Accessible, All-Polymer Metasurfaces: Low Effort, High Quality Factor

Abstract: Optical metasurfaces supporting resonances with high quality factors offer an outstanding platform for applications such as non-linear optics, light guiding, lasing, sensing, light-matter coupling, and quantum optics. However, their experimental realization typically demands elaborate multi-step procedures such as metal or dielectric deposition, lift-off, and reactive ion etching. As a consequence, accessibility, large-scale production and sustainability are constrained by reliance on cost-, time- and labor-intensive facilities. We overcome this fabrication hurdle by repurposing polymethyl methacrylate-which is usually employed as a temporary resist-as the resonator material, thereby eliminating all steps except for spin-coating, exposure and development. Because the low refractive index of the polymer limits effective mode formation, we present a bilayer recipe that enables the convenient fabrication of a freestanding membrane to maximize the index contrast with its surroundings. Since etching induced defects are circumvented, the membrane features high quality nanopatterns. We further examine the suspended membrane with scanning electron microscopy and extract its position-dependent spring constant and pretension with nanoindentation experiments applied by the tip of an atomic force microscope. Our all-polymer metasurface hosting Bound States in the Continuum experimentally delivers high quality factors (up to 523) at visible and near infrared wavelengths, despite the low refractive index of the polymer, and enables straightforward geometry-based tuning of both linewidth and resonance position. We envision this methodology to lay the groundwork for accessible, high performance metasurfaces with unique use cases such as material blending, angled writing and mechanically based resonance tuning.

• The paper demonstrates a simplified fabrication method using PMMA to achieve qBICs with Q factors up to 523.
• It employs bilayer electron beam lithography to create a freestanding metasurface with tunable optical resonances across visible and near-IR spectra.
• The platform shows robust mechanical stability and environmental benefits by eliminating multi-step, hazardous nanofabrication processes.

All-Polymer Metasurfaces: Simplified Fabrication and High-Q Resonances

Introduction

This work presents a significant advance in the fabrication and performance of optical metasurfaces by demonstrating an all-polymer, freestanding metasurface platform supporting quasi-bound states in the continuum (qBICs) with high quality factors (Q). The approach leverages polymethyl methacrylate (PMMA) as both the structural and functional material, eliminating the need for conventional multi-step nanofabrication processes involving deposition, etching, and hazardous chemicals. The resulting metasurfaces exhibit Q factors up to 523 in the visible and near-infrared, with robust mechanical and optical properties, and offer straightforward geometric tunability.

Background and Motivation

Metasurfaces, composed of subwavelength nanostructures, enable precise control over light’s phase, amplitude, polarization, and dispersion. High-Q resonances, particularly those arising from BICs and their accessible qBIC counterparts, are essential for applications in nonlinear optics, sensing, lasing, and quantum photonics. Traditionally, high-Q metasurfaces are realized in high-index dielectrics (e.g., Si, Ge), but these materials are challenging to process, especially at visible wavelengths, and their fabrication is resource-intensive and environmentally taxing.

Polymers, especially PMMA, are widely used as resists in lithography but are rarely employed as the final photonic material due to their low refractive index (RI ≈ 1.5), which limits mode confinement and Q. Previous attempts to use polymers have required metallic substrates (introducing ohmic losses and restricting operation to reflection mode) or complex fabrication steps. The need for a scalable, low-cost, and environmentally benign method for fabricating high-Q, all-polymer metasurfaces remains unmet.

Design and Numerical Analysis

The metasurface design consists of a periodic array of holes in a freestanding PMMA membrane. The unit cell is engineered to support both electric and magnetic BICs under normal incidence. To render the BICs accessible in the far field, Brillouin zone folding (BZF) is introduced by perturbing the radii of adjacent holes, parameterized by $a$. This perturbation breaks the symmetry, opening radiative leakage channels and converting the true BIC into a qBIC with a finite, yet high, Q.

