Highly efficient broadband absorbing surfaces covering the UV, visible and near-IR regions are of great importance for low-light imaging devices, optical devices and optoelectronic devices. In this work, we demonstrate the fabrication of remarkably efficient absorbing surfaces due to the formation of nanoflower-like cavity structures on a stainless steel (SS304) surface, along with micropatterning in a hierarchical fashion. The fabrication process is carried out using noncontact, programmable, single-step laser irradiation by an inexpensive and robust 532 nm nanosecond laser. The measured specular antireflection properties over a wide spectral region (250-1800 nm) are extremely low, less than 0.5%, over a large range of incident angles and for both orthogonal polarizations. These special hierarchical structures with nanorods, nanoparticles, and nanocavities, completely trap the photon incident on these surfaces due to multiple reflections. These surface structures evolve with time to give better nanostructured features with higher oxygen content on the surfaces, revealed by FESEM elemental analysis, which increases the ability to trap photons. We believe these antireflection surfaces, with high efficiencies and longterm stability, will play a vital role in many modern technological applications.

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Antireflective surfaces that are efficient over a broad spectral range with extremely low total reflectivity have abundant potential applications in defense, space, and industries. We successfully fabricated three-dimensional micro-to-nano hierarchical structures on a thin sheet of stainless steel (SS), 100 μm in thickness, by interference-based ultrafast laser patterning for near-perfect absorption. The Sagnac-based interferometer reduces the footprint of the laser on the substrate to smaller than the diffraction limit. In this work, we achieved a groove cut of 13 μm size using a 1030 nm ultrafast laser. We achieved an average total reflectivity of 1.2% over the spectral range of 400–2000 nm and an average specular reflectivity of 0.02% over a broad range of wavelengths of UV–visible–NIR (400–2000 nm) on thin low-weight SS sheets. Further, the average total reflectivity in the spectral region of 400 to 650 nm was 1.1% only. The average groove depth from the top of the surface was 8.5 μm, and most of the material was intact in this process. As these structures are modifications of the same material, they have high mechanical strength and thermal stability and also exhibit self-cleaning ability. The patterned surface is insensitive to incident polarization and has a wide range of acceptance to the incidence cone angle of ±60°. These are the best anti-reflectivity values on an SS surface fabricated via laser surface patterning.

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