Surface plasmon resonance (SPR) in metal nanostructures strongly localizes the optical energy in the near field by coupling photons with free electrons, and the interaction between light and matter near the nanostructures can be significantly enhanced for ultrasensitive molecular transduction and enhanced spectroscopies. SPR modes depend highly on the features of metal nanostructures, and the design and fabrication of SPR-tunable nanostructures present a significant challenge to large-scale metasurface devices. Ultraviolet (UV) laser interference lithography has garnered considerable attention because of its advantages such as low cost, high efficiency, no mask, large scale, and high flexibility. This study proposes angle-twisted interference lithography using dual UV laser beams followed by ion sputtering, by which a large-scale Au nanofilm Moire-grating (AuNF Moire-grating) with tunable SPR and fixed localized surface plasmon resonance (LSPR) with respect to orthogonal excitation polarization can be achieved. The AuNF Moire-grating can be easily designed and fabricated to match a specific wavelength for efficient SPR excitation. It demonstrates the use of polarization-independent surface-enhanced Raman spectroscopy (SERS) for ultrasensitive detection.

The fabrication procedure is shown in Fig. 1. The photoresist film is first obtained via dropwise addition of a photoresist to a clean quartz glass substrate, followed by spin curing. Subsequently, angle-twisted interference lithography is performed using dual laser beams on the photoresist with a single-exposure grating period of 300 nm, by which the period of the Moire grating can be modulated from 439 nm to 864 nm by adjusting the twisted rotation angle between the two exposures from 20° to 40°. Subsequently, AuNF is deposited onto the surface of the Moire grating structure via vacuum ion sputtering, by which a narrow-gap AuNF Moire-grating with a tunable SPR wavelength is obtained. The wavelength of the SPR absorption can be regulated from 540 nm to 875 nm. The surface morphology of the AuNF Moire-grating is examined using scanning electron microscope (SEM). The effect of the AuNF Moire-grating period on the tunability of the SPR wavelength as well as the mechanism of depolarization by the cooperation of the Moire-grating SPR with narrow-gap LSPR are revealed experimentally and theoretically. Finally, an AuNF Moire-grating SERS substrate with an SPR wavelength matching that of the excitation laser is fabricated for polarization-independent Raman sensing.

As the twisted rotation angle increases from 20° to 40°, the fabricated Moire-grating period modulates from 439 nm to 864 nm. The experimental results are consistent with the theoretical calculations. The width of the narrow gap remains unchanged, as shown in Fig. 2. The reflectance spectra of the AuNF Moire-gratings with different Moire periods are measured under circularly polarized excitation. Clear SPR absorption peaks are observed in the reflectance spectra. The SPR absorption wavelength is tunable between 540 nm and 875 nm, which is consistent with numerical-simulation results (Fig. 3). When the excitation wavelength matches the SPR of the AuNF Moire-grating, a significant electromagnetic enhancement in the near field can be achieved for s- and p-polarization owing to SPR and LSPR, respectively. The corresponding polarization-independent absorption is shown in Fig. 4. Finally, a AuNF Moire-grating SERS substrate is fabricated, which demonstrates polarization-independent Raman detection with a limit of 10^-10 mol/L, as shown in Figs. 5?7.

This study proposes a novel technique of angle-twisted interference lithography using dual UV laser beams followed by ion sputtering to obtain a narrow-gap AuNF Moire-grating with a tunable SPR wavelength. The Moire-grating period can be modulated from 439 nm to 864 nm by adjusting the twisted rotation angle from 20° to 40°, which regulates the wavelength of the SPR absorption from 540 nm to 875 nm. Experiments and numerical simulations indicate that when the SPR of the AuNF Moire-grating structure is matched with the excitation wavelength, the Moire-grating SPR with narrow-gap LSPR enhances the near-field electromagnetic field regardless of the polarization state, by which polarization-independent strong absorption is achieved. The AuNF Moire-grating can be used as a polarization-independent SERS substrate for trace detection with a limit of 10^-10 mol/L. This study provides opportunities for the design and fabrication of polarization-insensitive SERS substrates with tunable excitation wavelengths for practical applications.