Integrated nonlinear optical devices are one of the fundamental building blocks of any modern optical integrated circuits, especially those operating in the quantum optical regime.1, 2 However, dielectric photonics platforms are limited by diffraction, hence they are not compact enough to make these devices resource and energy efficient. In order to operate beyond the diffraction limit, these devices could implement plasmonic waveguide-based architectures. In this presentation we show how novel and superior performing plasmonic waveguide configurations could be the next step toward compact and energy efficient nonlinear optical devices. Furthermore, we show how these plasmonic waveguides can be fabricated and integrated with modern integrated dielectric waveguides.

Interest and excitement in nanophotonics—the study and control of light-matter interactions at the nanoscale—are driven by the ability to confine light to volumes well below a cubic wavelength, and, thereby, achieve extremely high intensities. This leads to light-matter interactions of unprecedented localization and strength. Such extreme behavior—both in terms of field enhancement and localization—can be achieved using plasmonic nanostructures, which concentrate light in regions much smaller than the wavelength of light, reducing the excitation power and, under certain conditions, removing phase-matching requirements in the nonlinear regime.

Here, we perform a global comparison of plasmonic waveguide configurations to identify the most performing configurations in terms of nonlinear optical signal generation.3-5 We show experimental demonstration of hybrid plasmonic waveguides and how to efficiently couple light from industry standard integrated photonic waveguides into these plasmonic structures.6-8 Furthermore, we theoretically demonstrate that metal–dielectric–metal (MDM) slot waveguides, consisting of a thin dielectric layer embedded between metal films, provide the strongest confinement and that integrating epsilon-near-zero (ENZ) materials into the slot significantly improves the nonlinear conversion efficiency of these structures9. The results show that the degenerate four-wave mixing (DFWM) conversion efficiency of these ENZ-MDM structures surpasses that of regular plasmonic structures and their dielectric counterparts, even under low pump power conditions, and remain robust despite higher losses in the ENZ material (Figure 1a).

Creating such extremely compact devices requires advanced nanofabrication methods capable of overcoming the limitations of conventional approaches; in response to this, we have developed a novel lithographic process flow based on Carbon Electron Beam Induced Deposition (C-EBID)10. Using a carbon hard mask prepared via C-EBID (Figure 1b), we overcome focused ion beam resolution limits and fabricate close-toideal slot waveguides with high aspect ratios. The effectiveness of this approach has been demonstrated by successfully preparing MDM slot waveguides with 10 nm gaps and straight side walls (Figure 1c), showing a significant advancement in the preparation of optically active nanostructures. Overall, these results underscore the potential for applications, for example, in quantum optics, where extreme compactness and high efficiency in harnessing pure and strong nonlinearities become crucial.

Figure 1: (a) Comparison of DFWM conversion efficiency of M–ENZ–M (using two ENZ materials: ITO and AZO), MDM, and Si-Slot waveguides for two different spacer thicknesses (ts) when each structure is driven at the maximum threshold power of MDM1 (Pth =0.32) for fairness. (b) Schematic illustrating important features of nanofabrication workflow. (c) SEM image of Au dimer structure with ∼10 nm gap.