Integrated Plasmonic Detectors and Mixers for Microwave and Terahertz Applications
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Date
2019
Publication Type
Doctoral Thesis
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yes
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Abstract
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.
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published
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Contributors
Examiner: Leuthold, Juerg
Examiner: Grange, Rachel
Book title
Journal / series
ETH Zürich Series in Electromagnetic Fields
Volume
11
Pages / Article No.
Publisher
ETH Zurich
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Date collected
Date created
Subject
Electro-optics; Plasmonics; Microwave Photonics; Terahertz; Photodetection; Graphene; Germanium; Nonlinear Optics; Wireless communication; Fiber-wireless link
Organisational unit
03974 - Leuthold, Juerg / Leuthold, Juerg