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Transparent conductive oxides (TCOs) are gaining increasingly high research interest for integrated photonic devices due to the strong plasma dispersion effect and process compatibility with versatile optoelectronic platforms. In this perspective article, the authors gave a brief review of research efforts both on theoretical modeling and experimental demonstration of integrated photonic devices, especially on high-efficiency electro-optic modulators through the integration with plasmonics and silicon photonics. In addition, the authors discussed the challenge and opportunity associated with TCO photonic devices and the application in photonic integrated circuits (PICs) with emphasis on high mobility materials, high-speed E-O modulators, and large-scale integration. Finally, we conclude that collaboration with existing silicon photonics foundry is a necessary route to incorporate TCOs into existing PIC ecosystems. 

Abstract Silicon microring modulator plays a critical role in energy-efficient optical interconnect and optical computing owing to its ultra-compact footprint and capability for on-chip wavelength-division multiplexing. However, existing silicon microring modulators usually require more than 2 V of driving voltage (V_pp), which is limited by both material properties and device structures. Here, we present a metal-oxide-semiconductor capacitor microring modulator through heterogeneous integration between silicon photonics and titanium-doped indium oxide, which is a high-mobility transparent conductive oxide (TCO) with a strong plasma dispersion effect. The device is co-fabricated by Intel’s photonics fab and our in-house TCO patterning processes, which exhibits a high modulation efficiency of 117 pm/V and consequently can be driven by a very low V_ppof 0.8 V. At a 11 GHz modulation bandwidth where the modulator is limited by the RC bandwidth, we obtained 25 Gb/s clear eye diagrams with energy efficiency of 53 fJ/bit. 

Abstract Silicon microring resonators (Si-MRRs) play essential roles in on-chip wavelength division multiplexing (WDM) systems due to their ultra-compact size and low energy consumption. However, the resonant wavelength of Si-MRRs is very sensitive to temperature fluctuations and fabrication process variation. Typically, each Si-MRR in the WDM system requires precise wavelength control by free carrier injection using PIN diodes or thermal heaters that consume high power. This work experimentally demonstrates gate-tuning on-chip WDM filters for the first time with large wavelength coverage for the entire channel spacing using a Si-MRR array driven by high mobility titanium-doped indium oxide (ITiO) gates. The integrated Si-MRRs achieve unprecedented wavelength tunability up to 589 pm/V, or V_πL of 0.050 V cm with a high-quality factor of 5200. The on-chip WDM filters, which consist of four cascaded ITiO-driven Si-MRRs, can be continuously tuned across the 1543–1548 nm wavelength range by gate biases with near-zero power consumption. 

We demonstrated efficient gate-tuning on-chip wavelength division multiplexing filters using a silicon microring resonator array driven by high-mobility titanium-doped indium oxide gates. It shows extensive wavelength coverage for entire channel spacing over 5 nm.