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 (TCOs) are getting increasing attention due to their unique epsilon-near-zero (ENZ) effect. The optical properties of TCOs can be dramatically changed from dielectric-like to metallic-like by controlling the carrier concentration in the telecommunication wavelengths, resulting in a near zero permittivity. The carrier concentration can be electrically manipulated when TCOs are built in a metal-oxide-semiconductor structure. With applied electrical bias, an accumulation layer forms at the oxide/TCO interface. When the accumulation layer meets ENZ condition, optical field is concentrated in the very thin accumulation layer, and thus the light-matter interaction is greatly enhanced. Such property makes TCOs wonderful materials for building active electro-optical devices. To data, several TCOs based devices configurations have been reported, such as metal-insulator-metal (MIM) waveguide based modulator, Si waveguide based plasmonic MOS modulator, PlasMOStor, and TCOs based tunable metasurfaces. Here, we design and demonstrate a TCOs based tunable subwavelength grating for surface normal modulation in the telecommunication wavelengths. The device combines the TCOs MOS structure with plasmonic grating filter. When applying voltage, the light-matter interaction in the active TCOs region is further enhanced by the surface-plasmon resonances coupled guided-mode resonances (GMRs), which enables high efficient modulation for both transmission and reflection mode with only 10nm thick TCOs layer. At peak wavelength (~1.55 μm), the simulated modulation depth achieves as high as 32% for transmission mode and 56% for reflection mode.
The ever-growing demands to compute information, store, and communicate generate a continuous driving force for transformative photonic technologies. On chip photonic integrated circuits (PICs) founded a roadmap for scaling down photonic modules to meet the challenges of bandwidth enhancement and power reduction. However, it also produces an even critical need for state-of-the-art devices and better materials. Lately, the material group of transparent conductive oxides (TCO) has appealed attentions for on-chip photonic components. In specific, indium tin oxide (ITO) has been found to have active large refractive index variations, which creates possibilities to realize high speed Electro-Optic (E-O) modulation of sub-diffraction device scales. This ITO based tapered plasmonic waveguide devices will combine the large E-O absorption effects of ITO with the localized SPPs and the ultra-strong optical field confinement. In this paper, we design the device which is fabricated on commercial silicon-on-insulator (SOI) platform integrated with a 3μm long, 300nm wide gold plasmonic slot waveguide, which can dynamically switch the optical transmission from high absorption mode by enhanced plasmonic E-O absorption to low loss mode. The active E-O modulation region consists of a metal-HfO<sub>2</sub>-ITO capacitor that can electrically switch the ITO into ENZ with ultra-high modulation strength of 2.62dB/μm in simulation and 3.5dB extinction ratio in experiment. We also demonstrated the EA modulator a relative uniform E-O modulation within 1530~1600nm wavelength.
Nanoantenna is used for coupling free space radiation to subwavelength plasmonic waveguide. We provide a theoretical design of ultra-compact dipole nanoantennas --- Yagi-Uda antenna with a reflector in telecom range and experimentally demonstrate efficient optical coupling between lensed fiber and plasmonic slot waveguide by utilizing our designed nanoantenna. We also prove that the couple-in efficiency of 8% from the lensed fiber does not equal to the couple-out efficiency of 50% from the plasmonic slot waveguide using the same nanoantenna design, which is different than many published and experimental results. We also study the relationship between couple in efficiency and the incident light spot size, which is experimentally characterized.
We present theoretical design and experimental demonstration of electro-optic modulation in metallic slits infiltrated by polymer. Dynamic control over surface plasmon plaritons(SPPs) was achieved by modulating the refractive index of the polymer layer adjacent to the metal surface with interdigitated configuration. Intensity modulation of the Fano resonance of the sub-wavelength plasmonic structure was observed and expected to achieve high-speed operation.