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Metal-oxide-semiconductor (MOS capacitor-driven silicon modulators have demonstrated exceeding performance in driving voltage, energy efficiency, and bandwidth through heterogeneous integration with other active materials, including III-V compound semiconductors, transparent conductive oxides (TCOs, and graphene. However, a proper quantitative comparison is lacking among various device structures due to the difference in material properties and device designs. In this article, we first briefly reviewed state-of-the-art MOS capacitor-driven silicon modulators. Following that, we modeled the modulation efficiency and optical loss of two most representative device structures by incorporating the optical properties such as plasma dispersion, band-filling, and bandgap shrinkage effects for several heterogeneously integrated materials. The comparative analysis based on simulated results shows that Ge, III-Vs, and TCOs all offer larger index modulation than Si, whereas graphene has the largest index modulation. At the end of this article, we discussed the strategy of heterogeneous integration based on device design and scalable fabrication.

Transparent conductive oxides (TCOs) such as indium-tin oxide (ITO) have attracted increasing interests in integrated photonics and silicon photonics, owing to their large plasma dispersion and epsilon-near-zero (ENZ) effect. The nonlinear change of refractive index induced by free carrier modulation leads to a large electro-optic modulation with ultra-compact device footprint and unprecedented energy efficiency. However, high-speed modulation result is rare, mainly due to the lack of high-speed device design and fabrication quality. In this article, we characterize the fundamental electro-optic modulation structure consisting of Au/ITO/oxide/p-Si capacitor, showing that the property of ITO is greatly affected by the process condition. We also report an 8-μm-long hybrid plasmonic-silicon modulator driven by an ENZ ITO capacitor, achieving 100fJ/bit energy efficiency, 3.5 GHz modulation bandwidth, and 4.5 Gb/s data rate. The electro-absorption modulator covers a broad optical bandwidth from 1515 to 1580 nm wavelength. For future development of such modulators, we point out that by replacing ITO with higher mobility TCO materials, we can achieve 40 GHz modulation bandwidth and 0.4 fJ/bit energy efficiency using a 3-μm-long device.

We experimentally demonstrated a novel silicon microring modulator driven by transparent conductive oxide capacitor, achieving a large tuning efficiency of 95 pm/V. Analysis indicates that a potential high-speed operation above 44GHz can be reached.

We demonstrated a compact broadband plasmonic-silicon electro-absorption modulator driven by epsilon-near-zero conductive oxide, covering the entire C-band from 1515nm to 1580nm wavelength. It achieved 3.5 GHz modulation bandwidth and 4.5 Gb/s data rate.

We designed and demonstrated a high-speed plasmonic-conductive oxide-silicon modulator using epsilon-near-zero electro-absorption, achieving modulation bandwidth of 3.5GHz and 4.5Gb/s data rate. The electro-absorption modulator covers the entire C-band from 1515 nm to 1580 nm wavelength.

We designed a novel microring modulator based on hybrid transparent conductive oxide-silicon metal-oxide-semiconductor (MOS) capacitor. Experimental results achieved large wavelength tunability of 100 pm/V and 3 dB modulation at 3V of V pp. Analysis indicates that 44 GHz sneed can be achieved by optimizing the electrode design.

We experimentally demonstrate an ultra-compact transparent conductive oxide gated silicon photonic crystal nanocavity electro-optical modulator with 2.5GHz operation speed. Digital modulation is measured up to 5Gbps with 2V voltage swing and 11fJ/bit energy efficiency.

Transparent conductive oxides (TCOs) have emerged as materials for nanoscale electro-optic modulators. The free-carrier-induced epsilon-near-zero effect by applying a gate voltage is capable of achieving ultra-strong electro-absorption (EA) effect. As the EA rate has been theoretically proved to be dependent on the mobility of the TCO gate, we experimentally demonstrated a hybrid silicon-plasmonic EA modulator using high-mobility In2O3 as the gate. With an ultra-compact active region of only 5-μm long, we achieved a small voltage swing Vpp of only 2 V to obtain an EA rate of 1.2 dB / μm, resulting in very high energy efficiency of 110 femto-joule/bit. We also experimentally proved the correlation between the EA rate and the mobility of TCO materials through comparison with indium-tin-oxide gated modulators. In addition, the hybrid EA modulator is capable of covering more than 100-nm optical bandwidth in the telecommunication wavelength window, which is limited by the bandwidth of the grating coupler.

Transparent conductive oxides have attracted escalating research interest for integrated photonic devices and metasurfaces due to the extremely large electro-optic modulation of the refractive index by the free-carrier-induced plasma dispersion effect. In this paper, we have designed and fabricated a silicon microring resonator using an indium-tin oxide gate as the electric-tuning electrode. It achieved an ultralarge resonance wavelength tunability of 271 pm/V, which is obtained through the reduced width of the ring waveguide and a high-dielectric-constant HfO2 insulator. We demonstrated a broad resonance wavelength tuning range of over 2 nm with an ultrafast response time of less than 12 ns and near-zero static power consumption, which outperforms traditional thermal tuning.