Nanoimprinted polymer templates have emerged as a powerful tool for the mass production of plasmonic nanostructures, enabling high-throughput fabrication of devices with applications ranging from biosensing to optical coatings. The process typically involves the use of thermoplastic or UV-curable polymers such as polymethyl methacrylate (PMMA) or polydimethylsiloxane (PDMS) as imprinting materials, which replicate nanostructured patterns from a master mold. The resulting templates serve as scaffolds for metal deposition, ultimately yielding plasmonic nanostructures like antenna arrays or nanohole patterns with precise control over geometry and optical properties.

The fabrication process begins with the creation of a master mold, often produced via electron-beam lithography (EBL). EBL offers nanometer-scale resolution, making it ideal for defining intricate plasmonic nanostructures. A resist-coated substrate, typically silicon or glass, is exposed to a focused electron beam, which patterns the resist with sub-100 nm features. After development, the patterned resist serves as a mask for etching the underlying substrate, transferring the design into a durable master mold. The mold’s surface may also be treated with anti-adhesion layers, such as fluorinated silanes, to facilitate demolding during subsequent imprinting steps.

Once the master mold is prepared, nanoimprinting proceeds via thermal or UV-assisted methods. In thermal nanoimprinting, a thermoplastic polymer like PMMA is heated above its glass transition temperature, allowing it to become pliable. The mold is then pressed into the softened polymer under controlled pressure, replicating the nanostructures. After cooling, the mold is separated, leaving an inverse pattern in the polymer film. UV nanoimprinting follows a similar approach but employs a photocurable resin, such as a PDMS-based formulation, which is crosslinked under UV exposure while in contact with the mold. This method is advantageous for its rapid curing and compatibility with flexible substrates.

Following imprinting, the polymer template is used to fabricate plasmonic nanostructures through metal deposition and lift-off processes. A thin metal film, typically gold or silver due to their strong plasmonic responses in visible and near-infrared wavelengths, is deposited onto the patterned polymer via physical vapor deposition techniques such as electron-beam evaporation or sputtering. The metal coats both the raised and recessed features of the template. To isolate the desired plasmonic structures, a lift-off step is performed by dissolving the polymer template in an appropriate solvent, leaving behind only the metal deposited on the recessed regions. This results in well-defined nanostructures such as nanodisc arrays, bowtie antennas, or nanohole arrays, depending on the original mold design.

The resolution of nanoimprinted plasmonic structures is influenced by several factors, including the master mold’s feature size, polymer flow characteristics during imprinting, and metal deposition uniformity. State-of-the-art nanoimprinting can achieve sub-20 nm resolutions, though practical limitations often arise from polymer shrinkage, mold wear, and edge roughness. Despite these challenges, the technique offers significant throughput advantages over direct-write methods like EBL, enabling large-area patterning in a single step. A single master mold can be reused for hundreds of imprints, making the process cost-effective for industrial-scale production.

Plasmonic nanostructures fabricated via nanoimprinting find extensive use in biosensing applications. Metallic nanohole arrays, for example, exhibit extraordinary optical transmission (EOT), where specific wavelengths of light are strongly transmitted due to surface plasmon resonance. This phenomenon is highly sensitive to refractive index changes in the surrounding medium, allowing for label-free detection of biomolecules. Functionalizing the metal surface with receptors enables selective binding of target analytes, which shifts the resonant wavelength and provides quantitative sensing. Similarly, nanoantenna arrays enhance local electromagnetic fields, improving sensitivity in techniques like surface-enhanced Raman spectroscopy (SERS).

Optical coatings represent another key application, where nanoimprinted plasmonic structures manipulate light at subwavelength scales. Metasurfaces composed of precisely arranged nanostructures can control phase, amplitude, and polarization of light, enabling ultrathin lenses, beam splitters, and anti-reflective coatings. The ability to mass-produce such coatings with nanoimprinting makes them viable for integration into consumer electronics, photovoltaics, and display technologies.

While nanoimprinting offers substantial benefits in scalability and cost, challenges remain in achieving defect-free patterning over large areas and maintaining resolution uniformity across imprints. Advances in mold materials, such as the use of diamond-like carbon or silicon carbide, have improved durability and reduced feature degradation over multiple imprint cycles. Additionally, hybrid approaches combining nanoimprinting with other lithographic techniques help address resolution limits for more complex multi-scale patterns.

In summary, nanoimprinted polymer templates provide a versatile platform for the high-volume fabrication of plasmonic nanostructures. From master mold fabrication via EBL to thermal or UV imprinting and subsequent metal deposition, the process enables precise control over nanostructure geometry and optical properties. Applications in biosensing and optical coatings highlight the technology’s potential, while ongoing improvements in resolution and throughput continue to expand its utility in both research and industrial settings. The balance between scalability and nanoscale precision positions nanoimprinting as a critical tool in the advancement of plasmonics and nanophotonics.