Multimode Slot Waveguide: Fabrication & Applications

• Multimode slot waveguides are integrated photonic devices featuring a low-index slot that confines light tightly while supporting multiple spatial optical modes.
• They employ advanced fabrication strategies—including ultrafast laser inscription, subwavelength metamaterials, and multilayer modulation—to achieve precise modal control with low loss.
• Applications span high-capacity communications, quantum photonics, and astronomical instrumentation, leveraging enhanced bandwidth, efficient nonlinear processes, and robust simulation techniques.

A multimode slot waveguide is an integrated photonic structure engineered to support and manipulate multiple spatial optical modes within a geometry that incorporates a low-index slot region for tight field confinement. These devices leverage layered and compositional strategies, advanced refractive index engineering, precise boundary control, and optimized layout to fulfill demanding requirements in high-capacity communications, nonlinear photonics, astronomical instrumentation, and quantum information, while navigating fundamental challenges related to mode management, loss, dispersion, and robust simulation.

1. Composite Architectures and Fabrication Strategies

Multimode slot waveguides are realized using a variety of composite approaches, the most prominent of which are stacks of single-mode waveguide tracks fabricated by ultrafast laser inscription (ULI), subwavelength metamaterial engineering, and multilayer boundary modulation (Jovanovic et al., 2012, Halir et al., 2016, Badri et al., 2019, Wu et al., 2018). In ULI, femtosecond lasers induce localized refractive index changes in glass substrates, which are translated precisely by air-bearing stages to achieve lattices of single-mode (SM) tracks that collectively form the multimode (MM) guide. Overlapping SM tracks maximize effective index contrast, enhancing numerical aperture (NA); non-overlapping tracks, placed with strong evanescent coupling but no direct overlap, yield a lower effective index but more uniform mode profiles.

Subwavelength-engineered designs introduce anisotropy into the core region, wherein the effective refractive index tensor $\mathrm{diag}(n_{xx}, n_{zz})$ yields strong direction-dependent modal properties (Halir et al., 2016). Meta-surface approaches utilize shallow non-uniform gratings on silicon, spatially shaping refractive index profiles to enable ultra-sharp bends and mode matching between bent and straight sections (Wu et al., 2018). Eaton lens-style multilayer techniques implement a radial gradient index via concentric cylindrical layers, with annular widths tailored to enforce the effective index profile necessary for mode bending in low-index contrast polymer waveguides (Badri et al., 2019).

These fabrication strategies permit aggressive engineering of modal confinement, interference, and coupling in devices with footprints reduced to the regime of $10-30\,\mu$m bending radii and $3.8 \times 3.8\,\mu$m$^2$ crossings, with low loss (<1 dB) and controlled crosstalk, facilitating dense photonic integration (Wu et al., 2018, Wang et al., 2022).

2. Modal Properties, Refractive Index Control, and Interference Mechanisms

Detailed modal control in multimode slot waveguides is achieved via tuning the refractive index profile, either in step-index, graded-index, or engineered discontinuous forms. The normalized frequency (V-number) $V = \pi d\,\mathrm{NA}/\lambda$ predicts the number of supported modes $N_m \sim V^2$ in the high-V regime (Jovanovic et al., 2012). Overlapping structures enhance the core-cladding index contrast $\Delta n$, thus supporting higher-order modes and larger angular spreads. Non-overlapped structures trade off modal range for uniformity and reduced scattering.

In advanced designs, the waveguide's boundaries are rendered "self-adaptive" using graded index profiles $n(y)$ (e.g. power laws or SWG composites), so that each mode experiences a unique effective width $L_m$ defined by $n(L_m) = n_{\text{eff},m}$ (Zhang et al., 2019). This ensures nearly equispaced modal frequencies and shared propagation constants, automatically meeting energy conservation and phase matching for nonlinear mixing processes. Metamaterial-based and graphene lattice-based devices exhibit anomalous bulk states, with mode profiles and interference cycles (governed by an interference factor $n$ independent of waveguide width), yielding robust, width-invariant MMI patterns (Liu et al., 12 Dec 2024).

Interference and mode conversion are controlled in converters and couplers by constraining spatial perturbation periods to match mode propagation constant differences, employing two-dimensional shallowly etched metastructures with optimized hexagonal taper profiles or quasi-periodic arrangements (Yao et al., 2019). This enables simultaneous conversion across multiple mode pairs, with low insertion loss ($0.4-1.0$ dB) and acceptable crosstalk ($-14$ to $-16.5$ dB) within compact devices ($\sim 16\,\mu$m) (Yao et al., 2019).

