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Ultracompact, High-Speed Field-Effect Optical Modulators

Conductive oxides-based modulator devices could provide promising candidates for ultra-compact and ultra-fast optical interconnects in future integrated photonic circuits.

The major goals of this research project included two parts. First, an ultracompact plasmonic electro-optical (EO) modulator was to be developed and investigated for efficient intensity modulation. Second, an ultracompact and high-speed EO modulator based on a dielectric platform was to be developed for straightforward integration with existing CMOS technology. Both modulators were targeted to facilitate next-generation interconnects for integrated photonic circuits.

This work performed on this project explored novel conductive oxide-based slot waveguides based on the unique properties of indium-tin-oxide (ITO). This research was one of the first experimental attempts to demonstrate optical modulators at nanoscale, and one of the first systematic explorations of conductive oxide-based modulation at GHz level. The research results contribute towards the advancement of nanophotonic technology and on-chip optical interconnects, and will support fundamental theory and techniques for field-effect electro-absorption modulators.

Illustration of the working modes of the metal-insulator-CO-insulator-metal structure: (a) Without bias. (b) Depletion mode, where the ITO is less absorptive and the waveguide has lower attenuation. (c) Accumulation mode, where the ITO is more absorptive and the waveguide has higher attenuation. (d) The mode profile for the no bias case. (e) The mode profile for the depletion case. (f) The mode profile for the accumulation case.

Ultra-Compact Field Effect Plasmonic Modulator

A metal-insulator-conductive oxide-insulator-metal (MICIM) waveguide was proposed and investigated. It was showed that light absorption in the gap between two gold films is controlled by the electric-field-induced charge in an intermediate ITO layer. The MICIM structure may be biased such that the ITO layer is either in electron depletion or accumulation, thus changing the absorption of the waveguide. Thus, the structure can switch between high and low absorptive states.

MICIM modulators were designed and fabricated, consisting of a series of layer-by-layer processes. Photolithography, thin film deposition, and liftoff processes were used for precise pattern definitions. Modulators of different waveguide lengths as small as 800 nm were characterized. The modulation performance of the 800 nm (length) modulator was measured with a DC-coupled photodetector using an applied 14 Vpp RF sine wave at 10 MHz, with a resulting extension ratio of 3.04 dB/μm. An AC-coupled photodetector was used to demonstrate modulator operation at frequencies up to 500 MHz.

Ultra-Compact High-Speed Dielectric Modulator

This project also investigated a doped Si-ITO-HfO2 dielectric modulator, which can provide straightforward photonic integration. In this device, TiO2 serves as a dielectric slot waveguide for guiding light to interact with ITO. External electric signals are applied on n+ doped Si and ITO electrodes, which stimulates the field effects in the active ITO layer at the ITO-HfO2 interface. The device was fabricated on SOI substrates. Gratings on the U-shaped waveguide ends were used for light coupling from angled fiber arrays. Gold was used for electrical contact pads. The coupling efficiency was measured to be relatively low, with a peak value of 2% at 1510 nm. For comparison, the FDTD simulation results based on the measured film parameters of the fabricated device resulted in a peak 5.5% output transmission at the wavelength of 1510 nm. An AC modulation depth of 2.5 dB/μm was realized on an 8 μm long modulator waveguide at 100 MHz. The modulation depth decays with increasing frequency, showing that the device has a RC circuit-limited operation speed. Nevertheless, successful modulation at frequencies as high as 2 GHz was demonstrated.

This work was done by Karl Hirschman, Rochester Institute of Technology, for the Army Research Office. ARL-0214

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Ultracompact, High-Speed Field-Effect Optical Modulators

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    Aerospace & Defense Technology Magazine

    This article first appeared in the September, 2018 issue of Aerospace & Defense Technology Magazine.

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    Overview

    The document is the final report for a research project titled "Ultracompact, High-Speed Field-Effect Optical Modulators," conducted by the Rochester Institute of Technology (RIT) under the sponsorship of the U.S. Army Research Office. The project, identified by Agreement Number W911NF-16-1-0357, was carried out from June 14, 2016, to August 31, 2017, and was submitted by Karl Hirschman.

    The primary objective of the research was to develop two types of ultracompact electro-optical (EO) modulators aimed at enhancing integrated photonic circuits. The first part focused on a plasmonic EO modulator designed for efficient intensity modulation, while the second part involved a dielectric platform modulator intended for seamless integration with existing CMOS technology. These modulators are crucial for advancing next-generation interconnects in photonic circuits.

    The report highlights significant accomplishments, including the exploration of novel conductive oxide-based slot waveguides utilizing indium-tin-oxide (ITO). This research represents one of the pioneering experimental efforts to demonstrate optical modulators at the nanoscale and to systematically investigate conductive oxide-based modulation at gigahertz frequencies. The findings contribute to the development of nanophotonic technology and on-chip optical interconnects, providing foundational theories and techniques for field-effect electro-absorption modulators.

    Key innovations include the design and investigation of a metal-insulator-conductive oxide-insulator-metal (MICIM) waveguide. The research demonstrated that light absorption in the waveguide can be modulated by controlling the electric-field-induced charge in the ITO layer, allowing the structure to switch between high and low absorption states. The performance of modulators as small as 800 nm was characterized, achieving modulation at frequencies up to 500 MHz.

    Additionally, the project explored a doped Si-ITO-HfO2 dielectric modulator, which facilitates straightforward photonic integration. This device uses TiO2 as a dielectric slot waveguide, enabling effective interaction with ITO through applied electric signals.

    Overall, the report underscores the importance of developing ultracompact, high-speed EO modulators to overcome technical barriers in integrated photonic circuits, ultimately enhancing the capability to convert electronic signals into high bit-rate photonic data. The research findings are expected to have significant implications for the future of optoelectronics and integrated photonic technologies.

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