Zhizhen Ma

Two-Dimensional Material-Based Mode Confinement Engineering in Electro-Optic Modulators

The ability to modulate light using 2-dimensional (2D) materials is fundamentally challenged by their small optical cross-section leading to miniscule modal confinements in diffraction-limited photonics despite intrinsically high electro-optic absorption modulation (EAM) potential given by their strong exciton binding energies. However the inherent polarization anisotropy in 2D materials and device tradeoffs lead to additional requirements with respect to electric field directions and modal confinement. A detailed relationship between modal confinement factor and obtainable modulation strength including definitions on bounding limits are outstanding. Here, we show that the modal confinement factor is a key parameter determining both the modulation strength and the modulator extinction ratio-to-insertion loss metric. We show that the modal confinement and hence the modulation strength of a single-layer modulated 2D material in a plasmonically confined mode is able to improve by more than 10× compared to diffraction-limited modes. Combined with the strong-index modulation of graphene, the modulation strength can be more than 2-orders of magnitude higher compared to Silicon-based EAMs. Furthermore, modal confinement was found to be synergistic with performance optimization via enhanced light-matter-interactions. These results show that there is room for scaling 2D-material EAMs with respect to modal engineering toward realizing synergistic designs leading to high-performance modulators.

Indium-Tin-Oxide for High-performanceElectro-optic Modulation

Abstract: Advances in opto-electronics are often led by discovery and development of materials featuring unique properties. Recently, the material class of transparent conductive oxides (TCO) has attracted attention for active photonic devices on-chip. In particular, indium tin oxide (ITO) is found to have refractive index changes on the order of unity. This property makes it possible to achieve electrooptic modulation of sub-wavelength device scales, when thin ITO films are interfaced with optical light confinement techniques such as found in plasmonics; optical modes are compressed to nanometer scale to create strong light-matter interactions. Here we review efforts towards utilizing this novel material for high performance and ultra-compact modulation. While high performance metrics are achieved experimentally, there are open questions pertaining to the permittivity modulation mechanism of ITO. Finally, we review a variety of optical and electrical properties of ITO for different processing conditions, and show that ITO-based plasmonic electro-optic modulators have the potential to significantly outperform diffractionlimited devices.

Active material, optical mode and cavity impact on nanoscale electro-optic modulation performance

Abstract Electro-optic modulation is a key function in optical data communication and possible future optical compute engines. The performance of modulators intricately depends on the interaction between the actively modulated material and the propagating waveguide mode. While a variety of high-performance modulators have been demonstrated, no comprehensive picture of what factors are most responsible for high performance has emerged so far. Here we report the first systematic and comprehensive analytical and computational investigation for high-performance compact on-chip electro-optic modulators by considering emerging active materials, model considerations and cavity feedback at the nanoscale. We discover that the delicate interplay between the material characteristics and the optical mode properties plays a key role in defining the modulator performance. Based on physical tradeoffs between index modulation, loss, optical confinement factors and slow-light effects, we find that there exist combinations of bias, material and optical mode that yield efficient phase or amplitude modulation with acceptable insertion loss. Furthermore, we show how material properties in the epsilon near zero regime enable reduction of length by as much as by 15 times. Lastly, we introduce and apply a cavity-based electro-optic modulator figure of merit, Δλ/Δα, relating obtainable resonance tuning via phase shifting relative to the incurred losses due to the fundamental Kramers-Kronig relations suggesting optimized device operating regions with optimized modulation-to-loss tradeoffs. This work paves the way for a holistic design rule of electro-optic modulators for high-density on-chip integration.

