Electronic-photonic circuit crossings

Abstract

Electrical control of light in integrated photonics is central to a wide range of research and applications. It is conventionally achieved with thermo-optic tuning, but this suffers from high energy consumption and crosstalk. Nanoelectromechanical photonics could resolve these issues, but integrating this technology with conventional multilayer metal architectures is challenging, and conventional approaches do not allow crossings of electrical wires and photonic waveguides. Here, we use topology optimization to devise a single-layer electronic-photonic circuit crossing with up to 99.8 % optical transmission across a 20 nm electrical isolation trench. We focus our experiments on 100 nm trenches and measure an average transmission of 92.9 % over a 100 nm bandwidth, in excellent agreement with theory. We use these concepts to demonstrate a monolithic silicon nanoelectromechanical add-drop switch in which the flow of photons, electrons, and mechanical motions are fully integrated within the same layer. Our work addresses an important challenge in incorporating opto-electro-mechanical topologies into photonic integrated circuits and may lead to new functionalities in nano-opto-electro-mechanical systems, optomechanics, and integrated quantum photonics.

Figures (4)

• Figure 1: Electronic-photonic integration with electronic-photonic circuit crossings.(a) Circuit topology for generic single-layer electronic-photonic integration where electrical wires (purple) must cross photonic waveguides (red). (b) Schematic representation of our electronic-photonic circuit crossing. The photonic waveguide is suspended using silicon bridges, which connect the waveguide to the surrounding bulk, thus bridging current (purple arrows) across the electrical isolation trenches while keeping the optical in- and output electrically isolated. (c) Zoomed view of the topology-optimized EPCC. (d) Normalized transverse electric (TE) field ($E_\text{y}$) distribution across the optimized EPCC at 1550nm. (e) Magnitude and direction of the time-averaged power flow in the middle plane of the structure crossing a 100nm isolation gap at 1550nm. (f) Calculated transmission (red) and reflection (blue) of the optimized EPCC over 100nm wavelength span centered at 1550nm. Numerical calculations were performed for a device with a thickness of 240nm determined by the measured thickness of the device layer of the SOI wafer after fabrication.
• Figure 2: Experimental realization of the electronic-photonic circuit crossing.(a) Dark-field microscope image of a photonic circuit including 6 EPCCs fabricated on an SOI wafer. The red inset at the center shows a scanning electron micrograph (30° tilted) of a single EPCC. The yellow inset shows the center of the EPCC. (b) Measured optical transmission (green) of the optimized EPCC obtained by a cut-back method showing excellent agreement with theory (black). The green shaded area represents the standard deviation from the mean of six nominally identical sets of devices.
• Figure 3: Demonstration of a single-layer $2\times2$ add-drop switching network combining electronic, photonic, and mechanical degrees of freedom.(a) Dark-field microscope image of the fabricated $2\times2$ add-drop switch made of four electrostatically actuated directional couplers. (b) 45°-tilted scanning electron micrograph of one switching element acquired at a low accelerating voltage of 5kV in order to illustrate the charging effect, showing that excellent electrical isolation is obtained between the actuator electrodes via multiple EPCCs and isolation trenches. The inset in (b) shows a zoomed top view scanning electron micrograph of one of the EPCCs employed to route the actuation signal to the electrostatic actuators. (c) Electrical chip layout of the switch network. Electrical pads are placed surrounding the photonic network and the voltages are routed towards four nanoelectromechanical directional couplers through multiple EPCCs. (d) 45°-tilted scanning electron micrograph of the four switching elements taken at a low accelerating voltage of 5kV. The charging effect in the SEM clearly shows how the EPCCs route the voltages from the contact pads to their corresponding nanoelectromechanical directional coupler along mutually isolated electrical domains. (e-h) Measured normalized transmission to through ports (red) and drop ports (blue) at 1550nm over actuation voltages. The incident light is coupled to $I_\text{1}$ in (e) and (f) and to $I_\text{2}$ in (g) and (h). The driving voltage is applied to the actuators at the top-left (e), the bottom-left (f), the top-right (g), and the bottom-right (h).
• Figure 4: Topology-optimized EPCCs with near-unity transmission. The Magnitude and direction of the time-averaged power flow in the middle plane of the structure is shown for designs obtained with single-wavelength topology optimization with gaps of (a) 100nm, (b) 60nm, and (c) 20nm. The power transmission at 1550nm for each design is indicated in the figure and reaches 99.8% for a gap of 20nm.