Ultra-compact silicon nanophotonic modulator with broadband response

Abstract

Electro-optic modulators have been identifi ed as the key drivers for optical communication and signal processing. With an ongoing miniaturization of photonic circuitries, an outstanding aim is to demonstrate an on-chip, ultra-compact, electro-optic modulator without sacrifi cing bandwidth and modulation strength. While silicon-based electro-optic modulators have been demonstrated, they require large device footprints of the order of millimeters as a result of weak non-linear electro-optical properties. The modulation strength can be increased by deploying a high-Q resonator, however with the trade-off of signifi cantly sacrifi cing bandwidth. Furthermore, design challenges and temperature tuning limit the deployment of such resonance-based modulators. Recently, novel materials like graphene have been investigated for electro-optic modulation applications with a 0.1 dB per micrometer modulation strength, while showing an improvement over pure silicon devices, this design still requires device lengths of tens of micrometers due to the ineffi cient overlap between the thin graphene layer, and the optical mode of the silicon waveguide. Here we experimentally demonstrate an ultra-compact, silicon-based, electro-optic modulator with a record-high 1 dB per micrometer extinction ratio over a wide bandwidth range of 1 μm in ambient conditions. The device is based on a plasmonic metal-oxide-semiconductor (MOS) waveguide, which efficiently concentrates the optical modes ' electric fi eld into a nanometer thin region comprised of an absorption coefficient-tuneable indium-tin-oxide (ITO) layer. The modulation mechanism originates from electrically changing the free carrier concentration of the ITO layer which dramatically increases the loss of this MOS mode. The seamless integration of such a strong optical beam modulation into an existing silicon-on-insulator platform bears signifi cant potential towards broadband, compact and effi cient communication links and circuits.Keywords: Modulator; silicon-on-insulator; ultra-compact.A waveguide-integrated electro-optical modulator can be perceived as a transistor with an optical source and drain and an electrical gate [1]. This waveguide receiving a continuous wave laser beam converts electrical data arriving at the gate electrode into an optical encoded data stream. A widely used modulation mechanism is to change the free carrier concentration of the material overlapping with the propagating optical mode, which leads to a shift of the plasma frequency of the dispersion relation [2]. This modifi es both the real and imaginary parts of the refractive index of the material, and henceforth alters the index and loss of the optical propagating mode. While a Mach-Zehnder type modulator utilized a refractive index change in the real part [3], an electro-absorption type modulator deploys the altered loss of the optical mode [4]. With the promise of silicon-on-insulator (SOI) technology [5] for on-chip integrated photonics, the Intel team demonstrated a Mach-Zehnder modulator by c...

Plasmonic modulator optimized by patterning of active layer and tuning permittivity

“…Current state-of-the-art plasmonic modulators outperform among Si-based electro-optic modulators [6,21]. However, plasmonic modulators suffer very high propagation losses.…”

“…They can be undesirably excited at the same time. However, in contrast to [21] or [29], the second mode damps much faster and consequently neither pronounced interference nor beatings can appear. We characterize the quality of a mode by factor Q = β off /α off .…”

“…Two different layouts are possible. In the first design, a metal-oxidesemiconductor (MOS) stack is deposited on top of a silicon waveguide [17][18][19][20][21][22]. Such waveguideintegrated plasmonic modulators are reasonably simply to fabricate, their size is comparable with the silicon waveguide size, and propagation losses are originated only from one metal interface.…”

“…Such waveguideintegrated plasmonic modulators are reasonably simply to fabricate, their size is comparable with the silicon waveguide size, and propagation losses are originated only from one metal interface. An ultracompact electro-optic modulator based on Silicon-onInsulator waveguide with an ITO-SiO 2 -Au stack is reported in [21]. High extinction ratio 1 dB/μm, low insertion loss -1 dB and broadband operation due to the non-resonant MOS mode are shown.…”

“…Recently the concept of utilizing transparent conducting oxides, namely indium tin oxide (ITO), as an active material with varied carrier density has been studied [20][21][22][30][31][32][33][34]. A unity-order refractive index change is achieved in a metal-oxide-semiconductormetal multilayered stack [34].…”