Optical refractive index (RI) sensors exploiting selective co-integration of plasmonics with silicon photonics in Lab-on-achip configurations are expected to disrupt Point-of-Care (POC) diagnostics, delivering performance and economic breakthroughs. Propagating surface-plasmon-polariton modes offer superior sensitivity due to their extreme overlap with the surrounding medium. In parallel, low-loss photonics act as the hosting platform with which the plasmonic losses can be sustained while allowing for multiplexed layouts via in-plane SPP excitation schemes. However, merging plasmonics with silicon photonics in a cost-effective manner, requires a truly CMOS-compatible manufacturing process. Herein, we demonstrate experimentally, the highest bulk-sensitivity among all the plasmo-photonic interferometric RI sensors, while taking the leap forward in the development of a CMOS-manufactured plasmo-photonic sensing platform merging Si3N4 photonics and aluminum plasmonics. The proposed structure relies on a butt-coupled interface between Si3N4 waveguides and a 70 μm long plasmonic stripe, deployed in one branch of a Mach-Zehnder Interferometer (MZI) serving as the sensing transducer that detects local changes in the refractive index. The lower MZI arm (reference arm) exploits the low-loss Si3N4 platform to deploy a MZI-based variable optical attenuator followed by a thermo-optic phase shifter to optimize the sensor performance achieving resonance extinction ratio values at the MZI output of more than 35 dB. Experimental evaluation of a gold-based sensor revealed a bulk refractive index sensitivity of 1930 nm/RIU. In addition, we experimentally demonstrate that the proposed plasmo-photonic waveguide platform can migrate from gold (Au) to Aluminum (Al), demonstrating the first step towards a fully CMOS compatible plasmo-photonic interferometric sensor.
Plasmonic sensors, leveraging the profound exposure of propagating Surface-Plasmon-Polariton (SPP) modes over metal stripes to test analytes, became so far the “gold-standard” in plasmonic biosensing resulting in commercial available devices. However, a series of challenges associated with their bulky prism-based coupling configuration as well as their high optical losses need to be overcome in order to allow for miniaturized and multiplexed sensor layouts. In this context, selective co-integration of plasmonics with low-loss silicon-nitride photonics emerges as a promising solution towards addressing these challenges yet reaping the benefits from both technologies. In this work, we present an interferometric sensor based on a Mach-Zehnder device, where a “plasmo-photonic” waveguide branch is utilized to interrogate changes in the refractive index of a test analyte exploiting the accumulated phase change of the SPP mode being exposed in an aqueous solution. More specifically, the “plasmo-photonic” Mach-Zehnder sensor incorporates a gold plasmonic stripe with a length of 70 μm and a width of 7 μm that has been interfaced with Si3N4 waveguides by means of a butt-coupled interface. By conducting numerical simulations and considering the dispersion properties of the involved materials, we optimized the structural parameters of the sensor aiming at ultra-high bulk sensitivity in the order of micrometres per Refractive Index Unit (RIU).
Plasmonics have been identified as an ideal platform for ultra-sensitive, label-free biosensors mainly due to the high field confinement on a metal-dielectric interface and the resulting strong light-matter interaction offered by surface plasmon resonances (SPRs) that can be entirely exposed to test analytes. Well-established SPR-based biosensors exploiting propagating SPRs yield superior specifications regarding bulk sensitivity compared to localized counterparts leading to already commercial available sensor devices. However, most of these systems require bulky prism-based configurations to couple light into the Surface Plasmon Polariton (SPP) mode impeding system miniaturization. In addition, SPR-based sensors suffer from intrinsic high propagation losses restricting the potential for multiple on-chip functionalities. In this context, co-integration of plasmonics with a low-loss photonic platform emerges as a viable solution towards highly sensitive, low-loss and small footprint optical sensors. In this work, we present an ultra-compact, interferometric plasmonic sensor co-integrated on a TiO2 photonic waveguide platform. The device consists of two access TiO2 photonic waveguides separated by a gold-based metal stripe which is located on top of an appropriately shorter TiO2 waveguide layer. Two metal/insulator interfaces are formed at the top (sensing arm) and bottom surfaces (reference arm) of the metal able to support SPP modes which upon excitation through the input photonic waveguide propagate along the two metal surfaces and interfere at the output waveguide realizing a single-arm Mach-Zehnder Interferometer. After optimization of the device in aqueous environment, we achieved sensitivity values as high as 2430 nm/RIU at near-infrared spectrum region for a 65 um long plasmonic stripe.
