here, we demonstrate a fiber-coupled, integrated plasmonic terahertz field detector on a silicon-photonics platform. The detector consists of two terahertz antenna-coupled plasmonic phase shifters integrated in a single on-chip Mach-Zehnder interferometer. The electro-optic phase shifters modulate the phase delay of a guided optical probe upon an incident oscillating terahertz field. The terahertz field amplitude is retrieved by a direct measurement of the probe power after the interferometer. The success of the scheme relies on the confinement of the terahertz field to a small volume of 10^(-8) (λ_THz/2)^3 in a plasmonic cavity and on the resonant enhancement of a dual-antenna design. The strong confinement and resonant approach also result in an extremely short interaction length of only 5 um, which eliminates the need for phase matching. We demonstrate an electro-optic bandwidth of 2.5 THz with a 65 dB dynamic range. The frequency response of the detector can be custom tailored by the terahertz antenna design, showing the flexibility of this technology and its potential for future low-cost, scalable and hand-held terahertz systems.
  • Monolithic Integration of III-V Photonic Devices on Silicon
    Item type: Doctoral Thesis
    Mauthe, Svenja (2020)
    ETH Zürich Series in Electromagnetic Fields
  • III-V on Silicon Photonics for CMOS-Embedded On-Chip Light Sources
    Item type: Doctoral Thesis
    Seifried, Marc (2018)
    ETH Zürich Series in Electromagnetic Fields
    Data-rich applications, such as cloud computing, video streaming and social media fuel a growing demand for high-performance data centers. Their performance is based on interconnected server nodes and thus a high-throughput and low-latency network is crucial for their operation. To support the network, cost-efficient optical transceivers have become the technology of choice, since they provide the required bandwidth density, distance and power efficiency. However, optical transceivers require many components such as lasers, photodetectors, optical modulators and electrical circuits to be assembled. This assembly represents a substantial fraction of the total transceiver cost. Hence, a monolithic integration of these components is a well-known way to reduce this cost. Today, all building blocks of an optical transceiver can be fabricated in the monolithic Silicon Photonics platform – except the laser. Integrating III-V materials, required for building on-chip lasers on silicon is therefore key for further cost reduction. In this thesis, an integration technology is developed to embed III-V based light sources in a CMOS Silicon Photonics platform. This technology allows lasers, photonic components and electronic circuits to be fabricated on the same chip. With such tight integration, compact and cost-efficient optical transceivers can be realized, both key for future high-speed and high-volume optical interconnects.
  • Plasmonic-Organic Hybrid Modulators
    Item type: Doctoral Thesis
    Heni, Wolfgang (2019)
    ETH Zürich Series in Electromagnetic Fields
  • Materials and Waveguide Platforms for High-Speed Plasmonic Modulators
    Item type: Doctoral Thesis
    Messner, Andreas (2021)
    ETH Zürich Series in Electromagnetic Fields
  • Low Complexity Digital Signal Processing for Optical Communications – Chromatic Dispersion, Polarization and Timing Synchronization
    Item type: Doctoral Thesis
    Josten, Arne (2019)
    ETH Zürich Series in Electromagnetic Fields
Publications 1 - 10 of 10