Key simulation results include:

• Field Enhancement: Electric field enhancement up to $|E/E_0| \sim 18$ in the voids, indicating strong near-field localization suitable for enhanced light-matter interaction.
• Mode Topology: The electric qBICs exhibit integer winding numbers in the far-field polarization, confirming their topological origin.
• Angle and Polarization Dependence: Mode 1 displays pronounced angle-stability along $k_x=0$ and $k_y=0$, while mode 2 is more sensitive, enabling polarization-selective robustness or sensitivity.
• Substrate Effects: Simulations show that even low-index substrates introduce Rayleigh-Wood anomalies that suppress the qBIC resonance due to the low index contrast. This necessitates a freestanding membrane for high-Q operation in low-index polymers.

Fabrication Methodology

A bilayer resist process is developed to realize the freestanding PMMA metasurface:

1. Substrate Preparation: Fused silica substrates are coated with an adhesion promoter.
2. Sacrificial Layer: A thick ($\sim$1250 nm) CSAR resist layer is spin-coated and baked.
3. PMMA Layer: A 300 nm PMMA film is spin-coated atop the CSAR.
4. Patterning: Electron beam lithography defines the nanohole array in the PMMA, exploiting the differential sensitivity of PMMA and CSAR.
5. Development: The PMMA is developed, and the overexposed CSAR is selectively removed, releasing the patterned PMMA as a freestanding membrane.

This process eliminates deposition, etching, and lift-off, reducing fabrication complexity, cost, and environmental impact. The approach is robust, as the final structure’s fidelity is determined primarily by the lithography step.

Mechanical and Structural Characterization

• SEM Imaging: The fabricated membranes (30 μm diameter, 300 nm thick) exhibit high pattern fidelity and minimal defects. Circular geometries are favored to avoid stress concentrations and membrane rupture.
• AFM Nanoindentation: Mechanical testing reveals position-dependent spring constants (20–120 N/m) and high pretension, attributed to process-induced stress. The membranes withstand loads of at least 0.6 μN without rupture, indicating mechanical robustness suitable for device integration and sensing applications.

Optical Characterization and Performance

• Transmittance Spectra: Experimental measurements confirm the emergence of qBIC resonances upon introducing radius perturbations ($a > 0$), in agreement with simulations.
• Quality Factor: The highest measured Q is 523, extracted via Fano resonance fitting using temporal coupled mode theory. This value is notable for a low-index, all-polymer system.
• Geometric Tunability: Scaling the unit cell laterally shifts the resonance from 551 nm (visible) to 838 nm (near-IR), demonstrating broad spectral tunability via simple geometric modification.
• Linewidth Control: The linewidth and resonance position are both tunable by adjusting the perturbation parameter $a$ and the unit cell dimensions.

Implications and Future Directions

The demonstrated platform offers several advantages:

• Scalability and Accessibility: The process is compatible with high-throughput photolithography, enabling large-area, low-cost production.
• Environmental and Safety Benefits: The elimination of etching and hazardous chemicals reduces ecological footprint and safety risks.
• Integration and Functionality: The freestanding, all-polymer nature facilitates integration with flexible substrates, cascaded metasurfaces, and device-in-device architectures.
• Mechanical Tunability: The membrane’s mechanical compliance opens avenues for dynamic, strain-based tuning of optical properties, potentially in conjunction with 2D materials for enhanced photoluminescence or hybrid photonic systems.
• Material Blending and Chiral Photonics: The method is amenable to blending with functional materials (e.g., emitters, nanoparticles) and to angled writing for chiral or polarization-sensitive devices.

Strong numerical results include the achievement of Q factors exceeding 500 in a low-index, all-polymer system, and the demonstration of robust mechanical and optical performance with a minimal fabrication process. The work challenges the prevailing assumption that high-Q metasurfaces require high-index, hard-to-process materials and complex fabrication.

Conclusion

This study establishes a practical route to high-Q, all-polymer metasurfaces via a simplified, bilayer lithographic process. The resulting freestanding PMMA membranes support geometrically tunable qBICs with Q factors up to 523 in the visible and near-IR, and exhibit mechanical robustness suitable for sensing and integration. The approach significantly lowers the barrier to entry for high-performance metasurface fabrication and paves the way for new applications in flexible, scalable, and multifunctional photonic devices. Future work may focus on further increasing Q, integrating active materials, and exploiting mechanical tunability for reconfigurable photonics.