Plasmonic technology has attracted intense research interest enhancing the functional portfolio of photonic integrated circuits (PICs) by providing Surface-Plasmon-Polariton (SPP) modes with ultra-high confinement at sub-wavelength scale dimensions and as such increased light matter interaction. However, in most cases plasmonic waveguides rely mainly on noble metals and exhibit high optical losses, impeding their employment in CMOS processes and their practical deployment in highly useful PICs. Hence, merging CMOS compatible plasmonic waveguides with low-loss photonics by judiciously interfacing these two waveguide platforms appears as the most promising route towards the rapid and costefficient manufacturing of high-performance plasmo-photonic integrated circuits. In this work, we present butt-coupled plasmo-photonic interfaces between CMOS compatible 7μm-wide Aluminum (Al) and Copper (Cu) metal stripes and 360×800nm Si3N4 waveguides. The interfaces have been designed by means of 3D FDTD and have been optimized for aqueous environment targeting their future employment in biosensing interferometric arrangements, with the photonic waveguides being cladded with 660nm of Low Temperature Oxide (LTO) and the plasmonic stripes being recessed in a cavity formed between the photonic waveguides. The geometrical parameters of the interface will be presented based on detailed simulation results, using experimentally verified plasmonic properties for the employed CMOS metals. Numerical simulations dictated a coupling efficiency of 53% and 68% at 1.55μm wavelength for Al and Cu, respectively, with the plasmonic propagation length Lspp equaling 66μm for Al and 75μm for Cu with water considered as the top cladding. The proposed interface configuration is currently being fabricated for experimental verification.
Bringing photonics and electronics into a common integration platform can unleash unprecedented performance capabilities in data communication and sensing applications. Plasmonics were proposed as the key technology that can merge ultra-fast photonics and low-dimension electronics due to their metallic nature and their unique ability to guide light at sub-wavelength scales. However, inherent high losses of plasmonics in conjunction with the use of CMOS incompatible metals like gold and silver which are broadly utilized in plasmonic applications impede their broad utilization in Photonic Integrated Circuits (PICs). To overcome those limitations and fully exploit the profound benefits of plasmonics, they have to be developed along two technology directives. 1) Selectively co-integrate nanoscale plasmonics with low-loss photonics and 2) replace noble metals with alternative CMOS-compatible counterparts accelerating volume manufacturing of plasmo-photonic ICs. In this context, a hybrid plasmo-photonic structure utilizing the CMOS-compatible metals Aluminum (Al) and Copper (Cu) is proposed to efficiently transfer light between a low-loss Si3N4 photonic waveguide and a hybrid plasmonic slot waveguide. Specifically, a Si3N4 strip waveguide (photonic part) is located below a metallic slot (plasmonic part) forming a hybrid structure. This configuration, if properly designed, can support modes that exhibit quasi even or odd symmetry allowing power exchange between the two parts. According to 3D FDTD simulations, the proposed directional coupling scheme can achieve coupling efficiencies at 1550nm up to 60% and 74% in the case of Al and Cu respectively within a coupling length of just several microns.
Programmable switching nodes supporting Software-Defined Networking (SDN) over optical interconnecting technologies arise as a key enabling technology for future disaggregated Data Center (DC) environments. The SDNenabling roadmap of intra-DC optical solutions is already a reality for rack-to-rack interconnects, with recent research reporting on interesting applications of programmable silicon photonic switching fabrics addressing board-to-board and even on-board applications. In this perspective, simplified information addressing schemes like Bloom filter (BF)-based labels emerge as a highly promising solution for ensuring rapid switch reconfiguration, following quickly the changes enforced in network size, network topology or even in content location. The benefits of BF-based forwarding have been so far successfully demonstrated in the Information-Centric Network (ICN) paradigm, while theoretical studies have also revealed the energy consumption and speed advantages when applied in DCs. In this paper we present for the first time a programmable 4x4 Silicon Photonic switch that supports SDN through the use of BF-labeled router ports. Our scheme significantly simplifies packet forwarding as it negates the need for large forwarding tables, allowing for its remote control through modifications in the assigned BF labels. We demonstrate 1x4 switch operation controlling the Si-Pho switch by a Stratix V FPGA module, which is responsible for processing the packet ID and correlating its destination with the appropriate BF-labeled outgoing port. DAC- and amplifier-less control of the carrier-injection Si-Pho switches is demonstrated, revealing successful switching of 10Gb/s data packets with BF-based forwarding information changes taking place at a time-scale that equals the duration of four consecutive packets.