This work reports multimode low-threshold lasing in a high-refractive-index dielectric cavity based on a 2D periodic array of cylindrical holes patterned into a layered structure via a soft nanoimprinting lithography method.

High-refractive-index (HRI) dielectrics, virtually free from absorption losses, are gaining increasing attention as building blocks for compact lasers. Compared to plasmonic platforms, HRI nanostructures can simultaneously support electric and magnetic modes, providing a higher degree of tunability for the intensity, wavelength, and quality factor of the resulting resonances. These properties have been harnessed to achieve light emission with controlled directionality and polarization, as well as to enhance spectroscopy, sensing, and nonlinear optical processes, among other applications. In this work, we have reported a scalable soft nanoimprinting lithography method to create a series of HRI photonic architectures to achieve multimode low-threshold lasing. In particular, we have used a two-dimensional periodic square array of cylindrical holes patterned into a thick epoxy SU-8 layer, which we subsequently coat with an 80 nm layer of TiO2 and cover with a Rhodamine B-doped SU-8 layer. We have found that the optimal laser performance occurs when the optical resonances of the array align with the emission wavelength range of the dye. Furthermore, we have observed that the anisotropy in the TiO2 coating breaks the polarization degeneracy of the square arrays, leading to the emergence of new modes and enabling the simultaneous appearance of multiple lasing peaks. Our work demonstrates that, despite the simplicity of their fabrication process, the designed HRI structures exhibit a high degree of complexity that leads to a rich optical response. These results offer an innovative approach to building robust HRI platforms for lasing with improved control over their emission properties. This work, which is the result of a collaboration between Juan R. Deop-Ruano, Luis Cerdán, and Alejandro Manjavacas from the Theoretical Nanophotonics Group at IQF and the group of Agustín Mihi at the Institute of Materials Science of Barcelona (ICMAB-CSIC), has been published in Nanoscale Horizons. https://pubs.rsc.org/en/content/articlelanding/2025/nh/d4nh00574k