Customizing plasmonic diffraction patterns by laser interference

Abstract

This work reports a versatile and efficient production of periodic structures of alloy nanoparticles (NPs) with customized diffraction patterns by using the technique of phase mask laser interference. This technique uses interfering single nanosecond laser pulses to induce the periodic dewetting of bilayer (Ag/Au) films on glass produced by pulse laser deposition. Film breaks up into alloy NPs around the regions exposed to intensity maxima and the cold regions placed in the minimum laser intensity are non-transformed. This allows fringes to be produced with a period within the range of 1.7-6.8 μm. Periodic structures of squares, diamonds, rectangles or triangles are produced by accumulating two or three laser pulses with different fringe orientations. As a film parameter, we have analyzed the pattern properties by varying the thickness of the Au layer while keeping that of Ag constant. The diameter of the NPs, their number density, percentage of the transformed region, the interface between transformed and non-transformed region or the minimum period achievable can be tuned by varying the Au concentration. In that way, isolated and big NPs, which are optically characterized by a plasmon resonance, are produced for the thinnest film, whereas a bimodal size distribution of big and small NPs, whose optical transmittance is characterized by IR absorption related with multipolar interactions between the close small NPs, are produced for the highest Au concentration. However, the periodic structure still generates visible diffractive patterns whose diffraction efficiency can increase up to a factor of 4, while their spectral trend dependences can increase or decrease as a function of the Au concentration. These optical behaviors have been explained satisfactorily by taking into account the optical contrast between the regions transformed into NPs and the non-transformed regions. Altogether, this allows the position of the diffraction orders and their relative and absolute spectral efficiency to be customized in a broad range.