High-Efficiency Nanostructuring using Multi-Beam Interference by Consecutively Deposited Ultrashort Laser Pulses on Tool Steel

Laser direct patterning by multi-beam interference can create structures in the sub-mu m regime on a wide variety of materials. As the structuring process relies on the interference of 2 or more coherent beams, the reachable dimension of a single structure can be comparable to the applied laser wave-length and the structure is replicated simultaneously over the whole interference volume. To obtain a stable interference pattern a classic approach namely spot-by-spot is adopted. In this approach, several laser pulses are deposited at each position on tool steel and there is no pulse overlap between two neighbored pulse positions. Since the surface is patterned spot by spot, a typical average processing rate of the spot-by-spot approach for a submicron periodicity on tool steel is 21 mm(2)/min. Aiming to structuring a large scale surface on tool steel with high efficiency and homogenous morphology, we present a patterning strategy, in which laser pulse are deposited with a constant consecutive pulse overlap. In this paper, two patterning methods and periodic nanostructures fabricated by both methods are investigated. As a demonstrator, a 140x140 mm(2) area with structural periodicity of 690 nm is fabricated on tool steel by using consecutively overlapped deposition of interfering ultrashort laser pulses. A processing rate of 360 mm(2)/min can be achieved with a single spot size of 120 mu m and laser fluence of 0.42 J/cm(2). The productivity of continuous structuring can be enhanced by two orders of magnitude comparing with the classic spot-to-spot approach. In addition, we characterized the local and long range morphological homogeneity of the structured surface by means of quantitative analysis of periodicity and depth of patterned nanostructures.