[Paper Review] Modular Hybrid Plasmonic Integrated Circuit -- Rotation, Nanofocusing and Nonlinear Enhancement

Alessandro Tuniz, Oliver Bickerton+6 more|Figshare|Aug 30, 2019

Photonic and Optical Devices14 citations

TL;DR

This paper presents a modular hybrid plasmonic integrated circuit that enables efficient coupling from standard silicon-on-insulator (SOI) waveguides to deep-subwavelength plasmonic modes via a post-fabrication process. By integrating a plasmonic rotator and nanofocuser, the circuit achieves >200× intensity enhancement in a 50 nm² mode area at 1320 nm, experimentally demonstrating nonlinear second-harmonic generation, marking the first fully integrated, PIC-compatible, nonlinear plasmonic nanosource.

ABSTRACT

We introduce a modular approach for efficiently interfacing photonic integrated circuits with deep-sub-wavelength hybrid plasmonic functionality. We demonstrate that an off-the-shelf silicon-on-insulator waveguide can be post-processed into an integrated hybrid plasmonic circuit by evaporating a silica and gold nanolayer. The circuit consists of a plasmonic rotator and a nanofocusser module, which together result in nano-scale, nonlinear wavelength conversion. We experimentally characterize each module, and demonstrate an intensity enhancement of $>200$ in a calculated mode area of $50\,{ m nm}^2$ at $λ= 1320\,{ m nm}$ using second harmonic generation. This work opens the door to customized plasmonic functionalities on industry-standard waveguides, bridging conventional integrated photonic circuits with hybrid plasmonic devices. This approach promises convenient access to nanometre-scale quantum information processing, nonlinear plasmonics, and single-molecule sensing.

Motivation & Objective

• To bridge conventional photonic integrated circuits (PICs) with deep-subwavelength plasmonic functionalities using a modular, post-fabrication approach.
• To overcome the challenge of inefficient coupling between standard SOI waveguides and plasmonic nanostructures due to mode profile mismatches and high losses.
• To demonstrate a fully integrated, chip-based system that enables nonlinear wavelength conversion via second-harmonic generation (SHG) at the nanoscale.
• To enable practical, scalable access to nanoscale field enhancement for applications in quantum photonics, single-molecule sensing, and nonlinear optics.

Proposed method

• Post-process standard SOI waveguides by electron-beam lithography to define plasmonic structures, followed by thermal evaporation of a 20 nm silica and 50 nm gold bilayer.
• Implement a plasmonic rotator module to convert transverse electric (TE) photonic modes into transverse magnetic (TM) hybrid-plasmonic modes with high efficiency.
• Integrate a nanofocuser module that compresses the TM plasmonic mode to a 50 nm² mode area, enhancing local intensity.
• Use a grating coupler to couple a free-space Gaussian beam into the SOI waveguide, enabling full chip-level integration with standard PIC components.
• Characterize the system using second-harmonic generation (SHG) to quantify nonlinear enhancement, with detection via an imaging spectrometer and NIR/VIS cameras.
• Measure coupling efficiency (14%) and propagation loss (0.12 dB/μm) to validate performance and optimize design parameters.

Experimental results

Research questions

• RQ1 Can a modular, post-fabrication approach enable efficient, on-chip integration of hybrid plasmonic components with standard SOI photonic integrated circuits?
• RQ2 What is the achievable intensity enhancement and mode confinement in a hybrid plasmonic circuit when combining a mode converter and nanofocuser?
• RQ3 To what extent can nonlinear optical effects like second-harmonic generation be enhanced in a fully integrated, PIC-compatible plasmonic circuit?
• RQ4 How does the performance scale with device geometry, such as gold thickness or taper design, and what improvements are possible?

Key findings

• The plasmonic rotator achieves efficient TE-to-TM mode conversion with a measured coupling efficiency of 14% into the SOI waveguide.
• The hybrid plasmonic nanofocuser compresses the mode to a 50 nm² area at 1320 nm, resulting in an intensity enhancement of >200×.
• Second-harmonic generation (SHG) is experimentally observed, confirming the nonlinear enhancement and validating the circuit’s functionality.
• The measured propagation loss in the SOI waveguide is 0.12 dB/μm, indicating good optical quality prior to plasmonic integration.
• Theoretical analysis suggests that increasing gold thickness or using multi-section tapers could boost the TE-to-TM conversion efficiency by up to 9×, leading to an 80-fold increase in nonlinear efficiency.
• The design is scalable and compatible with existing PIC technology, enabling future integration of additional modules like bowtie antennas or nonlinear materials for enhanced performance.