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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