Attojoule-efficient graphene optical modulators

Electro-optic modulation is a technology-relevant function for signal keying, beam steering, or neuromorphic computing through providing the nonlinear activation function of a perceptron. With silicon-based modulators being bulky and inefficient, here we discuss graphene-based devices heterogeneously integrated. This study provides a critical and encompassing discussion of the physics and performance of graphene. We provide a holistic analysis of the underlying physics of modulators including graphenes index tunability, the underlying optical mode, and discuss resulting performance vectors for this novel class of hybrid modulators. Our results show that reducing the modal area and reducing the effective broadening of the active material are key to improving device performance defined by the ratio of energy-bandwidth and footprint. We further show how the waveguides polarization must be in-plane with graphene, such as given by plasmonic-slot structures, for performance improvements. A high device performance can be obtained by introducing multi- or bi-layer graphene modulator designs. Lastly, we present recent results of a graphene-based hybrid-photon-plasmon modulator on a silicon platform and discuss electron beam lithography treatments for transferred graphene for the relevant Fermi level tuning. Being physically compact, this 100  aJ/bit modulator opens the path towards a novel class of attojoule efficient opto-electronics.

Low-loss tunable 1D ITO-slot photonic crystal nanobeam cavity

Tunable optical material properties enable novel applications in both versatile metamaterials and photonic components including optical sources and modulators. Transparent conductive oxides (TCOs) are able to highly tune their optical properties with applied bias via altering their free carrier concentration and hence plasma dispersion. The TCO material indium tin oxide (ITO) exhibits unity-strong index change and epsilon-near-zero behavior. However, with such tuning the corresponding high optical losses, originating from the fundamental Kramers–Kronig relations, result in low cavity finesse. However, achieving efficient tuning in ITO-cavities without using light–matter interaction enhancement techniques such as polaritonic modes, which are inherently lossy, is a challenge. Here we discuss a novel one-dimensional photonic crystal nanobeam cavity to deliver a cavity system offering a wide range of resonance tuning range, while preserving physical compact footprints. We show that a vertical silicon-slot waveguide incorporating an actively gated-ITO layer delivers ~3.4 nm of tuning. By deploying distributed feedback, we are able to keep the Q-factor moderately high with tuning. Combining this with the sub-diffraction limited mode volume (0.1 (λ/2n)3) from the photonic (non-plasmonic) slot waveguide, facilitates a high Purcell factor exceeding 1000. This strong light–matter-interaction shows that reducing the mode volume of a cavity outweighs reducing the losses in diffraction limited modal cavities such as those from bulk Si3N4. These tunable cavities enable future modulators and optical sources such as tunable lasers.

Temperature dependence of a sub-wavelength compact graphene plasmon-slot modulator

We investigate a plasmonic electro-optic modulator with an extinction ratio exceeding 1 dB/μm by engineering the optical mode to be in-plane with the graphene layer, and show how lowering the operating temperature enables steeper switching.

Graphene-based plasmonic slot electro-optic modulator (Conference Presentation)

Graphene, as the first identified two dimensional material, has shown great electro-optic response via Pauli-blocking for near IR frequencies and modulating functionality. However, this ability to modulate light is fundamentally challenged by its small optical cross-section leading to miniscule modal confinement factors in diffraction-limited photonics despite intrinsically high electro-optic absorption modulation (EAM) potential given by its strong index change. Yet the inherent polarization anisotropy in graphene and device tradeoffs lead to additional requirements with respect to electric field directions and modal confinement. The extinction ratio of graphene based EAM has, so far, been limited due to the small light matter interaction given the monolayer structure nature. Here we report an ultra-compact graphene based EAM by integrating graphene with a plasmonic slot waveguide. We show that the modal confinement and hence the modulation strength of a single-layer modulated graphene in this plasmonically confined mode is able to improve by more than 10x compared to diffraction-limited modes. Combined with the strong-index modulation of graphene the modulation strength could achieve more than 1dB/um, which is more than 2-orders of magnitude higher compared to Silicon platform graphene modulators. Furthermore, the modal confinement was found to be synergistic with performance optimization via enhanced light-matter-interactions. These results show that there is room for scaling 2D material EAMs with respect to modal engineering towards realizing synergistic designs leading to high-performance modulators.

Sub 1-Volt Graphene-based Plasmonic Electroabsorption Modulator on Silicon

We experimentally show a sub-volt graphene-based hybrid plasmonic waveguide-integrated electro-absorption modulator showing an extinction ratio (0.25 dB/μm) compact footprint (5 μm2) enabled by improved both electrostatics and mode overlap factor.