Laser-Induced Periodic Surface Structures on Layered GaSe Crystals: Structural Coloring and Infrared Antireflection.

We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.

Plasmonic Nanolenses Produced by Cylindrical Vector Beam Printing for Sensing Applications.

Interaction of complex-shaped light fields with specially designed plasmonic nanostructures gives rise to various intriguing optical phenomena like nanofocusing of surface waves, enhanced nonlinear optical response and appearance of specific low-loss modes, which can not be excited with ordinary Gaussian-shaped beams. Related complex-shaped nanostructures are commonly fabricated using rather expensive and time-consuming electron- and ion-beam lithography techniques limiting real-life applicability of such an approach. In this respect, plasmonic nanostructures designed to benefit from their excitation with complex-shaped light fields, as well as high-performing techniques allowing inexpensive and flexible fabrication of such structures, are of great demand for various applications. Here, we demonstrate a simple direct maskless laser-based approach for fabrication of back-reflector-coupled plasmonic nanorings arrays. The approach is based on delicate ablation of an upper metal film of a metal-insulator-metal (MIM) sandwich with donut-shaped laser pulses followed by argon ion-beam polishing. After being excited with a radially polarized beam, the MIM configuration of the nanorings permitted to realize efficient nanofocusing of constructively interfering plasmonic waves excited in the gap area between the nanoring and back-reflector mirror. For optimized MIM geometry excited by radially polarized CVB, substantial enhancement of the electromagnetic near-fields at the center of the ring within a single focal spot with the size of 0.37λ2 can be achieved, which is confirmed by Finite Difference Time Domain calculations, as well as by detection of 100-fold enhanced photoluminescent signal from adsorbed organic dye molecules. Simple large-scale and cost-efficient fabrication procedure offering also a freedom in the choice of materials to design MIM structures, along with remarkable optical and plasmonic characteristics of the produced structures make them promising for realization of various nanophotonic and biosensing platforms that utilize cylindrical vector beam as a pump source.