3. Layout Engineering, Loss, and Bandwidth Optimization

Mode filtering techniques in multimode slot waveguides underpin bandwidth enhancement and loss management. The use of bends (with radii down to $5\,$mm in polymer systems or sub-$20\,\mu$m in engineered silicon/polymer) and multiple crossings selectively attenuates higher-order modes responsible for modal dispersion, boosting the bandwidth-length product (BLP) to above $40$ GHz$\cdot$m without launch conditioning (Chen et al., 2016). For bends, reducing the radius increases filtering and bandwidth, but at the expense of added loss ($\sim 1.9$ dB for $5\,$mm radius bends). Crossings filter out dispersive modes in the first several repetitions, with diminishing returns and increasing total loss beyond saturation (>10 crossings) (Chen et al., 2016, Chen et al., 2017).

Loss and bandwidth studies under varied launch conditions and waveguide profiles reveal the need for design rules—minimum bend radii, two-slope crossing loss models, and RI profile optimization—to meet stringent power budgets at high data rates ($\geqslant$40 Gb/s) (Chen et al., 2017). The interplay between layout-induced mode filters and optimized waveguide geometries governs modal content, throughput, and dispersion performance across complex photonic interconnects and on-board routers.

4. Nonlinear and Quantum Functionality

Multimode slot waveguides are pivotal in emerging nonlinear and quantum applications. "Self-adaptive boundary" designs provide wideband multi-mode four-wave mixing with bandwidths exceeding $300\,$nm and phase matching among modes separated by $>400\,$nm (Zhang et al., 2019). The graded-index "photon well" approach produces nearly uniform modal spacings, overcoming intrinsic material dispersion limits and facilitating all-optical processing, mid-infrared generation, and Brillouin scattering.

In quantum photonics, coupled waveguide arrays allow manipulation of multimode squeezing and tripartite entangled states. By injecting single-mode squeezed light into an elliptical waveguide array, the noise reduction can be transferred between single-mode or multimode (joint quadrature) states, toggled by input polarization and propagation length (Rojas-Rojas et al., 2019). The theory, summarized by the evolution of squeezing coefficients $Z_a$, $Z_b$, $Z_{ab}$ in dimers and $T_a$, $T_b$, $T_c$, $T_{ij}$ in trimers, predicts the conditions for exclusive multimode squeezing and robustness against losses—a property relevant to quantum metrology applications.

5. Astronomical and Communications Applications

Multimode slot waveguides are integral to astrophotonic instrumentation, particularly in slit-reformatting devices for telescope focal plane coupling (Jovanovic et al., 2012). A composite MM waveguide collects seeing-limited light, redistributes it into SM tracks via a lantern-like transition, and reformats it to a linear slit for feeding a diffraction-limited spectrograph. This architecture enables smaller, more stable spectrographs with reduced FRD and enhanced throughput, matching system focal ratios (often $F/8$) and supporting point spread functions appropriate for telescope aperture and atmospheric seeing.

In optical communications and data-center interconnects, the high modal bandwidth and integration density resulting from advanced slot waveguide and converter design facilitate simultaneous mode- and polarization-division multiplexing, with transmission losses and crosstalk controlled well below application thresholds (Wang et al., 2022, Badri et al., 2019, Halir et al., 2016).

The recent demonstration of width-independent MMI splitters using anomalous bulk states in multilayer graphene lattices points to robust, scalable splitters, filters, couplers, and multiplexers for photonic, phononic, or electronic integrated circuits (Liu et al., 12 Dec 2024). Such devices exhibit stable MMI performance across stepped-width interconnects and are insensitive to geometric perturbations, enabling flexible system architectures.

6. Simulation, Design, and Optimization Techniques

Ultra-fast, accurate simulation frameworks are necessary for the design and optimization of large-scale multimode slot waveguide devices. The dataset-based eigenmode expansion (EME) method precomputes eigenmode properties and overlap matrices over a discretized geometric parameter space (e.g., grid over width and curvature), transforming simulation from repeated eigenproblem-solving to rapid matrix multiplications (Song et al., 16 Apr 2025). This method achieves $10^2$–$10^3\times$ speedup compared to conventional FDTD, with validated accuracies for devices such as $30\,\mu$m radius silicon bends and $20\,\mu$m arbitrary ratio splitters, making large-scale optimization (>$8{,}000$ iterations) feasible on standard CPU hardware. This paradigm is extendable to photonic structures with more parameter dimensions (slot width, sidewall angles, etc.), enabling efficient exploration of complex design spaces.

7. Future Directions and Open Research Areas

Open directions for multimode slot waveguide research include further miniaturization and integration (e.g. via advanced boundary engineering with anomalous bulk state designs), expansion of modal conversion architectures, refinements in layout-induced bandwidth enhancement, and scalable quantum/optical functionality. Experimental validation under fabrication variability, broadening mode support and bandwidth, cascading with other photonic components, and hybrid integration (e.g., SWG-assisted Euler curves) are ongoing challenges (Wang et al., 2022, Liu et al., 12 Dec 2024). Extension to new material platforms (silicon, graphene, III–V, polymers) and across photonic, phononic, and electronic domains will enable more robust, efficient, and versatile integrated circuits for next-generation multiplexing, quantum metrology, nonlinear optics, and astronomical instrumentation.